Airfield Pavement Design - Hercules Environmentalherculesenvironmental.net/ResourseFolder/30Airfield_Pavement_Design.pdf · 12-4 Airfield Pavement Design. runway and parking apron

5-430-00-2/AFJPAM 32-8013, Vol II

AIRFIELD PAVEMENT DESIGN CHAPTER

This chapter provides information to help select design, and con-struct airfield structures and landing facilites. Close battle support,and rear area TO air-fields are designed according to specific aircraftcharacteristics and requirements governing thickness, strength, andquality of materials. Airfield location and soil strength determinethe different minimum pavement thicknesses and design procedures.The proper placement of the base, subbase, and subgrade determinethe effectiveness of the airfield under all climatic and seasonalconditions.

AIRFIELD STRUCTURE TYPE

Airfield structures fall into three categories:expedient-surfaced, aggregate-surfaced, andflexible-pavement. Expedient-surfaced andaggregate-surfaced airfields are used primar-ily in the close battle and support areas.Flexible-pavement airfields are primarilyconstructed in the rear area.

PRELIMINARY INFORMATION

Field condition, soil strength, and soil behaviorare the three most important pieces of informa-tion used to determine the feasibility of con-structing an airfield at a particular location.

Field Condition

Knowledge of the current field condition isimportant when designing and constructingan airfield. A proper description of the fieldcondition at a proposed construction site in-cludes the following elements:

Ground cover (vegetation).

Natural slopes.

Soil density.

Moisture content.

Soil consistency (soft or hard).

Existing drainage.

Natural soil strength (in terms of Califor-nia Bearing Ratio (CBR)).

Information about the kind and distributionof ground cover, slopes, moisture content,and natural strength is used to estimate theconstruction effort required for a specific typeof airfield. The surface condition and thesoil type must be known to predict potentialdust problems at the site. Moisture contentdata is required to determine the effect oftraffic on soil strength and to estimate waterneeds during construction. Soil strengthdata is needed to determine the surfacing re-quirements as well as the thickness design.

Soil Strength

From an engineering viewpoint, shearing re-sistance (or shear strength) is one of themost important properties that a soil pos-sesses. A soil’s shearing resistance undergiven conditions is related to its ability towithstand a load. The shearing resistanceis especially important in its relation to thesupporting strength or bearing capacity of asoil used as a base or subgrade beneath a

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road, runway, or other structure. For mostmilitary pavement applications, the CBRvalue of a soil is used as an empirical meas-ure of shear strength. The CBR is deter-mined by a standardized penetration sheartest and is used with empirical curves fordesigning and evaluating unsurfaced aggre-gate-surfaced, and flexible pave ments formilitary airfields. The CBR test is usuallyperformed on laboratory compacted testspecimens when used in pavement design.When used in pavement evaluations, de-structive test pits are usually dug to deter-mine pavement layer thicknesses, and in-place field CBR tests are conducted on thebase course, subbase, and subgrade materi-als. In-place CBR tests are time-consumingto run and are usually impractical for usein the TO.

For expedient-surfaced airfields in the closebattle and support areas, the laboratoryCBR test (which usually takes about fourdays to complete) is inappropriate due totime and equipment constraints. Therefore,several field-expedient methods of determin-ing CBR are available in the TO.

The Unified Soil Classification System(USCS) correlation is the quickest meansavailable for estimating CBR. For each soilclassification, empirical studies have deter-mined a range of CBR values. Theseranges can be found in FM 5-410 (Table 5-3, page 5-11). Since the CBR ranges areonly estimates, use the lowest CBR value inthe range. The soil type usually variesacross the entire airfield.

A better method for determining the CBRfor in-place soils is with a penertrometer.There are currently three types of pene-trometers available for airfields: the airfieldcone penetrometer, the trafficability pene-trometer, and the dual-mass dynamic conepenetrometer (DCP).

The airfield cone penetrometer described inAppendix I is used to determine an index ofsoil strengths (Fenwick 1965) for variousmilitary load applications. The airfield pene-trometer consists of a 30-degree cone witha 0.2-square-inch base area. The force re-

quired to penetrate to various depths in thesoil is measured by a spring, and the air-field index (AI) is read directly from thepenetrometer. The airfield cone penetrome-ter has a range of 0 to 15 (CBR value of 0to approximately 18). (The AI-CBR correla-tion is shown in Figure 12-1.) The airfieldcone penetrometer is compact and sturdy.Its operation is simple enough that inexperi-enced military personnel can use it to deter-mine soil strength. A major drawback tothe airfield cone penetrometer is that it willnot penetrate many crusts, thin basecourse, or gravel layers that may lie oversoft layers. Relying only on the surface AItest results could cause the loss of vehiclesor aircraft.

The airfield cone penetrometer must not beconfused with the trafficability penetrome-ter, a standard military item in the soil testset. The trafficability penetrometer has adial-type load indicator (0 to 300 range)and is equipped with two cones: one is 1/2inch in diameter with a cross-sectional areaof 0.2 square inch, and the other is 0.8

Figure 12-1. Correlation of CBR and Al

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inch in diameter with a cross-sectional areaof 0.5 square inch. It also will not pene-trate gravelly soils or aggregate layers, butit may be useful for subgrades. If the traffi-cabiIity penetrometer is used to measure AI,the readings obtained with the 0.2-square-inch cone must be divided by 20; the read-ing with the 0,5-square-inch cone must bedivided by 50. Use the same testing proce-dures as discussed in Appendix I for the air-field cone penetrometer.

The dual-mass DCP described in AppendixJ will overcome some of the shortfalls asso-ciated with traffic ability and airfield conepenetrometers. The DCP was originally de-signed and used for determining thestrength profile of flexible-pavement struc-tures. It will penetrate soil layers havingCBR strengths in excess of 100 and willalso measure soil strengths less than 1CBR. The DCP is a powerful, relatively com-pact, sturdy device that can be used by in-

experienced military personnel to determinesoil strength. The DCP relation to CBR isshown in Figure 12-2. Presently, the DCPis not in the Army inventory. It was re-cently modified and studied by the UnitedStates Army Engineer (USAE) Waterways Ex-periment Station (WES). Information onprocurement and use of the DCP should bedirected to USAE WES, Pavement SystemsDivision, Geotechnical Laboratory, 3909Halls Ferry Road, Vicksburg, MS 39180-6199.

Soil Behavior

Soil is compacted to improve its load-carry-ing capacity and to prevent differential set-tlement (rutting) under aircraft traffic loads.High soil strength is usually associatedwith a high degree of compaction. However,attaining and maintaining a desiredstrength in soils is contingent upon thewater content at the time of constructionand throughout the period of use. Some

Figure 12-2. Correlation plot of CBR versus DCP index


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basic moisture content-density relations forcohesive and cohesionless soils are dis-cussed in FM 5-410. Generally, it is desir-able for the soil to be compacted to Ameri-can Society of Test Materials (ASTM) 1557or to compactive effort, 55 blows per layer(CE 55), while it is within the desired mois-ture content range. The 4 percent moisturerange and the 5 percent density range arederived from initial soil tests, and theymake up the specification block. Soils aretreated to improve their strength or to re-duce the effects of plasticity and high liquidlimits. Stabilizing a soil can also providedust control and waterproofing. During con-struction, the type of soil treatment is deter-mined by the soil characteristics and avail-ability of stabilizing materials. (See FM 5-410 for additional information.)

DESIGN CONSIDERATIONS

The design of airfield structures is basedon—

Airfield location/mission.

Using aircraft and associated grossweight.

Strength of subgrade and available con-struction materials.

Susceptibility of geographic area andconstruction materials to frost action.

Traffic areas.

Expected number of passes of aircraft.

Airfield Location/Mission

The location of the airfield within the TO isbroken down into three major areas, as de-scribed in Chapters 10 and 11:

Close battle area.

Support area.

Rear area.

These areas areof the airfield.

Design AircraftWeight

designated by the mission

and Associated Gross

In TO airfield design, the design aircraft isbased solely on the airfield location, asshown in Table 12-1. The gross weight isthe maximum allowable weight during take-off (worst case) and is the basis for thethickness design. Of the aircraft listed inTables 11-1 and 11-2, pages 11-2 and 11-3,that can possibly use the airfield, the de-sign aircraft is the one that presets the mostextreme load distribution characteristics.

Expected Number of Passes

For a runway, passes are determined by thenumber of aircraft movements across animaginary traverse line placed within 500feet of the end of the runway. More simply,a pass on a runway is equivalent to a take-off and landing of an aircraft similar inweight to the design aircraft. For taxiwaysand aprons, passes are determined by thenumber of aircraft cycles across a line onthe primary taxiway that connects the

Table 12-1. Design aircraft


runway and parking apron. At single run-way airfields, the pass level of the runway,taxiway, and apron should be the same.

For expedient-surfaced airfields, the in-place soil strength determines the numberof passes. If the mission requires a longerservice life, the designer must adjust the de-sign so measures are taken to improve thein-place soil. When designing aggregateand flexible-pavement surfaces, there is a di-rect correlation between the number ofpasses and the thickness of the design.

Traffic Areas

On expedient-surfaced airfields in the closebattle and support areas, traffic areas are

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Type A. The airfield is capable of support-ing missions as soon as the runway is con-structed. The layout of the runway andhammerhead turnaround areas are shownin Figure 12-3. Specific dimensions for theentire runway are shown in Chapter 11.The 63-foot turnaround area is required forthe design aircraft (C-130). When C-17sare anticipated, the turnaround area doesnot increase the airfield width, which is 90feet. Ensure that an extra 90-foot sectionis added to both ends of the runway be-cause turnaround procedures can be detri-mental to an airfield surface. Taxiways andaprons should then be continually devel-oped to support continuous traffic.

Figure 12-3. Typical layout for expedient-surfaced airfield in close battle and support areas


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On aggregate-surfaced airfields in the sup-port area, traffic areas are designated asshown in Figure 12-4. Type A areas in-clude primary taxiways, parking aprons,washrack areas, power check pads, and1,000 feet on both ends of the runway.The interior portion of the runway and theladder taxiway are considered Type C areas.Since the lift on the wings accounts forsome of the aircraft load, Type C areas aredesigned for only 75 percent of the totalload.

On pavement airfields in the rear area, pave-ments can be grouped into four traffic ar-eas designated as Types A, B, C, and D.They are defined below and shown in Fig-ure 12-5.

Type A traffic areas include all primary taxi-ways, including straight sections, turns,and intersections. The ends (1,000 feet) arealso considered Type A since the aircraftload is still fully transferred to the pave-ment. Although traffic tends to channelize

Figure 12-4. Typical layout of aggregate-surfaced airfields in the support area


in the center lane on long, straight taxiwaysections, it is not practical in the TO to con-struct pavement sections of varying thick-nesses. Type B areas include all apronsand hardstands. Type C areas include thecenter, 75-foot width of runway interior be-tween the 1,000-foot runway ends and atthe runway edges adjacent to intersectionswith ladder taxiways. Washrack pavementsare also included in Type C areas. Type Dareas include those areas where traffic vol-ume is extremely low, and/or the applied

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weight of the operating aircraft is muchlower than the design weight. Type D areasinclude the edges of the entire runway ex-cept for the approach and exit areas at taxi-way intersections.

In designing flexible-pavement structures,the area type determines the actual load onthe pavement. Type A and B areas supportthe entire design weight, while Types C andD should be designed for only 75 percent ofthe design weight.

Figure 12-5. Typical layout of flexible-pavement airfields in the rear area


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Soil Strength

The strength of construction materials canbe determined in terms of CBR by usingthe laboratory CBR test, airfield cone pene-trometer, trafficability penetrometer, orDCP, as discussed earlier in this chapter.The strength of both the subgrade and avail-able construction materials can be deter-mined in terms of CBR based on proce-dures outlined in Chapter 5, FM 5-430-00-1/AFPAM 32-8013, Vol 1. Strength of thein-place soil or subgrade will determine thetype of surface and the number of passesfor expedient-surfaced airfields. It also willdetermine the total thickness design in ag-gregate and flexible-pavement surfaces.

Frost Action

In regions subject to frost action, the de-sign of aggregate-surfaced and flexible-pave-

ment airfields must give consideration tomeasures that will prevent serious damagefrom frost action. Three conditions must ex-ist simultaneously for detrimental frost ac-tion to occur: (1) soil must be frost-suscep-tible, (2) temperature must remain belowfreezing for a considerable period time, and(3) ample supply of groundwater must beavailable. Precise methods for estimatingthe depth of freeze and thaw in soils arecontained in AFR 88-19, Vol 1. In addition,Chapter 4, Air Force Manual (AFM) 88-6(TM 5-818-2), contains the criteria and pro-cedures for design and construction of pave-ments subject to seasonal frost action.

Specific design procedures for frost are dis-cussed in detail in aggregate-surfaced andflexible-pavement airfield design sections,pages 12-22 and 12-35, respectively.

EXPEDIENT-SURFACED AIRFIELDS

Unsurfaced deserts, dry lake beds, and flatvalley floors serve as possible airfield sites.Normally, expedient-surfaced airfields areused for very short periods of time (zero tosix months) and support C-130s, C-17s,and Army aircraft operations. Although ex-pedient-surfaced airfields require very littleinitial construction, they may require exten-sive daily maintenance.

Expedient-surfaced airfields are primarilyused for the movement of troops and sup-plies in the close battle and support areas.Only those Army and Air Force aircraft con-figured for expedient surfaces will be al-lowed to use the airfields. The C-130 hasbeen the primary aircraft for missions inthe close battle area because it can land onunpaved or semiprepared surfaces. The C-17, which is used primarily for strategic mo-bility, can also land on austere airfields.Therefore, expedient-surfaced airfields aredesigned for the C-130 or the C-17.

Since the close battle area is expected tochange quickly, minimal resources shouldbe committed to airfields in this area. Al-though the design life of expedient surfaces

ranges from zero to six months (initial con-struction), the airfield is usually only re-quired from zero to two weeks, unless it isupgraded to a support area. If a soil willsupport an unsurfaced airfield for the de-sign aircraft, do not surface the airfieldwith matting unless the service life becomessignificant. Use the following design proce-dure to determine the expedient surfacetype and its expected service life:

DESIGN STEPS

1. Determine the airfield location.

2. Determine the design aircraft and asso-ciated gross weight.

3. Determine the in-place soil strength.

4. Determine the required number ofpasses (service life).

5. Determine the allowable number ofpasses and surface type.

6. Outline corrective actions to increaseservice life as necessary.


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STEP 1. DETERMINE THE AIRFIELD cise determination of gradients should be

LOCATION surveyed by Army engineering teams usingtheodolites, auto levels, and Philadelphia

The general area (close battle, support area) rods.will be given in the mission statement. Inthis case, expedient-surfaced airfields only After a potential airfield site has been se-

occur in the close battle and support areas. lected, it must be tested to ensure its suit-

Determining the best location should be ability. The reconnaissance leader mustbased on a thorough reconnaissance of the first determine the alignment of the airfieldarea, if possible. and the location of the runway, hammer-

head turnaround, taxiway, and parkingSite Reconnaissance apron (if any). Airfield approach zones also

Potential LZ areas fall into three basic cate- must be evaluated for satisfactory glide an-

gories: gles. (See Chapter 2, FM 5-430-00-1/AFPAM 32-8013, Vol 1.) The criteria may

Existing. Roads, highways, and other dictate the airfield alignment even before -

paved surfaces that can be used for soil testing begins.

cargo aircraft (beyond the scope of thischapter). STEP 2. DETERMINE THE DESIGNUnsurfaced. Natural areas such as de- AIRCRAFTserts, dry lake beds, and flat valleyfloors that may or may not include amembrane (geosynthetic covering thatdoes not contribute strength).

Surfaced. Unsurfaced airfields requir-ing a matting or membrane surface be-cause of the in-place soil or to increasethe service life of the airfield.

USAF combat control teams (CCTs) aretrained to perform airfield surveys in sup-port of C-130 and C-17 aircraft operations.CCTs gather all available data on the air-field and perform site visits to evaluate ap-proach-zone obstruction clearances andweight bearing. CCTs are equipped withhand-held pocket transits, clinometers, andlevels to check approach-zone clearance.Airfield and DCPs are used to check weightbearing of unsurfaced LZs. CCTs are notqualified to evaluate deteriorating existingpavements for traffic cycles and weight bear-ing.

Design aircraft, as discussed previously, aremerely a function of the area, which is de-termined by the mission. The design air-craft for expedient surfaces is the C-130 orC-17. When a C-17 is expected, it will be-come the design aircraft. The C-17 has agreater load capacity in the close battle andsupport areas. The gross weights for bothaircraft are shown in Table 12-1, page 12-4.

STEP 3. DETERMINE THE IN-PLACESOIL STRENGTH

This design step is significant in determin-ing the thickness design and the servicelife. Therefore, it is important that accu-rate readings are taken from one of the ex-pedient CBR methods. Use these proce-dures for determining soil strength for auniform soil, which has equivalent CBRreadings and soil characteristics (Atterberglimits, gradation) to a depth of 24 inchesafter organics and loose granular soil have

CCTs gather data from an on-site survey, been removed. Special cases of varyingpresent an LZ survey package, and recom- subgrades are discussed later in this chap-

mend approval/disapproval for use of a pro- ter.

posed airfield. Airlift force commanders atthe Numbered Air Force/Airlift Control Cen- Potentially soft or dangerous areas shouldter/Air Force Special Operations Base make be tested first. Areas with poor drainage,the final decision. Airfields that require pre- with moist or discolored soil, or where vege-

tation is growing well may indicate a


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problem. Additionally, animal burrow holes,areas prone to flash flooding, previously for-ested areas, and dry lake beds may all posepotential problems. The airfield may need tobe realigned, taking into consideration anarea that will not lend itself to traffic.

Once the initial alignment of the airfield hasbeen decided, determine critical CBR for thesites. To do this, test each aspect of the air-field to ensure accurate coverage in locatingpotential problem area. (Appendices I and Jdetail the recommended testing intervals forthe airfield cone penetrometer and DCP, re-spectively.) Many soils may not be a uniformclassification throughout the depth concerned(usually 24 inches.) Cases where specific lay-ers have different CBRs pose special concernsin determining the critical CBR These casesare discussed in detail in following sections.

STEP 4. DETERMINE THE REQUIREDNUMBER OF PASSES

From the mission statement or an estimateof the situation, determine the minimumnumber of design aircraft passes that willaccomplish the mission. Remember, a passis considered one takeoff and one landing.Given the design aircraft, the in-place soilCBR, and the number of required aircraftpasses, you can determine the airfield sur-face type needed. While unsurfaced air-fields are favorable in minimizing resourcesinvolved in construction, some soils in theirnatural state cannot support traffic withouta surface. For more information about spe-cific mats and membranes, see Appendix N.

STEP 5. DETERMINE THE ALLOWABLENUMBER OF PASSES AND SURFACE

TYPE

The service life is a function of taking thedesign aircraft and the in-place soil CBR en-tering into Figure 12-6 and determining thenumber of allowable passes. The surfacetype (unsurfaced, light-duty mat, or me-dium-duty mat) is also a variable in deter-mining the allowable number of passes. Itdoes not directly increase the strength ofthe soil, but a surface does increase a soil’sservice life. Determine the surface type bychecking the least resource-intensive

method first. For example, if the intersec-tion of the soil CBR and the unsurfacedcurve does not meet the required number ofpasses, use the light-duty mat curve. Usea medium-duty mat if the number of re-quired passes is still not met. If the soilCBR cannot support the required numberof passes for any surface type, go to Step 6.

STEP 6. OUTLINE CORRECTIVEACTIONS TO INCREASE SERVICE LIFE

After determining the allowable number ofpasses, compare it to the number of passesrequired by the mission or construction direc-tive. There are certain courses of action avail-able to increase the allowable number ofpasses. Each course of action involves in-creasing the strength of the in-place soil: (1)compact the in-place soil, (2) stabilize the in-place soil using mechanical, chemical, or geo-synthetic stabilization, or (3) add a basecourse. Each of these methods is discussedin more detail later in this chapter.

DESIGN EXAMPLES

Example 1

Design an airfield for 200 passes in the closebattle area given an in-place soil with AI=13.No C-17s are expected to use the airfield.

NOTE: Multiply the number of requiredpasses by two to account for the aircrafttaxiing down the runway to take off orunload.

Solution 1

Step 1. The airfield location is the closebattle area.

Step 2. From Table 12-1, page 12-4, the de-sign aircraft is a C-130, which has a grossweight of 130 kips.

Step 3. The soil strength is given as an AI.It can be converted to a CBR value throughFigure 12-1, page 12-2. AI = 13 is equiva-lent to CBR = 14.1.

Step 4. The required number of passes is 200.

Step 5. Determine the allowable passesfrom Figure 12-6. Enter the chart withCBR = 14.1. Read the number of passes onthe horizontal axis where the CBR inter-sects the C-130 curve for the appropriate


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surface. In this case, the unsurfaced curveexceeds the required number of passes fora CBR = 14.1 soil; therefore, the soil willcarry the 200 required passes.

Example 2

Design an airfield for logistics missions of aC-17 in the support area. The in-place soilhas a DCP index of 60. The division AirForce liaison estimates the need for 600passes.

Solution 2

Step 1. The airfield is located in the sup-port area (given).

Step 2. Design aircraft is a C-17, whichhas a gross weight of 430 kips in the sup-port area (Table 12-1, page 12-4).

Step 3. DCP index is 60. From Figure 12-2, page 2-3, CBR = 3.

Step 4. The required number of passes is600.

Step 5. From Figure 12-6, page 12-11, theintersection of the unsurfaced curve andthe soil CBR yeilds zero passes. The onlysurface available for this low CBR is a me-dium-duty mat. The allowable number ofpasses for a C-17 weighing 430 kips on amedium-duty treat is 540. Since the allow-able number of passes exceeds the requirednumber, proceed to Step 6.

Step 6. Outline corrective actions to in-crease service life. Since the DCP index re-flects the soil strength in an undisturbedstate, first determine the DCP index afterseveral passes with a roller suitable to thesoil type. If you improve the index onlyslightly, then you can meet the service lifewith compaction only. Consider stabiliza-tion or adding a base course as other meth-ods if compaction alone is not enough.

EXPEDIENT AIRFIELDDESIGN—SPECIAL CASES

The previous discussion of soil-strength de-termination was adequate for a soil thathas a uniform CBR and soil characteristics(Atterberg criteria, gradation) to a depth of24 inches after organics and any loose,granular soil is moved aside. It is possible

to determine a critical CBR for soils withvarying strengths by evaluating each caseseparately. This changes the method of de-termining the in-place CBR but not the ac-tual design procedure.

Soil-Strength Profile-Increasing withDepth

If a soil-strength profile increases withdepth, the critical CBR is the average CBRfor the upper 12 inches. Soil strength usu-ally increases with depth so the weakest 12inches are considered critical, and they con-trol the evaluation. If the average CBR ofthe top 12-inch layer yields a CBR thatdoes not meet any surfacing requirement inFigure 12-6, consider stabilizing the sub-grade. (See Chapter 9, FM 5-410, for de-tails on the type and depth of stabilization.)

Example 3

Determine the number of allowable trafficpasses for a C-130 aircraft in the close bat-tle area. Gross weight is 130 kips. Thesoil-strength profile is shown below.


Solution 3

Step 1. Airfield location is the close battle area.

Step 2. Design aircraft is the C-130, grossweight = 130 kips.

Step 3. From the soil profile, the CBR in-creases with depth. The critical CBR is an av-erage of the top 12 inches; therefore, CBR =5.5.

Step 4. The required number of passes isnot given in the mission statement, so go toStep 5.

Step 5. While the required number of passesis not given, use Figure 12-6, page 12-11, todetermine the surface type and allowablenumber of passes. For a CBR = 5.5, an un-surfaced airfield allows only 42 passes. If alight-duty mat is used, however, the servicelife increases to 5,000 passes (use the largestnumber if it runs off the scale). Either sur-face would be correct, depending on the tacti-cal situation; but if time and resources exist,use the light-duty mat.

Soil-Strength Profile-Very Soft Layer ona Hard Layer

Determining the critical CBR on a soil witha very soft layer over a hard layer can besubjective, depending on the AI or CBRvalue and design aircraft weight. A softlayer can be a thin layer in which there isan extreme contrast between the upper fewinches and the lower level.

If a very soft layer is 4 inches thick or less,discard the CBR values from the soft layer,and determine the critical CBR from the 12-inch layer below the soft layer. For unsur-faced airfields, the allowable number of traf-fic passes may be reduced due to rutting inthe top 6 inches, which causes excessivedrag on aircraft during takeoff. This mustbe carefully monitored by airfield personnel.The maximum rutting depths (Table 12-2)are based on the orientation of the ruts aswell as the soil strength.

If the very soft layer is more than 4 inchesthick, the soft layer should be reduced bygrading to at least 4 inches. If you cannotred uce the depth of the soft layer because oftime or equipment constraints, determine the

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Table 12-2. Maximum rutting depth

critical CBR as an average of the top 12inches. The resulting low CBR will pre-scribe matting, which reduces the effects ofrutting. Generally, the area will not be suit-able as an airfield without placing mattingon the traffic area or blading the soft mate-rial off and waste it, if the equipment isavailable.

Example 4

Determine the surface type needed to sup-port 2,000 passes of a C-130 aircraft in thesupport area. Gross weight is 130 kips;soil-strength profile is indicated below.


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Solution 4

Step 1. Airfield location = support area.

Step 2. Design aircraft = C-130; grossweight = 130 kips.

Step 3. The soil profile shows a soft layerthat is roughly 4 inches deep. It is fol-lowed by a hard layer. Discard the datafrom the soft layer since the critical AI isan average of the 12-inch layer below thesoft layer. The average AI from the 6.2-inchdepth to the 16.7-inch depth is 7. Theequivalent CBR from Figure 12-1, page12-2, is 5.4.

Step 4. The required number of passes is2,000.

Step 5. The surface type is a light-dutymat, capable of supporting 5,000 passes(Figure 12-6, page 12-11).

NOTE: If the CBR was high enough tojustify an unsurfaced airfield, check Ta-ble 12-2, page 12-13, for maximum rut-ting depth. Airfield personnel shouldcarefully monitor the runway to ensureruts do not exceed the maximum.

Hard Layer Over a Softer Layer

Some soils may yield a profile that shows ahard layer over a soft layer. This type pro-file is generally exhibited by a soil that hasa gravel surface over a natural or fill soil,or by a natural soil that has a hard crustin the upper layer. If the top layer of soilis adequate to support the desired aircraftpasses, then the strength of the weaker soillayers beneath the top layer is used tocheck for the critical CBR.

The airfield cone penetrometer cannot beused to determine soil strength in a grav-elly soil, but the DCP can be used. If theDCP is not available, dig a test hole or testpit to determine the thickness of the hardlayer.

If the hard layer is less than 4 inchesthick, the hard layer is discarded, and thecritical CBR is determined by the averageCBR of the 12-inch layer profile below thehard layer. The number of traffic passes isdetermined as before (Figure 12-6).

If the hard layer is greater than 4 inchesthick, the critical layer is the 12 inches di-rectly beneath the hard layer. If the hardlayer is greater than 12 inches, simply aver-age the CBR values of the 12- and 24-inchlayers.

Example 5

Determine the surface type and the numberof allowable traffic passes for a C-130 air-craft in the close battle area. Gross weightis 130 kips. The soil-strength profile yields5 inches of gravel, and the 12-inch soil pro-file below the gravel layer has an averageCBR = 6. The commander indicated thathe needed 60 passes to accomplish the mis-sion.

Solution 5

Steps 1-3. Close battle area; C-130 (130kips); CBR = 6 (given).

Step 4. The required number of passes is60 (given).

Step 5. Allowable passes = 70 (Figure 12-6)for an unsurfaced airfield.

Example 6

Design an airfield in the support area foruse by both C-130s (130 kips) and C-17s(430 kips) for 1,000 passes. The soil ana-lysts used a DCP to determine the soilstrength profile below.


Solution 6

Step 1. Airfield location = close battle area(given).

Step 2. Although both aircraft will use theairfield, the C-17 is the design aircraftwhen both are present.

Step 3. The soil profile above shows that asoft layer exists under a hard layer. TheAIs are consistently above 17 until the 18-inch depth, when they drop significantly to10. Since the hard layer is greater than 12inches and the soil is only evaluated to 24inches, calculate the average of the bottom12 inches or the 12.4-inch to the 24.3-inchlayer:

The critical AI is 12.4, which yields a CBR= 13 (Figure 12-1, page 12-2).

Step 4. The required number of passes is1,000 (given).

Step 5. From Figure 12-6, page 12-11, anunsurfaced airfield allows 130 passes for asoil CBR = 13. Since this does not meetthe commander’s guidance, check other sur-faces. A light- or a medium-duty mat canbe used in this situation.

A hard soil layer over a soft layer can usu-ally be found in dry lake beds having ahigh evaporation rate and a high water ta-ble. The upper crust is often 2 to 6 inchesthick, and the soil beneath it generally can-not support an aircraft.

Soil-Strength Profile Decreasing withDepth

This type profile is similar to a hard layerover a soft layer. Generally, the soil exhibitsa weakening with depth without a verystrong surface layer. This type profile canreadily be seen in areas of dry lake beds orwhere the groundwater can be found close tothe surface. Areas such as these also maybe subjected to seasonal fluctuation if thewater table causes the soil profile to change.

Determine the critical CBR for this type pro-file by evaluating various layers to a depthof 24 inches. Determine the profile’s criti-

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cal CBR by choosing the lowest averageCBR from the following layers: 6-18inches, 8-20 inches, 10-22 inches, and 12-24 inches.

Example 7

Determine the number of allowable C-130traffic passes on an airfield in the close bat-tle area, gross weight 130 kip-pounds, anda soil strength profile shown below:

Solution 7

Step 1. Airfield location = close battle area(given).

Step 2. Design aircraft = C-130; grossweight = 130 kips.

Step 3. Determine the in-place soilstrength by calculating averages for the fol-lowing layers:


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Since the lowest average CBR for the differ-ent layers is 7.2, it is the critical CBR.

Step 4. The required number of passes isnot specified.

Step 5. From Figure 12-6, page 12-11, theallowable number of passes = 180 (unsur-faced) and 5,000 (light-duty mat).

SMOOTHNESS REQUIREMENTS FORUNSURFACED AIRFIELDS

While unsurfaced airfields require littlepreparation, both the C-130 and the C-17require relatively smooth surfaces for take-off and landing. The overall grades, gradechanges, and slopes must be within the lim-its indicated in Table 11-3, page 11-4. Therandom surface deviations and obstacles al-lowed depend on the strength, hardness,and size of items that cause roughness.They should not exceed the following limits:

Rocks in traffic areas must be removed,embedded, or interlocked in a mannerthat will preclude displacement whentraversed by aircraft. Tree stumps mustbe cut to within 2 inches of the ground.

Dried, cohesive dirt clods (clay ex-cluded) and soil balls (as much as 6inches in diameter) that will burst upontire impact are allowed. Because hard-ened clay clods may have charac-teristics similar to those of rocks, theymust be pulverized or removed from traf-fic areas.

Contours of dirt patterns are allowed whenthey result from plowing to reduce erosion,aid water drain off, and prepare the soil forplanting. These contours contain a softcore that does not require removal.

Limitations on rutting are a function ofthe orientation, depth of ruts, and soilbearing strength. Maximum ruts thatcan be traversed safely are shown in Ta-ble 12-2, page 12-13.

Potholes must be filled if they exceed 15inches across their widest point and 6inches deep. Potholes are circular oroval and are distinguished from depres-sions by their smaller size and sharp-an-


gled corners. Distance between repairsshould be at least 20 feet apart.

Ditches more than 6 inches deep mustbe eliminated from traffic areas. Whenditches are filled, the bearing strengthmust approximate that of the surround-ing soil.

If it is decided (after final analysis of thesubgrade strength) to change the alignmentof the airfields, test the new area as re-quired.

Remember, these evaluations do not guaran-tee risk-free operation; and evaluations areaffected by airfield condition, weather, andaircraft use. The commander must keepthese risks in mind when making decisionson airfield use.

DESIGN REQUIREMENTS FORMEMBRANE- AND MAT-SURFACED

AIRFIELDS

Many high-performance Air Force aircraftcannot operate on the degree of surfaceroughness permitted by unsurfaced criteria.Heavy cargo aircraft will rarely operate onunsurfaced airfields because of their sensi-tivity to foreign object damage and soilstrength requirements. Matting and mem-branes can alleviate some of these prob-lems. A thorough discussion on mem-branes and matting placement is containedin Appendices L, M, and N.

Smoothness Requirement for Mats andMembrane Airfields

Membrane-surfaced airfields. Membrane cov-erings are impermeable nylon fabrics thatprotect an airfield from harmful drainage ef-fects and act as a rustproofing agent. Mem-branes are used on both types of expedientsurfaces (unsurfaced and surfaced) with alight- or medium-duty mat. Although themembranes do not increase the strength ofin-place soil, they may increase the servicelife in many geographic areas. The sur-face smoothness requirements for this air-field category apply to the subgrade sur-face before placement of membrane or to

an existing unsurfaced airfield where sus-tained operations of the C-130 are ex-pected. Smooth grade the runway and taxi-way to a crown or transverse slope thatmeets design standards. The overall gradesand slopes will not exceed that required forthe unsurfaced airfield either longitudinallyor transversely at any location on the sur-face of the runway, taxiway. or apron.

Mat-surfaced airfields. Mat surfacing pro-vides a very smooth, well-drained, fine-graded surface free of local depressions orpotholes. Surface smoothness requirementsfor this air-field category apply to the sur-face of subgrade before placement of mem-brane and landing mat. For satisfactoryperformance, the landing mat must be sup-

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ported by the subgrade and must not be re-quired to bridge over depressions or pot-holes. Prepare a satisfactory surface forthe landing mat by compacting and finegrading to a predetermined grade. Tables12-3 and 12-4, page 12-18, list some matcharacteristics.

Grade runways and the taxiway to providea crown section or transverse slope thatmeets the design standards. Overall gradesand slopes must be within the limits givenin Table 11-3, page 11-4. Random surfacedeviations in grade will not exceed 1 incheither longitudinally or transversely from a12-foot straightedge or string line placed atany location on the surface of the taxiway,runways, or aprons.

Table 12-3. Characteristics of M19 and ancillary items


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Table 12-4. Mat dimensions

DESIGN IMPROVEMENTS FOREXPEDIENT-SURFACED AIRFIELDS

When suitable in-place soils cannot befound to support expedient-surfaced air-fields, improve the in-place soil of the de-sired location as a last resort. The extratime and resources involved in improving in-place soil is minimal when compared to re-configuring missions based on finding asuitable subgrade.

The easiest way to increase the allowablenumber of passes is by compacting the in-place soil or subgrade. Through compac-tion, soil particles orient themselves in adenser formation, which increases the soilCBR. Compaction will only be effective ifdone for the entire critical layer. For uni-formly distributed soil profiles, that meansthe top 12 inches. Since most rollers onlycompact to a depth of 6 to 8 inches, scarifyand windrow the top 6 inches to the side inorder to compact the bottom 6-inch layer.Specific guidelines for the type of roller touse can be found in FM 5-410. After youincrease the soil CBR, go back through thedesign steps to determine the new allowable

number of passes. Depending on the un-compacted CBR and the amount that itchanged by compaction, the surface type orthe need for a surface altogether also mayhave changed.

Normally, compaction wiIl improve thestrength of soils. However, there are somespecial cases where working a soil may actu-ally decrease its strength. Specifically, thefine-grained soils, types CH and OH, canhave high strengths in an undisturbed con-dition; but scarifying, grading, and compact-ing may reduce their shearing resistance.For more information on these soils, seeChapter 8, FM 5-410.

Another way to increase the strength of thesubgrade is through soil stabilization.There are many methods of stabilizationavailable to increase soil CBR. The threemajor types of stabilization are stabilizationexpedients (or geosynthetics), mechanicalstabilization, and chemical stabilization.Choosing the best one depends on the soilcharacteristics as well as available re-sources. Specific information on each typeof stabilizer can be found in Chapter 9,


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FM 5-410. Stabilizing an in-place soil ismost commonly done to increase the soil’sCBR, but it can also be used to negate theharmful effects of dust and water. Table12-5 summarizes the possible functions ofstabilizers in traffic and nontraffic areas onexpedient surfaces.

Dust Control and Waterproofing

Much information needs to be developed toform comprehensive criteria for selectingand using dust-control agents and soil wa-terproofers on expedient airfields. Thereare many possible choices. Until one ortwo vastly superior dust-control agents orsoil waterproofers are developed, the engi-neer should be aware of the potentially ac-ceptable systems and some of their charac-teristics.

Dust Control. The presence of dust-sizedparticles in a soil surface may not indicatea dust problem. An external force imposedon a ground surface will generate dust.Dust may be generated as a result of ero-sion by an aircraft’s propeller wash, engineexhaust blast, jet-blast impingement, or thedraft of moving aircraft. The kneading andabrading action of tires can loosen particlesfrom the ground surface. These particlesmay become airborne as dust.

On unsurfaced airfields, the source of dustmay be the runway, taxiways, shoulders,overruns, or parking areas. In areas ofopen terrain and prevailing winds, soil parti-cles may be blown in from distant locationsand deposited on an airfield. This can con-tribute to dust potential despite adequate in-itial control measures of the soil within theconstruction area. Where blowing dust is aproblem, it may be necessary to apply addit-ional dust-control agents to an airfield.

The primary objective of a dust-controlagent is to prevent soil particles from be-coming airborne. These agents may beneeded on traffic and nontraffic areas. Ifprefabricated landing mat, membrane, orconventional pavement surfacing is used inthe traffic areas of an airfield, dust-controlagents are needed only on nontraffic areas.The substance used in these areas must re-sist the maximum intensity of air blast im-pingement of aircraft.

Dust-control agents used for traffic areasmust withstand the abrasion of wheels andblast impingement. Although dust-controlagents may provide resistance against airimpingement, they may be unsuitable as awearing surface. An important factor

Table 12-5. Stabilization functions


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limiting the applicability of a dust-controlagent in traffic areas is the extent of sur-face rutting that occurs under traffic. Un-der these conditions, the effectiveness of ashallow dust-control treatment could be de-stroyed rapidly by breakup and subsequentstripping from the ground surface. Somedust-control agents will tolerate deforma-tions better than others. Normally, ruts inexcess of 1/2 inch will result in the destruc-tion of any thin layer or shallow dust-con-trol treatment.

Waterproofing. Water may enter a soil bythe (1) leaching of precipitation or pondedsurface water, (2) capillary action of underly-ing groundwater, (3) a rise in the water ta-ble, or (4) condensation of water vapor andthe accumulation of moisture under a vapor-impermeable surface.

As a general rule, areas with an existingshallow water table will have a low soil bear-ing strength and should be avoided when-ever possible.

The objective of a soil surface waterprooferis to protect soil against water and preserveits strength during wet-weather operations.The use of soil waterproofers is limited totraffic areas except where excessive soften-ing of nontraffic or limited traffic areassuch as shoulders or overruns must be pre-vented.

Soil waterproofer may prevent soil erosionresulting from surface-water runoff. Likedust-control agents, a thin or shallow soilwaterproofing treatment loses its effective-ness when damaged by excessive rutting.These treatments can be used efficientlyonly in areas that are initially firm.

Many soil waterproofers also function wellas dust-control agents. A single materialmay be used as a treatment in areas withboth wet- and dry-soil surface conditions.

Materials. Many materials for dust controland soil waterproofing are available. Noone choice, however, can be singled out asacceptable for all problem situations. Tosimplify the discussion, materials are

grouped into five general classifications:Group I, bituminous materials: Group II, ce-menting materials: Group III, resinous andlatex systems; Group IV, salts; and GroupV, miscellaneous materials.

A summary of the various materials and aguide to their applications as a dust-controlagent or soil waterproofer are given in Table12-6. This summary is the best estimate ofthe applicability of the materials based onexisting information. Two materials in thetable (asphalt penetrative soil binder (APSB)(Peneprime) and polyvinyl acetate dust-con-trol agent (DCA 1295) warrant special men-tion.

APSB (Peneprime) is a special cutbackasphalt having good penetration capabil-ity and rapid curing characteristics.This material is effective in sand, gravel,silt, and lean clay. It is not effective inheavy clay or clay with excessive shrink-age or swelling characteristics. Surfaceapplication assures good penetration ingranular soils. In clay, silt, and granu-lar soils that are highly compacted, thesurface should be scarified to a shallowdepth before the material is applied.

Compaction should be initiated whenpenetration is complete. In traffic ar-eas, compaction can be accomplishedby normal traffic. These materials areeffective in traffic and nontraffic.When the material is applied on un-scarified areas of well-compacted soil,reapplication may be necessary if thetraffic is moderate to heavy.

Polyvinyl acetate (DCA 1295) is an emul-sion that is applied to the surface usinga fiberglass scrim (screen) fabric to rein-force it. This material can be used onall types of soil, and it cures in fourhours or less. This system is applicableto shoulders and overruns and is effec-tive as a waterproofing agent. It will notsupport heavy fixed-wing aircraft traffic.

The following information is provided in Ta-ble 12-6:


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Table 12-6. Dust-control and waterproofing applications


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Column 1 identifies the material.

Column 2 indicates the usual form inwhich the material is supplied.

Column 3 indicates the most acceptablemethod of application. Where a mate-rial may be applied either as an admix-ture or as a surface penetration treat-ment, the preferred and most generallyused method is indicated first.

Column 4 shows applicable soil ranges.The range of soils indicated will nor-mally result in reasonably satisfactoryresults with the particular material.Sometimes the materials may be usedoutside this range with decreased effec-tiveness. In general, granular soils(gravel to coarse sand) may or may notrequire treatment for dust control or wa-terproofing, depending on the amountof fines present. Fine sands (such asdune or windblown sands) will probablyrequire a dust-control treatment butwill not need to be waterproofed. Soilsranging from silty sand to highly plasticclay may require a dust-control agent ora soil waterproofer.

Columns 5, 6, and 7 show the primaryfunction of the materials as either a

dust-control agent or soil water-proofer, and where known, therelative degree of effectivenessthat can be expected. Rarely willnontraffic areas require water-proofing because there is usuallyno need to maintain soil strengthin nontraffic areas. If such a re-quirement exists, materials suit-able for traffic areas can be con-sidered acceptable for use in non-traffic areas.

Columns 8 and 9 reflect thequantity requirements applicableto the soil range indicated in col-umn 4. The lower quantity of therange generally is suitable forcoarse soils, and the greaterquantity is needed for fine soils.These quantity requirements aregiven only as a general guide,and in some cases, effective re-sults may be achieved with lesseror greater amounts than thosegiven in the table. (Detailed infor-mation on dust control is in TM5-830-3.)

Column 10 indicates the mini-mum curing time requirements.

AGGREGATE-SURFACED AIRFIELDS

While time and resources are limited inthe close battle area, it may be possible tocommit resources in the support area to ag-gregate-surfaced airfields. Most airfieldsin the support area initially have expedientsurfaces, which may be upgraded to aggre-gate surfaces for sustained operations.Sometimes a former close battle area is re-designated as a support area and up-graded to an aggregate surface for ensuingoperations.

The design of aggregate-surfaced airfields issimilar to the design of expedient-surfacedairfields. In aggregate-surfaced airfields,however, a layer of high-quality material isplaced on the compacted subgrade to

improve its strength. The thicknessdesign is a function of the CBR ofthe in-place soil and the design air-craft. Instead of determining thenumber of allowable passes based onthe CBR, use the required number ofpasses to determine the total thick-ness design. For a given CBR, thethickness design increases with in-creased number of passes. Normally,aggregate-surfaced airfields are usedfrom one to six months and supportC-17 and C- 130 sorties.

Design the layout of aggregate-sur-faced airfields like expedient-sur-faced airfields. The runway withturnarounds should be constructedfirst as shown in Figure 12-3, page


12-5. As time permits, complete the airfieldlayout according to Figure 12-4, page 12-6.

MATERIALS

Materials used in aggregate airfields mustmeet the requirements stated in Chapter 5,FM 5-430-00-1/AFPAM 32-8013, Vol l, andin the following paragraphs. The materialsshould have greater strength than the sub-grade and should be placed so the higherquality material is on top of the lower qual-ity material. All layers in an airfield designrequire a minimum layer thickness of 6inches and should conform to the CBR andcompaction criteria shown in Table 12-7.

Table 12-7. Compaction criteria and CBR re-quirements for aggregate-surfaced airfields

(MIL STD 621 method 100 CE 55)

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Subgrade

The in-place soil or subgrade requires moreattention in aggregate-surfaced airfieldstructures. Before developing the thicknessdesign, determine the compacted CBR ofthe subgrade. Since laboratory CBR testsare impractical for initial construction, usethe penetrometers discussed earlier.

Determine the soil’s CBR profile as dis-cussed previously for expedient surfaces.Like road design, the CBR of the subgradedetermines the thickness of the whole de-sign. If you can improve the CBR throughcompaction, the thickness of theaggregate airfield structure will decrease.The depth to which an in-place soil shouldbe compacted is normally 6 inches, but thedepth is determined in the design procedure.

Select and Subbase Materials

Select and subbase materials used in aggre-gate airfields provide granular fill to meetthe thickness design based on the subgradeCBR. Select materials and subbase coursesmust meet the Atterberg limits and grada-tion requirements of Table 12-8, which arethe same criteria used for roads.

Base Course

Only good quality materials should be usedin base courses of aggregate airfields.Since the base course is also the surfacecourse, it must meet specifications for bothstrength anti gradation. The minimum CBR

Table 12-8. Maximum permissible values for subbases and select materials


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for an airfield base course is 80 (Table 12-7, page 12-23). Since CBR tests requiretime, use one of the base-course materialsshown in Table 12-9, if possible. They arematerials of known strength. If a materialnot listed is more easily obtained, use atest strip to determine its compacted CBRwith a DCP.

Gradation requirements for aggregate-sur-faced layers are given in Table 12-10, wherethe specifications depend on the maximumsize aggregate (MSA). These are the same gra-dation requirements as given in Chapter 5,FM 5-430-00-1/AFPAM 32-8013, Vol 1, forbase courses for aggregate-surfaced roads.

FROST CONSIDERATIONS

Aggregate-surfaced airfields, unlike roads orexpedient surfaces, require much more re-strictive tolerances in construction and gen-eral maintenance. For this reason and thepotential for catastrophic accidents in thecase of structural failure, frost must be con-sidered in the design of aggregate airfields.The specific areas where frost has an im-pact on the design are discussed in the fol-lowing paragraphs:

Table 12-9. Assigned’CBR ratings for base-course materials

Table 12-10.rock or slag,

aggregates

Desirable gradation for crushedand uncrushed sand and gravelfor nonmacadam base courses

As discussed earlier, three conditions mustexist for detrimental frost action to occur:(1) the subgrade must be frost susceptible,(2) the temperature must remain belowfreezing for a considerable amount of time,and (3) an ample supply of groundwatermust be available. Since aggregate-sur-faced airfields have a design life up to sixmonths, the effects of frost may not be rele-vant because of the time of year. In anycase, evaluate the frost effects during thedesign process in the event the airfield isneeded for sustained operations.

In general, frost-susceptible soils are thosewith considerable amount of fines or withat least 6 percent of materials finer than0.02-millimeter grain size by weight. Youdo not have to relocate or find another soilwhen faced with one of these situations;however, you need to adjust the thicknessdesign to account for the frost action.When water in a subgrade freezes, addi-tional water travels by capillary rise and in-creases the ice lense. The ice lenses candisturb the compacted layers enough to cre-ate large voids during the next thaw cycle.


Defense Printing Service

THICKNESS DESIGN PROCEDURE

The design procedure for aggregate-surfacedairfields in the support area is very similarto expedient-surfaced airfields. The majordifference is that the outcome is the thick-ness of the aggregate structure, which is afunction of the subgrade CBR, the designCBR, and the number of passes.

Step 1. Determine the Airfield Location

The airfield location is always the supportarea. While aggregate-surfaced airfields aretoo resource intensive for the close battlearea (unless they are existing airfields),they do meet the surface requirements forthe rear area.

Step 2. Determine the Design Aircraftand Gross Weight

The C-17 and C-130 are the only possibili-ties for design aircraft in the support area.Aggregate surfaces are considered a semi-prepared surface where only the C-17 andC-130 can land. Since the support area isprimarily a connector of the rear and closebattle areas, it is logical that the design air-craft be able to land in all three areas. Theaircraft also have the same design weightsfor the support and close battle areas asshown in Table 12-1, page 12-4.

5-430-00-2/AFJPAM 32-8013, Vol II

Step 3. Check Soils and Construction Ag-gregates

This design step has three parts: (1) checkthe local area for possible borrow sites tobe used as select materials and subbases,(2) check the strength and gradation of apossible base course, and (3) check thefrost susceptibility of all materials, if neces-sary.

Check construction aggregates for useas select and subbase materials. Thisis similar to road design discussed inChapter 9, FM 5-430-00-1/AFPAM 32-8013, Vol 1. Conduct soil tests on anyborrow sites to determine the soil’sCBR, gradation, and liquid and plasticlimits. Compare these values to Table12-8, page 12-23, to determine if theborrow material can be used in a layerof the design.

Check the strength and gradation of thebase course. The base course must havea CBR = 80 or higher and meet the gra-dation criteria of Table 12-10.

Check frost susceptibility of materials.If detrimental frost action (as definedearlier) is a concern, evaluate each layersoil below the base course for frost sus-ceptibility. A soil is frost susceptible ifit has 6 percent fines and/or 6 percent(by weight) 0.02 millimeter grain size.

For frost design purposes, soils have beendivided into seven groups (Table 12-11,page 12-26). Only the nonfrost-susceptible(NFS) group is suitable for base course.Soils are listed in approximate order of de-creasing bearing capability during periodsof thaw.

The percentage of fines should be restrictedin all the layers to facilitate drainage and re-duce the loss of stability and strength dur-ing thaw periods.

Do not use a soil above the compacted sub-grade if it is frost susceptible. For example,a borrow material that meets the criteria fora subbase should not be used in the designif it has more than 6 percent finer than0.02 millimeter by weight.


FM 5-430-00-2/AFJPAM 32-8013, Vol II

Table 12-11. Soil classification for frost design

If a subgrade is frost susceptible, determine Step 4. Determine the Number of Passesits frost group from Table 12-11, and findthe frost area soil support index from Table12-12. This value is needed to adjust thethickness design in Step 8.

Table 12-12. Frost area soil support index

Required

Unlike design for expedient surfaces, youcan control the thickness design by thenumber of passes required. As the numberof passes increases, so does the thicknessdesign and, consequently, the constructioneffort. You may be given the required num-ber of passes in a mission statement or youcan adjust it based on the thickness design.

Step 5. Determine the Total SurfaceThickness and Cover Requirements

The total thickness of the aggregate struc-ture is a function of the subgrade CBR, thedesign aircraft, and the number of passes.Since the thickness design is usually


greater than 6 inches (the minimum layerthickness), multiple soil layers are used.For example, if the thickness required overa subgrade was 18 inches, it would be ex-pensive and wasteful to fill the entire 18inches with a high-quality base course. In-stead, use borrow materials to fill all butthe top 6 inches. The CBR of each soilused in the design determines the requiredcover.

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After evaluating the available soils and de-termining the number of passes, enter Fig-ure 12-7 or 12-8, page 12-28, with the sub-grade CBR until it intersects the grossweight of the design aircraft. Trace a linehorizontally until you intersect the desirednumber of passes and determine theminimum cover required over the subgradefrom the horizontal axis. Determine theminimum cover required over each soil thatcould be used in the design.

Figure 12-7. C-130 design curves for gravel-surfaced airfields


FM 5-430-00-2/AFJPAM 32-8013, Vol II


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Step 6. Complete the Temperate Thick-ness Design

After finding the cover requirements, youcan complete the thickness design withoutconsidering the effects of frost. The methodis similar to road design in that you deter-mine the layer thicknesses that satisfy theminimum cover requirements for each layer.Remember that each layer must be a mini-mum of 6 inches thick. Also, do not usesoil layers in the design if they are not nec-essary to satisfy the cover requirements.For example, if a subgrade only requires 5inches of granular fill, then the base courseis the only aggregate layer required. Eventhough you may have determined that sub-bases and select materials are readily avail-able nearby, they are not necessary for theairfield design. Figure 12-9 illustrates therelationship between minimum cover andlayer thickness.

Step 7. Adjust Thickness Design forFrost Susceptibility

If you are not designing in a frost area or ifthe subgrade in a frost area is not frost sus-ceptible (see Step 1), go to Step 9.

Since the freeze-thaw cycles associated withfrost areas weaken soils, you now have toconsider frost and how it will affect thethickness design. Since the subgrade is theonly frost-susceptible material at this point,retrieve the subgrade information fromStep 1.

Determine the frost-area soil-support indexfrom Table 12-12, page 12-26. Use the in-dex to enter Figure 12-7, page 12-27, or Fig-ure 12-8 instead of the compacted CBR.

For example, for a C-130 airfield, if the com-pacted subgrade CBR was found to haveCBR = 8 and the subgrade was found to bean F2 type soil, enter Figure 12-7 with CBR= 6.5 instead of 8. Since the lower value in-creases the thickness design for the samenumber of passes, choose the thicker of thetwo designs.

The frost design will not always increasethe thickness design. For instance, if Step7 indicates a total thickness design of 14inches over a subgrade with a CBR = 3 andthe soil is an F3 soil, use Table 12-12, page12-26, to determine the soil support indexof 3.5. Since 3.5 is greater than 3, it re-quires thinner design (determined by Figure12-7).

After choosing the thicker of the two de-signs, you must add a frost filter to the de-sign and adjust the layer thicknesses. Afrost filter is sand or a uniformly graded, co-hesionless material that allows the lateralmovement of water. Place a 4-inch layer di-rectly on the compacted subgrade and com-pact it to the specifications outlined inStep 8.

Geotextiles may be used over F3 and F4subgrade materials in seasonal frost areasto help prevent intrusion of fines into baselayers during periods of thaw. The geotex-tile should provide at least 110 pounds at10 percent strain when the material istested by the Grab Strength Test (ASTM D-5034 and D-5035). If the material exhib-its different strengths in perpendicular di-rections, the lowest value is used. If longi-tudinal seams are required, they mustmeet the requirements in ASTM D-1683.End overlap at transverse joints should be

Figure 12-9. Thickness design procedure


FM 5-430-00-2/AFJPAM 32-8013, Vol II

a minimum of 2 feet. The fabric will beplaced directly on the subgrade and mustextend laterally to within 1 foot of the toeof slope on each side. A frost-filter layer isnot required when a geotextile is placed di-rectly on the compacted subgrade.

Step 8. Determine Subgrade Depth andCompaction Requirements

The layer thickness of an in-place soil isthe depth to which you must ensure ade-quate compaction. Determine the depth byentering Table 12-13 with the appropriatetraffic area and soil information.

The actual depth of subgrade compaction isthe difference between the total thicknessabove the subgrade and the value from Ta-ble 12-13 or 6 inches, whichever is greater.For example, if the thickness above the co-hesive subgrade (Type B traffic area) is 16inches, then the depth of subgrade compac-tion is 21 inches (Table 12- 13) - 16 inches= 5 inches. However, since 5 inches is lessthan 6 inches, compact to a depth of 6inches. Since the equipment effort for com-paction is about the same for depths of 1 to6 inches, the minimum depth of subgradecompaction is 6 inches.

Because most road construction missions re-quire cut-and-fill operations, the subgradedepth requirement is only significant in cutsections since the soil in fill sections isplaced and compacted in lifts (usually 6inches). In cut sections, however, the sub-grade must be scarified and compacted in

Table 12-13. Depth of compaction requiredfor subgrades

place to the depth required after the cut ismade.

Compaction requirements for subgradeand granular layers are expressed as apercent of maximum CE 55 density as de-termined by using Military Standard (MIL-STD) 621 Test Method 100 (ASTM 1557).The specifications for each layer in thedesign are listed in Table 12-7, page 12-23.

Remember, there are special cases for sub-grades that lose strength when being re-molded. These are generally soil types CHand OH. See Chapter 8, FM 5-410, formore information on these soils.

Step 9. Draw the Final Design Profile

This step is a culmination of the pre-vious eight steps into a picture that thebuilder can understand. It shows thelayer thicknesses, soil CBRs, and compac-tion requirements. It also shows thecompactive effort of any fill sections,which is the same soil as the subgrade.Figure 12-10 shows the specific detail in-cluded in the profile.

1 These are optional layers depending on the materials

available and the thickness design.

2

3

Scarifiy and compact in place.Total depth of fill.

Figure 12-10. Final design profile


Example 8

Design the taxiways and ends of the run-way (Type B area) for an aggregate-surfacedairfield in the support area (Honduras) for1,000 passes of a C-130. The in-place soilis a well-graded, sandy clay with a PI = 6,and has 7 percent finer than 0.02 millime-ter by weight. Your soils analyst reports auniform CBR = 5. After he set up a teststrip, he found that the CBR increased to 7with compaction. From the reconnaissanceteams, you have one potential borrow sitewith the following soil characteristics:

Borrow A: GP-GC; CBR = 35, PI = 8, LL= 28; 60 percent passes Number 10 sieve;15 percent passes Number 200 sieve.

Base course: Nearby civilian batch planthas been leased by the US; well-graded,crushed limestone available with the fol-lowing gradation specifications:

Solution 8

Step 1. Airfield location = support area(given).

Step 2. Design aircraft = C- 130/130 kips(given).

Step 3. Check construction aggregates.

a. Check materials for use as select/sub-base. Since there is only one potentialsource, check it according to Table 12-9,page 12-24. Since PI = 6, the soil does notmeet the Atterberg criteria for a subbase.Therefore, determine whether it meets selectmaterial criteria. Since its LL < 25 and thePI < 12, it can be used as a select materialCBR = 20.

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b. Determine the base course CBR. FromTable 12-10, page 12-24, since the basecourse material is a well-graded, crushed ag-gregate (limestone), the CBR = 100.

c. Check materials for frost susceptibility.Since the location of the airfield is Hondu-ras, frost is not a concern.

Step 4. Determine the number of passes re-quired. Passes required = 1,000 (given).

Step 5. Determine the total surface thick-ness and cover requirements. Using CBRsfor each soil layer that requires cover, enterFigure 12-7, page 12-27, to determine thecover requirements.

Step 6. Complete the temperate thicknessdesign. Draw a figure to determine thelayer thicknesses based on the cover re-quirements.

Calculate the layer thicknesses from thesurface down. First, look at the cover re-quired above the select layer. It requires aminimum of 4 inches above it. The basecourse has a layer thickness of 6 inches be-cause the minimum layer thickness in anairfield is 6 inches. Next, look at the coverrequired above the CBR = 7 subgrade.While 10 inches are required, you alreadyhave 6 inches in the base. Therefore, thesubgrade requires an additional 4 inches ofcover. Again, since the minimum layer.thickness is 6 inches, round the selectlayer thickness up to 6 inches.


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Step 7. Not applicable since the airfield islocated in a nonfrost area.

Step 8. Determine the subgrade depth andcompaction requirements. (See Table 12-13, page 12-30, to find the minimum depthof compaction below the surface.) Becausethe subgrade soil has a PI = 6, it is a cohe-sive soil. For a cohesive soil in a Type Carea, the required depth of compaction is17 inches below the surface. Since the to-tal thickness design is 12 inches, the actualdepth of subgrade compaction is 17 - 12 =5 (rounded up to 6 inches.) The compactionrequirements (from Table 12-7, page 12-23)for the three layers is shown below:

Step 9. Draw the final design profile.

Example 9

Design a Type B area for an aggregate-sur-faced airfield in northeastern Turkey thatcan withstand 10,000 passes of a C-17(gross weight = 430 kips). The area is sub-jected to seasonal frost conditions (assumethat seasonal frost will occur during the air-field service life). Below is a summary ofsoil and construction aggregate data.

Subgrade: CL: PI = 14: natural CBR =3: compacted CBR = 5: 7 percent finerthan 0.02 millimeter by weight.

Borrow A: GP: CBR = 35: PI = 8; LL =28: 10 percent pass Number 80 sieve; 5percent pass Number 200 sieve; NFS.

Borrow B: GW-GC; CBR = 45; PI = 5;LL = 23; 65 percent pass Number 10sieve; 12 percent pass Number 200sieve; NFS.

Base course: Limestone; meets grada-tion limits for 2-inch MAS (Table 12-10,page 12-24).

Solution 9

Steps 1 and 2. Support area is the onlychoice for aggregate-surfaced airfields; C-17(430 kips) is the design aircraft.

Step 3. Check soils and construction aggre-gates.

a. Check possible subbases and select ma-terials.

Borrow A: Fails as a subbase due to At-terberg criteria, but meets select mate-rial criteria. Therefore, it can be usedas a select material CBR = 20.

Borrow B: Meets criteria for a subbaseCBR = 50; therefore, use it as a subbaseCBR = 45.

b. Check strength and gradation of thebase course. Since the base course is lime-stone, the CBR = 80 (Table 12-9, page 12-24). The soils analyst already checked thegradation information and said it met thespecifications.

c. Check for frost susceptibility. No mate-rials above the compacted subgrade arefrost susceptible. Since the subgrade hasgreater than 6 percent finer than 0.02 milli-meter by weight, it is frost susceptible.From Table 12-11, page 12-26, the soil fallsinto frost group F3. The soil support indexfrom Table 12-12, page 12-26, is 3.5.

Step 4. The number of passes required is10,000 (given).


Step 5. Determine the cover requirementsfrom Figure 12-8, page 12-28.

Step 6. Complete the temperate thicknessdesign.

The required cover above the select materialCBR=20 is only 6 inches. Since the basecourse already has a layer thickness of 6inches, the select’s cover requirement is sat-isfied. Therefore, there is no need for thesubbase layer. The cover required over thesubgrade is 23 inches; consequently, the se-lect material must be 23 - 6 = 17 inches.This is the most cost-effective design undernormal conditions because there are fewerrestrictions on select materials than on sub-bases. Keeping a subbase layer would beacceptable if the material is readily avail-able and usable in its borrowed or quarriedstate.

Step 7. Adjust thickness design for frost.Since the subgrade is a frost-susceptiblesoil (frost group F3) and the area is sub-jected to frost, the total thickness designmust be derived from the soil-support in-dex, which is 3.5 (Table 12-12, page 12-26).Entering Figure 12-8 with 3.5 yields a mini-mum required cover of 29.2 inches(rounded up to 30 inches.) Since this thick-

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ness is greater than the 23

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inches requiredfor the temperate design, use this designfor the airfield. Also, you must add a 4-inch frost filter. This changes the thicknessdesign shown below.

Step 8. Determine subgrade depth andcompaction requirements. From Table 12-13, page 12-30, the required depth of com-paction below the surface is 21 inches.Since the actual thickness design is greaterthan 21 inches, use the minimum depth ofsubgrade compaction = 6 inches. To findthe compaction requirements for the soil lay-ers, see Table 12-7, page 12-23.


SPECIAL DESIGN CONSIDERATIONS

Stabilized Soil Design

The use of stabilized soil layers for aggre-gate-surfaced pavement structures (as de-scribed in Chapter 5, FM 5-430-00-l/AFPAM


FM 5-430-00-2/AFJPAM 32-8013, Vol II

32-8013, Vol 1, and FM 5-410) provides theopportunity to reduce the overall thicknessrequired to support a given load. Designingan airfield with stabilized soil layers re-quires the application of equivalency factorsto a layer or layers of a conventionally de-signed structure.

To qualify for the application of equivalencyfactors, the stabilized layer must meet ap-propriate strength and durability require-ments. An equivalency factor representsthe number of inches of a conventionalbase or subbase that can be replaced by 1inch of stabilized material. Equivalency fac-tors are determined as shown in Table 9-21, page 9-76, FM 5-430-00-l/AFPAM 32-8013, Vol 1, for bituminous-stabilized mate-rials and in Figures 9-55 and 9-56, page 9-76, FM 5-430-00-1/AFPAM 32-8013, Vol 1,for materials stabilized with cement, lime,or fly ash mixed with cement or lime. Se-lecting an equivalency factor from the tabu-lation depends on the classification of thesoil to be stabilized. Selecting an equiva-lency factor from Figures 9-55 and 9-56 re-quires the unconfined compressive strength(as determined by ASTM D 1633) be known.Figure 9-55 shows equivalency factors forsubbase materials, and Figure 9-56 showsequivalency factors for base materials.

Minimum thickness. The minimum thick-ness requirement for a stabilized base orsubbase is 6 inches.

Application of equivalency factors. The useof equivalency factors requires that a roador airfield be designed to support the de-sign load conditions. If a stabilized base orsubbase course is desired, the thickness ofa conventional base or subbase is dividedby the equivalency factor for the applicablestabilized soil. (See page 9-77, FM 5-430-00-1/AFPAM 32-8013, Vol 1, for examplesof applying equivalency factors to base andsubbase thicknesses.)

Drainage Requirements

Adequate surface drainage should be pro-vided in order to minimize moisture dam-age. Expeditiously removing surface waterreduces the potential for absorption and en-

sures more consistent strength and reducedmaintenance. Drainage, however, must beprovided in a manner to preclude damageto the aggregate-surfaced airfield from ero-sion of fines or the entire surface layer.Also, ensure the change in the overall drain-age regime, as a result of construction, canbe accommodated by the surrounding topog-raphy without damage to the environmentor to the newly constructed airfield.

The surface geometry of an airfield shouldbe designed to provide drainage at allpoints. Depending on surrounding terrain,surface drainage of the roadway can beachieved by a continual cross slope or by aseries of two or more interconnecting crossslopes. Judgment is required to arrangethe cross slopes in a manner to removewater from the airfield at the nearest possi-ble points while taking advantage of thenatural surface geometry.

It is also essential to provide adequatedrainage outside the airfield area to accom-modate maximum flow from the area to bedrained. One or more such provisions willbe required if they do not already exist. Ad-ditionally, adjacent areas and their drainageprovisions should be evaluated to determineif rerouting is needed to prevent water fromother areas flowing across the airfield.

Drainage should be considered a critical fac-tor in aggregate-surface airfield design, con-struction, and maintenance. Therefore,drainage should be considered before con-struction and, when necessary, serve as abasis for site selection.

Maintenance Requirements

Environment and surface migration of mate-rials as the result of traffic are the primaryreasons that an aggregate surface requiresfrequent maintenance. Also, rainfall andwater running over the aggregate surfacetend to reduce cohesiveness by washing thefines from the surface course. Maintenanceshould be performed at least weekly and, ifrequired, more frequently. Experience withaggregate surfaces indicates that the fre-quency of maintenance is initially high, butit will decrease over time to a constant


value. Although the design life of an aggre-gate-surfaced airfield is only 6 months, thedecreasing maintenance allows the designlife to be easily increased for sustained op-erations in the support area. Most mainte-nance consists of replacing fines and grad-ing periodically to remove ruts and potholescreated by passing traffic and the environ-ment. During the lifetime of the airfield, oc-casionally scarifying the surface layer mightbe required to bring fines back to the sur-face. Additional aggregate must be addedto restore the thickness, and the wearingsurface must be recompacted to thespecified density. Additional maintenanceinformation is provided in Chapter 8, FM 5-430-00-1/AFPAM 32-8013, Vol 1.

Dust Control

The primary objective of a dust palliative isto prevent soil particles from becoming air-borne as a result of wind or traffic. Wheredust palliative are considered for traffic ar-

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eas, they must withstand the abrasion ofwheels and tracks. An important factor lim-iting the applicability of the dust palliativein traffic areas is the extent of surface rut-ting or abrasion that occurs under traffic.Some palliative tolerate deformations bet-ter than others, but normally ruts in excessof 1/2 inch will result in the virtual destruc-tion of any thin layer or shallow penetrationdust-palliative treatment. The abrasive ac-tion of aircraft landing gear may be too se-vere for the use of some dust palliative ina traffic area.

A wide selection of materials for dust con-trol is available to the engineer. No onechoice, however, can be singled out as be-ing the most universally acceptable for allproblem situations that may be encoun-tered. However, several materials havebeen recommended for use and are dis-cussed in TM 5-830-3.

FLEXIBLE-PAVEMENT AIRFIELDS

Bituminous (flexible)-pavement designs per-mit the maximum use of readily available lo-cal construction materials. They are easierto construct and upgrade than rigid-pave-ment designs. Thus, they permit greaterflexibility in responding to changes in thetactical situation.

Each type airfield in the basic airfield com-plex has a specific purpose. The type, vol-ume, composition, and character of antici-pated traffic is much greater in the reararea than in the close battle or support ar-eas. Therefore, a different pavement struc-ture and a resilient, waterproof, load-distrib-uting medium that protects the base coursefrom detrimental effects of water and theabrasive action of traffic may be required inthe rear area. In designing a flexible-pave-ment structure, the design values for vari-ous layers are determined and applied tothe curves and criteria in this chapter to de-termine the best structure. Generally, sev-eral designs are possible for a specific site.

Only the most economical, practical designshould be selected. Because the decisionmay be largely a matter of judgment, full de-tails regarding the selection of the final de-sign should be included in the analysis.

Circumstances may warrant the evaluationof an airfield pavement for aircraft otherthan the controlling aircraft. In this case,the design evaluation curves in Appendix Mmay be used for the pass level required.These evaluation curves can be used for de-sign by entering them in reverse order andmay be used when estimating the numberof passes for unknown (captured) airfields.Evaluation of pavements is discussed laterin this chapter.

Pavement Types and Uses

The descriptions, uses, advantages, and dis-advantages of bituminous pavements andsurfaces presented in TM 5-337 are applica-ble to TO construction except as modifiedin the following paragraphs:


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Hot-mix bituminous-concrete pavements.Dense-graded, hot-mix bituminous-concretemixtures are suited for paving airfields withvolumes of 1,000 or more aircraft passes.Where conditions warrant, use these mix-tures to pave airfields having traffic vol-umes of less than 1,000 aircraft passes. Se-lect exact percentages of bituminous materi-als on the basis of design tests described inTM 5-337 and Chapter 9, TM 5-822-8/AFM88-6.

Cold-laid bituminous-concrete plant mix.Where hot-mix bituminous-concrete mix-tures are not available, use cold-plant bitu-minous concrete to pave areas subject topneumatic-tired traffic only.

Bituminous road mix. Use road mix as awearing course for TO roads or as the firststep in stage construction for airfields.When the existing subgrade soil is suitableor satisfactory aggregates are nearby, roadmixing saves time in handling and trans-porting aggregates as compared with plantmixing. When properly designed and con-structed, the quality of road mix ap-proaches that of cold-laid plant mix.

Flexible-Pavement StructureA typical flexible-pavement structure isshown in Figure 9-32, page 9-34, FM 5-430-00-1/AFPAM 32-8013, Vol 1, and illustratesthe terms used to refer to the various layers.

A bituminous pavement may consist of oneor more courses depending on stage con-struction features, job conditions, and eco-nomical use of materials. The pavementshould consist of a surface course, an inter-mediate (binder) course, and when needed,a leveling course. These courses should bethick enough to (1) prevent displacement ofthe base course because of shear deform-ation, (2) provide long life by resisting theeffects of wear and traffic abrasion and act-ing as a waterproofing agent, and (3) mini-mize differential settlements.

Sources of Supply

If time and conditions permit, investigatesubgrade conditions, borrow areas, and allsources of select materials, subbase, base,and paving aggregates before designing the

pavement. When determining subgradeconditions in cut sections of roads, con-duct test borings deeper than the frostpenetration depth. The minimum boringshould never be less than 4 feet below thefinal grade.

NOTE: Not all layers and coats arepresent in every flexible-pavementstructure. Intermediate courses may beplaced in one or more lifts. Tack coatsmay be required on the surface of eachintermediate course while a prime coatmay be required on the uppermostaggregate course.

MATERIALS

Select Materials and SubbasesThe criteria for aggregate layers in a flexible-pavement structure are the same as pre-viously discussed for aggregate-surfaced air-fields. Local materials used to satisfy thesubgrade’s minimum required cover mustsatisfy all the requirements for a given layerthat are listed in Tables 12-8 and 12-9,pages 12-23 and 12-24. (See Chapter 5,FM 5-430-00-1/AFPAM 32-8013, Vol 1, formore specific information.)

Base CoursesAlthough a base course can be either bitu-minous or aggregate, the latter is more com-mon because of its availability and the re-sources involved to work with it. The speci-fications for an aggregate base course in aflexible-pavement structure are the same asthe base course in an aggregate-surfacedairfield. Since a flexible pavement transfersmost of any load to the underlying basecourse, aggregate strength, gradation andcompaction are essential. The CBRstrength of a base course can be deter-mined by the material type in Table 12-10,page 12-24; gradation specifications arelisted in Table 12-11, page 12-26. (SeeChapter 5, FM 5-430-00-1, AFPAM 32-8013, Vol 1, for more information.)

Bituminous PavementsBituminous pavements may be made up ofone or more courses, depending on the totalpavement thickness, economic use of materi-als, stage construction features. availability

12-36 Airfield Structural Design

of equipment, and job conditions. Usually,flexible-pavement airfields in the TO resem-ble aggregate structures with an asphalticconcrete (AC) wear surface. For most air-craft in the rear area, an aggregate struc-ture is suitable only when it has a smooth,water-shedding surface like AC.

If time and resources exist, a flexible pave-ment with a surface course and one or moreintermediate courses is preferred. Once thetotal thickness is known, you can design forintermediate courses based on Table 12-14.Table 12-15 shows the recommended pave-ment thicknesses based on the traffic areaand the strength of the base course.

Generally, if the thickness of the bitumi-nous layer is greater than 2 inches, itshould be placed in two lifts to ensurethat each is properly compacted. Com-pacting a lift greater than 2 inches mayresult in the asphalt cooling before it iscompacted to the required density. Thecompaction requirement for AC is 98-100 percent CE 55. After the pavementmeets the required density, it must beproof rolled. A proof roller is a heavy,rubber-tired roller having four tires,each loaded with 30,000 pounds ormore and inflated to at least 150pounds per square inch (psi). Type Aand Type C areas require a proof rollerto make at least 30 coverages, where asingle coverage is the application of onetire print over each point on the surface.

Designing the actual bituminous pavementmix consists of (1) selecting the bitumenand aggregate gradation, (2) blending aggre-

Table 12-14. Intermediate asphalt courses

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gates to conform to the selected gradation,(3) determining the optimum AC content,and (4) calculating the job mix formula.Mix design is further discussed in Chapter4, TM 5-337, and Chapter 9, TM 5-822-8/AFM88-6.

TRAFFIC AREAS

For previous airfield designs, only Types Band C were considered. Since rear area air-fields have a design life up to two years, itis practical to consider all traffic areas of afull-service airfield. (See Figure 12-5, page12-7, for the following descriptions.)

Type A. Primary taxiways, through taxilanes, and portions of the 1,000-foot endsof the runway are all Type A areas andare designed for the full gross weight ofthe design aircraft. Although the effectsof channelization are evident in the cen-ter lane of taxiways, it is impractical intemporary construction to construct pave-ments of alternating variable thicknesses.

Type B. These areas are also designed forthe gross weight of the design aircraft, butthe repetition of such stress is less thanType A areas. Essentially, all aprons andhardstands are considered Type B.

Type C. These areas are characterizedby a low volume of traffic or a decreasein the applied weight of the operatingaircraft due to lift on the wings. The 75-foot width of the interior portion of therunway (excluding 1,000-foot end sec-tions), ladder taxiways, hangar accessaprons and floors, and washrack pave-

Table 12-15. Minimum thicknesses, pavementand base course

Airfield Structural Design 12-37

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ments are all Type C areas. Decreasethe design gross weight by 25 percentwhen designing a Type C area.

Type D. The outside edges of the entirelength of runway except for the approachand exit areas at taxiway intersections areType D areas. Expected traffic volume inthese areas is extremely low, or the ap-plied weight of the aircraft is considerablyless. Therefore, like Type C areas, de-crease the design gross weight by 25 per-cent when designing these structures.

Overruns. Overruns are generally sur-faced with a multiple surface treatment.The thickness design is usually thesame as the runway design, but it maybe decreased based on design loading.If the airfield is designed for jet aircraft,an overrun blast area may be desirable.This 150-foot strip of overrun is immed-ately adjacent to the runway and is forthe full width of the runway, excludingshoulders. Surface this area with 2inches of hot-mix AC.

Shoulders. The thickness of the shouldersis determined by Figure 12-11, which isvalid for all design aircraft. Enter the figurewith the compacted CBR of the subgradeand insect the curve. From the intersec-tion point, draw a line to the left-hand sideof the figure. The result will be the thicknessof the shoulder after compaction.

THICKNESS DESIGN PROCEDURE

The design procedure for flexible-pavement sur-faces is almost identical to that of aggregatesurface. Since flexible pavements exist onlyin the rear area, however, they are subjected

to fighter and cargo aircraft. While some ofthe curves and specifications may be differ-ent, the design steps are exactly the same.

Step 1. Determine the Airfield LocationFlexible pavements are only constructed in therear area. While existing airfields may provideflexible-pavement surfaces in the support oreven close battle areas, the rear area requiresa flexible-pavement surface to support fighteraircraft as well as large cargo aircraft.

Step 2. Determine the Design Aircraftand Gross WeightWhile previous airfield types limited aircraftbased on their landing capability, the reararea has no constraints about the type ofaircraft that can land. Flexible-pavementstructures have the capability to supportlarge cargo aircraft with tremendous grossweights as well as small fighter aircraftwith large tire pressures. Because of therear area’s diverse mission and the servicelife (six months to two years), it is logical todesign the airfield for only the most con-straining aircraft, the C-141 Starlifter.While it is not the heaviest aircraft in therear area, its gross weight (345 kips) is notdistributed like that of the C-5 or C-17.

Step 3. Check Soils and ConstructionAggregatesThe procedure for evaluating materials forflexible-pavement structures is the same asfor aggregate-surfaced airfield structures.First, locate borrow sites and evaluate themfor suitability as select and subbasecourses. Use Table 12-8, page 12-23, tocheck soil characteristics and strengthagainst the specifications for each layer.Second, check the strength and gradationof the base course. The strength of aknown material is determined from Table12-9, page 12-24, while the gradation of asoil must meet the specifications in Table12-10, page 12-24, based on the MSA.Third, check the materials for frost suscepti-bility. Any frost-susceptible borrow materi-als cannot be used in the design. If thesubgrade is frost susceptible, determine thefrost group and soil support index from Ta-bles 12-11 and 12-12, page 12-26.


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Step 4. Determine the Number of PassesRequiredSince the rear area is considered temporaryconstruction (6 to 24 months), design flex-ible-pavement airfields to sustain an appro-priate number of passes.

Step 5. Determine the Total SurfaceThickness and Cover RequirementsThe procedure for designing the total thick-ness for flexible pavements differs in twoways. First, since the design aircraft is dif-ferent, you must use a different curve. En-ter the curve for the appropriate traffic areawith the soil CBR and number of requiredpasses. The thickness design curve for theC-141 is found in Figure 12-12. The result-ing thickness is the cover required abovethat particular soil layer to protect it fromshear failure. Second, the asphalt thick-ness is a function of the traffic area andthe strength of the base course, and it canbe determined from Table 12-15, page 12-37.

Steps 6 and 7. These design steps are thesame as previously discussed for aggregate-surfaced airfields. See pages 12-25through 12-30 for a review.

Step 8. Determine the CompactionRequirements and Subgrade DepthWhile compaction requirements are thesame as previously discussed, the requireddepth of subgrade compaction changes be-cause of the significant loads in the reararea. Table 12-16 shows the depth of re-quired compaction below the surface of thepavement. Choose the depth for the typesubgrade or 6 inches, whichever is greater.

Step 9. Draw the Final Design ProfileDraw the final design profile as previouslyshown for aggregate airfields.

Example 10Design a rear area airfield in Central Amer-ica, Type B traffic area, capable of handling100,000 passes of a C-141 aircraft. Soil lay-ers have already been determined by thesoils analyst, as follows:

Table 12-16. Depth (inches) of required sub-grade compaction below the surface of rear

area flexible-pavement airfields

Subgrade: Clay, PI = 12, LL = 20, natu-ral CBR = 4, compacted CBR = 5.

Borrow A: Select material CBR = 15, PI= 7.

Borrow B: Subbase material CBR = 40,PI = 4.

Base course (limestone): CBR = 80, PI =4. Meets gradation specifications for 2-inch MSA (Table 12-11, page 12-26).

Solution 10Step 1. Airfield location (given) = reararea/Type B traffic area.

Step 2. Design aircraft (always) =C-141/345 kips.

Step 3. Check soils and construction aggre-gates:

Select and subbase (given): Borrow A,select material CBR = 15: borrow B, sub-base CBR = 40.

Base course: Limestone, CBR = 80;meets gradation.

Frost is not a concern in Central Amer-ica.

Step 4. Number of passes (given) =100,000.


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Step 5. Determine the thickness require-ments from Figure 12-11, page 12-39 (TypeB traffic area):

Step 6. Complete the temperate thicknessdesign.

See Table 12-15, page 12-37, with the traf-fic area (B) and the base course CBR (80)to find that the thickness of the AC pave-ment = 4 inches. See Table 12-14, page12-37, for a further breakdown of the spe-cific course in the pavement design. Next,from Step 4, calculate the layer thick-nesses. For instance, the cover requiredover the select material is 20 inches. Withthe base course and the AC pavement com-bined, the thickness is already 10 inches.To meet the cover requirement over the se-lect material, the thickness of the subbasemust be at least 10 inches.

Step 7. Frost adjustment not applicable.

Step 8. Determine subgrade depth andcompaction requirements. From Table 12-16, page 12-40, determine the requireddepth of subgrade compaction. Since thesubgrade is cohesive (P = 15) and the trafficarea is a Type B, the depth required = 42inches. The total design thickness is 45inches; therefore, the depth of subgradecompaction is 6 inches since 6 inches >3

inches. Next, determine the compaction re-quirements for each layer from Table 12-17.

Table 12-17. Compaction criteria and CBR re-quirements for a flexible-pavement structure



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SPECIAL DESIGN CONSIDERATIONS

STABILIZED SOIL DESIGN

The use of stabilized soil layers within a flex-ible-pavement airfield structure provides theopportunity to reduce the overall thickness re-quired to support a given level. The sectionon stabilized soil design (described in detailin Chapter 9, FM 5-430-00-1/AFPAM 32-8013, Vol 1) pertains to airfield flexible pave-ments as well. As such, only examples ofprocedural applications for flexible-pavementairfields are discussed below:

Design ExampleAssume a conventional flexible-pavement air-field has been designed that requires a totalthickness of 10 inches above the subgrade.The minimum thicknesses of AC and baseare 2 and 6 inches, respectively. The thick-ness of the subbase is 6 inches, the mini-mum layer thickness. Replace the baseand subbase with a cement-stabilized, grav-elly soil having an unconfined compressivestrength of 890 psi. From Figure 9-55,page 9-76, FM 5-430-00-l/AFPAM 32-8013,Vol 1, the equivalency factor for a subbasehaving an unconfined compressive strengthof 890 is 2; and from Figure 9-56, page 9-76, FM 5-430-00-l/AFPAM 32-8013, Vol 1,the equivalency factor for the base is 1.

Therefore, the thickness of the stabilizedsubbase is 6 inches/2 = 3 inches, and thethickness of the stabilized base course is 6inches/ 1 = 6 inches. The final sectionwould be 2 inches of AC and 9 inches of ce-ment-stabilized, gravelly soil. The subgradestill has an equivalent cover of 11 incheswithin the newly designed 2 inches of ACand 9 inches of cement-stabilized, gravellysoil. The savings of 3 inches of aggregatemay prove to be more economical and effi-cient, depending on material, equipment,and time constraints.

The other alternative would be to increasethe base course thickness to 9 inches. Ifmaterial and resources are available, thismay be the most efficient method. How-ever, if the base course material is comingfrom a batch plant or leased contractor, you

may save time by stabilizing a soil and us-ing equivalency factors to reduce the thick-ness design.

FROST REGIONS

Pavements frequently break up or are se-verely damaged when subgrades and materi-als within the flexible-pavement structurefreeze in the winter and thaw in the spring.Besides the physical damage suffered bypavements during periods of freezing andthawing and the high cost of time, equip-ment, and personnel required in mainte-nance, the military loss to the using agencymay be very great (or intolerable) from thestrategy standpoint. The design engineerfor TO rear area airfields must decidewhether to design for frost, given the in-creased thickness and material quality re-quirements.

Investigational Procedures for Frost ActionField and laboratory investigations con-ducted in accordance with FM 5-410 usu-ally provide sufficient information to deter-mine whether a given combination of soiland water conditions beneath the pavementwill be conducive to frost action. The proce-dures for determining whether the condi-tions necessary for ice segregation are pre-sent at a proposed site are described in thefollowing paragraphs:

Soil. Inorganic soils containing 6 percentor more (in the TO) by weight of grainsfiner than 0.02 millimeter are generally con-sidered susceptible to ice segregation.Thus, examination of the fines portion ofthe gradation curves obtained from hydrome-ter analysis or recantation process forthese materials indicates whether they arefrost susceptible. In borderline cases orwhere unusual materials are involved, slowlaboratory freezing tests may be performedto measure the relative frost susceptibility.

Depth of frost penetration. The depth towhich freezing temperatures penetrate be-low the surface of a pavement kept clearedof snow and ice depends principally on the


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magnitude and duration of below-freezingair temperatures, the properties of the un-derlying materials, and the amount of waterthat becomes frozen. Methods are de-scribed in Engineering Manual (EM) 1110-3-138 and TM 5-818-2.

Water. A potentially troublesome water sup-ply for ice segregation is present if the high-est groundwater at anytime of the year iswithin 5 feet of the proposed subgrade sur-face or the top of any frost-susceptible basematerials. When the depth to the upper-most water table measured from the sub-grade surface is in excess of 10 feetthroughout the year, a source of water forsubstantial ice segregation is usually notpresent. In silts or homogeneous clay soils,the water content of the subgrade underpavement is usually sufficient to providewater for ice segregation even with a remotewater table. Additional water may enter afrost-susceptible subgrade by surface infil-tration through pavement and shoulderareas.

Consider all reliable information concerningpast frost heaving and performance duringthe frost-melting period of airfield pave-ments previously constructed in the area.Place emphasis toward modifying or increas-ing frost design requirements.

Counteractive Techniques for Frost ActionThe military engineer cannot prevent the ba-sic condition of temperature affecting frostaction. If a runway is constructed in a cli-mate where freezing temperatures occur, inall probability the soil beneath the pave-ment will freeze unless the period of low-ered temperature is very short. There are,however, several construction techniquesthat may be applied to counteract the pres-ence of water and frost-susceptible soil.

Lowering water table. Try to lower thegroundwater table in relation to the eleva-tion of the runway. This may be accom-plished by installing subsurface drains oropening side ditches if suitable outlets areavailable and the subgrade soil drains.(See Chapter 5, FM 5-430-00-l/AFPAM 32-8013, Vol 1.) It also may be accomplished

by raising the grade line in relation to thewater table. Whatever means are employed,the distance from the top of the proposedsubgrade surface to the highest probableelevation of the water table should not beless than 5 feet. Distances greater than 5feet are desirable if they can be obtained atreasonable cost.

Preventing upward water movement. Inmany cases, it may not be practical tolower the water table. In swampy areas, forexample, an outlet for subsurface drainsmay not be present. Treatments that suc-cessfully prevent the rise of water includeplacing a 4- to 6-inch layer of pervious,coarse-grained soil between the maximumexpected frost depth and the water table.This layer must be designed as a filter toprevent clogging the pores with fine mate-rial. If the depth of frost penetration is nottoo great, it may be cheaper to backfill withgranular material.

Another method (successful, though expen-sive) is to excavate to the frost line, lay pre-fabricated bituminous surfacing (PBS), andbackfill with granular material. In somecases, soil-cement and asphalt-stabilizedmixtures, 6 inches thick, have been used ef-fectively to cut off the upward movement ofwater. Waterproof membranes also may beused.

Removing frost-susceptible soil. Eventhough the site selected may be on idealsoil, long or wide expanses of runways prob-ably will have localized areas containingfrost-susceptible soils. These must be iden-tified, removed, and replaced with selectgranular material. Unless this procedure ismeticulously carried out, differential heav-ing or frost boils may result.

Insulating subgrade against frost. The mostwidely accepted method of preventing pave-ment failure due to frost action is to pro-vide adequate thickness of pavement, base,and subbase over the subgrade. This pre-vents excessive frost heave and providesnecessary load-carrying capacity duringthawing periods. Extruded polystyrene


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thermal insulation has been successfullyused to replace a substantial portion of thebase and subbase.

Snow removal. During freezing weather, ifthe wearing surface is cleared of snow, it isimportant that the shoulders also be keptfree of snow. When this precaution is notfollowed, freezing will set in first beneaththe wearing surface. This permits water tobe drawn into and accumulated in the sub-grade from the unfrozen shoulder area,which is protected by the insulating snow.

If both areas are free of snow, freezing willbegin in the shoulder area because it is notprotected by pavement. Under this condi-tion, water is drawn from the subgrade tothe shoulder area. As freezing progressesto include the subgrade, there will be littlefrost action unless more water is availablefrom groundwater or seepage.

Base Composition Requirements forPreventing Frost Action In FlexiblePavementsBase courses may be made of granular, un-bound materials; bound materials; or a com-bination of both. However, an unboundbase course will not be placed between twoimpervious, bound layers. If the combinedthickness, in inches, of pavement and con-tiguous, bound base course is less than0.09 multiplied by the design freezing in-dex, and the pavement is expected to havea life exceeding one year, not less than 4inches of free-draining material should beplaced directly beneath the lower layer ofbound base. If there is no bound base, ma-terial should be placed directly beneath thepavement slab or surface course.

Frost filter. The free-draining materialshould contain 2 percent or less, by weight,of grains that can pass the Number 200sieve. To meet this requirement, the mate-rial probably will need to be screened andwashed. The material in the 4-inch layeralso must conform to the filter require-ments prescribed in the following para-graphs. If the structural criteria for designof the pavement does not require granular,unbound base other than the 4 inches offree-draining material, the material in the

4-inch layer must be checked for confor-mance with the filter requirements. If itfails the test of conformance, an additionallayer meeting those requirements must beprovided.

Other granular unbound base course. If thestructural criteria for design of the pave-ment requires more granular, unbound basethan the 4 inches of free-draining material,the material should meet the applicable re-quirements of current guide specificationsfor base and subbase materials. In addi-tion, the top 50 percent of the total thick-ness of granular unbound base must beNFS and must contain not more than 5 per-cent, by weight, of particles passing a Num-ber 200 sieve. The lower 50 percent of thetotal thickness of granular, unbound basemay be NFS or partially frost-susceptible(PFS) material (S1 or S2). (See Table 12-11, page 12-26.) If the subgrade soil isPFS material meeting the requirements ofcurrent guide specifications for base or sub-base, the lower 50 percent of granular basewill be omitted. If subgrade freezing will oc-cur, an additional requirement is that eitherthe bottom 4-inch layer in contact with thesubgrade must meet the filter requirements,or a geotextile fabric meeting the filter re-quirements must be placed in contact withthe subgrade. The dimensions and perme-ability of the base should satisfy the basecourse drainage criteria given in Chapter 2,TM 5-820-2/AFM 88-5, and the thicknessrequirements for frost design. If necessary,thicknesses indicated by frost criteriashould be increased to meet subsurfacedrainage criteria. Base course materials ofborderline quality should be tested fre-quently after compaction to ensure theymeet these design criteria. Subbase andbase materials must meet applicable com-paction requirements.

Use of F1 and F2 Soils for Base Coursesin Roads and Airfields with Short LifespansA further alternative is the use of PFS basematerials permitted for all roads and air-fields with short life spans (less than oneyear).


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Materials of frost groups F1 and F2 may beused in the lower part of the base over F3and F4 subgrade soils. F1 materials maybe used in the lower part of the base overF2 subgrades. The thickness of F2 basematerial should not exceed the difference be-tween the reduced subgrade strength thick-ness requirements over F2 and F3 sub-grades. The thickness of F1 base shouldnot exceed the difference between the thick-ness requirements over F1 and F2 sub-grades. Any F1 or F2 material used in thebase must meet the applicable requirementsof the guide specifications for base, sub-base, or select materials. The thickness ofthe F1 and F2 materials and the thicknessof pavement and base above the F1 and F2materials must meet the nonfrost criteria.

Filter Over SubgradeGranular filters. For both flexible and rigidpavements where subgrade freezing will oc-cur, at least the bottom 4 inches of granu-lar unbound base should consist of sand,gravelly sand, screenings, or similar mate-rial. It should be designed as a filter be-tween the subgrade soil and overlying basecourse material to prevent mixing of thefrost-susceptible subgrade with the baseduring and immediately following the frost-melting period. This filter is not intendedto serve as a drainage course. The grada-tion of this filter material should be deter-mined in accordance with the following crite-ria to prevent movement of particles of theprotected soil into or through the filter(s):

The percent size in these equations is usedto determine a particle size. For example,the 15 percent size refers to a grain size inmillimeters at which 15 percent passes on aGrain Size Distribution Graph.

To offset the tendency of segregation of thefilter material, a coefficient of uniformity ofnot more than 20 will be required.

The filter material must be NFS or PFS. Ex-perience shows that a fine-grained subgradesoil will work up into a coarse, open-gradedoverlying gravel or crushed stone basecourse under the kneading action of trafficduring the frost-melting period if a filtercourse is not provided between the sub-grade and the overlying material. Experi-ence and tests indicate that well-gradedsand is especially suitable for this filtercourse. The 4-inch minimum filter thick-ness is dictated primarily by construction re-quirements and limitations. Greater thick-nesses should be specified when required tosuit field conditions. Over weak subgrades,a 6-inch or greater thickness may be neces-sary to support construction equipment andto provide a working platform for placementand compaction of the base course.

Geotextile fabric filters. The use of geotex-tile filters in lieu of a granular filter is en-couraged. No structural advantage will beattained in the design when a geotextile fab-ric is used; it serves as a separation layeronly. Filter criteria for geotextile filtersfound in Chapter 2, AFM 88-5, is as follows:

85 percent size of material adjacent to fabric 1

equivalent s ize of fabr ic openings

DESIGN OF PAVEMENTS FOR FROSTACTION

In the reduced subgrade strength method ofdesign, the design curves for the C-141 (Fig-ure 12-12, page 12-41) should be used todetermine the combined thickness of flex-ible pavement and base required for aircraftloads. The curves should not be enteredwith subgrade CBR values determined bytests or estimates but with one of the appli-cable frost area soil support indexes shownin Table 12-11, page 12-26.


The soil support index for PFS soils meetingcurrent specifications for base and subbasewill be determined by conventional CBRtests in the unfrozen state.

FIELD CONTROL FOR FROSTCONDITIONS

Inspection of airfield and road pavementconstruction in areas of seasonal freezingand thawing should emphasize looking forconditions and materials that promote detri-mental frost action. Remove unsuitable ma-terials where such conditions exist. Person-nel assigned to quality control must be ableto recognize unsuitable materials.

Subgrade PreparationWhere laboratory and field investigations in-dicate that the soil and groundwater condi-tions will not result in ice segregation inthe subgrade soils, the pavement design isbased on the assumption that the inspec-tion personnel must check the validity ofthe design assumptions and take correctiveaction if pockets of frost-susceptible mate-rial and wet subgrade conditions are re-vealed.

The subgrade is to be excavated and scari-fied to a predetermined depth, windrowed,and bladed successively to achieve adequateblending. It is then relaid and compacted.The purpose of this work is to achieve ahigh degree of uniformity of the soil condi-tions by mixing stratified soils, eliminatingisolated pockets of soil of higher or lowerfrost susceptibility, and blending the vari-ous types of soils into a single, homogeneousmass. It is not intended to eliminate soilsfrom the subgrade in which detrimentalfrost action will occur, but it is intended toproduce a subgrade of uniform frost suscep-tibility and thus create conditions tendingto make both surface heave and subgradethaw weakening as uniform as possible overthe paved area.

The depth of subgrade preparation, meas-ured downward from the top of the sub-grade, should be the lesser of the following:

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24 inches.

72 inches, less the actual combinedthickness of pavement, base, and sub-base.

Prepared subgrade must meet the compac-tion requirements stated in Step 8 of theflexible-pavement airfield design procedure,page 12-40. At transitions from cut to fill,the subgrade in the cut section should beundercut and backfilled with the same mate-rial as the adjacent fill. (See TM 5-818-2.)

Exceptions to the basic requirement for sub-grade preparation in the preceding para-graph are limited to the following:

Subgrades known to be NFS to thedepth prescribed for subgrade prepara-tion and known to contain no frost-sus-ceptible layers or lenses, as demon-strated and verified by extensive andthorough subsurface investigations andby the performance of nearby existingpavements.

Fine-grained subgrades containing mois-ture well in excess of the optimum forcompaction, with no feasible means ofdrainage nor of otherwise reducingwater content. Consequently, it is notfeasible to scarify and recompact thesubgrade. If wet, fine-grained soils ex-ist at the site, it is necessary to achieveequivalent frost protection with fill mate-rial. This may be done by raising thegrade by an amount equal to the depthof subgrade preparation that would oth-erwise be prescribed, or by undercut-ting and replacing the wet, fine-grainedsubgrade to the same depth. In eithercase, the fill or backfill material may beNFS or frost-susceptible material meet-ing specified requirements. If the fill orbackfill material is frost-susceptible, itshould be subjected to the same sub-grade preparation procedures pre-scribed above.

Correction for gradation changes. Performgradation tests on all questionable materi-als found during grading operations. In an


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otherwise NFS subgrade, remove all pocketsof frost-susceptible soils to the full depth offrost penetration. Replace frost-susceptiblesoils with NFS material when possible.

Wherever the design indicates that frost ac-tion may be a problem, the construction en-gineer must ensure that special frost protec-tion measures are adequate and provisionsin this chapter for base composition designare strictly followed.

Correction for special subgrade conditions.Besides abrupt variations in soil charac-teristics, frequent sources of trouble includesudden changes in groundwater conditions;changes from cut to fill; and location of un-der-pavement pipes, drains, or culverts.The top soil and humus materials at thetransition between cut-and-fill sectionsshould be completely removed for the fulldepth of frost penetration in otherwise NFSmaterials, even though the specificationsmay not require stripping of the subgradein fill areas.

Carefully check wet areas in the subgrade.and install special drainage facilities as re-quired. The most frequent special need inairfield construction is to provide intercept-ing drains. These drains prevent infiltra-tion of water into the subgrade from higherground adjacent to the road.

Preparation of rock subgrades. In areaswhere rock excavation is required, examinethe character of the rock and seepage condi-tions. The excavation should always pro-vide transverse drainage to ensure no pock-ets are left in the rock surface that permitpending of water within the maximumdepth of freezing. The irregular presence ofgroundwater may result in heaving of thepavement surface under freezing conditions.It may be necessary to fill drainage pocketswith lean concrete. Stones larger that 12inches in diameter should be removed fromfrost-susceptible subgrades to prevent boul-der heaves from damaging the pavement.This boulder removal must be accomplishedto the depth of subgrade preparation out-lined in the preceding paragraphs.

Where seepage is great, cover the rock sub-grade with a high pervious gravel materialso water can escape. Fractures and jointsin the rock surface frequently contain frost-susceptible soils. Clean these soils out ofthe joints to the depth of frost penetrationand replace them with NFS material. Ifthis is impractical, it may be necessary toremove the rock to the full depth of frostpenetration. Blasting the rock in place toprovide additional cracks for the downwardand lateral movement of water has alsobeen successful. If blasting is used, rockshould be broken to the full depth of frostpenetration.

Base-Course ConstructionWhere available, base-course materials (in-cluding any select and subbase layers) areclearly NFS; base-course construction con-trol should be in accordance with normalpractices. When the selected base-coursematerial is borderline frost susceptibility(usually having as much as 3 percent byweight of grains finer than 0.02 millimeter),make frequent gradation checks to ensurematerials meet design criteria.

If pit selection of base material is required,inspect the materials at the pit. It is easierto reject unsuitable material at the sourcewhen large volumes of base course are be-ing placed.

It is frequently desirable to check the grada-tion of materials taken from the base aftercompaction; for example, check gradationson density test materials. This proceduredetermines whether fines are being manufac-tured in the base under the passage of thebase course compaction equipment.

Avoid mixing base-course materials withfrost-susceptible subgrades by ensuring thesubgrade is properly graded and compactedbefore placing the base course. Also, en-sure the first layer of base course or sub-base is thick enough and provides sufficientfilter action to prevent penetration of sub-grade fines under compaction. Excess wet-ting by hauling equipment may cause mix-ing of subgrade and base materials. This


can be greatly reduced by frequently rerout-ing hauling equipment.

After completing each layer of the aggregatecourse, carefully inspect them before permit-ting placement of additional material. Thisensures there are no areas with a high per-centage of fines. These areas may fre-quently be recognized by visual examinationof the materials and by observation of theiraction under compaction equipment, particu-larly when the materials are wet.

Remove materials that do not meet the re-quirements or specifications and replacethem with suitable material.

FROST DESIGN FOR STABILIZEDRUNWAY OVERRUNS

A runway overrun pavement must be de-signed to withstand occasional short land-ings, aborted takeoffs, long landings, andpossible barrier engagements. The pave-ment also must give service under the traf-fic of various maintenance vehicles such ascrash trucks and snowplow equipment.

Frost Condition Requirements

The design of an overrun must provide forthe following frost conditions:

Adequate structure for infrequent air-craft loading during the frost-melting pe-riod.

Adequate structure for normal traffic ofsnow-removal equipment and othermaintenance vehicles during frost-melt-ing periods.

Sufficient thickness of frost-free, base-or subbase-course materials to protectagainst objectionable heave during freez-ing periods.

Frost Design CriteriaTo provide adequate strength during frost-melting periods, the combined thickness offlexible pavement and NFS base and sub-base course must be equal to 75 percent ofthe thickness required for normal frost de-

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sign, based on reduced subgrade strength.The thickness established by this procedurewill not be less than that required for con-ventional flexible-pavement design.

Arid regions. In regions where the annualprecipitation is less than 15 inches and thewater table (including perched water table)is at least 15 feet below the finished pave-ment surface, the danger of high moisturecontent in the subgrade is reduced. Whereinformation on existing structures in theseregions indicates that the water content ofthe subgrade will not increase above opti-mum (as determined by the CE 55 compac-tion test), the total thickness above the sub-grade (as determined by CBR tests onsoaked samples) may be reduced by 20 per-cent. The reduction is made in the selectmaterial or in the subbase courses havingthe lowest CBR value. The reduction ap-plies to the total thickness dictated by thesubgrade CBR.

If only limited rainfall records are availableor the annual precipitation is near the 15-inch criterion, before any reduction in thick-ness is made, carefully consider such fac-tors as the number of consecutive years inwhich the annual precipitation exceeds 15inches and the sensitivity of the subgradeto small increases in moisture content.

Arctic regions. Airfield construction in arc-tic regions will be a rare occurrence. Whenconstruction is called for, engineer unitswill find construction in extreme environ-mental conditions difficult at best. Snowpavements requiring strength charac-teristics above CBR = 40 are difficult to pro-duce with practical construction methods.With specialized equipment, the strength ofsnow pavement can be achieved with dry-processing methods (milled or mixed snow,compacted with tractor tracks, vibrators,and rollers) if temperatures during construc-tion are in the -12° to -1° Celsius (C) range.As the temperature decreases, particularlybelow -18°C, the compaction effectivenessdecreases, the rate of age hardening (or sin-tering) decreases, and equipment opera-tional problems increase.


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Due to the low rate of the age-hardeningprocess in arctic temperature conditions, aone-year waiting period after constructionmay be required before C-141 aircraft opera-tions could be considered. That is, if run-way construction can be successfully com-pleted during one season, the hardeningprocess may require a full season to pro-gress to a stage where the snow strength ap-proaches that required for C-141 aircraft.A design-and-testing manual for the con-struction of compressed-snow runway pave-ment can be found in Appendix B of theCorps of Engineers Cold Regions Researchand Engineering Laboratory Special Report89-10, April 1989. Appendix C contains aconstruction manual for compacted snowrunways. The report is available from theUS Army Cold Regions Research and Engi-neering Laboratory, 72 Lyme Road, Hano-ver, NH 03755-1290.

Airfields also can be constructed on glacialice. Unlike cold, dry snow, blue ice (typicalof most ice found on Antarctica) has suffi-cient bearing capacity to support heavywheel loads without rutting, even at thehighest imaginable inflation pressures foraircraft tires. Blue ice is solid glacier ice.It is usually hundreds of meters thick andrests on solid rock (on an ice shelf, it maybe afloat). By contrast, typical sea ice is athin (<3 meters), viscoelastic plate floatingfreely on a liquid foundation and shouldnot be considered for airfield construction.If blue ice is smooth and level over a suffi-cient distance and approaches are unob-structed by high terrain, then it is highlysuitable for use as an airfield. Unfortu-nately, there are few blue-ice ablation areasin arctic regions. Most of the existing areasare unsuitable for use as airfields becausethey are not smooth, level, and unob-structed.

EVALUATION OF AIRFIELD PAVEMENTS

The design of airfield pavement is based oncovering a subgrade of given strength withadequate thickness of a suitable basecourse and pavement to prevent the sub-grade from being overstressed under a givenload. This same principle applies to anylayer in the system—the base, subbase, se-lect material, or subgrade.

The evaluation of an expedient, aggregate,or flexible pavement is the reverse of thisprocess. It determines the allowable load-ing when the in-place thickness, strength,and quality of the materials in the variouslayers have been established. The thick-ness of the pavement and soil layers is de-termined by actual measurement. Thestrength of the subgrade and overlaying sub-base and base courses is determined usingCBR tests. Also, the purpose of a pave-ment evaluation may be much differentthan that of a design. You should designthe airfield for the design aicraft, but to en-sure the airfield’s suitability to changingconditions, you may evaluate an airfield foruse by only one particular aircraft for oneor more missions. Since the C-130 and

C-17 are the predominant aircraft in theclose battle and support areas, evaluationbecomes significant only in rear areas,where existing pavements may support mis-sions short of the large cargo aircraft. Forthis reason, Appendix K includes flexible-pavement curves for most aircraft that nor-mally operate on flexible pavements in therear area.

The quality of the bituminous pavementstructure is the ability of the various layersto support traffic and withstand jet-fuelspillage and blast. The quality of materialsin the various layers is determined by vis-ual observation, tests on in-place materials,laboratory tests on samples of the materi-als, and construction data.

The load-carrying capacity of a flexible pave-ment is limited by the strength of its weak-est component—the bituminous pavement,base, subbase, select, or subgrade. Theability of a given subsurface layer to with-stand the loads imposed on it depends onthe thickness of material above it and itsstrength in its weakest condition.


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An evaluation must consider possible futureincreases in moisture content, increases ordecreases in density, and the effects of freez-ing and thawing. The expected use governsthe amount of time and effort spent on theevaluation of an existing airfield. If timepermits and the required life of the pave-ment is two years, the evaluation effortshould be thorough with appropriate modifi-cation for lesser life requirement.

Pavement evaluation may be based on exist-ing design- and construction-control data,data obtained in tests performed especiallyfor evaluation, or a combination of the two.Condition surveys record the existing pave-ment condition of the facilities being evalu-ated.

NEW PAVEMENTS

If the airfield to be evaluated was built byUS forces, construction-control test dataadequate for an evaluation may be secured.Extract data from these records on condi-tions and materials pertinent to the evalu-ation. The number of test values neededfor an evaluation cannot be prescribed, butobtain enough data to establish a reason-ably good statistical probability curve.Where data is insufficient and time permits,perform supplemental tests in accordancewith the discussion in the following section:

OLD PAVEMENTS

The following paragraphs describe proce-dures for evaluating an existing field forwhich no design or construction-controldata is available and for which field andlaboratory tests must be made. The condi-tion survey mentioned above is made first.Then, test locations are selected, in-placetests are made, samples for laboratory testsare secured, and test pits are backfilled.Laboratory tests are the final phase in dataprocurement.

Selection of Test LocationsSelecting the most representative test loca-tions is essential for an accurate evalu-ation. Also, hold the number of test

locations to a minimum to reduce interrup-tions in normal aircraft operations on the fa-cilities being tested.

The first step in the selection of test loca-tions is to prepare longitudinal profiles ofthe runways, taxiways, and aprons. Thisdevelops a general picture of subgrade, sub-base, base, and pavement conditions. Fromthese profiles, select test pit locationswhere more detailed tests can be per-formed.

The profiles should show information regard-ing thickness and types of pavement, base(includes all aggregate layers), and sub-grade soil classification. Obtain this databy cutting small holes in the pavementthrough which thickness measurements canbe made. Then, sample the base course,subbase course, and subgrade. Space thesmall holes about 500 feet apart. A widerspacing may be adequate when uniform con-ditions exist. Classify the samples in ac-cordance with the USCS (see FM 5-410).Determine the moisture content becausevariations in moisture content often indi-cate variations in soil strength.

After profiles have been developed, studythem to determine representative condi-tions. Test pits should be located in typicalbase and subgrade conditions or where sig-nificant strength or traffic intensity exists.Ensure test pits are placed where maximuminformation can be obtained with minimumtesting.

After typical areas of high and lowstrengths in each material have been deter-mined by observing pavement condition orby studying soil profile, place test pits in ar-eas where traffic intensity is high. This per-mits determination of the soil strength un-der traffic conditions.

When no weak areas are found, place testpits where traffic is heavy and loading conditions are most severe. These cond it ionsare usually found in the 1,000-foot sectionsat runway ends, in the entire lenght of taxiways, and in taxi lanes on aprons. An airfield having a runway, an apron, and


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connecting taxiways usually requires 12 to20 test pits for adequate overall evaluation.A minimum of two pits (one at each end)should be required on aprons and on thetaxiway system. More pits may be neces-sary when weak or failed areas are encoun-tered.

Test pits. Test pits (approximately 4 feetwide by 6 feet long) are dug through thepavement to permit the performance of in-place tests and to obtain samples for labora-tory tests. Record a description of generalconditions in each test pit and visuallyclassify the materials from pit to pit. Meas-ure the pavement thickness to the nearestquarter of an inch. Make several measure-ments around the sides of the pit to obtainrepresentative values.

Describe each soil course, giving color, insitu conditions, texture, and visual classifi-cations. Perform in-place moisture content,CBR (except when moisture conditions arenot satisfactory), and density tests on thebase course and subgrade. Use judgmentwhen selecting test locations in the pit.Place the CBR piston or penetrometer sothe surface to be penetrated represents anaverage condition of the surface beingtested. The piston should not be set on un-usually large pieces of aggregate or othermaterials.

Space CBR tests in the pit so that areascovered by the surcharge weights of the in-dividual tests do not overlap. Performthese tests on the surface and at each full6-inch depth in the base and subbasecourses, on the surface of the subgrade,and on underlying layers in the subgradeas needed.

Determine the density and moisture contentat 1 -foot intervals to a total depth of 4 feetbelow the surface of the subgrade. Use theresults of density and moisture tests atthese depths to decide whether additionalCBR tests are needed. Locate the tests inthe pit so the density determinations areperformed between adjacent CBR tests.

Moisture content determination. Whencoarse material makes up 40 percent ormore of the base course, the moisture con-tent of the fine portion may influence thebehavior of the base course more, with re-spect to strength, than the moisture con-tent of the total sample.

The critical portion of material consideredis that part passing sieve sizes rangingfrom Number 200 to Number 4. The Num-ber 40 sieve is the sieve on which separa-tions for the Atterberg liquid and plasticlimit determinations are made. The mate-rial passing the Number 40 sieve is used todetermine the soil’s plastic and liquid limits.

CBR tests. Perform three in-place CBRtests (as described in FM 5-5301) or equiva-lent tests with one of the CBR expedientmethods at each elevation tested. However,if the results of these three tests do notshow reasonable agreement, make three ad-ditional tests. A reasonable agreementamong three tests permits a tolerance of 3where the CBR is less than 10, 5 where itranges from 10 to 30, and 10 where itranges from 30 to 60. Where the CBR isabove 60, variations in individual readingsare not important.

For example, test results of 6, 8, anti 9 arereasonable and can be averaged as 8. Re-sults of 18, 20, and 23 are reasonable andgive an average of 20. If the first threetests do not fall within this tolerance, makethree additional tests at the same locationand use the numerical average of the sixtests as the CBR for that location.

Generally, CBR values below 20 arerounded to the nearest point. Values above20 are rounded to the nearest 5 points.Obtain a moisture-content sample at thepoint of each penetration.

Density determination. Make three densitydeterminations at each elevation tested ifsamples of about 0.05 cubic foot volumeare taken. If somewhat larger samples aretaken, decrease the number of density deter-minations to two. If a reasonable agree-ment is not found among the test results,


perform two additional tests. A reasonableagreement is considered to provide for a tol-erance of about 5 pounds per cubic foot(pcf) wet density. For example, test resultsof 108, 111, and 113 pcf wet density are inreasonable agreement and can be averagedas 111 pcf.

Sampling. Obtain samples of typical pave-ment, base-course, and subgrade materialsfor laboratory tests. Take the base and sub-grade samples to ensure representative ma-terials.

Backfilling. Holes cut in flexible pavementscan be backfilled satisfactorily if a few pre-cautions are followed. Backfill the sub-grade with a material similar to that re-moved. Place the material in about 3-inchlifts and compact them to the required den-sity with a pneumatic tamper. The backfillfor the base course should consist of a ma-terial similar to that removed and should becompacted to a high density. The surfaceof the base course should be primed andthe sides of the adjacent pavement swabbedwith a liquid asphalt. Use an RC-70 incold weather and an MC-80 in hot weather.

A hot-mix AC is best for patching pave-ment, but many successful patches havebeen made with a cold mix. Avoid a coldmix when it will be subjected to jet blast orfuel spillage. It is not necessary to heat acold mix in hot weather unless it has hard-ened. In cold weather, however, the mate-rial must be heated until it can be handledsatisfactorily.

Compact the patching material thoroughlywith a pneumatic tamper. If cold mix isused, swab the surface with liquid asphaltand cover it with small aggregate. Use asmooth-wheeled or pneumatic roller overthe surface.

LABORATORY TESTS

Laboratory tests provide data with which toclassify materials and determine theirstrength characteristics. In the laboratory,materials can be reworked or their moistureconditions adjusted to arrive at an estimate

FM 5-430-00-2/AFJPAM 32-8013, Vol II

of the strength expected under future condi-tions of increased density or moisture. Thefollowing tests apply to both satisfactoryand failed areas.

Pavement TestsWhere a pavement consists of more thanone course, the cores obtained for testingshould be split at the interfaces of the vari-ous courses so that each course can betested separately. Test the cores of eachcourse in the laboratory for Marshall stabil-ity; flow; percentage of asphalt; aggregatetype, shape, and gradation; specific gravityof bitumen and aggregate; and density.Compute the voids in the total mix and thepercentage of voids filled with asphalt fromthe test results.

Use parts of the chunk samples to deter-mine aggregate gradation; specific gravityof asphalt and aggregate; and penetra-tion, ductility, and softening point of theasphalt. Disintegrate and recompactother chunk samples, and test the recom-pacted specimens for Marshall stability,flow, and density. Compute their void re-lationships.

The stability of cores cut from the pavementmay be lower than that of the recompactedsample. Part of this difference is due to dif-ferences in density because field cores sel-dom have a density as high as the labora-tory-compacted samples. Most variation instability is attributed to differences in struc-ture of the field and laboratory samples.Another factor is that the asphalt hardensduring reheating.

Remove and replace the mix if results aretotally unfavorable (for example, if stabilityis under 500 based on 50 blows or if flowis greater than 20 based on 50 blows).Sometimes, additional compaction increasesthe stability. (For voids total mix, toleranceis within 1 percent of specifications; forvoids filled with asphalt, tolerance is within5 percent of specifications.)

No standard tests have been developed todetermine resistance to spillage. However,a small amount of jet fuel should be spilled


FM 5-430-00-2/AFJPAM 32-8013, Vol II

on one chunk from each test pit to see ifthe fuel penetrates the sample quickly or ifit puddles on the surface.

Base-Course, Subbase, and Subgrade TestsObtain classification data consisting of Atter-berg limits, gradation, and specific gravitydeterminations from design and construc-tion-control tests or tests performed on sam-ples of base-course, subbase, and subgradematerials remolded at three compaction ef-forts. Develop the moisture density andCBR relationships for the CE 55 compac-tion test. Where available, include resultsof tests made on the soaked and unsoakedcondition for possible future use.

MAKING THE EVALUATION

Evaluation of expedient pavement requiresless effort than evaluation of flexible pave-ment. Expedient-pavement evaluation proce-dures are similar to expedient-pavement de-sign procedures covered previously in thischapter.

Evaluation of a flexible pavement consistsof two principle determinations—the load-carrying capacity of the entire pavementstructure and the quality of the bituminouspavement. The load-carrying capacity isevaluated by applying the proper criteria tothe factors of pavement thickness; the CBRof the base course, CBR of subbase, or com-bined thickness of all courses above thesubgrade; and the CBR of the subgrade.The quality of bituminous pavement isjudged by its ability to withstand trafficloads, fuel spillage, and jet blast.

Evaluation of rigid pavements requires anunderstanding of its characteristics, whichare beyond the scope of this chapter. Theactual procedure, however, is very similar toflexible-pavement evaluation. Essentially,the strength of the subgrade and the condi-tion of the rigid pavement determinewhether an existing airfield requires addi-tional overlays to support certain aircraft.(Rigid pavement evaluation is covered inChapter 3, TM 5-826-3/AFM 88-24.)

EXPEDIENT- ANDAGGREGATE-SURFACED AIRFIELDS

The evaluation of unsurfaced, mat-surfaced,and aggregate-surfaced pavements to deter-mine the number of allowable traffic cyclesis conducted using the appropriate designaircraft (C-130 or C-17). Since these arethe only major aircraft that can operate onsmall, semiprepared, austere airfields, thecurves used in the design process can beused in the evaluation. The procedure isvery similar to the actual design procedure.

Example 11Given an unsurfaced airfield with a CBR of9 and soil type ML, determine the numberof allowable aircraft passes for a C-130 air-craft with a weight of 130,000 pounds.

Solution 11The aircraft, weight, and CBR are all given.Using Figure 12-6, page 12-11, enter theleft side at CBR = 9 and read right (horizon-tally) to the C-130 curve. Follow downwardand determine that there are 740 allowableaircraft passes.


FLEXIBLE-PAVEMENT AIRFIELDS

The evaluation of flexible pavements shouldbe based on existing conditions. Do notconsider the minimum allowable designthickness and maximum allowable designCBR values in Table 12-8, page 12-23, inthe evaluation. The evaluation to determinethe number of passes allowable is con-ducted using Figures K-1 through K-40,pages K-1 through K-40, because the evalu-ation may or may not be based on the C-141 (the design aircraft for flexible pave-ments). Use the following procedure:

Determine the aircraft type and weightthat will use the pavement. If the air-craft operating weight is unknown, usethe weight for the design aircraft(C-141/345 kips).

Determine the traffic area to evaluate(see Figure 12-5, page 12-7). Do not in-clude overrun areas, Type D traffic ar-eas, blast pads, and other nonload-car-rying pavements in the evaluation.However, prepare a statement of theircondition (as determined by visual in-spection) and record the thickness andquality of the various layers.

Determine the layer thickness of the se-lect, subbase, base, and AC surface ac-cording to the procedure for the DCP orthis chapter.

Determine the CBR of the subgrade, se-

FM 5-430-00-2/AFJPAM 32-8013, Vol II

Another method for determining sub-grade CBR is to use the DCP (AppendixJ). The CBR of the select, subbase, andbase also could be established from con-struction drawings, the AFCESA, or theCorps of Engineers’ evaluation reports.The CBR also can be estimated basedon soil classification (see FM 5-410).The physical properties on which theevaluation is based are presented in Ta-ble 12-18. The evaluation for the run-way is based on the pavement thicknessin the central 100-foot width for all gearconfigurations.

Select the appropriate evaluation curvefrom Figures K-1 through K-40, pagesK-2 through K-41. If there is nocurve for the aircraft considered, usethe curve for the controlling aircraft(C-141).

Use the appropriate curve by enteringthe top with the cover thickness abovethe subgrade (total thickness of higherCBR material existing above the top ofthe subgrade). Follow downward to thegross aircraft weight. Then, move hori-zontally (left or right) to the subgradeCBR. Finally, move downward to deter-mine the number of allowable aircraftpasses. Repeat this procedure for eachpavement layer (select, subbase, andbase) using the cover thickness on topof the layer being considered. Once thenumber of allowable aircraft passes hasbeen determined for each pavement

lect, subbase, and base. The CBR testcan be conducted using test pits de-scribed previously in this chapter.

layer, the most conservative (that is, thelowest) number will control the evalu-ation.

Table 12-18. Summary of physical property data


MISPRINT

No figure K-40.

FM 5-430-00-2/AFJPAM 32-8013, Vol II

Example 12Determine the number of allowable aircraftpasses for all F-4 aircraft with a weight of62,000 pounds. The test pit evaluation of acaptured enemy airfield indicates the follow-ing conditions:

Type B traffic area (primary taxiway).

4 inches of AC.

6 inches of base course, CBR = 80 (GW).

4 inches of subbase course, CBR = 40(GM).

Subgrade, CBR = 10 (CL).

Solution 12The aircraft type (F-4), weight (62 kips), traf-fic area (B), layer thicknesses, and CBR val-ues have all been provided. Using Figure K-38, page K-38, enter the top with a coverthickness (above the subgrade) of 14inches. Read downward to a gross aircraftweight of 62 kips. Then, read right toreach the subgrade CBR value of 10. Fi-nally, read downward to determine a sub-grade allowable pass level of 10 , or 10,000passes. Repeat these steps for the sub-base. Enter the cover thickness = 10inches, gross aircraft weight = 62 kips, andsubbase CBR = 40. The allowable subbasepass level is greater than 100,000. Now,evaluate the base course. Enter the coverthickness = 4 inches, weight = 62 kips, andbase CBR = 80. The allowable base passlevel is greater than 100,000. The sub-grade controls the evaluation with 10,000passes.

Example 13Evaluate the airfield described in the pre-vious example for a C-5A weighing 700,000pounds.

Using the same steps as in the previous ex-ample, the allowable passes are as follows:

Subgrade = 140.

Subbase = 100,000+.

Solution 13The subgrade controls the evaluation with140 allowable passes.

Selection of Strength and ThicknessvaluesCarefully select CBR values for use in anevaluation. Thickness values are selectedfrom design or actual measured thicknessfor the base and subbase layers.

CBR test results from an individual test pitare seldom uniform. Therefore, study thedata carefully to determine reasonable val-ues for the evaluation. There are no rulesor formulas for the number of valuesneeded. This is a matter of engineeringjudgment. The following guidelines may as-sist in determining the number needed.A minimum of five CBR values per facilityis required even when the material isknown to be uniform, when control tests in-dicate that placement is uniform, and whenavailable values cover a narrow range.When the uniformity of material andconstruction are not known, the number oftest values should be sufficient to establisha good statistical distribution.

To select values for an evaluation, plot testresults on profiles or arrange them in tabu-lar form to show the range of the data. Inmost cases, the value selected should be alow average, but it should not be the lowestvalue in a range.

When conditions are uniform, the lowerquartile value from a cumulative distribu-tion plot may be used. Where conditionsare not uniform, the following example maybe helpful.

The subgrade material beneath a facility be-ing evaluated varies so that the facility maybe divided into several large areas of differ-ing subgrade material. The in-place CBRvalues for the entire facility, arranged in as-cending order, are as follows: 7, 7, 8, 9, 9,10, 14, 14, 15, 16, 20, 21, 21, 22, 28, 30,30, and 31. A study of in-place conditionsreveals the degree of saturation of the sub-

Base = 100.000+.grade is about the same for the entire areacovered by the facility, and the degree of


saturation is high enough that in-placeCBR values can be used for evaluation.

Preliminary analysis of this data shows thestatistical distribution for the whole facilityis not good, and the values logically fallinto four groups. Each group representsone of the areas of different material. Themost critical area is that represented by therange of values from 7 to 10 because moreof the values fall in that group than in anyother. Thus, the evaluation should bebased on this area.

Because the range is narrow, a formal sta-tistical analysis is not necessary. A visualinspection of the figures indicates a valueof 8 or 9 should be selected.

Regardless of the number of values avail-able and the method used to select theevaluation figure, the number of values andthe analytical process used should be de-scribed in the evacuation report in sufficientdetail to be easily understood later.Because of certain inherent difficulties inprocessing samples for laboratory tests andin performing in-place tests on base coursematerials, it is advisable to assign arbitraryCBR values to certain materials based ontheir service behavior (see Table 5-3, page 5-12, FM 5-430-00-l/AFPAM 32-8013, Vol 1).Use these CBR values for base-course mate-rial when the material meets quality require-ments of the specification.

When evaluation tests are made less thanthree years after construction and indicateplasticity index values greater than 5, con-sider in-place CBR values but assign novalue greater than 50. When tests aremade three years or more after constructionand indicate plasticity index values greaterthan 5, use in-place values.

When evaluation tests on subbase materialsare made less than three years after con-struction and tested materials meet the sug-gested requirements, consider in-place CBRvalues, but assign no value greater than50. When tests are made three years ormore after construction, use in-place val-ues.

FM 5-430-00-2/AFJPAM 32-8013, Vol II

Sometimes, CBR tests tend to underrate cer-tain cohesionless, nonplastic materials thatare not confined. If records show adequateperformance and service behavior for thesematerials, use judgment to assign an arbi-trary CBR value for evaluation.

Quality Of Bituminous PavementThe condition of a bituminous pavement,either surface or binder course, is evaluatedat the time of sampling by comparing thetest data from the core samples with designcriteria in TM 5-337. Future behavior ofthe pavement under additional traffic is pre-dicted by comparing the test data from therecompacted laboratory specimens with thedesign criteria. The following exampleshows the prediction of behavior from testson cores and on recompacted laboratory sur-face course specimens.

Assume the thickness and aggregate grada-tion are satisfactory. The current density(cores) is relatively low, the flow is ap-proaching the upper limit, the voids’ rela-tionship is outside the acceptable ranges,and the stability is satisfactory.

Data from the recompacted specimens indi-cate additional compaction from traffic willtend to improve the quality of the pave-ment. Thus, the pavement probably will ad-just itself to heavier loads and tire pres-sures than it has sustained in the past andwill be satisfactory under either high- orlow-pressure traffic. At CE 55, the voids’ to-tal mix value is below the midpoint of theacceptable range, and the flow is at the up-per limit, indicating a mix slightly richerthan ideal. However, no danger from flush-ing (bleeding) is expected.

Ability to withstand fuel spillage. ACs arereadily soluble in jet fuels, but tars are not.Maximum distress is caused to AC pave-ments by fuel frequently dripping on agiven area or by the pavement mix being sopervious that it allows considerable fuelpenetration. Voids in the total mix controlthe rate at which penetration occurs. Fuelswill penetrate very little into pavementswith about 3 percent voids but will rapidly


FM 5-430-00-2/AFJPAM 32-8013, Vol II

penetrate pavements with high (over 7 per-cent) voids.

Weathering appears to increase the pave-ment’s resistance to penetration of jet fuels.Pavements about one year old or older usu-ally perform better in this respect than newpavements.

Tar concretes and rubberized-tar concretesare not readily soluble in jet fuel, but satu-ration with jet fuel is detrimental to the lifeof such pavements. A low-void, total-mixvalue in a surface course indicates that itis sufficiently impervious to forestall dam-age.

Determine the type binder in the surfacecourse, and study the surface course char-acteristics for resistance to jet fuel. Notepoorly bonded thin layers. Use Table 12-19as a guide for evaluating the types of bitu-minous pavements from the standpoint offuel spillage for use in areas throughout theairfield.

Ability to withstand jet blast. Tests haveshown that about 300°F is the critical tem-perature for AC and rubberized-tar con-crete. About 250°F is the critical tempera-ture for tar concrete. Field tests simulatingpretakeoff checks at the ends of runways in-dicate the maximum temperatures inducedin pavement tests; simulating maintenancecheckups were 315°F. Rubberized-tar con-cretes usually withstand these tempera-tures. No bituminous pavement resists ero-sion if afterburners are turned on when theplane is standing still.

Thin surface courses that are not bondedwell to the underlying layers may be picked

up or flayed by high-velocity blasts eventhough the binder is not melted. All jet air-craft currently in use produce blasts of suf-ficient velocity to flay such courses. Sur-face courses less than 1 inch thick withpoor bond to the underlying layers are,therefore, rated as unsatisfactory for all jetaircraft. This rating applies only to parkingareas and the ends of runways.

Effects of traffic compaction. When evaluat-ing effects of future traffic on the behaviorof the paving mix, compare existing condi-tions with results of laboratory tests men-tioned previously. If the pavement is con-structed so voids fall at or about the lowerlimit of the specified allowable range,planes with high-pressure tires probablywill produce sufficient densification to re-duce voids in the total mix. When voidsfall below the specified minimum, there isno internal air in the asphalt mix for the as-phalt +o flow into. Such pavement is consid-ered to be in a critical condition. Theseconditions cannot be translated into numeri-cal evaluations, but they should be dis-cussed and summarized in the evaluationreport.

To evaluate the base, subbase, and sub-grade from the standpoint of future compac-tion, compare in-place densities (in percent-age of CE 55 maximum density) with designrequirements for the various loads and gearconfigurations the pavement is expected tosupport. If the in-place density of a layeris appreciably lower than required, removethe surface, base course, and subbasecourses and apply proper compaction.

Low-density materials combined with lowmoisture content permit densification.

Table 12-19. Guidance for evaluating pavement types


Include statements of the possible amountof settlement due to densification in theevaluation of pavements subjected to chan-nelized traffic.

If cohesive materials develop pore pres-sures, study the possible loss in strengthand estimate the lowest probable CBR. Con-sider this estimated value when selectingthe evaluation CBR for that material.

Actual and estimated pavement behavior.Study the traffic history to learn theweights of planes that have been using thefield, then compare the behavior of the fa-cilities under actual plane weights with thatindicated by the evaluations of the pave-ment’s load-carrying capacities. In makingthese comparisons, consider the number ofcoverages produced by each type of planeand the effects of mixed traffic.

No criteria exists for judging the effects ofmixed traffic. However, flexible pavementsprobably can withstand a few applicationsof loads well in excess of the load they canwithstand for full operation. Also, numer-ous applications of loads below the full op-eration load (50 percent or less) are not det-rimental; in fact, they are probably benefi-cial.

Exact agreement between behavior of facili-ties as shown by the evaluation and behav-ior that occurs under traffic is not ex-pected. This is primarily caused by the dif-ficulties in determining the exact trafficthat produced the behavior and becauseconditions change with time. Study majordifferences in the evaluation based on thetest data and data indicated from behavioru rider traffic. Discuss the differences inthe evaluation report.

As a minimum, the evaluation of an airfieldshould allow loads and intensity of trafficequal to those previously sustained, pro-vided this traffic does not produce distress.As an operating procedure, frequently in-spect the pavements after heavier planesare introduced during the frost-melting pe-riod. Limit loads or reduce traffic intensitywhen high deflections are observed duringtraffic.

FM 5-430-00-2/AFJPAM 32-8013, Vol II

Evaluation for Arid RegionsThe danger of saturation beneath flexiblepavements is reduced when the annual rain-fall is less than 15 inches, the water table(including perched water table) is at least15 feet below the surface, and the watercontent of the subgrade does not increaseabove the optimum determined by the CE55 compaction test. Under such condi-tions, reduce the total design thickness ofthe pavement, base, and subbase coursesby 20 percent. Apply this reduction to theselect material or to the subbase course hav-ing the lowest design CBR value.

Conversely, when evaluating flexible pave-ments under these conditions, increase thetotal thickness above the subgrade by 25percent before entering the evaluationcurves. Apply this increase to the selectmaterial or the subbase course having thelowest bearing ratio or to the same layer inwhich the reduction was made in the de-sign analysis.

FLEXIBLE OVERLAY OVER FLEXIBLEPAVEMENTS

An evaluation of a flexible-pavement airfieldmay determine that the traffic areas do notmeet the cover requirements for a particularmission. In this case, it is possible to over-lay the existing pavement with additionalmaterial to make it satisfactory for use.

An example of the design procedure for ap-plying a flexible overlay to flexible pavementfollows.

Example 14The evaluation of an existing airfield pave-ment indicates the conditions shown below.Tests on the AC indicate that it is ade-quate. The directive states the field will beused as a rear area 10,000-foot airfield for1,000 passes.


FM 5-430-00-2/AFJPAM 32-8013, Vol II

Solution 14The critical aircraft for the rear area 10,000-foot airfield is the C-141, which has a grossweight of 345,000 pounds. The pavementbeing considered is a Type B traffic area.To design the overlay, check the existing air-field against the thickness design require-ments in Figure 12-12, page 12-41. This in-dicates whether the airfield is satisfactoryas is or whether an overlay is needed.

Enter Figure 12-12 with the CBR of eachsoil layer of the pavements, and read thethickness required above that layer fromthe curve. The value from the curve is com-pared with the existing thickness.

If the thickness from the curve is less thanthe existing thickness, the airfield pavementis satisfactory. If the required thickness isgreater than the existing thickness, an over-lay is required. The overlay thickness mustbe equal to the difference between the de-sign thickness and the existing thickness.The results of this example are shown be-low:

Use the largest overlay thickness require-ment. An overlay thickness of 8 inches willsatisfy the thickness requirement for operat-ing on this pavement for 1,000 passes as aheavy lift, rear-area pavement. It is possi-ble to overlay 8 inches of AC on the exist-ing surface, but this would be prohibitivelyexpensive. A better alternative is to overlay6 inches (minimum lift thickness) of CBR =100 base course and 3 inches of AC. Theoverlay is shown following:

Another method to determine an overlay re-quirement is to use the evaluation curves,Figures K-1 through K-40, pages K-1through K-40. Enter these curves at thebottom with the number of aircraft passesand read upward to the layer’s CBR value.Then, read horizontally to the aircraft grossweight. Finally, read upward to determinethe required thickness above that soil layer.Using the same example as above yields thefollowing information:

Thus, this method also would result in anoverlay.

NONRIGID OVERLAYS OVER RIGIDPAVEMENTS

In the rear areas of the TO, it may be neces-sary to evaluate existing rigid pavementsand to bring them to required strengths byadding nonrigid overlays. Use the followingdesign procedure to determine the nonrigidoverlay thickness needed to increase theload-carrying capacity of existing concretepavements. This design procedure is also


MISPRINT

No figure K-40.

FM 5-430-00-2/AFJPAM 32-8013, Vol II

used to evaluate existing concrete pave-ments with nonrigid overlay.

Nonrigid overlays may be AC or flexible.The type of nonrigid overlay used for agiven condition depends on the requiredoverlay thickness. In general, the flexibleoverlay is used when the required overlay isof sufficient thickness to incorporate a mini-mum 4-inch compacted layer of high-qual-ity, base-course material, plus the requiredthickness of AC surface course. The ACoverlay will be used when less overlay thick-ness is needed.

The method used assumes the nonrigid over-lay on rigid pavement to be a flexible pave-ment, with the rigid-base pavement as-sumed to be a high-quality base coursewith CBR = 100. This is a very conserva-tive assumption. The nonrigid overlay onrigid pavement is designed and evaluated inthe same manner as a flexible pavement,the procedure for which was described ear-lier in this chapter. Thus, when designingand evaluating, it will be necessary to deter-mine the physical constants that are re-quired for flexible pavements.

If an existing flexible overlay has alreadybeen placed, the quality of the AC portionof the overlay and the CBR values of thesubgrade and base course beneath the rigidbase pavement will have to be established.As mentioned above, the rigid-base pave-ment will be assumed to have CBR = 100.

Example 15Assume a runway with uniform thickness ofnonrigid overlay on rigid pavement through-

out its entire width and length must beevaluated. The overlay composed of AC forfull depth is 2 inches, the thickness of therigid base pavement is 6 inches, the basecourse thickness under rigid pavement is 8inches, the base course CBR is 40, and thesubgrade CBR is 7.

This sample airfield is to be used by C-141aircraft for 5,000 passes. The design loadis 345,000 pounds. Tests of the AC indi-cate that it meets design requirements forstability, density, gradation, voids relations,and other design requirements.

Solution 15A design curve for traffic area Type B willbe required. Enter Figure 12-12, page12-41, with this design load (in kips). It isfound that the CBR = 7 subgrade requires28 inches of cover, CBR = 40 requires 7.3inches of cover, and CBR = 100 (Portland ce-ment (PC) concrete) requires no cover.

The total cover over the CBR = 7 subgradeis only 16 inches, whereas 28 inches is re-quired. Therefore, the overlay must be 28 -16 inches, or 12 inches thick.

PAVEMENT AND AIRFIELD CLASSIFICATION NUMBERS

After an airfield pavement has been de-signed, constructed, or evaluated, aircraftother than the critical aircraft probably willland on the pavement. (These can includeforeign national aircraft.) Due to these con-straints, it will be extremely difficult to ac-count for all traffic loads in relation to thedesign life of the airfield. One method toaccount for this is to set a pavement classi-

fication number (PCN) based on the designaircraft, assign an aircraft classificationnumber (ACN) to aircraft based on theirload, and then compare the two. The ACNexpresses the relative structural effect of anaircraft on different pavement types forspecified standard subgrade strengths interms of a standard single-wheel load. ThePCN expresses the relative load-carrying


FM 5-430-00-2/AFJPAM 32-8013, Vol II

capacity of a pavement in terms of a stand-ard single-wheel load.

The system is structured so that a pave-ment with a particular PCN value can sup-port, without weight restrictions, an aircraftthat has an ACN value equal to or less thanthe pavement’s PCN value. This is possiblebecause ACN and PCN values are computedusing the same technical basis.

DETERMINATION OF VALUES

Pavement Classification NumbersThe PCN numerical value for a particularpavement is determined from the allowableload rating, which is usually based on thedesign aircraft. Once the allowable load rat-ing is established, determining the PCNvalue js a process of converting that ratingto a standard relative value. Curves for con-verting allowable load ratings to PCN valuesare presented in Appendix O. The PCNvalue is usable for reporting the pavementstrength only.

Rigid pavement PCN—allowable load curves.For rigid pavements, aircraft landing gearflotation requirements are determined bythe Westergaard solution for a loaded elas-tic plate on a dense liquid foundation (inte-rior load case), assuming a concrete work-ing stress of 399 psi. Four different sub-grade strengths are considered: high, 554pounds per cubic inch (pci); medium, 296pci: low, 148 pci: and ultralow, 74 pci. Us-ing these parameters, a standard single-wheel load at a tire pressure of 181 psi iscomputed for each subgrade strength. Thestandard single-wheel load is expressed inkilograms and divided by 500 to obtain thePCN. Division by 500 is a rounding-offprocess to make the numbers smaller andmore manageable.

Flexible-pavement PCN—allowable loadcurves. For flexible pavements, aircraftlanding gear flotation requirements are de-termined by the CBR method. As with therigid pavement, four different subgradestrengths are considered: high, CBR = 15;medium, CBR = 10; low, CBR = 6; and ul-tralow, CBR = 3. A standard single-wheel

load at a tire pressure of 181 psi is com-puted for each of these subgrade strengths.The standard single-wheel load is expressedin kilograms and divided by 500 to obtainthe PCN.

Reporting the PCN. The PCN should be re-ported in whole numbers, rounding off anyfractional parts to the nearest whole num-ber. For pavements of variable strength,the controlling PCN numerical value for theweakest feature of the pavement should bereported as the strength of the pavement.Besides their PCN number, data coded inTable 12-20 must be provided.

ACN values are determined the same wayas PCN values because they are relative tothe aircraft load and subgrade strength. Aset value has not been selected for aircraftsince this can vary based on the aircraftload and can be different from takeoff andlanding due to full expenditure.

Guidance on Overload OperationsPavement overload can result from loadsthat are too large, from a substantially in-creased application rate, or from both.Loads larger than the defined (design orevaluation) load shorten the design lifewhile smaller loads extend it. Except formassive overloading, pavements in theirstructural behavior are not subject to a par-ticular limiting load above which they sud-denly or catastrophically fail. Their behav-ior is such that a pavement can sustain adefinable load for an expected number ofrepetitions during its design life. As a re-sult, occasional minor overloading is accept-able, when expedient, with only limited lossin pavement life expectancy and small accel-eration of pavement deterioration. Forthose operations in which the magnitude ofoverload or the frequency of use does notjustify a detailed analysis, the following cri-teria are suggested:

Ž For flexible pavements, occasional move-ments by aircraft with ACN not exceed-ing 10 percent above the reported PCNshould not adversely affect the pave-ment.


Ž

•

•

FM 5-430-00-2/AFJPAM 32-8013, Vol II

For rigid or composite pavements wherea rigid pavement layer provides a pri-mary element of the structure, occa-sional movements by aircraft with ACNnot exceeding 5 percent above thereported PCN should not adversely af-fect the pavement.

If the pavement structure is unknown,the 5-percent limitation should apply.

The annual number of overload move-

or when the strength of the pavement or itssubgrade could be weakened by water. Ex-cessive repetition of overloads can cause se-vere shortening of pavement life or requiremajor rehabilitation of pavement. There-fore, where overload operations are con-ducted, the appropriate authority should re-view the relevant pavement condition regu-larly and review the criteria for overload op-erations periodically.

ments should not exceed approximately NOTE: Thickness design for rigid, flexible,5 percent of the total annual aircraft and unsurfaced pavement (with ormovements. without matting) can be accomplished

with computer programs developed bySuch overload movements should not nor- WES. The programs are available from themally be permitted on pavements exhibiting US Army Transportation Center, ATTN:signs of distress or failure. Furthermore, CEMRD-ED-TT, 12565 West Center Road,overloading should be avoided during any Omaha, NE 68144-3869.periods of thaw following frost penetration

Table 12-20. PCN five-part code


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