TM 5-818-7

TECHNICAL MANUAL

FOUNDATIONSIN

EXPANSIVE SOILS

H E A D Q U A R T E R S , D E P A R T M E N T O F T H E A R M YSEPTEMBER 1983

This manual has been prepared by or for the Government and, except to the ex-tent indicated below, is public property and not subject to copyright.

Copyrighted material included in the manual has been used with the knowledgeand permission of the proprietors and is acknowledged as such at point of use.Anyone wishing to make further use of any copyrighted material, by itself andapart from this text, should seek necessary permission directly from the pro-prietors.

Reprints or republications of this manual should include a credit substantially asfollows: “Department of the Army USA, Technical Manual TM 5-818-7, Founda-tions in Expansive Soils, 1 September 1983.”

If the reprint or republication includes copyrighted material, the credit shouldalso state: “Anyone wishing to make further use of copyrighted material, by itselfand apart from this text, should seek necessary permission directly from the pro-prietors.”

By Order of the Secretary of the Army:

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FOUNDATIONS IN EXPANSIVE SOILS

INTRODUCTIONPurpose, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Background, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Causes and patterns of heave .. . . . . . . . . . . . . . . . . . . . .Elements of design. . . . . . . . . . . . . . . . . . . . . . .RECOGNITION OF PROBLEM AREASSite selection, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Hazard maps, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FIELD EXPLORATIONScope, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surface examination... . . . . . . . . . . . . . . . . . . . .Subsurface exploration . . . . . . . . . . . . . . . . . . . .Groundwater . ., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LABORATORY INVESTIGATIONSIdentification of swelling soils... . . . . . . . . . . . . . . . . . . . .Testing procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .METHODOLOGY FOR PREDICTION OF VOLUME

CHANGESApplication of heave predictions . . . . . . . . . . . . . . . . . . . . .Factors influencing heave . . . . . . . . . . . . . . . . . . . . .Direction of soil movement . . . . . . . . . . . . . . . . . . . . . . . . . .Potential total vertical heave.., . . . . . . . . . . . . . . . . . . . . .Potential differential heave...,. . . . . . . . . . . . . . . . . . . . .Heave with time, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DESIGN OF FOUNDATIONSBasic considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Shallow individual or continuous footings . . . . . . . . . . . . . .Reinforced slab-on-grade foundations . . . . . . . . . . . . . . . . .Deep foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MINIMIZATION OF FOUNDATION MOVEMENTPreparation for construction. . . . . . . . . . . . . . . . . . . . . .Drainage techniques, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Stabilization techniques. . . . . . . . . . . . . . . . . . . . . . . . .CONSTRUCTION TECHNIQUES AND INSPECTIONMinimization of foundation problems from construction . . .Stiffened slab foundations . . . . . . . . . . . . . . . . . . . . . . . . . .Drilled shaft foundations . . . . . . . . . . . . . . . . . . . . .REMEDIAL PROCEDURESBasic considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Evaluation of information . . . . . . . . . . . . . . . . . . . . . . . . . .Stiffened slab foundations,.... . . . . . . . . . . . . . . . . . . . . .Drilled shaft foundations. . . . . . . . . . . . . . . . . . . . .REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CHARACTERIZATION OF SWELL BEHAVIOR FROM SOIL SUCTION .FRAME AND WALL CONSTRUCTION DETAILS . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Table Title

CHAPTER 1

INTRODUCTION

1-1. PurposeThis manual presents guidance and information forthe geotechnical investigation necessary for the selec-tion and design of foundations for heavy and lightmilitary-type buildings constructed in expansive claysoil areas. The information in this manual is generallyapplicable to many types of structures such as resi-dences, warehouses, and multistory buildings. Empha-sis is given to the maintenance of an environment thatencourages constant moisture conditions in thefoundation soils during and following construction.Special attention must always be given to specific re-quirements of the structure such as limitations on al-lowable differential movement.

a. The guidance and information provided in thismanual can significantly reduce the risk of undesirableand severe damages to many structures for numerousexpansive soil conditions. However, complete solutionsfor some expansive soil problems are not yet available;e.g., the depth and amount of future soil moisture

- changes may be difficult to predict.

b. This manual presents guidance for selecting eco-nomical foundations on expansive soil to minimizestructural distress to within tolerable levels and guid-ance for minimizing problems that may occur in struc-tures on expansive soils.

1-2. Scopea. Guidelines of the geotechnical investigation and

analysis necessary for selection and design of military-type buildings constructed in expansive clay soil areas,as outlined in chapters 2 to 5, consist of methods forthe recognition of the relative magnitude of the swell-ing soil problem at the construction site, field explora-tion, laboratory investigations, and application ofmethodology for prediction of volume changes inswelling foundation soils. Chapter 6 presents guidancefor selection of the type of foundation with structuraldetails of design procedures provided for reference.Chapters 7 to 9 discuss methods of minimizing founda-tion movement, construction techniques and inspec-tion, and considerations for remedial repair of dam-aged structures.

b. Guidance is not specifically provided for designof highways, canal or reservoir linings, retainingwalls, and hydraulic structures. However, much of the

basic information presented is broadly applicable tothe investigation and analysis of volume changes insoils supporting these structures and methods forminimizing potential soil volume changes. Guidance isalso not specifically provided for the design of struc-tures in areas susceptible to soil volume changes fromfrost heave and chemical reactions in the soil (e.g., oxi-dation of iron pyrite), although much of the informa-tion presented can be useful toward these designs.

1-3. BackgroundThis manual is concerned with heave or settlementcaused by change in soil moisture in nonfrozen soils.Foundation materials that exhibit volume changefrom change in soil moisture are referred to as expan-sive or swelling clay soils. Characteristic expansive orswelling materials are highly plastic clays and clayshales that often contain colloidal clay minerals suchas the montmorillonites. Expansive soils as used inthis manual also include marls, clayey siltstones, sand-stones, and saprolites.

a. Damages from differential movement. The differ-ential movement caused by swell or shrinkage of ex-pansive soils can increase the probability of damage tothe foundation and superstructure. Differential ratherthan total movements of the foundation soils are gen-erally responsible for the major structural damage.Differential movements redistribute the structuralloads causing concentration of loads on portions of thefoundation and large changes in moments and shearforces in the structure not previously accounted for instandard design practice.

b. Occurrence of damages. Damages can occur with-in a few months following construction, may developslowly over a period of about 5 years, or may not ap-pear for many years until some activity occurs to dis-turb the soil moisture. The probability of damages in-creases for structures on swelling foundation soils ifthe climate and other field environment, effects ofconstruction, and effects of occupancy tend to promotemoisture changes in the soil.

c. Structures susceptible to damages. Types ofstructures most often damaged from swelling soilinclude foundations and walls of residential and light(one- or two-story) buildings, highways, canal andreservoir linings, and retaining walls. Lightly loaded

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TM 5-818-7

one- or two-story buildings, warehouses, residences,and pavements are especially vulnerable to damage be-cause these structures are less able to suppress the dif-ferential heave of the swelling foundation soil thanheavy, multistory structures.

(1) Type of damages. Damages sustained by thesestructures include: distortion and cracking of pave-ments and on-grade floor slabs; cracks in grade beams,walls, and drilled shafts; jammed or misaligned doorsand windows; and failure of steel or concrete plinths(or blocks) supporting grade beams. Lateral forces maylead to buckling of basement and retaining walls, par-ticularly in overconsolidated and nonfissured soils.The magnitude of damages to structures can be exten-sive, impair the usefulness of the structure, and de-tract aesthetically from the environment. Mainte-nance and repair requirements can be extensive, andthe expenses can grossly exceed the original cost of thefoundation.

(2) Example of damages. Figure 1-1 illustratesdamages to a building constructed on expansive soilwith a deep water table in the wet, humid climate ofClinton, Mississippi. These damages are typical ofbuildings on expansive soils. The foundation consistsof grade beams on deep drilled shafts. Voids were notprovided beneath the grade beams above the expansivefoundation soil, and joints were not made in the wallsand grade beams. The floor slab was poured on-gradewith no provision to accommodate differential move-ment between the slab and grade beams. The heave ofthe floor slab exceeded 6 inches. The differential soilmovement and lack of construction joints in the struc-ture aggravated cracking.

14. Causes and patterns of heavea. Causes. The leading cause of foundation heave or

settlement in susceptible soils is change in soil mois-ture, which is attributed to changes in the field envi-ronment from natural conditions, changes related toconstruction, and usage effects on the moisture underthe structure (table 1-1). Differential heave may becaused by nonuniform changes in soil moisture, varia-tions in thickness and composition of the expansivefoundation soil, nonuniform structural loads, and thegeometry of the structure. Nonuniform moisturechanges occur from most of the items given in table1-1.

b. Patterns of heave.(1) Doming heave. Heave of foundations, although

often erratic, can occur with an upward, long-term,dome-shaped movement that develops over manyyears. Movement that follows a reduction of naturalevapotranspiration is commonly associated with adoming pattern of greatest heave toward the center ofthe structure. Evapotranspiration refers to the evapo-ration of moisture from the ground surface and trans-

piration of moisture from heavy vegetation into the at-mosphere. Figure 1-2 schematically illustrates somecommonly observed exterior cracks in brick walls fromdoming or edgedown patterns of heave. The pattern ofheave generally causes the external walls in the super-structure to lean outward, resulting in horizontal,vertical, and diagonal fractures with larger cracksnear the top. The roof tends to restrain the rotationfrom vertical differential movements leading to addi-tional horizontal fractures near the roofline at the topof the wall. Semiarid, hot, and dry climates and deepwater tables can be more conducive to severe and pro-gressive foundation soil heaves if water become avail-able.

(2) Cyclic heave. A cyclic expansion-contractionrelated to drainage and the frequency and amount ofrainfall and evapotranspiration may be superimposedon long-term heave near the perimeter of the struc-ture. Localized heaving may occur near water leaks orponded areas. Downwarping from soil shrinkage (fig.1-2) may develop beneath the perimeter during hot,dry periods or from the desiccating influence of treesand vegetation located adjacent to the structure. Theseedge effects may extend inward as much as 8 to 10feet. They become less significant on well-drainedland. Heavy rain periods may cause pending adjacentto the structure with edge lift (fig. 1-3) and reversal ofthe downwarping.

(3) Edge heave. Damaging edge or dish-shapedheaving (fig. 1-3) of portions of the perimeter maybeobserved relatively soon after construction, particu-larly in semiarid climates on construction sites withpreconstruction vegetation and lack of topographic re-lief. The removal of vegetation leads to an increase insoil moisture, while the absence of topographic reliefleads to ponding (table 1-1). A dish-shaped pattern canalso occur beneath foundations because of consolida-tion, drying out of surface soil from heat sources, orsometimes lowering of the water table. Changes in thewater table level in uniform soils beneath uniformlyloaded structures may not contribute to differentialheave. However, structures on a deep foundation, suchas drilled shafts with a slab-on-grade, can be adverselyaffected by a changing water table or changes in soilmoisture if the slab is not isolated from the perimetergrade beams and if internal walls and equipment arenot designed to accommodate the slab movement.

(4) Lateral movement. Lateral movement may af-fect the integrity of the structure.

(a) Lateral thrust of expansive soil with a hori-zontal force up to the passive earth pressure can causebulging and fracture of basement walls. Basementwalls and walls supporting buildings usually cannottolerate the same amount of movement as free-stand-ing retaining walls. Consequently, such walls must bedesigned to a higher degree of stability.

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a . V e r t i c a l c r a c k s

1-5. Elements of design

the structure are simply supported on-grade or at-tached to the structure, they can contribute to futuremaintenance problems.

(2) Potential problems that could eventually af-fect the performance of the structure are best deter-mined during the predesign and preliminary designphases when compromises can be made between thestructural, architectural, mechanical, and other as-pects of the design without disrupting the design proc-ess. Changes during the detailed design phase or dur-ing construction will probably delay construction andpose economic disadvantages.

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TM 5-818-7

— CHAPTER 2

RECOGNITION OF PROBLEM AREAS

2-1. Site selectionThe choice of the construction site is often limited. Itis important to recognize the existence of swelling soilson potential sites and to understand the problems thatcan occur with these soils as early as possible. A sur-face examination of the potential site as discussed inparagraph 3-2 should be conducted and available soildata studied during the site selection.

a. Avoidance of potential problems. If practical,the foundation should be located on uniform soils sub-ject to the least swelling or volume change. Discon-tinuities or significant lateral variations in the soilstrata should be avoided. Swampy areas, backfilledponds, and areas near trees and other heavy vegetationshould be avoided, Special attention should be given toadequate compaction of filled areas, types of fill, andleveling of sloped sites (para 7-1).

(1) Undeveloped sites. Undeveloped sites general-ly have little or no subsurface soil information avail-able and require subsurface exploration (para 3-3).

(a) Substantial differential heave may occur be-neath structures constructed on previously undevel-oped sites where trees and other heavy vegetation hadbeen removed prior to construction, Soil moisture willtend to increase since loss of heavy vegetation reducesthe transpiration of moisture. Construction of thefoundation over the soil will tend to further increasesoil moisture because of reduced evaporation of mois-ture from the ground surface.

(b) Swampy or ponded areas may contain great-er quantities of plastic fine particles with a greatertendency to swell than other areas on the site.

(c) Future irrigation of landscaped areas andleakage from future sewer and other water utility linesfollowing development of the site may substantiallyincrease soil moisture and cause a water table to rise orto develop if one had not previously existed. Filledareas may also settle if not properly compacted.

(2) Developed sites. Subsurface explorationshould be conducted if sufficient soil data from earlierborings are not available for the site selection and/orproblems had occurred with previous structures. Somesubsurface exploration is always necessary for site se-lection of any structure of economic significance, par-ticularly multistory buildings and structures with spe-cial requirements of limited differential distortion.

(a) An advantage of construction on developed

sites is the experience gained from previous construc-tion and observation of successful or unsuccessful pastperformance. Local builders should be consulted to ob-tain their experience in areas near the site. Existingstructures should be observed to provide hints of prob-lem soil areas.

(b) The soil moisture may tend to be much closerto an equilibrium profile than that of an undevelopedsite. Differential movement may not be a problem be-cause previous irrigation, leaking underground waterlines, and previous foundations on the site may havestabilized the soil moisture toward an equilibrium pro-file. Significant differential movement, however, isstill possible if new construction leads to changes insoil moisture. For example, trees or shrubs planted tooclose to the structure or trees removed from the site,change in the previous irrigation pattern followingconstruction, lack of adequate drainage from the struc-ture, and improper maintenance of drainage provi-sions may lead to localized changes in soil moistureand differential heave. Edge movement of slab-on-grade foundations from seasonal changes in climatemay continue to be a problem and should be minimizedas discussed in chapter 7.

(3) Sidehill or sloped sites. Structures construct-ed on sites in which the topography relief is greaterthan 5 degrees (9 percent gradient) may sustain dam-age from downhill creep of expansive clay surface soil.Sidehill sites and sites requiring split-level construc-tion can, therefore, be expected to complicate the de-sign. See chapter 7 for details on minimization of foun-dation soil movement.

b. Soil surveys, Among the best methods availablefor qualitatively recognizing the extent of the swellingsoil problem for the selected site is a careful examina-tion of all available documented evidence on soil condi-tions near the vicinity of the site. Local geological rec-ords and publications and federal, state, and institu-tional surveys provide good sources of information onsubsurface soil features. Hazard maps described inparagraph 2-2 document surveys available for esti-mating the extent of swelling soil problem areas.

2-2. Hazard mapsHazard maps provide a useful first-order approxi-mation of and guide to the distribution and relative ex-pansiveness of problem soils. These maps should be

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TM 5-818-7

used in conjunction with local experience and locally due to expansive materials. The stratigraphy and min-available soil surveys and boring data. The maps dis- eralogy are key elements in the classification.cussed in a and b below are generally consistent with (1) Classification. The soils are classified intoeach other and tend to delineate similar areas of categories of High, Medium, Low, and Nonexpansive moderately or highly expansive soil. as shown in figure 2-1. The distribution of expansive

a. Waterways Experiment Station (WES) Map. This materials is categorized by the geologic unit on the ba-map, which was prepared for the Federal Highway Ad- sis of the degree of expansiveness that relates to theministration (FHWA), summarizes the areas of the expected presence of montmorillonite and the fre-United States, except Alaska and Hawaii, where swell- quency of occurrence that relates to the amount of claying soil problems are likely to occur (fig. 2-1). The ba- or shale. The amount refers most significantly to thesis for classification depends primarily on the esti- vertical thickness of the geologic unit, but the areal ex-mated volume change of argillaceous or clayey mate- tent was also considered in the classification. Therials within the geologic unit, the presence of mont- premises in table 2-1 guide the categorization of soils.morillonite, the geologic age, and reported problems

(2) Physiographic provinces. Table 2-2 summar-izes the potentially expansive geologic units on the ba-sis of the 20 first-order physiographic provinces. Fig-ure 2-1 shows the locations of the physiographic prov-inces.

b. Other maps.(1) Area map of susceptible soil expansion prob-

lems. A hazard map was developed by M, W. Witczak(Transportation Research Board, Report 132) on thebasis of the occurrence and distribution of expansivesoils and expansive geologic units, the pedologic analy-sis, and climatic data to delineate areas susceptible toexpansion problems. Some geologic units for whichengineering experiences were not available may havebeen omitted, and the significance of pedological soilon expansion was not shown on the map.

(2) Assessment map of expansive soils within theUnited States. The major categories for classificationof the severity of the swelling soil problem presentedby J. P. Krohn and J. E. Slosson (American Society ofCivil Engineers, Proceedings of the Fourth Interna-tional Conference on Expansive Soils, Volume 1 (seeapp. A) correspond to the following modified shrink-swell categories of the Soil Conservation Service (SCS)of the U. S. Department of Agriculture:

High: Soils containing large amounts of montmorilloniteand clay (COLE >6 percent)

Moderate: Soils containing moderate amounts of clay with

Low: Soils containing some clay with the clay consist-ing mostly of kaolinite and/or other low swelling clay minerals (COLE <3 percent).

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These categories of classification use the coefficient oflinear extensibility (COLE), which is a measure of thechange in linear dimension from the dry to a moiststate, and it is related to the cube root of the volumechange. Premises guiding the categorization of theKrohn and Slosson map include: degree of expansionas a function of the amount of expandable clay; coverof nonexpansive glacial deposits; and low-rated areaswith nonexpansive and small quantities of expansivesoils. Environmental factors, such as climatic effects,vegetation, drainage, and effects of man, were not con-sidered.

(3) Soil Conservation Service county soil surveys.Survey maps by SCS provide the most detailed surfi-cial soil maps available, but not all of the UnitedStates is mapped. Soil surveys completed during the1970’s contain engineering test data, estimates of soilengineering properties, and interpretations of proper-ties for each of the major soil series within the givencounty. The maps usually treat only the upper 30 to 60inches of soil and, therefore, may not fully define thefoundation soil problem.

(4) U.S. and State Geological Survey maps. TheU.S. Geological Survey is currently preparing hazardmaps that will include expansive soils.

c. Application of hazard maps. Hazard maps providebasic information indicative of the probable degree ofexpansiveness and/or frequency of occurrence of swell-ing soils. These data lead to initial estimates for the lo-cation and relative magnitude of the swelling problemto be expected from the foundation soils. The SCS

count y survey maps prepared after 1970, if available,provide more detail on surface soils than do the othermaps discussed in b above. The other maps used in con-junction with the SCS maps provide a better basis for election of the construction site.

(1) Recognition of the problem area at the construc-tion site provides an aid for the planning of field ex-ploration that will lead to the determination of theareal extent of the swelling soil formations and sam-ples for the positive identification and evaluation ofpotential swell of the foundation soils and probablesoil movements beneath the structure.

(2) Problem areas that rate highly or moderatelyexpansive on any of the hazard maps should be ex-plored to investigate the extent and nature of theswelling soils. Structures in even low-rated areas of po-tential swell may also be susceptible to damages fromheaving soil depending on the ability of the structureto tolerate differential foundation movement. Theselow-rated areas can exhibit significant differential soilheave if construction leads to sufficiently largechanges in soil moisture and uneven distribution ofloads. Also, low-rated areas on hazard maps may in-clude some highly swelling soil that had been neglect-ed.

(3) Figure 2-1 indicates that most problems withswelling soils can be expected in the northern central,central, and southern states of the continental UnitedStates. The Aliamanu crater region of Fort Shafter, Hawaii, is another example of a problem area.

TM 5-818-7

CHAPTER 3

FIELD EXPLORATION

3-1. ScopeThe field study is used to determine the presence, ex-tent, and nature of expansive soil and groundwaterconditions. The two major phases of field explorationare surface examination and subsurface exploration.The surface examination is conducted first since theresults help to determine the extent of the subsurfaceexploration. In situ tests may also be helpful, particu-larly if a deep foundation, such as drilled shafts, is tobe used.

3-2. Surface examinationa. Site history. A study of the site history may re-

veal considerable qualitative data on the probable fu-ture behavior of the foundation soils. Maps of the pro-posed construction site should be examined to obtaininformation on wooded areas, ponds and depressions,water-courses, and existence of earlier buildings. Sur-face features, such as wooded areas, bushes, and otherdeep-rooted vegetation in expansive soil areas, indi-cate potential heave from accumulation of moisturefollowing elimination of these sources of evapotran-spiration. The growth of mesquite trees, such as foundin Texas, and other small trees may indicate subsur-face soil with a high affinity for moisture, a character-istic of expansive soil. Ponds and depressions are oftenfilled with clayey, expansive sediments accumulatedfrom runoff. The existence of earlier structures on ornear the construction site has probably modified thesoil moisture profile and will influence the potentialfor future heave beneath new structures.

b. Field reconnaissance. A thorough visual examina-tion of the site by the geotechnical engineer is neces-sary (table 3-1). More extensive subsurface explora-tion is indicated if a potential for swelling soil is evi-dent from damages observed in nearby structures. Theextent of desiccation cracks, plasticity, slickensides,and textures of the surface soil can provide a relativeindication of the potential for damaging swell.

(1) Cracking in nearby structures. The appearanceof cracking in nearby structures should be especiallynoted. The condition of on-site stucco facing, joints ofbrick and stone structures, and interior plaster wallscan be a fair indication of the possible degree of swell-ing that has occurred. The differential heave that mayoccur in the foundation soil beneath the proposedstructure. however, is not necessarily equal to the dif-

ferential heave of earlier or nearby structures. Differ-ential heave depends on conditions such as variation ofsoils beneath the structure, load distribution on thefoundation, foundation depth, and changes in ground-water since construction of the earlier structures.

(2) Soil gilgai. The surface soil at the site shouldalso be examined for gilgai. Soil gilgai are surfacemounds that form at locations where the subsurfacesoil has a greater percentage of plastic fines and isthus more expansive than the surface soil. Gilgai beginto form at locations where vertical cracks penetrateinto the subsurface soil. Surface water enters andswelling takes place around the cracks leaving frac-tured zones where plastic flow occurs. These moundsusually have a higher pH than the adjacent low areasor depressions and may indicate subsurface soil thathad extruded up the fractures.

(3) Site access and mobility. Indicators of site ac-cess and mobility (table 3-1) may also influence behav-ior of the completed structure. For example, nearbywater and sewer lines may alter the natural moistureenvironment. Flat land with poor surface drainage, asindicated by ponded water, may aggravate differentialheave of the completed structure if drainage is not cor-rected during construction. Construction on land withslopes greater than 5 degrees may lead to structuraldamage from creep of expansive clay surface soils.Trees located within a distance of the proposed struc-ture of 1 to 1.5 times the height of mature trees maylead to shrinkage beneath the structure, particularlyduring droughts.

c. Local design and construction experience. Localexperience is very helpful in indicating possible designand construction problems and soil and groundwaterconditions at the site. Past successful methods of de-sign and construction and recent innovations shouldbe examined to evaluate their usefulness for the pro-posed structure.

3-3. Subsurface explorationSubsurface exploration provides representative sam-ples for visual classification and laboratory tests. Clas-sification tests are used to determine the lateral andvertical distribution and types of foundation soils. Soilswell, consolidation, and strength tests are needed toevaluate the load/displacement behavior and bearingcapacity of the foundation in swelling soil. The struc-

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ture interaction effects in swelling soil are complicatedby the foundation differential movement caused bysoil heave. Sufficient samples should be available to al-low determination of the representative mean of theswell and strength parameters of each distinctive soilstratum. The lower limit of the scatter in strengthparameters should also be noted.

a. Sampling requirements. The design of lightlyloaded structures and residences can often be madewith minimal additional subsurface investigations andsoil testing if the site is developed, if subsurface fea-tures are generally known, and if the local practice hasconsistently provided successful and economical de-signs of comparable structures. Additional subsurfaceinvestigation is required for new undeveloped sites,multistory or heavy buildings, structures with pre-viously untested or new types of foundations, and spe-cial structures that require unusually limited differen-tial movements of the foundation such as deflec-tion/length ratios less than 1/1000. Where the localpractice has not consistently provided satisfactory de-signs, a careful review of the local practice is neces-

sary. Corrections to improve performance comparedwith earlier structures may prove difficult to deviseand implement and may require evaluation of the be-havior of the subsurface foundation soils and ground-water conditions.

b. Distribution and depth of borings. The distribu-tion and depth of borings are chosen to determine thesoil profile and to obtain undisturbed samples requiredto evaluate the potential total and differential heave ofthe foundation soils from laboratory swell tests, aswell as to determine the bearing capacity and settle-ment. Consequently, greater quantities of undisturbedsamples may be required in swelling soils than nor-mally needed for strength tests.

(1) Borings should be spaced to define the geologyand soil nonconformities. Spacings of 50 or 25 feet andoccasionally to even less distance may be requiredwhen erratic subsurface conditions (e.g., soils of differ-ent swelling potential, bearing capacity, or settlement)are encountered. Initial borings should be located closeto the corners of the foundation, and the number should not be less than three unless subsurface condi-

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tions are known to be uniform. Additional boringsshould be made as required by the extent of the area,the location of deep foundations such as drilled shafts,and the encountered soil conditions.

(2) The depth of sampling should be at least asdeep as the probable depth to which moisture changesand heave may occur. This depth is called the depth of

down about 10 to 20 feet below the base of the founda-tion or to the depth of shallow water tables, but it maybe deeper (para 5-4c). A shallow water table is definedas less than 20 feet below the ground surface or belowthe base of the proposed foundation. The entire thick-ness of intensely jointed or fissured clays and shalesshould be sampled until the groundwater level is en-countered because the entire zone could swell, provid-ed swelling pressures are sufficiently high, when givenaccess to moisture. Continuous sampling is requiredfor the depth range within the active zone for heave.

(3) Sampling should extend well below the antici-pated base of the foundation and into strata of ade-quate bearing capacity. In general, sampling shouldcontinue down to depths of 1.5 times the minimumwidth of slab foundations to a maximum of 100 feetand a minimum of three base diameters beneath thebase of shaft foundations. The presence of a weak,compressible, or expansive stratum within the stressfield exerted by the entire foundation should be de-tected and analyzed to avoid unexpected differentialmovement caused by long-term volume changes in thisstratum. Sampling should continue at least 20 feet be-neath the base of the proposed foundation. Determi-nation of the shear strength and stress/strain behaviorof each soil stratum down to depths of approximately100 feet below the foundation is useful if numericalanalysis by the finite element method is considered.

c. Time of sampling. Sampling may be done whensoil moisture is expected to be similar to that duringconstruction. However, a design that must be adequatefor severe changes in climate, such as exposure to peri-ods of drought and heavy rainfall, should be based onmaximum levels of potential soil heave. Maximum po-tential heaves are determined from swell tests usingsoils sampled near the end of the dry season, which of-ten occurs toward the end of summer or early fall.Heave of the foundation soil tends to be less if samplesare taken or the foundation is placed following the wetseason, which often occurs during spring.

d. Sampling techniques. The disturbed samples andthe relatively undisturbed samples that provide mini-mal disturbance suitable for certain laboratory soiltests may be obtained by the methods described in ta-ble 3-2. Drilling equipment should be well maintainedduring sampling to avoid equipment failures, whichcause delays and can contribute to sample disturbance.

Personnel should be well trained to expedite propersampling, sealing, and storage in sample containers.

(1) Disturbed sampling. Disturbed auger, pit, orsplit spoon samplers may be useful to roughly identifythe soil for qualitative estimates of the potential forsoil volume change (para 4-1). The water content ofthese samples should not be altered artificially duringboring, for example, by pouring water down the holeduring augering.

(2) Undisturbed sampling. Minimization of sam-ple disturbance during and after drilling is importantto the usefulness of undisturbed samples. This fact isparticularly true for expansive soils since smallchanges in water content or soil structure will signifi-cantly affect the measured swelling properties.

(a) The sample should be taken as soon as pos-sible, after advancing the hole to the proper depth andcleaning out the hole, to minimize swelling or plasticdeformation of the soil to be sampled.

(b) The samples should be obtained using a pushtube sampler without drilling fluid, if possible, tominimize changes in the sample water content. Drill-ing fluids tend to increase the natural water contentnear the perimeter of the soil sample, particularly forfissured soil.

(c) A piston Denisen or other sampler with acutting edge that precedes the rotating outer tube intothe formation is preferred, if drilling fluid is neces-sary, to minimize contamination of the soil sample bythe fluid.

e. Storage of samples. Samples should be immedi-ately processed and sealed following removal from theboring hole to minimize changes in water content.Each container should be clearly labeled and stored un-der conditions that minimize large temperature andhumidity variations. A humid room with relativehumidity greater than 95 percent is recommended forstorage since the relative humidity of most naturalsoils exceeds 95 percent.

(1) Disturbed samples. Auger, pit, or other dis-turbed samples should be thoroughly sealed in water-proof containers so that the natural water content canbe accurately measured.

(2) Undisturbed samples. Undisturbed samplesmay be stored in the sampling tubes or extruded andpreserved, then stored. Storage in the sampling tube isnot recommended for swelling soils even though stressrelief may be minimal, The influence of rust and pene-tration of drilling fluid or free water into the sampleduring sampling may adversely influence the labora-tory test results and reduce the indicated potentialheave. Iron diffusing from steel tubes into the soilsample will combine with oxygen and water to formrust. Slight changes in Atterberg limits, erosion resist-ance, water content, and other physical properties mayoccur. In addition, the outer perimeter of a soil sample

3-3

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stored in the sampling tube cannot be scraped to re-move soil contaminated by water that may have pene-trated into the perimeter of the sample during sam-pling. The sample may also later adhere to the tubewall because of rust. If samples are stored in tubes, thetubes should be brass or lacquered inside to inhibit cor-rosion. An expanding packer with a rubber O-ring inboth ends of the tube should be used to minimize mois-ture loss. The following procedures should be followedin the care and storage of extruded samples.

(a) Expansive soil samples that are to be ex-trubed and stored should be removed from the sam-pling tubes immediately after sampling and thorough-ly sealed to minimize further stress relief and moistureloss. The sample should be extruded from the samplingtube in the same direction when sampled to minimizefurther sample disturbance.

(b) Samples extruded from tubes that were ob-tained with slurry drilling techniques should be wipedclean to remove drilling fluid adhering to the surfaceof the sample prior to sealing in the storage con-tainers. An outer layer of 1/8 to 1/4 inch should betrimmed from the cylindrical surface of the samples sothat moisture from the slurry will not penetrate intothe sample and alter the soil swelling potential andstrength. Trimming will also remove some disturbanceat the perimeter due to sidewall friction. The outerperimeter of the soil sample should also be trimmedaway during preparation of specimens for laboratorytests.

(c) Containers for storage of extruded samplesmay be either cardboard or metal and should beapproximately 1 inch greater in diameter and 1.5 to 2inches greater in length than the sample to be encased.Three-ply, wax-coated cardboard tubes with metal bot-toms are available in various diameters and lengthsand may be cut to desired lengths.

(d) Soil samples preserved in cardboard tubesshould be completely sealed in wax. The wax and card-board containers provide an excellent seal againstmoisture loss and give sufficient confinement to mini-mize stress relief and particle reorientation. A goodwax for sealing expansive soils consists of a 1 to 1 mix-ture of paraffin and microcrystalline wax or 100 per-cent beeswax. These mixtures adequately seal the sam-ple and do not become brittle when cold. The temper-ature of the wax should be approximately 20 degreesFahrenheit above the melting point when applied tothe soil sample, since wax that is too hot will penetratepores and cracks in the sample and render it useless, aswell as dry the sample. Aluminum foil or plastic wrapmay be placed around the sample to prevent penetra-tion of molten wax into open fissures. A small amountof wax (about 0.5-inch thickness) should be placed inthe bottom of the tube and allowed to partly congeal.The sample should subsequently be placed in the tube,

completely immersed and covered with the moltenwax, and then allowed to cool before moving.

(e) When the samples are being transported,they should be protected from rough rides and bumpsto minimize further sample disturbance.

f. Inspection. A competent inspector or engineershould accurately and visually classify materials asthey are recovered from the boring. Adequate classifi-cation ensures the proper selection of samples for lab-oratory tests. A qualified engineering geologist orfoundation engineer should closely monitor the drillcrew so that timely adjustments can be made duringdrilling to obtain the best and most representativesamples. The inspector should also see that all openborehoes are filled and sealed with a proper grout,such as a mixture of 12 percent bentonite and 88 per-cent cement, to minimize penetration of surface wateror water from a perched water table into deeper stratathat might include moisture deficient expansive clays.

3-4. GroundwaterMeaningful groundwater conditions and engineeringproperties of subsurface materials can often best bedetermined from in situ tests. In situ tests, however,are not always amenable to simple interpretation. Thepore water conditions at the time of the test may differappreciably from those existing at the time of con-struction. A knowledge of groundwater and the nega-tive pore water pressure are important in evaluatingthe behavior of a foundation, particularly in expansivesoil. Every effort should be made to determine the po-sition of the groundwater level, its seasonal variation,and the effect of tides, adjacent rivers, or canals on it.

a. Measurement of groundwater level. The most re-liable and frequently the only satisfactory method fordetermining groundwater levels and positive porewater pressures is by piezometers with tips installed atdifferent depths. Ceramic porous tube piezometerswith small diameters (3/8-inch) risers are usually ade-quate, and they are relatively simple, inexpensive, andsufficient for soils of low permeability.

b. Measurement of in situ negative pore water pres-sure, Successful in situ measurements of negative porewater pressure and soil suction have been performedby such devices as tensiometers, negative pore pres-sure piezometers, gypsum blocks, and thermocouplepsychrometer. However, each of these devices hascertain limitations, The range of tensiometers andnegative pore pressure piezometers has been limited tothe cavitation stress of water under normal conditions,which is near one atmosphere of negative pressure.The fluid-filled tensiometer is restricted to shallowsoils less than 6 feet in depth. The useable range of thetensiometer is reduced in proportion to the pressureexerted by the column of fluid in the tensiometer. Gyp-

3-5

sum blocks require tedious calibration of electricalresistivity for each soil and dissolved salts greatly in-fluence the results. Thermocouple psychrometer can-not measure soil suctions reliably at negative pres-sures that are less than one atmosphere and require aconstant temperature environment. Psychrometeralso measure the total suction that includes an osmoticcomponent caused by soluble salts in the pore water, aswell as the matrix suction that is comparable with the

negative pore water pressure. Tensiometers requireconstant maintenance, while gypsum blocks and psy-chrometer tend to deteriorate with time and may be-come inoperable within one year. A routine field meas- urement of soil suction is not presently recommendedbecause of the limitations associated with these de-vices. Alternatively, laboratory measurements of soilsuction can be easily performed (para 4-2a).

3-6

CHAPTER 4

LABORATORY INVESTIGATIONS

4-1. Identification of swelling soils

Soils susceptible to swelling can be identified by classi-fication tests. These identification procedures were de-veloped by correlations of classification test resultswith results of one-dimensional swell tests performedin consolidometers on undisturbed and compacted soilspecimens. Classification data most useful for identi-fying the relative swell potential include the liquidlimit (LL), the plasticity index (PI), the COLE (para

chemical tests. Several of the more simple and success-ful methods recommended for identifying swelling soilfrom classification tests described below were devel-oped from selected soils and locations combined withthe results of limited field observations of heave.These procedures assume certain environmental condi-tions for surcharge pressure (e.g., 1 pound per squareinch) and changes in moisture from the initial watercontent (e.g., to saturation or zero final pore waterpressure),

a. WES classification. Consolidometer swell tests

were performed on 20 undisturbed clays and clayshales from the states of Mississippi, Louisiana, Texas,Oklahoma, Arizona, Utah, Kansas, Colorado, Wyo-ming, Montana, and South Dakota. Results of thesetests for a change in moisture from natural water con-tent to saturation at the estimated in situ overburdenpressure (pressures corresponding to depths from 1 to8 feet) indicated the degrees of expansion and poten-

sents the percent increase in the vertical dimension orthe percent potential vertical heave. The classificationmay be used without knowing the natural soil suction,but the accuracy and conservatism of the system arereduced. Soils that rate low may not require furtherswell tests, particularly if the LL is less than 40 per-cent and the PI is less than 15 percent. Soils with theseAtterberg limits or less are essentially nonexpansive.However, swell tests may be required for soils of lowswelling potential if the foundation of the structure isrequired to maintain small differential movementsless than 1 inch (para 4-2c).

b. Texas Department of Highways and Public c. Van Der Merwe method. This method evolvedTransportation (TDHPT) method. This procedure, from empirical relationships between the degree of ex-which is known as Tex-124-E of the TDHPT Manual pansion, the PI, the percent clay fraction, and the sur-of Testing Procedures, is based on the swell test results charge pressure, The total heave at the ground surfaceof compacted soils from Texas. Field heaves of each is found from —soil stratum in the profile are estimated from a familyof curves using the LL, PI, surcharge pressure on thesoil stratum, and initial water content. The initial wa-ter content is compared with maximum (0.47 LL + 2) whereand minimum (0.2 LL + 9) water contents to evaluate AH =the percent volumetric change. The potential vertical D =rise (PVR) of each stratum is found from a chart usingthe percent volumetric change and the unit load bear-ing on the stratum. These PVRs for depths of as muchas 30 feet or more are summed to evaluate the total PE =PVR. This method may overestimate the heave of lowplasticity soils and underestimate the heave of highplasticity soils. and 1 inch/foot for low, medium, high, and very high.

levels, respectively, of potential expansiveness, de-fined in figure 4-1 as functions of the PI and the mi-

meter swell test results and field observations. Thismethod does not consider variations in initial moistureconditions.

d. Physiochemical tests. These tests include iden-tification of the clay minerals, such as montmorillo-nite, illite, attapulgite, and kaolinite, with kaolinitebeing relatively nonexpansive, cation exchange capac-ity (CEC), and dissolved salts in the pore water. TheCEC is a measure of the property of a clay mineral toexchange ions for other anions or cations by treatmentin an aqueous solution. The relatively expansive mont-morillonite minerals tend to have large CEC exceeding80 milliequivalents per 100 grams of clay, whereas theCEC of nonexpansive kaolinite is usually less than 15milliequivalents. The presence of dissolved salts in thepore water produces an osmotic component of soil suc-tion that can influence soil heave if the concentrationof dissolved salts is altered. In most cases, the osmoticsuction will remain constant and not normally influ-ence heave unless, for example, significant leaching ofthe soil occurs.

e. Other methods. Other methods that have beensuccessful are presented in table 4-2. These methods

heave assuming that all swell is confined to the verti-

cal direction, and they require an estimate of the depth

Van Der Merwe methods do not require estimates of

the computed heaves become negligible. The Van DerMerwe, McKeen-Lytton, and Johnson methods tend togive maximum values or may overestimate heave,whereas the remaining methods tend to give minimumvalues or may underestimate heave when comparedwith the results of field observations at three WEStest sections.

f. Application. These identification tests along withthe surface examination of paragraph 3-2 can indicateproblem soils that should be tested further and canprovide a helpful first estimate of the expected in situheave.

(1) More than one identification test should beused to provide rough estimates of the potential heavebecause limits of applicability of these tests are notknown. In general, estimates of potential heave at theground surface of more than 1/2 inch may require fur-ther laboratory tests, particularly if local experiencesuggests swelling soil problems. Soil strata in whichthe degree of expansion is medium or high should alsobe considered for further swell tests (para 2-2c).

(2) The McKeen-Lytton method of table 4-2 hasbeen applied to the prediction of potential differentialheave for average changes in moisture conditions bythe Post-Tensioning Institute (PTI) for design and con-

struction of stiffened slabs-on-grade in expansive soils.The PTI structural design procedure is described inparagraph 6-3b.

4-2. Testing procedures

Quantitative characterization of the expansive soil—from swell tests is necessary to predict the anticipated

potential soil heave devaluation of swell behavior andpredictions of total and differential heave are deter-mined from the results of tests on undisturbed speci-mens. Strength tests may be performed to estimatethe bearing capacity of the foundation soil at the finalor equilibrium water content. A measure of shearstrength with depth is also needed to evaluate soil sup-

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port from adhesion along the perimeter of shaftfoundations or the uplift that develops on the shaftwhen swelling occurs.

a. Swell tests. Laboratory methods recommendedfor prediction of the anticipated volume change or po-tential in situ heave of foundation soils are consoli-dometer swell and soil suction tests, The WES expan-sive soil studies show that consolidometer swell testsmay underestimate heave, whereas soil suction testsmay overestimate heave compared with heaves meas-ured in the field if a saturated final moisture profile isassumed (chap 5). The economy and simplicity of soilsuction tests permit these tests to be performed at fre-quent intervals of depth from 1 to 2 feet.

(1) Consolidometer. Recommended consolidom-eter swell tests include swell and swell pressure testsdescribed in Appendix VIII of EM 1110-2-1906. Theswell test may be performed to predict vertical heaveAH of soil thickness H when the vertical overburdenand structural pressures on thickness H are knownprior to the test. The total vertical heave at the groundsurface is the sum of the AH for each thickness H inthe soil profile. Figure 5-4 illustrates the applicationof swell test data. The swell pressure test is performed

quired for prediction of vertical heave by equation

(5-8) discussed in paragraph 5-4e. The confining pres-

little is known about swell behavior or groundwaterconditions, an appropriate swell testis given in (a) and(b) below.

—-

(a) An initial loading pressure, simulating fieldinitial (preconstruction) vertical pressure &, should beapplied to determine the initial void ratio e., point 1 of

(i.e., the lowest possible load) prior to adding distilledwater, point 2. The specimen is allowed to expand atthe seating pressure until primary swell is complete,point 3, before applying the consolidation pressures.

(b) The swell test of figure 4-2 can eliminatethe need for additional tests when behavior is differ-ent than that anticipated (e.g., the specimen consoli-dates rather than swells following addition of water atloading pressures greater than the seating pressure).The void ratio-log pressure curve for final effectivepressures, varying from the seating to the maximumapplied pressure, can be used to determine heave or

settlements will occur for final effective pressures ex-

with respect to the initial vertical pressure&.

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pressure that must be applied to the soil to reduce thevolume expansion down to the (approximated) in situ

in appendix VIII of EM 1110-2-1906 tend to providelower limits of the in situ swell pressure, while thesimple swell test, figure 4-2, tends to provide upperlimits. The maximum past pressure is often a useful

(2) Soil suction. Soil suction is a quantity that alsocan be used to characterize the effect of moisture onvolume changes and, therefore, to determine theanticipated foundation soil heave. The suction is a ten-sile stress exerted on the soil water by the soil massthat pulls the mass together and thus contributes tothe apparent cohesion and undrained shear strength ofthe soil. The thermocouple psychrometer and filterpaper methods, two of the simplest approaches forevaluation of soil suction and characterization of swell-ing behavior, are described in appendix B. The suctionprocedure, which is analogous to the procedure forcharacterization of swell from consolidometer swelltests, is relatively fast, and the results can increaseconfidence in characterization of swell behavior.

b. Strength tests. The results of strength tests areused to estimate the soil bearing capacity and load/de-flection behavior of shaft or other foundations. Thecritical time for bearing capacity in many cases isimmediately after completion of construction (firstloading) and prior to any significant soil consolidationunder the loads carried by the foundation. The long-term bearing capacity may also be critical in expansivefoundation soils because of reductions in strengthfrom wetting of the soil.

c. Application. Sufficient numbers of swell and

strength tests should be performed to characterize thesoil profiles. Swell tests may not be necessary on speci-mens taken at depths below permanent deep ground-water levels.

(1) The representative mean of the swell andstrength parameters (and lower limit of the scatter instrength parameters) of each distinctive soil stratumshould be determined down to depths of 1.5 times theminimum width of mat slabs to a maximum of 100feet and to at least three base diameters beneath thebase of shaft foundations.

(2) One consolidometer swell and one strengthtest should be performed on specimens from at leastfive undisturbed samples at different depths withinthe depth of the anticipated active zone (e.g., within 10to 20 feet beneath the base of the foundation). Suctiontests may also be performed at relatively frequentdepth intervals (e.g., l-foot increments) to better char-acterize swell behavior and thereby increase confi-dence in prediction of potential heave discussed inchapter 5.

(3) One consolidometer swell and one strengthtest should be performed on specimens from eachundisturbed sample (or at intervals of 2.5 feet. forcontinuous sampling) at depths above the base of deepshaft foundations to permit evaluation of the adjacentsoil heave and uplift forces exerted on the shaft/soilinterface, Suction tests may also be performed to fur-ther characterize swell behavior and increase confi-dence in prediction of potential heave.

(4) Suction test results can characterize the porepressure profile by indicating depths of desiccationand wetting, which are useful for minimizing potentialfoundation problems from soil movement and for eval-uating remedial measures to correct problems.

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CHAPTER 5

METHODOLOGY FOR PREDICTION OF VOLUME CHANGES

5-1. Application of heave predictions

Reasonable estimates of the anticipated vertical andhorizontal heave and the differential heave are neces-sary for the following applications.

a. Determination of adequate designs of structuresthat will accommodate the differential soil movementwithout undue distress (chap 6). These predictions arealso needed to estimate upward drag from swellingsoils on portions of deep foundations such as drilledshafts within the active zone of moisture change andheave. Estimates of upward drag help determine anoptimum design of the deep foundation.

5-2. Factors influencing heave

Table 5-1 describes factors that significantly influ-ence the magnitude and rate of foundation movement.The difficulty of predicting potential heave is compli-cated beyond these factors by the effect of the typeand geometry of foundation, depth of footing, and dis-tribution of load exerted by the footing on the magni-tude of the swelling of expansive foundation soil.Additional problems include estimating the exact loca-tion that swelling soils will heave or the point sourceof water seeping into the swelling soil and the final orequilibrium moisture profile in the areas of heavingsoil.

b. Determination of techniques to stabilize the foun-dation and to reduce the anticipated heave (chap 7).

5-3. Direction of soil movement

The foundation soil may expand both vertically andlaterally. The vertical movement is usually of primaryinterest, for it is the differential vertical movementthat causes most damages to overlying structures.

a. Vertical movement. Methodology for predictionof the potential total vertical heave requires an as-sumption of the amount of volume change that occursin the vertical direction. The fraction of volumetricswell N that occurs as heave in the vertical directiondepends on the soil fabric and anisotropy. Verticalheave of intact soil with few fissures may account forall of the volumetric swell such that N = 1, whilevertical heave of heavily fissured and isotropic soilmay be as low as N = 1/3 of the volumetric swell.

b. Lateral movement. Lateral movement is very im-portant in the design of basements and retainingwalls. The problem of lateral expansion against base-ment walls is best managed by minimizing soil volumechange using procedures described in chapter 7. Other-wise, the basement wall should be designed to resistlateral earth pressures that approach those given by

(5-1)

horizontal earth pressure, tons per squarerootlateral coefficient of earth pressure at restsoil vertical or overburden pressure, tonsper square footcoefficient of passive earth pressure

order of 1 to 2 in expansive soils and often no greaterthan 1.3 to 1.6.

5-4. potential total vertical heave

Although considerable effort has been made to developmethodology for reliable predictions within 20 percentof the maximum in situ heave, this degree of accuracywill probably not be consistently demonstrated, par-ticularly in previously undeveloped and untestedareas. A desirable reliability is that the predicted po-tential total vertical heave should not be less than 80percent of the maximum in situ heave that will even-tually occur but should not exceed the maximum insitu heave by more than 20 to 50 percent. Useful pre-dictions of heave of this reliability can often be ap-proached and can bound the in situ maximum levels ofheave using the results of both consolidometer swelland soil suction tests described in paragraph 4-2a. Thefraction N (para 5-3a) should be 1 for consolidometerswell test results and a minimum of 1/3 for soil suctiontest results. The soil suction tests tend to provide anupper estimate of the maximum in situ heave (N = 1)in part because the soil suction tests are performed

without the horizontal restraint on soil swell thatexists in the field and during one-dimensional consoli-dometer swell tests.

a. Basis of calculation. The potential total vertical —heave at the bottom of the foundation, as shown in fig-ure 5-1, is determined by

i= NELAH= DELTA(i)

i= NBX

i= NEL(5-2)

i= NBXwhere

AH=

N =

DX =NEL =NBX =

DELTA(i) =

potential vertical heave at thebottom of the foundation, feetfraction of volumetric swell thatoccurs as heave in the vertical di-rectionincrement of depth, feettotal number of elementsnumber of nodal point at bottomof the foundationpotential volumetric swell of soilelement i, fractionfinal void ratio of element iinitial void ratio of element i

The AH is the potential vertical heave beneath a flex-ible, unrestrained foundation. The bottom nodal pointNNP = NEL + 1, and it is often set at the active depth

(1) The initial void ratio, which depends on geo-logic and stress history (e.g., maximum past pressure),the soil properties, and environmental conditionsshown in table 5-1 may be measured on undisturbedspecimens using standard laboratory test procedures.It may also be measured during the laboratory swelltests as described in EM 1110-2-1906. The final voidratio depends on changes in the foundation conditionscaused by construction of the structure.

(2) The effects of the field conditions listed in ta-ble 5-1 may be roughly simulated by a confinementpressure due to soil and structural loads and an as-sumption of a particular final or equilibrium porewater pressure profile within an active depth of heave

pore water pressure profiles are related to the finalvoid ratio by physical models. Two models based on re-sults of consolidometer swell and soil suction tests areused in this manual (para 4-2a).

b. Pore water pressure profiles. The magnitude ofswelling in expansive clay foundation soils depends onthe magnitude of change from the initial to the equi-librium or final pore water pressure profile that will beobserved to take place in a foundation soil because of

5-2

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the construction of the foundation.(1) Initial profile. Figure 5-1 illustrates relative

initial dry and wet profiles. The wet initial profile isprobably appropriate following the wet season, whichtends to occur by spring, while the dry initial profiletends to occur during late summer or early fall. Theinitial pore water pressure profile does not need to beknown if the consolidometer swell model is used be-cause the heave prediction is determined by the differ-

ratios (fig. 4-2). The initial void ratio is a function ofthe initial pore water pressure in the soil. The initialpore water pressure profile, which must be known ifthe soil suction model is used, may be found by themethod described in appendix B.

(2) Equilibrium profile. The accuracy of the pre-diction of the potential total vertical heave in simulat-ing the maximum in situ heave depends heavily on theability to properly estimate the equilibrium pore waterpressure profile. This profile is assumed to ultimatelyoccur beneath the central portion of the foundation.The pore water pressure profile beneath the founda-tion perimeter will tend to cycle between dry and wetextremes depending on the field environment andavailability of water. The three following assumptionsare proposed to estimate the equilibrium profile. Afourth possibility, the assumption that the ground-water level rises to the ground surface, is most con-servative and not normally recommended as beingrealistic. The equilibrium profile may also be esti-mated by a moisture diffusion analysis for steady-stateflow, which was used to predict differential heave aspart of the procedure developed by the Post-Tension-ing Institute (PTI) for design and construction of slabs-on-grade (para 6-3b). The results, which should beroughly compatible with the hydrostatic profilesdiscussed in (b) and (c) below, lead to predictions ofheave smaller than the saturated profile.

(a) Saturated. The saturated profile, Method 1

in figure 5-1, assumes that the in situ pore water pres-

change and heave(5-3)

foot at any depth X in feet within the active zone. Al-though a pore water pressure profile of zero is not inequilibrium, this profile is considered realistic formost practical cases and includes residences and build-ings exposed to watering of perimeter vegetation andpossible leaking underground water and sewer lines.Water may also condense in a layer of permeable sub-grade soil beneath foundation slabs by transfer ofwater vapor from air flowing through the cooler sub-grade. The accumulated water may penetrate intounderlying expansive soil unless drained or protectedby a moisture barrier. This profile should be used if’other information on the equilibrium pore water pres-sure profile is not available.

(b) Hydrostatic I. The hydrostatic I profile,Method 2 in figure 5-la, assumes that the pore waterpressure becomes more negative with increasing verti-cal distance above the groundwater level in proportionto the unit weight of water

(5-4)

cubic foot).

This profile is believed to be more realistic beneathhighways and pavements where drainage is good,pending of surface water is avoided, and leaking un-derground water lines are not present. This assump-tion will lead to smaller predictions of heave than thesaturated profile of Method 1.

(c) Hydrostatic II. This profile, Method 3 in figure 5-lb, is similar to the hydrostatic I profile exceptthat a shallow water table does not exist. The negativepore water pressure of this profile also becomes morenegative with increasing vertical distance above the

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weight of water(5-5)

(d) Example application. Figure 5-2 illustrateshow the saturated (Method 1) and hydrostatic II(Method 3) profiles appear for a suction profile with-out a shallow water table at a sampling site nearHayes, Kansas. The initial in situ soil suction or nega-tive pore water pressure was calculated from the givennatural soil suction without confining pressure To by

T (5-6)where

mean normal confining pressure, tons persquare foot

was assumed to be unity. The initial in situ soil suction

that of the corresponding negative pore water pressure

the hydrostatic equilibrium profile is nearly verticalwith respect to the large magnitude of soil suction ob-served at this site. Heave will be predicted if the satur-ated profile occurs (Method 1 as in fig. 5-1), whileshrinkage will likely be predicted if the hydrostatic II(Method 3) profile occurs. The availability of water tothe foundation soil is noted to have an enormous im-pact on the volume change behavior of the soils. There-fore, the methods of chapter 7 should be used as muchas practical to promote and maintain a constant mois-ture environment in the soil.

c. Depth of the active zone. The active zone depth

changes in water content and heave occur because ofclimate and environmental changes after constructionof the foundation.

be assumed equal to the depth of the water table forgroundwater levels less than 20 feet in clay soil (fig.

ro for the hydrostatic I equilibrium profile in the pres-ence of such a shallow water table.

deep groundwater levels may often be determined byevaluating the initial pore water pressure or suctionwith depth profile as described in appendix B, Themagnitude of u., is then determined after the depth

(a) If depths to groundwater exceed 20 feet be-neath the foundation and if no other information is

10 feet (for moist profiles or soil suctions less than 4tons per square foot) and 20 feet (for dry profiles orsoil suctions greater than 4 tons per square foot) belowthe base of, the foundation (fig. 5-lb). However, the

the base diameter of a shaft foundation. Sources ofmoisture that can cause this active zone include theseepage of surface water down the soil-foundation in-terface, leaking underground water lines, and seepagefrom nearby new construction.

(b) The pore water pressure or soil suction is of-ten approximately constant with increasing depth be-

below which the water content/plastic limit ratio orsoil suction is constant.

(c) If the soil suction is not approximately con-stant with increasing depth below depths of 10 to 20

to 2 feet below the first major change in the magni-tude of the soil suction, as shown in figure 5-2.

d. Edge effects. Predictions of seasonal variations invertical heave from changes in moisture between ex-treme wet and dry moisture conditions (fig. 5-1) arefor perimeter regions of shallow foundations. These

TM 5-818-7

calculations require a measure or estimate of both sea-sonal wet and dry pore water pressure or suction pro-files. It should be noted from figure 5-lb that perime-ter cyclic movement from extremes in climaticchanges can exceed the long-term heave beneath thecenter of a structure.

(1) Soil-slab displacements. A slab constructed onthe ground surface of a wet site may in time lead todownwarping at the edges after a long drought orgrowth of a large tree near the structure (fig. 5-3a).Edge uplift may occur following construction on aninitially dry site (fig. 5-3b). The AH in figure 5-3 isrepresentative of the maximum differential verticalheave beneath the slab, excluding effects of restraintfrom the slab stiffness, but does consider the slabweight.

(2) Edge distance. The edge lift-off distance e oflightly loaded thin slabs at the ground surface oftenvaries from 2 to 6 feet but can reach 8 to 10 feet.

(3) Deflection/length ratio. The deflection/lengthratio of the slab is A/L, where A is the slab deflectionand L is the slab length. The angular deflection/span

5-3).

(5-8)

thickness of expansive soil layer, feetswell index, slope of the curve betweenpoints 3 and 4, figure 4-2swell pressure, tons per square footfinal vertical effective pressure, tons persquare foot

The final effective pressure is given by(5-9)

4-2. A simple hand method and an example of predict-ing potential total vertical heave from consolidometerswell tests assuming a saturated equilibrium profile,equation (5-3), are given in TM 5-818-1 and in figure5-4. However, hand calculations of potential heavecan become laborious, particularly in heterogeneousprofiles in which a variety of loading conditions needto be evaluated for several different designs,

(2) Computer applications. Predictions of poten-tial total heave or settlement can be made quickly withthe assistance of the computer program HEAVE avail-able at the U. S. Army Corps of Engineers Waterways

Experiment Station. The program HEAVE is applica-ble to slab, long continuous, and circular shaft founda-tions. This program considers effects of loading andsoil overburden pressures on volume changes, hetero-geneous soils, and saturated or hydrostatic equilibri-um moisture profiles (equations (5-3) to (5-5)). Resultsof HEAVE using the saturated profile, equation (5-3),are comparable with results of manual computationsdescribed in figure 5-4.

5-5. Potential differential heave

Differential heave results from edge effects beneath afinite covered area, drainage patterns, lateral varia-tions in thickness of the expansive foundation soil, andeffects of occupancy. The shape and geometry of thestructure also result in differential heave. Examples ofeffects of occupancy include broken or leaking waterand sewer lines, watering of vegetation, and pondingadjacent to the structure. Other causes of differentialheave include differences in the distribution of loadand the size of footings.

a. Unpredictability of variables. Reliable predic-tions of future potential differential heave are oftennot possible because of many unpredictable variablesthat include: future availability of moisture fromrainfall and other sources, uncertainty of the exact lo-cations of heaving areas, and effects of human occu-pancy.

b. Magnitude of differential heave.(1) Potential differential heave can vary from zero

to as much as the total heave. Differential heave is of-ten equal to the estimated total heave for structuressupported on isolated spot footings or drilled shafts be-cause some footings or portions of slab foundations of-ten experience no movement. Eventually, differentialheave will approach the total heave for most practicalcases and should, therefore, be assumed equal to thetotal potential heave, unless local experience or otherinformation dictates otherwise.

(2) The maximum differential heave beneath alightly loaded foundation slab may also be estimatedby the procedure based on the moisture diffusion theo-ry and soil classification data developed by the PTI.Heave predictions by this method will tend to be lessthan by assuming that the differential heave is the to-tal potential heave.

5-6. Heave with time

Predictions of heave with time are rarely reliable be-cause the location and time when water is available tothe soil cannot be readily foreseen. Local experiencehas shown that most heave (and the associated struc-tural distress) occurs within 5 to 8 years following con-struction, but the effects of heave may also not be ob-served for many years until some change occurs in the

5-5

●

foundation conditions to disrupt the moisture regime. tant engineering problems are the determination ofPredictions of when heave occurs are of little engineer- the magnitude of heave and the development of waysing significance for permanent structures. The impor- to minimize distress of the structure.

5-7

DESIGN OF FOUNDATIONS

6-1. Basic considerations

a. Planning. Swelling of expansive foundation soilsshould be considered during the preliminary designphase and the level of structural cracking that will beacceptable to the user should be determined at thistime.

(1) The foundation of the structure should be de-signed to eliminate unacceptable foundation and struc-tural distress. The selected foundation should also becompatible with available building materials, con-struction skills, and construction equipment.

(2) The foundation should be designed and con-structed to maintain or promote constant moisture inthe foundation soils. For example, the foundationshould be constructed following the wet season if pos-sible. Drainage should be provided to eliminate pondedwater. Excavations should be protected from drying.Chapter 7 describes the methods of minimizing soilmovement.

b. Bearing capacity. Foundation loading pressuresshould exceed the soil swell pressures, if practical, butshould be sufficiently less than the bearing capacity tomaintain foundation displacements within tolerableamounts, Present theoretical concepts and empiricalcorrelations permit reasonably reliable predictions ofultimate capacity, but not differential movement ofthe foundation. Factors of safety (FS) are therefore ap-plied to the ultimate bearing capacity to determinesafe or allowable working loads consistent with tolera-ble settlements. Further details on bearing capacityare presented in TM 5-818-1.

c. Foundation systems. An appropriate foundationshould economically contribute to satisfying the func-tional requirements of the structure and minimize dif-ferential movement of the various parts of the struc-ture that could cause damages. The foundation shouldbe designed to transmit no more than the maximumtolerable distortion to the superstructure. The amountof distortion that can be tolerated depends on the de-sign and purpose of the structure. Table 6-1 illustratesfoundation systems for different ranges of differential

selection of the foundation. Figure 6-1 explains the

not a satisfactory basis of design in situations such as a5-foot layer of highly swelling soil overlying nonswell-

ing soil, rock, or sand. Pervious sand strata may pro-vide a path for moisture flow into nearby swelling soil.

(1) Shallow individual or continuous footings.Shallow individual or long continuous footings are of-ten used in low swelling soil areas where the predictedfooting angular deflection/span length ratios are onthe order of 1/600 to 1/1000 or 0.5 inch or less ofmovement.

(2) Stiffened mats (slabs). Stiffened mat founda-tions are applicable in swelling soil areas where pre-dicted differential movement AH may reach 4 inches.The stiffening beams of these mats significantly re-duce differential distortion. The range provided in ta-ble 6-1 for beam dimensions and spacings of stiffenedslabs for light structures normally provides an ade-quate design.

(3) Deep foundations. A pile or beam on a drilledshaft foundation is applicable to a large range of foun-dation soil conditions and tends to eliminate effects ofheaving soil if properly designed and constructed (para6-4). The type of superstructure and the differentialsoil movement are usually not limited with properlydesigned deep foundations. These foundations shouldlead to shaft deflection/spacing ratios of less than1/600.

d. Superstructure systems. The superstructureshould flex or deform compatibly with the foundationsuch that the structure continues to perform its func-tions, contributes aesthetically to the environment,and requires only minor maintenance. Frame construc-tion, open floor plans, and truss roofs tend to minimizedamage from differential movement. Load bearingwalls tend to be more susceptible to damage fromshear than the relatively flexible frame construction.Wood overhead beams of truss roof systems providestructural tension members and minimize lateralthrust on walls. Table 6-2 illustrates the relative flexi-bility provided by various superstructure systems.

(1) Tolerable angular deflection/length ratios. Theability of a structure to tolerate deformation dependson the brittleness of the building materials, length toheight ratio, relative stiffness of the structure in shearand bending, and mode of deformation whether heave(dome-shaped, fig. 1-2) or settlement (dish-shaped, fig.

that can be tolerated, therefore, varies considerably

PI = 25

PI =50

PI =40

footings or about twice the A/L ratio of the slab (fig.5-3). Only rough guidance of the range of tolerable

ferent framing systems.(a) Propagation of cracks depends on the degree

of tensile restraint built into the structure and itsfoundation. Thus, frame buildings with panel walls areable to sustain larger relative deflections without se-vere damage than unreinforced load-bearing walls.Structural damage is generally less where the dish-shaped pattern develops than in the case of centerheaving or edge downwarping because the foundationis usually better able to resist or respond to tensionforces than the walls.

avoid cracking in single and multistory structures.Plaster, masonry or precast concrete blocks, and brick

1/600 to 1/1000. However, cracks may not appear inthese walls if the rate of distortion is sufficiently slowto allow the foundation and frame to adjust to the new

distortions. The use of soft bricks and lean mortar alsotend to reduce cracking. Reinforced masonry, rein-forced concrete walls and beams, and steel frames can

pear in the structure. Deflection ratios exceeding1/250 are likely to be noticed in the structure and

1/150 usually lead to structural damage.(2) Provisions for flexibility. The flexibility re-

quired to avoid undesirable distress may be providedby joints and flexible connections. Joints should beprovided in walls as necessary, and walls should not betied into the ceiling. Slabs-on-grade should not be tiedinto foundation walls and columns but isolated usingexpansion joints or gaps filled with a flexible, imper-vious compound. Construction items, such as rein-forced concrete walls, stud frames, paneling, andgypsum board, are better able to resist distortions andshould be used instead of brick, masonry blocks, orplaster walls. The foundation may be further rein-forced by making the walls structural members capa-

6-3

TM 5-818-7

ble of resisting bending such as reinforced concreteshear walls. Several examples of frame and wall con-struction are provided in appendix C.

6-2. Shallow individual or continuousfootings

a. Susceptibility y to damage. Structures supportedby shallow individual or continuous wall footings aresusceptible to damages from lateral and vertical move-ment of foundation soil if provisions are not made toaccommodate possible movement. Dishing or substan-tial settlement may occur in clays, especially in initial-ly wet soil where a well-ventilated crawl space is con-structed under the floor. The crawl space preventsrainfall from entering the soil, but the evaporation ofmoisture from the soil continues. Center heave or edgedownwarping (fig. 1-2) can occur if the top layer ofsoil is permeable and site drainage is poor. Fracturesmay appear in walls not designed for differential

movement exceeds about 0.5 inch.

b. Applications. Shallow footings may be usedwhere expansive strata are sufficiently thin to allowlocation of the footing in a nonexpansive or low-swell-ing stratum (fig. 6-2).

(1) A structural floor slab should be suspended ontop of the footing (fig. 6-2a) or the slab-on-gradeshould be isolated from the walls (fig. 6-2b). The slab-on-grade should be expected to crack.

(2) Figure 6-3 illustrates examples of interior con-struction for a slab-on-grade. Interior walls may besuspended from the ceiling or supported on the floor.A flexible joint should be provided in the plenum be-tween the furnace and the ceiling. Sewer lines andother utilities through the floor slab should be permit-ted to slip freely.

(3) Swelling of deep expansive soil beneath a non-expansive stratum may cause differential movementof shallow footings if the moisture regime is altered inthe deep soil following construction (e.g., change ingroundwater level, or penetration of surface water into deep desiccated soil). Excavations for crawl spaces

or basements decrease the vertical confining pressureand pore water pressure, which can cause the underly-ing expansive foundation soil to heave from adjust-ment of the moisture regime back to the natural porewater pressures.

c. Basements. Basements and long continuous foot-ings constructed in excavations are subject to swellpressures from underlying and adjacent expansive soil.

(1) Walls. Basement walls of reinforced concretecan be constructed directly on the foundation soilwithout footings provided foundation pressures areless than the allowable bearing capacity (fig. 6-4a).However, placing heavy loads on shallow footings maynot be effective in countering high swell pressures be-cause of the relative small width of the footings. Thestress imposed on the soil is very low below a depth ofabout twice the width of the footing and contributeslittle to counter the swell pressure unless the expan-sive soil layer is thin.

(2) Voids. Voids can also be spaced at intervals be-neath the walls to increase loading pressures on thefoundation soil and to minimize flexing or bowing ofthe walls (fig. 6-4b). The voids may be made with re-movable - wood forms,

INTERIOR

commercially available card-

board, or other retaining forms that deteriorate andcollapse (para 6-4d).

(3) Joints. Joints should be provided in interiorwalls and other interior construction if slab-on-groundis used (fig. 6-3). The slab should be isolated from thewalls with a flexible impervious compound.

(4) Lateral earth pressure on wall. The coefficientof lateral earth pressure can exceed one if the backfillis heavily compacted and expansive, or the natural soiladjacent to the wall is expansive. Controlled backfillsare recommended to minimize lateral pressures and in-crease the economy of the foundation (para 7-3a).Steel reinforcement can provide the necessary re-straint to horizontal earth pressures, Unreinforcedmasonry brick and concrete blocks should not be usedto construct basement walls.

d. Design. Standard design procedures for founda-tions of buildings and other structures are presented inTM5-818-1.

6-3. Reinforced slab-on-grade founda-tions

a. Application. The reinforced mat is often suitablefor small and lightly loaded structures, particularly if

TM 5-818-7

the expansive or unstable soil extends nearly continu-ously from the ground surface to depths that excludeeconomical drilled shaft foundations. This mat is suit-able for resisting subsoil heave from the wetting ofdeep desiccated soil, a changing water table, laterallydiscontinuous soil profiles, and downhill creep, whichresults from the combination of swelling soils and thepresence of slopes greater than 5 degrees. A thick, re-inforced mat is suitable for large, heavy structures.The rigidity of thick mats minimizes distortion of thesuperstructure from both horizontal and verticalmovements of the foundation soil.

(1) Effects of stiffening beams. Concrete slabswithout internal stiffening beams are much more sus-ceptible to distortion or doming from heaving soil.Stiffening beams and the action of the attached super-structure with the mat as an indeterminate structureincrease foundation stiffness and reduce differentialmovement. Edge stiffening beams beneath reinforcedconcrete slabs can also lessen soil moisture loss and re-duce differential movement beneath the slab. How-ever, the actual vertical soil pressures acting on stif-fened slabs can become very nonuniform and cause lo-calized consolidation of the foundation soil.

(2) Placement of nonswelling layer. Placement ofa nonswelling, 6-inch-(or more) thick layer of (prefer-ably) impervious soil on top of the original ground sur-face before construction of lightly loaded slabs is rec-ommended to increase the surcharge load on thefoundation soil, slightly reduce differential heave, andpermit the grading of a slope around the structureleading down and away from it. This grading improvesdrainage and minimizes the possibility that the layer(if pervious) could be a conduit for moisture flow intodesiccated foundation expansive soils. The layershould have some apparent cohesion to facilitatetrench construction for the stiffening beams.

6-6

b. Design of thin slabs for light structures. Stiff-ened slabs may be either conventionally reinforced orposttensioned. The mat may be inverted (stiffeningbeams on top of the slab) in cases where bearing capac-ity of the surface soil is inadequate or a supported firstfloor is required. The Department of Housing and Ur-ban Development, Region IV, San Antonio Area Of-fice, has documented a series of successful conven-tionally reinforced and posttensioned slabs for thesouthern central states. Successful local practiceshould be consulted to help determine suitable designs.

(1) Conventionally reinforced. The conventionalreinforced concrete waffle type mat (table 6-1), whichis used for light structures, consists of 4- to 5-inch-thick concrete slab. This slab contains temperaturesteel and is stiffened with doubly reinforced concretecrossbeams. Figure 6-5 illustrates an engineered rebarslab built in Little Rock, Arkansas. Appendix C pro-vides details of drawings of reinforced and stiffenedthin mats. The 4-inch slab transmits the self-weightand first floor loading forces to the beams, which re-sist the moments and shears caused by differentialheave of the expansive soil. Exterior walls, roof, andinternal concentrated loads bear directly on the stiff-ening beams. Clearance between beams should be lim-ited to 400 square feet or less. Beam spacings may bevaried between the limits shown in table 6-1 to allowfor concentrated and wall loads. Beam widths varyfrom 8 to 12 or 13 inches but are often limited to aminimum of 10 inches.

(a) Concrete and reinforcement. Concrete com-pressive strength f ‘c should be at least 2500 psi andpreferably 3000 psi. Construction joints should beplaced at intervals of less than 150 ft and cold jointsless than 65 ft. About 0.5 percent reinforcing steelshould be used in the mat to resist shrinkage and tem- perature effects.

(b) Preliminary design, The three designs for re-inforced and stiffened thin mats presented in table6-1 differ in the beam depth and spacing depending

ings for each of the light, medium, and heavy slabs are

servative in view of still undetermined fully acceptabledesign criteria and relatively high repair cost of rein-forced and stiffened slabs. Stirrups may be added, par-ticularly in the perimeter beams, to account for con-centrated and exterior wall loads.

(2) Post-tensioned. Figure 6-6 illustrates an ex-ample of a posttensioned slab. Properly designed andconstructed posttensioned mats are more resistant tofracture than an equivalent section of a conventionalrebar slab and use less steel. However, post-tensionedslabs should still be designed with adequate stiffeningbeams to resist flexure or distortion from differentialheave of the foundation soil, Experienced personnelare necessary to properly implement the posttension-ing.

(3) Assumptions of design parameters. Designparameters include effects of climate, center and edgemodes of differential swelling, perimeter and uniformloads, and structural dimensions.

(a) The effects of climate and differential swell-- - ing are accounted for by predictions of the maximum

differential heave AH and the maximum edge lift-off

related with the Thornthwaite Moisture Index (TMI) infigure 6-7. The TMI, a climate related parameterroughly estimated from figure 6-8, represents theoverall availability of water in the soil. The TMI canvary 10 to 20 or more (dimensionless) units from year

exceed the range given in figure 6-7, depending on theactivity of the soil or extreme changes in climatic con-ditions (e.g., long droughts and heavy rainfall), the

(b) The loading distribution depends on thearchitectural arrangement of the building and oftencannot be significantly altered. Perimeter and concen-trated loads should be supported directly on the stiff-ening beams.

(c) The length and width of the slab are usuallyfixed by the functional requirement. Beam spacing de-pends on the slab geometry and varies between 10 and20 feet. The depth of stiffening beams is controlled bythe moment and shear capacity. The beam depth is ad- Ijusted as needed to remain within the allowable limits.The width of the stiffening beam is usually controlledby the excavation equipment and soil bearing capacity.

(4) Structural design procedure, The design proce-

6 - 7

(Department of Housing and Urban

Development, Region IV)

Figure 6-6. Post-tensioned slab in Lubbock, Texas, for single-family, single-story, minimally loaded frame residence.

dure should provide adequate resistance to shear, mo-ment, and deflections from the structural loadingforces, while overdesign is minimized. An economical-ly competitive procedure that builds on the early workof the Building Research Advisory Board of the Na-tional Academy of Sciences is that developed for thePost-Tensioning Institute (PTI).

(a) The PTI procedure is applicable to both con-

ventionally reinforced and posttensioned slabs up to18 inches thick. It considers the previously discussedassumptions of the design parameters.

of the unloaded soil determined by the PTI procedurereflect average moisture conditions and may be ex-ceeded if extreme changes in climate occur.

(c) Material parameters required by the PTI pro-

CLIMATE

cedure are the compressive strength of concrete; allow-able tensile and compressive stresses in concrete; type,grade, and strength of the prestressing steel; gradeand strength of the mild steel reinforcement; and slabsubgrade friction coefficient, The amount of reinforc-ing steel recommended by this procedure should beconsidered a minimum. The slab-subgrade coefficientof friction should be 0.75 for concrete cast on poly-ethylene membranes and 1.00 if cast on-grade.

This ratio may be as large as 1/360 for center heave

criterion is recommended by the PTI because edge liftis usually much less than center lift deflections and thestems of the beams resisting the positive bending mo-ment may be unreinforced.

c. Design of thick mats. The state of the art for esti-mating spatial variations in soil pressures on thickmats is often not adequate. These mats tend to beheavily overdesigned because of the uncertainty in theloading and the relatively small extra investment ofsome overdesign.

(1) Description. Concrete mats for heavy struc-tures tend to be 3 feet or more in thickness with a con-tinuous two-way reinforcement top and bottom. An 8-foot-thick mat supporting a 52-story structure inHouston, Texas, contains about 0.5 percent steel,while the 3-foot-thick mat of the Wilford Hall Hospitalcomplex at Lackland Air Force Base in Texas also con-tains about 0.5 percent steel. The area of steel is 0.5percent of the total area of the concrete distributedequally each way both top and bottom. The steel isoverlapped near the concentrated loads, and a 3-inch

cover is provided over the steel. The depth of the exca-vation that the mats are placed in to achieve bearingcapacity and tolerable settlements eliminates seasonaledge effects such that the edge lift-off distance is notapplicable.

(2) Procedure. The thick mat is designed to deter-mine the shear, moment, and deflection behaviorusing conventional practice, then modified to accom-modate swell pressures and differential heave causedby swelling soils. The analyses are usually performedby the structural engineer with input on allowable soilbearing pressures, uplift pressures (hydrostatic andswell pressures from expansive soils) and estimates ofpotential edge heave/shrinkage and center heave fromthe foundation engineer. Computer programs are com-monly used to determine the shear, moments, and de-flections of the thick mat.

(a) Structural solutions. The structural solutionmay be initiated with an estimate of the thickness of aspread footing that resists punching shear andbending moments for a given column load, concretecompressive strength, and soil bearing capacity. Fol-lowing an estimation of the initial thickness, handsolutions of mat foundations for limited applicationbased on theory of beams on elastic foundations areavailable from NAVFAC DM-7. More versatile solu-tions are available from computer programs based ontheory of beams on elastic foundations such asBMCOL 2, which is available at the U.S. Army Corpsof Engineer Waterways Experiment Station, and fi-nite element analysis.

(b) Foundation soil/structure solutions. TheBMCOL 2 soil-structure interaction program permitsnonlinear soil behavior. Finite element programs are

also available, but they are often burdened with hardto explain local discontinuities in results, time-con-suming programming of input data, and need of expe-rienced personnel to operate the program. The finiteelement program originally developed for analysis ofPort Allen and Old River Locks was applied to theanalysis of the Wilford Hall Hospital mat foundationat Lackland Air Force Base in Texas. Figure 6-9 com-pares predicted with observed movement of the 3.5-foot-thick mat at Wilford Hall. Foundation soils in-clude the fissured, expansive Navarro and upper Mid-way clay shales. These computer programs help refinethe design of the mat and can lead to further cost re-ductions in the foundation.

6-4. Deep foundations

The deep foundation provides an economical methodfor transfer of structural loads beyond (or below) un-stable (weak, compressible, and expansive) to deeperstable (firm, incompressible, and nonswelling) strata.Usually, the deep foundation is a form of a pile founda-tion. Numerous types of pile foundations exist ofwhich the most common forms are given in table 6-3.Occasionally when the firm-bearing stratum is toodeep for the pile to bear directly on a stable stratum,the foundation is designed as friction or floating pilesand supported entirely from adhesion with the sur-rounding soil and/or end bearing on underreamedings.

foot-

a. General applications. Each of the types of pilingis appropriate depending on the location and type ofstructure, ground conditions (see table 3-1 for exam-ples), and durability. The displacement pile is usually appropriate for marine structures. Any of the piles intable 6-3 may be considered for land applications. Ofthese types the bored and cast in situ concrete drilledshaft is generally more economical to construct thandriven piles.

b. Application of drilled shafts. Table 6-4 describesdetailed applications of drilled shaft foundations in-cluding advantages and disadvantages. Detailed dis-cussion of drilled shaft foundations is presented belowbecause these have been most applicable to the solu-tion of foundation design and construction on expan-sive clay soils.

(1) A drilled shaft foundation maybe preferred toa mat foundation if excavating toward an adequatebearing stratum is difficult or the excavation causessettlement or loss of ground of adjacent property.

(2) A drilled shaft foundation 20 to 25 feet deeptends to be economically competitive with a ribbedmat foundation,

(3) Drilled shafts may be preferred to mat founda-

ratios exceed 1/250, Mat foundations under such con-ditions may tilt excessively leading to intolerable dis-tortion or cracking.

Figure 6-9. Settlement and deflection of a mat foundation.

pared with traditional strip footings, particularly inopen construction areas and with shaft lengths lessthan 10 to 13 feet, or if the active zone is deep, such aswithin areas influenced by tree roots.

c. General considerations.(1) Causes of distress. The design and construc-

tion of drilled shaft foundations must be closely con-trolled to avoid distress and damage. Most problemshave been caused by defects in construction and by in-adequate design considerations for effects of swelling soil (table 6-5). The defects attributed to constructiontechniques are discontinuities in the shaft, which mayoccur from the segregation of concrete, failure to com-plete concreting before the concrete sets, and early setof concrete, caving of soils, and distortion of the steelreinforcement. The distress resulting from inadequatedesign considerations are usually caused by wetting ofsubsoil beneath the base, uplift forces, lack of an airgap beneath grade beams, and lateral movement fromdownhill creep of expansive clay.

(2) Location of base. The base of shafts should belocated below the depth of the active zone, such as be-low the groundwater level and within nonexpansivesoil. The base should not normally be located withinthree base diameters of an underlying unstable stra-tum.

(a) Slabs-on-grade isolated from grade beamsand walls are often used in light structures, such asresidences and warehouses, rather than the more cost-ly structural slabs supported by grade beams andshafts. These slabs-on-grade will move with the expan-sive soil and should be expected to crack.

(b) To avoid “fall-in” of material from the granu-

lar stratum during underreaming of a bell, the basemay be placed beneath swelling soil near the top of agranular stratum.

(3) Underreams. Underreams are often used to in-crease anchorage to resist uplift forces (fig. 6-10). Thebelled diameter is usually 2 to 2.5 times the shaft

45- or 60-degree bells may be used, but the 45-degreebell is often preferred because concrete and construc-tion time requirements are less. Although the 45-de-gree bell may be slightly weaker than the 60-degreebell, no difference has been observed in practice. Thefollowing considerations are necessary in comparingunderreamed shafts with straight shafts.

(a) Straight shafts may be more economicalthan underreams if the bearing stratum is hard or ifsubsoils are fissured and friable. Soil above the under-ream may be loose and increase the upward movementneeded to develop the bell resistance.

(b) The shaft can often be lengthened to elimi-nate the need for an underream, particularly in soilswhere underreams are very difficult to construct. Fric-tion resistance increases rapidly in comparison withend bearing resistance as a function of the relativeshaft-soil vertical movement.

(c) Underreams reduce the contact bearing pres-sure on potentially expansive soil and restrict the min-imum diameter that may be used.

(4) Uplift forces. If bells or underreams are notfeasible, uplift forces (table 6-5) may be controlled bythe following methods:

(a) The shaft diameterrequired for downloads andand control.

should be the minimumconstruction procedures

6-11

(b) The shaft length may be extended furtherinto stable, nonswelling soil to depths of twice the

(c) Widely spaced shafts may be constructed

exceeds the maximum uplift thrust (fig. 6-11) ex-

The point n in figure 6-11 is the neutral point. The

equal to the soil allowable bearing capacity. Widespans between shafts also reduce angular rotation ofthe structural members. The minimum spacing ofshafts should be 12 feet or 8 times the shaft diameter(whichever is smaller) to minimize effects of adjacentshafts.

(d) The upper portion of the shaft should bekept vertically plumb (maximum variation of 1 inch in6 feet shown in fig. 6-10) and smooth to reduce adhe-sion between the swelling soil and the shaft. Frictionreducing material, such as roofing felt, bitumen sliplayers, polyvinyl chloride (PVC), or polyethylenesleeves, may be placed around the upper shaft to re-

6-13

TM 5-018-7

duce both uplift and downdrag forces. Vermiculite,pea gravel, or other pervious materials that will allowaccess of water to the expansive material should beavoided.

d. Design. The heave or settlement of the founda-tion usually controls the design and should not exceedspecified limits set by usage requirements and toler-ances of the structure. The design of drilled shafts, inaddition to bearing capacity, should consider the meth-od of construction, skin resistance, and uplift forces.The computer program HEAVE (WES MiscellaneousPaper GL-82-7) may be used to help determine themovement of drilled shafts for different lengths anddiameters of the shaft, and the diameter of the under-ream for different loading forces.

(1) Skin resistance. Skin resistance develops fromsmall relative displacements between the shaft and theadjacent soil. Positive (upward directed) skin friction,which helps support structural loads, develops whenthe shaft moves down relative to the soil. Uplift of ad-jacent swelling soils also transfers load to the shaft

foundation by positive skin friction and can causelarge tensile stresses to develop in the shaft. Negativeskin friction, which adds to the structural loads and in-creases the end bearing force, develops when the sur-rounding soil moves down relative to the shaft, Nega-tive skin friction is associated with the settling of theadjacent fill, loading of surrounding compressiblesoils, or lowering of the groundwater level.

ated by the equation

(6-2)

adhesion, tons per square footratio of horizontal to vertical effectivestressvertical effective stress, tons per squarefootangle of friction between the soil andshaft, degrees

TM 5-818-7

soil against concrete. The skin resistance, which is afunction of the type of soil (sand, clay, and silt), isusually fully mobilized with a downward displacementof 1/2 inch or less or about 1 to 3 percent of the shaftdiameter. These displacements are much less thanthose required to fully mobilize end bearing resistance.

(b) The fully mobilized skin resistance has beencompared with the undrained, undisturbed shear

(6-3)

found to vary between 0.25 and 1.25 depending on thetype of shaft and soil conditions. The reduction factoris the ratio of mobilized shearing resistance to the un-

be independent of soil strength. Also, the in situ reduc-tion factor may appear greater than one depending onthe mechanism of load transfer. For example, theshaft load may be transferred over some thickness ofsoil such that the effective diameter of the shaft isgreater than the shaft diameter D,. The reduction fac-tor concept, although commonly used, is not fully

of 0.25 is recommended if little is known about the soilor if slurry construction is used.The reduction factor approaches zero near the top and

bottom of the shafts in cohesive soils, reaching a maxi-

attributed to soil shrinkage during droughts and lowlateral pressure, while the reduction at the bottom isattributed to interaction of stress between end bearingand skin resistance.

(c) Skin resistance may also be evaluated interms of effective stress from results of drained directshear tests

(6-4)

angle of internal friction. The effective cohesion is as-sumed zero in practical applications and eliminatedfrom equation (6-4). Most of the available field data

mally consolidated soils, while it is about 0.8 for over-

be calculated for normally consolidated soils by(6-5a)

and in overconsolidated soils by

(6-5b)

cohesion is often assumed to be zero.(2) Uplift forces. Uplift forces, which area direct

function of swell pressures, will develop against sur-

6-15

faces of shaft foundations when wetting of surround-ing expansive soil occurs. Side friction resulting in up-lift forces should be assumed to act along the entiredepth of the active zone since wetting of swelling soilcauses volumetric expansion and increased pressureagainst the shaft. As the shaft tends to be pulled up-ward, tensile stresses and possible fracture of concretein the shaft are induced, as well as possible upward dis-placement of the entire shaft.

(a) The tension force T (a negative quantity)may be estimated by

(6-6)

includes the weight of the shaft. Limited observations

and (6-4)) varies between 1 and 2 in cohesive soils forshafts subject to uplift forces. The same swelling re-sponsible for uplift also increases the lateral earthpressure on the shaft. Larger K values increase thecomputed tension force.

(b) The shaft should be of proper diameter,length, and underreaming, adequately loaded, andcontain sufficient reinforcing steel to avoid both ten-sile fractures and upward displacement of the shaft.ASTM A 615 Grade 60 reinforcing steel with a mini-

The minimum percent steel required if ASTM A 615Grade 60 steel is used is given approximately by

T(6-7)

where T is the tension force in tons and the shaft diam-

more may be required. The reinforcing steel should behooked into any existing bell as shown in figure 6-10,and it may also be hooked into a concrete grade beam.

Maximum concrete aggregate size should be one thirdof the openings in the reinforcement cage.

d. Grade beams. Grade beams spanning betweenshafts are designed to support wall loads imposed ver-tically downward. These grade beams should be iso-lated from the underlying swelling soil with a voidspace beneath the beams of 6 to 12 inches or 2 timesthe predicted total heave of soil located above the baseof the shaft foundation (whichever is larger). Steel isrecommended in only the bottom of the grade beam ifgrade beams are supported by drilled shafts above thevoid space. Grade beams resting on the soil withoutvoid spaces are subject to distortion from uplift pres-sure of swelling foundation soil and are not recom-mended.

(1) Preparation of void space. Construction ofgrade beams with void spaces beneath the beams mayrequire overexcavation of soil in the bottom of thegrade beam trench between shafts. The void space maybe constructed by use of sand that must later be blownaway at least 7 days after concrete placement, or byuse of commercially available cardboard or other re-tainer forms that will support the concrete. The card-board forms should deteriorate and collapse beforeswell pressures in underlying soil can deflect or dam-age the grade beams. The resulting voids should beprotected by soil retainer planks and spacer blocks.Figure 6-12 illustrates some void details.

(2) Loading. Interior and exterior walls and con-centrated loads should be mounted on grade beams.Floors may be suspended from grade beams at least 6inches above the ground surface, or they maybe placeddirectly on the ground if the floor slab is isolated fromthe walls. Support of grade beams, walls, and suspend-ed floors from supports other than the shaft founda-tion should be avoided. Figure 6-13 illustrates typicalexterior and interior grade beams.

b Thickness

Figure 6-13. Typical exterior and interior grade beams.

TM 5-818-7

CHAPTER 7

MINIMIZATION OF FOUNDATION MOVEMENT

7-1. Preparation for constructionThe foundation should always be provided with ade-quate drainage, and the soil properly prepared to mini-mize changes in soil moisture and differential move-ment.

a. Removal of vegetation. Existing trees and otherheavy vegetation should be removed. New plantings oflike items installed during postconstruction landscap-ing should not be located within a distance away fromthe structure ranging from 1 to 1.5 times the height ofthe mature tree.

b. Leveling of site. Natural soil fills compacted atthe natural water content and the natural density ofthe in situ adjacent soil minimize differential move-ment between cut and fill areas of sloping ground,trenches, or holes caused by removal of vegetation.The volume of cut portions should be kept to a mini-mum. Cut areas reduce the overburden pressure onunderlying swelling soil and lead to time-dependentheave.

c. Excavation.

(1) Construction in new excavations (within a fewyears of excavating) without replacement of a sur-charge pressure equal to the original soil overburdenpressure should be avoided where possible because thereduction in effective stress leads to an instantaneouselastic rebound plus a time-dependent heave. The re-duction in overburden pressure results in a reductionof the pore water pressure in soil beneath the excava-tion. These pore pressures tend to increase with timetoward the original or equilibrium pore pressure pro-file consistent with that of the surrounding soil andcan cause heave.

(2) Ground surfaces of new excavations, such asfor basements and thick mat foundations, should beimmediately coated with sprayed asphalt or other seal-ing compounds to prevent drying of or the seepage ofponded water into the foundation soil during construc-tion (fig. 7-1). Rapid-cure RC 70 or medium-cure MC30 cutback asphalts are often used as sealing com-pounds, which penetrate into the soil following com-paction of the surface soil and cure relatively quickly.

7-2. Drainage techniquesDrainage is provided by surface grading and subsur-

face drains.

a. Grading. The most commonly used technique isgrading of a positive slope away from the structure.The slope should be adequate to promote rapid runoffand to avoid collecting, near the structure, pondedwater, which could migrate down the foundation/soilinterface. These slopes should be, greater than 1 per-cent and preferably 5 percent within 10 feet of thefoundation,

(1) Depressions or water catch basin areas shouldbe filled with compacted soil (para 7-3a) to have apositive slope from the structure, or drains should beprovided to promote runoff from the water catch basinareas. Six to twelve inches of compacted, impervious,nonswelling soil placed on the site prior to construc-tion of the foundation can ensure the necessary gradeand contribute additional uniform surcharge pressureto reduce uneven swelling of underlying expansivesoil.

(2) Grading and drainage should be provided forstructures constructed on slopes, particularly forslopes greater than 9 percent, to rapidly drain offwater from the cut areas and to avoid pending of waterin cuts or on the uphill side of the structure. Thisdrainage will also minimize seepage through backfillsinto adjacent basement walls.

b. Subsurface drains. Subsurface drains (fig. 7-1)may be used to control a rising water table, ground-water and underground streams, and surface waterpenetrating through pervious or fissured and highlypermeable soil. Drains can help control the water tablebefore it rises but may not be successful in loweringthe water table in expansive soil. Furthermore, sincedrains cannot stop the migration of moisture throughexpansive soil beneath foundations, they will not pre-vent all of the long-term swelling.

(1) Location of subsurface drains, These drainsare usually 4- to 6-inch-diameter perforated pipesplaced adjacent to and slightly below the baseline ofthe external wall to catch free water (fig. 7-1).

(a) An impervious membrane should be placedbeneath the drain in the trench to prevent migrationof surface moisture into deeper soil. The membrane ad-jacent to the foundation wall should be cemented tothe wall with a compatible joint sealant to preventseepage through the joint between the membrane andthe foundation.

(b) If a 6- to 12-inch layer of granular material

was provided beneath a slab-on-grade, the granularmaterial in the drain trench should be continuous withthe granular material beneath the floor slab. The per-forated pipe should be placed at least 12 inches deeperthan the bottom of the granular layer. An imperviousmembrane should also be placed on the bottom andsides of the drain trench but should not inhibit flow ofmoisture into the drain from beneath the floor slab.Granular fills of high permeability should be avoidedwhere possible.

(c) Deep subsurface drains constructed to con-trol arising water table should be located at least 5feet below a slab-on-grade. An impervious membraneshould not be placed in the drain trench. These drainsare only partially effective in controlling soil heaveabove the drain trench, and they are relatively expen-sive. A more economical solution may be to place atemporary (or easily removable slab-on-grade) with apermanent slab after the groundwater table hasreached equilibrium.

(2) Outlets. Drains should be provided with out-lets or sumps to collect water and pumps to expelwater if gravity drainage away from the foundation isnot feasible. Sumps should be located well away fromthe structure. Drainage should be adequate to preventany water from remaining in the drain (i.e., a slope ofat least 1/8 inch per foot of drain or 1 percent should beprovided).

(3) Drain trench material. The intrusion of fines

7-2

in drains maybe minimized by setting the pipe in filterfabric and pea gravel/sand.

7-3. Stabilization techniquesTwo effective and most commonly used soil stabi-lization techniques are controlled backfilling andcontinuous maintenance involving drainage controland limited watering of surface soil adjacent to thestructure during droughts. Other techniques, such asmoisture barriers and lime treatment, are not widelyused in minimizing differential heave of single andmultistory buildings. Presetting or pending for peri-ods of a few months to a year prior to construction isoften effective but normally is not used because oftime requirements. Prewetting should not be used onfissured clay shales because swelling from water seep-ing into fissures may not appear until a much laterdate and delayed problems may result.

a. Controlled backfills. Removal of about 4 to 8 feetof surface swelling soil and replacement with nonex-pansive, low permeable backfill will reduce heave atthe ground surface. Backfills adjacent to foundationwalls should also be nonswelling, low permeable mate-rial. Nonswelling material minimizes the forces exert-ed on walls, while low permeable backfill minimizesinfiltration of surface water through the backfill intothe foundation soil. If only pervious, nonexpansive(granular) backfill is available, a subsurface drain at the bottom of the backfill is necessary to carry off in-

filtrated water (fig. 7-1) and to minimize seepage ofwater into deeper desiccated foundation expansivesoils.

(1) Backfill of natural soil. Backfill using naturalsoil and compaction control has been satisfactory insome cases if nonswelling backfill is not available.However, this use of backfill should be a last resort,

(a) In general, the natural soil should be com-pacted to 90 percent of standard maximum densityand should be wet of optimum water content. Founda-tion loads on fills should be consistent with the allow-able bearing capacity of the fill. Overcompactionshould be avoided to prevent aggravating potentiallyswelling soil problems such as differential heave of thefill. Compaction control of naturally swelling soil is us-ually difficult to accomplish in practice. Some soils be-come more susceptible to expansion following remold-ing, and addition of water to achieve water contentsnecessary to control further swell may cause the soil tobe too wet to work in the field.

(b) As an alternative, backfills of lime-treatednatural soil compacted to 95 percent standard maxi-mum density at optimum water content may be satis-factory if the soil is sufficiently reactive to the lime (dbelow), Lime treatment may also increase soil strengthand trafficability on the construction site.

(2) Backfill adjacent to walls. A IV on lH slopecut into the natural soil should dissipate lateral swellpressures against basement or retaining walls exertedby the natural swelling material. The nonswellingbackfill should be a weak material (sand fill with fric-tion angle of 30 degrees or lessor cohesive fill with co-hesion less than about 0.5 tons per square foot) to al-low the fill to move upward when the expansive natu-ral soil swells laterally. Restraining loads should notbe placed on the surface of the fill. A friction reducingmedium may be applied on the wall to minimize fric-tion between the wall and the backfill, TM 5-818-4discusses details on optimum slopes of the excavationand other design criteria.

b. Maintenance. Maintenance programs are di-rected toward promoting uniform soil moisture be-neath the foundation. A good program consists of thefollowing:

(1) Maintenance of a positive slope of about 5 per-cent around the structure for drainage and eliminationof water catch areas.

(2) Maintenance of original drainage channels andinstallation of new channels as necessary.

(3) Maintenance of gutters around the roof and di-version of runoff away from the structure.

(4) Avoidance of curbs or other water trapsaround flower beds.

(5) Elimination of heavy vegetation within 10 to15 feet of the foundation or 1 to 1.5 times the heightof mature trees.

TM 5-018-7

(6) Uniform limited watering around the struc-ture during droughts to replace lost moisture.

c. Moisture barriers. The purpose of moisture bar-riers is to promote uniform soil moisture beneath thefoundation by minimizing the loss or gain of moisturethrough the membrane and thus reducing cyclic edgemovement, Moisture may still increase beneath orwithin areas surrounded by the moisture barriers lead-ing to a steady but uniform heave of the foundation orslab-on-grade.

(1) Types of barriers. These barriers consist ofhorizontal and vertical plastic and asphalt membranesand granular materials. Concrete is an ineffectivemoisture barrier. Longlasting membranes includechlorinated polyethylene sheets, preferably placedover a layer of catalytically blown or sprayed asphalt.All joints, seams, and punctures should be sealed byplastic cements or concrete/asphalt joint sealants.ASTM D 2521 (Part 15) describes use of asphalt incanal, ditch, and pond linings (app A).

(2) Horizontal.(a) An impervious membrane on the ground sur-

face in a crawl space where rainfall does not enter mayhelp reduce shrinkage in clayey foundation soils withdeep groundwater levels by minimizing evaporationfrom the soil. A vapor barrier should not be placed inventilated crawl spaces if there is a shallow water ta-ble or if site drainage is poor because heave maybe ag-gravated in these cases. Figure 7-2 illustrates a usefulapplication of horizontal membranes,

(b) Other applications include the use of hori-zontal moisture barriers around the perimeter of struc-tures to reduce lateral variations in moisture changesand differential heave in the foundation soil. Plastic orother thin membranes around the perimeter should beprotected from the environment by a 6- to 12-inch-thick layer of earth.

(c) A disadvantage of these barriers is that theyare not necessarily reliable and may be detrimental insome cases. For example, most fabrics and plasticmembranes tend to deteriorate with time. Undetected(and hence unrepaired) punctures that allow water toget in, but not to get out, commonly occur in handlingon placement. Punctures may also occur during plant-ing of vegetation. If the barrier is a concrete slab, theconcrete may act as a wick and pull water out of thesoil.

(3) Vertical.(a) Plumbing or utility trenches passing

through the barrier may contribute to soil moisture be-neath the foundation.

(b) The vertical barrier (fig. 7-3) should extendto the depth of the active zone and should be placed aminimum of 3 feet from the foundation to simplifyconstruction and to avoid disturbance of the founda-tion soil. The barrier may not be practical in prevent-

1-3

ing migration of moisture beneath the bottom edge foractive zones deeper than 8 to 10 feet. The granular bar-

- rier may also help reduce moisture changes duringdroughts by providing a reservoir of moisture. Theplacement of a filter fabric around the trench to keepfine particles from entering the perforated pipe willpermit use of an open coarse aggregate instead of agraded granular filter. In some cases, the perforatedpipe could be eliminated from the drain trench.

d. Lime treatment. This treatment is the mostwidely used and most effective technique of chemicalalteration to minimize volume changes and to increasethe shear strength of foundation expansive soils.

(1) Applications. Lime treatment is applied to thestrengthening and minimization of volume change ofsoil in railroad beds, pavement subgrades, and slopes.When this treatment is applied to foundation soils ofsingle and multistory structures, it is not always suc-cessful because the usefulness depends on the reactive-ness of the soil to lime treatment and the thorough-ness of dispersion of lime mixed into the soil.

(a) Lime treatment is effective in the minimiza-tion of volume changes of natural soil for backfill.However, this treatment increases the soil permeabili-ty and the soil strength. The soil permeability shouldbe kept low to restrict seepage of surface waterthrough the backfill. The backfill strength should be as

— low as possible compatible with economical design tominimize the transfer of lateral swell pressures from

the natural in situ soil through the backfill to the base-ment and retaining walls.

(b) Lime treatment may be used to stabilize a 6-to 12-inch layer of natural expansive soil compacted onthe surface of the construction site to provide a posi-tive slope for runoff of water from the structure and alayer to reduce differential heave beneath the floorslab.

(c) Lime treatment may be applied to minimizedownhill soil creep of slopes greater than 5 degrees (9percent) by increasing the stiffness and strength of thesoil mass through filling fractures in the surface soils.If lime slurry pressure injection (LPSI) can cause alime slurry to penetrate the fissures in the soil mass toa sufficient depth (usually 8 to 10 feet), then the lime-filled seams will help control the soil water content, re-duce volumetric changes, and increase the soilstrength. However, LSPI will probably not be satisfac-tory in an expansive clay soil that does not contain anextensive network of fissures because the lime will notpenetrate into the relatively impervious soil to any ap-preciable distance from the injection hole to form acontinuous lime seam moisture barrier.

(d) LSPI may be useful for minimization ofmovement of fissured foundation expansive soils downto the depth of the active zone for heave or at least 10ft. The lime slurry is pressure injected on 3- to 5-footcenter to depths of 10 to 16 feet around the perimeterof the structure 3 to 5 feet from the structure.

(2) Soiloughly andcient depth

mixture preparation. Lime should be thor-intimately mixed into the soil to a suffi-to be effective. For stabilization of expan-

sive clay soils for foundations of structures, mixingshould be done down to depths of active zone forheave. In practice, mixing with lime is rarely donedeeper than 1 to 2 feet. Therefore, lime treatment isnormally not useful for foundations on expansive soilexcept in the above applications. Moreover, poor mix-ing may cause the soil to break up into clods from nor-mal exposure to the seasonal wetting/drying cycles.The overall soil permeability is increased and providespaths for moisture flow that require rapid drainagefrom this soil. Lime treatment should be performed byexperienced personnel.

(3) Lime modification optimum content (LMO).The LMO corresponds to the percent of lime that maxi-mizes the reduction in the soil plasticity or the PI. Thereduction in plasticity also effectively minimizes thevolume change behavior from changes in water con-tent and increases the soil shear strength.

(a) A decision to use lime should depend on thedegree of soil stabilization caused by the lime. Lime

treatment is recommended if a 50 percent reduction inthe PI is obtained at the LMO content (table 7- 1). ThePI should be determined for the natural soil, LMO,LMO+ 2, and LMO - 2 percent content.

(b) The increase in strength of the lime-treated –

soil should be similar for soil allowed to cure at least 2or more days following mixing and prior to compactionto similar densities.

(c) The amount of lime needed to cause the opti-mum reduction in the PI usually varies from 2 to 8 per-cent of the dry soil weight.

e. Cement treatment. Cement may be added to thesoil to minimize volume changes and to increase theshear strength of the foundation expansive soil if thedegree of soil stabilization achieved by lime alone isnot sufficient. The amount of cement required willprobably range between 10 to 20 percent of the drysoil weight. A combination of lime-cement or lime-ce-ment-fly ash may be the best overall additive, but thebest combination can only be determined by a labora-tory study. TM 5-822-4 presents details on soil sta-bilization with cement and cement-lime combinations.

TM 5-818-7

CHAPTER 8

CONSTRUCTION TECHNIQUES AND INSPECTION

8-1. Minimization of foundation prob-lems from construction

Many problems and substandard performance of foun-dations observed in structures on expansive soils occurfrom poor quality control and faulty construction prac-tice. Much of the construction equipment and proce-dures that are used depends on the foundation soilcharacteristics and soil profiles. Careful inspectionduring construction is necessary to ensure that thestructure is built according to the specifications.

a. Important elements of construction techniques.Construction techniques should be used that promotea constant moisture regime in the foundation soils dur-ing and following construction. The following ele-ments of construction are important in obtaining ade-quate foundation performance in expansive soils.

(1) Excavations. The excavation should be com-pleted as quickly as possible to the design depth andprotected from drying. An impervious moisture barri-er should be applied on the newly exposed surfaces ofthe excavation to prevent drying of the foundationsoils immediately after excavating to the design depth.Sides of the excavation should be constructed on a lVon lH slope or an appropriate angle that will not trans-mit intolerable swelling pressures from the expansivesoil to the foundation. The foundation should be con-structed in the excavation as quickly as practical.

(2) Selection of materials. Selected materialsshould conform to design requirements.

(a) Backfills should be nonswelling materials.(b) Concrete should be of adequate strength and

workability.(c) Reinforcing steel should be of adequate size

and strength.(d) Moisture barriers should be durable and im-

pervious.(3) Placement of materials. All structural materi-

als should be positioned in the proper location of thefoundation.

(4) Compaction of backfills. Backfills of naturalexpansive soil should be compacted to minimize effectsof volume changes in the fill on performance of thefoundation. Backfills should not transmit intolerableswell pressures from the natural expansive foundationsoil to basement or retaining walls.

(5) Drainage during construction. The site shouldbe prepared to avoid ponding of water in low areas.Consideration should be given to compaction of 6 to 12

inches or more of impervious nonswelling soil on thesite prior to construction of the foundation to promotedrainage and trafficability on the site. Dehydratedlime may also be sprinkled on the surface of expansivesoil to promote trafficability. Sumps and pumpsshould be provided at the bottom of excavations if nec-essary to remove rainwater or subsurface drainage en-tering the excavation. Provision for after normal dutyoperation of the pumps should also be made.

(6) Permanent drainage. Grades of at least 1 per-cent and preferably 5 percent, to promote drainage ofwater away from the structure, should be providedaround the perimeter of the structure. Low areasshould be filled with compacted backfill. Runoff fromroofs should be directed away from the structure bysurface channels or drains. Subsurface drains shouldbe constructed to collect seepage of water through per-vious backfills placed adjacent to the foundation.

b. Considerations of construction inspection. Table8-1 lists major considerations of construction inspec-tion. Inspections related to concrete reinforced slaband drilled shaft foundations, the two most commonlyused foundations in expansive soil areas, are discussedbelow.

8-2. Stiffened slab foundationsItems in table 8-2 should be checked to minimize de-fective slab foundations.

a. The inspector should check for proper site preparation and placement of the moisture barrier, steel,and concrete. All drainage systems should be inspectedfor proper grade and connections to an outlet.

b. Posttensioned slabs require trained personneland careful inspection to properly apply the postten-sioning procedure. For example, anchors for the steeltendons should be placed at the specified depth (lowerthan the depth of the tensioning rods) to avoid pulloutduring tensioning. Tendons should be stressed 3 to 18days following the concrete placement (to eliminatemuch of the shrinkage cracking) such that the mini-mum compressive stress in the concrete exceeds 50pounds per square inch. Stressing should be completedbefore structural loads are applied to the slabs.

8-3. Drilled shaft foundationsItems in table 8-3 should be checked to minimize de-fective shaft foundations. The foundation engineer

TM 5-818-7

should visit the construction site during boring of thefirst shaft holes to verify the assumptions regardingthe subsurface soil profile, e.g., the nature and locationof the subsoils. Periodically, he or she should alsocheck the need for the designer to consider modifica-tions in the design.

a. Location of shaft base. The base of the shaft is lo-cated in the foundation soils to maintain shaft move-ments within tolerable limits. This depth depends onthe location and thickness of the expansive, compressi-ble or other unstable soil, sand lenses or thin perme-able zones, depth to groundwater, and depth to foun-dation soil of adequate bearing capacity. The designdepth may require modification to relocate the base inthe proper soil formation of adequate bearing capacityand below the active zone of heave. The purpose of lo-cating the base of the shaft in the proper soil forma-tion should be emphasized to the inspector during thefirst boring of the drilled shaft foundation. Under-reams may be bored in at least l.5-foot-diameter (pref-erably 2.5-foot) dry or cases holes where inspectionsare possible to ensure cleanliness of the bottom.

b. Minimization of problems, Long experience hasshown that drilled shaft foundations are reliable andeconomical. Nevertheless, many problems are asso-ciated with these foundations and can occur from in-adequate understanding of the actual soil profile andgroundwater conditions, mistakes made while drilling,inadequate flow of concrete, and improper reinforce-ment.

(1) Inadequate information.(a) Site conditions should be known to permit

optimum selection of equipment with the required mo-bility.

(b) Subsurface conditions should be known topermit selection of equipment with adequate boringcapacity.

(c) Type of soil (e.g., caving and pervious strata)may require slurry drilling. Specifications should per-mit sufficient flexibility to use slurry for those soilconditions where it maybe needed.

(d) Previously unnoticed sand lenses or thinpermeable zones in otherwise impervious clay maycause problems during construction of drilled shafts.Seepage through permeable zones may require casingor slurry and may render construction of an under-ream nearly impossible.

(e) Overbreak or the loss of material outside ofthe nominal diameter of the shaft due to caving soil isa serious problem that can cause local cavities or de-fects in the shaft. The construction procedure (boringdry, with casing, or using slurry) should be chosen tominimize overbreak.

(2) Problems with the dry method. Caving,squeezing soil, and seepage are the most common prob-

lems of this method. Stiff or very stiff cohesive soilswith no joints or slickensides are usually needed. Un-derreams are vulnerable to caving and should be con-structed as quickly as possible.

(3) Problems with the casing method. Slurryshould be used while drilling through caving soil priorto placement of the casing and sealing of the casing inan impervious layer. An impervious layer is necessaryto install the bottom end of the casing.

(a) Casing should not be pulled until the head ofconcrete is sufficient to balance the water head exter-nal to the casing; otherwise, groundwater may mixwith the concrete.

(b) Squeezing or localized reduction in the bore-hole diameter on removal of the casing can be mini-mized by using a relatively high slump concrete with asufficient pressure head.

(c) Casing sometimes tends to stick in place dur-ing concrete placement. If the concrete appears to besetting up, attempts to shake the casing loose shouldbe abandoned and the casing left in place to avoid theformation of voids in the shaft when the casing ispulled.

(d) Steel reinforcement should be full length toavoid problems in downdrag of the reinforcementwhile the casing is pulled. The reinforcement cageshould also be full length if uplift forces are expectedon the drilled shaft from swelling soil.

(4) Problems with the slurry method. Slurry ofsufficient viscosity is used to avoid problems with cav- ing soils. A rough guide to appropriate slurry viscosi-ties is given by a Marsh cone funnel test time of about30 seconds for sandy silts and sandy clays to 50 sec-onds for sands and gravels. The Marsh cone test timeis the time in seconds required to pour 1 quart ofslurry through the funnel. The workability of theslurry should also be adequate to allow complete dis-placement of the slurry by the concrete from theperimeter of the borehole and steel of the rebar cage.

(a) Slurries should be of sufficient viscosity toeliminate settling of cuttings. Loose cuttings adheringto the perimeter of the hole can cause inclusions and adefective shaft.

(b) The tremie sometimes becomes plugged,stopping the flow of concrete into the borehole. Thetremie should not be pulled above the concrete level inthe shaft before the concrete placement is completed,otherwise inclusions may occur in the shaft followingreinsertion of the tremie into the concrete.

(c) The reinforcement cage may move up if thetremie is too deep in the concrete or the concrete isplaced too rapidly.

c. Placement of concrete. Concrete strength of atleast 3,000 pounds per square inch should be used andplaced as soon as possible on the same day as drillingthe hole. Concrete slumps of 4 to 6 inches and limited

8-2

TM 5-818-7

aggregate size of one third of the rebar spacing are rec- (2) Tip of tremie always below the column ofommended to facilitate flow of concrete through the freshly placed concrete in wet construction; no segre-reinforcement cage and to eliminate cavities in the gation in a dry hole.shaft. Care should be exercised while placing the con- (3) Adequate strength ofcrete to ensure the following: mize distortion and buckling.

(1) Continuity while pulling the casing.

the rebar cage to mini-

Construction

CHAPTER 9

REMEDIAL PROCEDURES

9-1. Basic considerationsRemedial work for damaged structures is usually diffi-cult to determine because the cause of the problem(e.g., location of source or loss of soil moisture, andswelling or settling/shrinking soil) may not be readilyapparent, A plan to fix the problem is often difficult toexecute, and the work may have to be repeated becauseof failure to isolate the cause of the moisture changesin the foundation soil, An effective remedial proceduremay not be found until several attempts have beenmade to eliminate the differential movement. Require-ments for minimizing moisture changes (chap. 7) aretherefore essential. The foundation should have suffi-cient capacity to maintain all distortion within tolera-ble limits acceptable to the superstructure. This distor-tion occurs from differential heave for the most severeclimates and changes in the field environment.

a. Specialized effort. Investigation and repair aretherefore specialized procedures that usually requiremuch expertise and experience. Cost of repair workcan easily exceed the original cost of the foundation.The amount of damage that requires repair also de-pends on the attitudes of the owner and occupants totolerate distortion as well as damage that actually im-pairs the usefulness and safety of the structure.

fects of swelling soil tends to be cosmetic rather thanstructural, and repairs are usually more economicalthan rebuilding as long as the structure remainssound. At-early signs of distress, remedial action tominimize future distortion should be undertaken andshould be given a greater priority than the cosmetic re-pairs as this action will minimize maintenance workover the long term. Maintenance expenses and fre-quency of repairs tend to be greatest in lightly loadedstructures and residences about 3 to 4 years followingthe original construction. Overall maintenance can beminimized by taking remedial action to minimize fu-ture distortion before extensive repairs are required(e.g., breaking out and replacing sections of walls).

c. Examples of remedial procedures. The choice ofremedial measures is influenced by the results of siteand soil investigations as well as by the type of origi-nal construction. Table 9-1 illustrates common reme-dial measures that can be taken. Only one remedialprocedure should be attempted at a time so as to deter-mine its effect on the structure. The structure should

be allowed to adjust, following completion of remedialmeasures, for at least a year before cosmetic work isdone. The structure is seldom rebuilt to its originalcondition, and in some instances, remedial measureshave not been successful.

9-2. Evaluation of informationAll existing information on the foundation soils anddesign of the foundation and superstructure should bestudied before proceeding with new soil investiga-tions.

a. Foundation conditions. The initial soil moistureat time of construction, types of soil, soil swell poten-tials, depth to groundwater, type of foundation andsuperstructure, and drainage system should be deter-mined. The current soil moisture profile should also bedetermined. Details of the foundation, such as actualbearing pressures, size and length of footings, and slaband shaft reinforcing, should also be collected. Drillinglogs made during construction of shaft foundationsmay be used to establish soil and groundwater condi-tions and details of shaft foundations. Actual construc-tion should be checked against the plans to identifyany variances.

well as the time movements first became noticeable,should be determined, Most cracks caused by differen-tial heave are wider at the top than at the bottom.Nearly all lateral separation results from differentialheave. Diagonal cracks can indicate footing or drilledshaft movement, or lateral thrust from the domingpattern of heaving concrete slabs. Fractures in slabs-on-grade a few feet from and parallel with the perime-ter walls also indicate heaving of underlying soils. Lev-el surveys can be used to determine the trend of move-ment when prior survey records and reliable bench-marks are available. Excavations may be necessary tostudy damage to deep foundations, such as cracks inshafts from uplift forces.

c. Sources of moisture. The source of soil moisturethat led to the differential heave should be determinedto evaluate the cause of damage. Location of deeproot-ed vegetation, such as shrubs and trees, location andfrequency of watering, inadequate slopes and pending,seepage into foundation soil from surface or perchedwater, and defects in drain, water, and sewer lines can

9-1

TM 5-818-7

make important changes in soil moisture and can leadto differential heave.

9-3. Stiffened slab foundationsMost slab foundations that experience some distressare not damaged sufficiently to warrant repairs. Dam-age is often localized by settlement or heave of one sideof the slab. The cause of the soil movement, whethersettlement or heave, should first be determined andthen corrected.

a. Stabilization of soil moisture. Drainage improve-ments and a program to control soil moisture at theperimeter of the slab are recommended (chap 7) for alldamaged slab foundations.

b. Remedial procedures. Remedial work on slabs de-pends on the type of movement, Repair of a settledarea requires raising of that area, while repair of aheaved area often requires raising the entire unheavedportion of the slab up to the level of the heaved por-tion. Repair costs are consequently usually greater forheaving than settling cases.

(1) Repair of a damaged slab consists of a combi-nation of underpinning and mudjacking using acement grout. Mudjacking using a cement grout is re-quired simultaneously with underpinning to fill voidsduring leveling of the slab. Fractured slabs are usuallyeasier to repair than unfractured slabs that have beendistorted by differential movement because usuallyonly the fractured portion of the slab requires treat-ment. The distortion of unfractured slabs can alsocause considerable damage to the superstructure andinconvenience to the occupants.

(2) Underpinning and mudjacking are applied si-multaneously and usually clockwise around the slab

until all parts of the foundation are at the same eleva-tion. If a heaved area is lowered to the same elevationas the rest of the foundation, such as to repair a mush-roomed or dome-shaped heave pattern, the slab is first supported before digging out the soil to prevent theslab from creeping down on the work crew during thedigging. Attempts at leveling dome-shaped distortionby raising the perimeter may be unsuccessful becausemudjacking usually causes the entire slab to rise.

9-4. Drilled shaft foundationsMost damage to structures with shaft foundations con-sists of fractured slabs-on-grade. The shaft may con-tribute to the damage caused by migration of moisturedown the shaft/soil interface into swelling soil beneaththe shaft footing. The fracture pattern of open cracksin the floor slab parallel to and several feet from thewall often shows that the slab had not been free tomove near the walls. Damage to drilled shafts is oftencaused by upward movement of the shaft from swell-ing soil beneath its base and by uplift forces on theshaft perimeter from adjacent swelling soil.

ments and a program to control soil moisture aroundthe perimeter of the foundation are recommended(chap 7).

removal of the slab and underlying wet soil, replace-ment with nonswelling soil, and placement of a new slab isolated from the perimeter walls. Repair ofdrilled shafts consists of cutting down the top of theshaft and releveling the foundation. The tops of thedrilled shafts are cut to the elevation of the top of thelowest shaft where possible.

Table 9-1. Remedial Measures

Measure

Drainage

Moisture stabilization(maintenance ofconstant moisturewhether at high orlow levels)

Superstructureadjustments

Description

Slope ground surface (positive drainage) from structure; add drainsfor downspouts and outdoor faucets in areas of poor drainage,and discharge away from foundation soil; provide subdrains ifperched water tables or free flow of subsurface water are prob-lems; provide flexible, watertight utility connections.

Remove natural swelling soil and recompact with impervious, non-swelling backfill; install vertical and/or horizontal membranesaround the perimeter; locate deep-rooted vegetation outside ofmoisture barriers; avoid automatic sprinkling systems in areasprotected with moisture barriers; provide a constant source ofmoisture if a combination of swelling/shrinking soils is oc-curring; thoroughly mix 4 to 8 percent lime into soil to reducepotential for swell or pressure-inject line slurry aroundthe perimeter of the structure.

Free slabs from foundation by cutting along foundation walls; pro-vide slip joints in interior walls and door frames; reinforce ma-sonry and concrete block walls with horizontal and vertical tiebars or reinforced concrete beams; provide fanlights over doorsextended to the ceiling.

9-2

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Table 9-1. Remedial Measures–Continued

Measure

Spread footings anddeep foundationadjustments

Continuous wallfoundationadjustments

Reinforced andstiffened slab-on-grade adjustments

Description

Decrease footing size; underpin with deep shafts; mudjack using acement grout; reconstruct void beneath grade beams; eliminatemushrooms at top of shafts; adjust elevation by cutting the topof the shaft or by adding shims; increase footing or shaftspacing to concentrate loading forces and to reduce angular dis-tortion from differential heave between adjacent footings andshafts.

Provide voids beneath portions of wall foundation; posttension; rein-force with horizontal and vertical tie bars or reinforced concretebeams.

Mudjack using a cement grout; underpin with spread footings orshafts to jack up the edge of slabs,

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APPENDIX A

REFERENCES

Government Publications

Department of the Army

Technical ManualsTM 5-818-1 Procedures for Foundation Design of Buildings and Other Structures (Except

Hydraulic Structures)TM5-818-4 Backfill for Subsurface StructuresTM5-822-4 Soil Stabilization for Roads and Streets

Department of the Navy

Change 1 A-1

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APPENDIX B

CHARACTERIZATION OF SWELL BEHAVIOR FROM SOIL SUCTION

B-1. IntroductionSoil suction is a quantity that can be used to character-ize the effect of moisture on volume, and it is a meas-ure of the energy or stress that holds the soil water inthe pores or a measure of the pulling stress exerted onthe pore water by the soil mass. The total soil suctionis expressed as a positive quantity and is defined as the

to the geometrical configuration of the soil and struc-ture, capillary tension in the pore water, and watersorption forces of the clay particles. This suction is al-so pressure-dependent and assumed to be related to

(B-1)

(B-2)

matrix soil suction, tons per square footcompressibility factor, dimensionlesstotal mean normal confining pressure,tons per square footratio of total horizontal to vertical stressin situtotal vertical pressure, tons per squarefoot

The exponentout confining pressure except atmospheric pressure.Experimental results show that the in situ matrix suc-

ity factor is determined by the procedure in paragraphB-3d.

by the concentration of soluble salts in the pore water,and it is pressure-independent. The effect of the os-motic suction on swell is not well known, but an osmot-ic effect may be observed if the concentration of solu-ble salts in the pore water differs from that of the ex-ternally available water. For example, swell may occurin the specimen if the external water contains lesssoluble salts than the pore water. The effect of the os-motic suction on swell behavior is assumed small com-pared with the effect of the matrix suction. The osmot-ic suction should not significantly affect heave if thesalt concentration is not altered.

B-2. Methods of measurementTwo methods are recommended for determining thetotal soil suction: thermocouple psychrometer and fil-ter paper. The suction range of thermocouple psychro-meters usually is from 1 to 80 tons per square footwhile the range of filter paper is from 0.1 to more than1,000 tons per square foot. Two to seven days are re-quired to reach moisture equilibrium for thermocouplepsychrometer, while 7 days are required for filterpaper. The thermocouple psychrometer method is sim-ple and can be more accurate than filter paper afterthe equipment has been calibrated and the operatingprocedure established. The principal disadvantage isthat the suction range is much more limited than thefilter paper method, The filter paper method is techni-cally less complicated than the thermocouple psy-chrometer method; however, the weighing procedurerequired for filter paper is critical and vulnerable tolarge error.

a. Calibration. The total soil suction is given on the

(B-3)

pressuretons per

universal gas constant, 86.81 cubic cen-timetres-tons per square foot/mole-Kel-vinabsolute temperature, Kelvinvolume of a mole of liquid water, 18.02cubic centimetres/molerelative humiditypressure of water vapor, tons persquare footpressure of saturated water vapor, tonsper square foot

Equation (B-3) shows that the soil suction is related tothe relative humidity in the soil. Both thermocouplepsychrometer and filter paper techniques require cali-bration curves to evaluate the soil relative humidityfrom which the soil suction may be calculated usingequation (B-3). Calibration is usually performed withsalt solutions of various known molality (moles of saltper 1,000 grams of water) that produce a given rela-tive humidity. Table B-1 shows the modalities re-

B-1

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Table B-1. Calibration Salt Solutions

Measured Suction, tsf

temperature for cited molality of sodium chloride solution

0.053 0.100 0.157 0.273 0.411 0.550 1.000

15 3.05 4.67 7.27 12.56 18.88 25.29 46,5520 3.10 4.74 7.39 12.75 19.22 25.76 47.5025 3.15 4.82 7.52 13.01 19.55 26.23 48.4430 3.22 4.91 7.64 13.22 19.90 26.71 49.37

quired for sodium chloride salt solutions to provide thesoil suctions given as a function of temperature.

b. Thermocouple psychrometer technique. The ther-mocouple psychrometer measures relative humidity insoil by a technique called Peltier cooling. By causing acurrent to flow through a single thermocouple junctionin the proper direction, that particular junction willcool, then water will condense on it when the dew-point temperature is reached. Condensation of thiswater inhibits further cooling of the junction. Evapo-ration of condensed water from the junction after thecooling current is removed tends to maintain a differ-ence in temperature between the thermocouple andthe reference junctions. The microvoltage developedbetween the thermocouple and the reference junctionsis measured by the proper readout equipment and re-lated to the soil suction by a calibration curve.

(1) Apparatus. Laboratory measurements to eval-uate total soil suction may be made with the apparatusillustrated in figure B-1. The monitoring system in-cludes a cooling circuit with the capability of immedi-ate switching to the voltage readout circuit on termi-nation of the current (fig. B-2). The microvoltmeter(item 1, fig. B-2) should have a maximum range of atleast 30 microvolt and allow readings to within 0.01microvolt. The 12-position rotary selector switch(item 2) allows up to 12 simultaneous psychrometerconnections. The 0-25 millimeter (item 3), two1.5-volt dry cell batteries (item 4), and the variablepotentiometer (item 5) form the cooling circuit, Equip-ment is available commercially to perform these meas-urements of soil suction.

(2) Procedure.(a) Thermocouple psychrometer are inserted

into 1-pint-capacity metal containers with the soilspecimens, and the assembly is sealed with No. 13-1/2rubber stoppers. The assembly is inserted into a 1- by1- by 1.25-foot chest capable of holding six l-pintcontainers and insulated with 1.5 inches of foamedpolystyrene, Cables from the psychrometer arepassed through a 0.5-inch-diameter hole centered inthe chest cover, The insides of the metal containers arecoated with melted wax to inhibit corrosion of the con-tainers.

(b) The apparatus is left alone until equilibriumis attained. Temperature equilibrium is attained with-in a few hours after placing the chest cover. Time toreach equilibrium of the relative humidity in the air

measured by the psychrometer and the relative humid-ity in the soil specimen depends on the volume and ini-tial relative humidity in the container. Equilibriumtime may require up to 7 days, but may be reduced to 2or 3 days by repeated testing of soils with similar suc-tions.

(c) After equilibrium is attained, the microvolt-meter is set on the 10- or 30-microvolt range andzeroed by using a zeroing suppression or offset control.The cooling current of approximately 8 millimeters isapplied for 15 seconds and then switched to the micro-voltmeter circuit using the switch of item 6 in figureB-2, The maximum reading on the microvoltmeter isrecorded. The cooling currents and times should beidentical to those used to determine the calibrationcurves.

(d) The readings can be taken at room tempera-ture, preferably from 20 to 25 degrees Centigrade, andcorrected to a temperature of 25 degrees Centigradeby the equation

(B-4)

wheremicrovolt at 25 degrees Centigrademicrovolt at t degrees Centigrade

Placement of the apparatus in a constant temperatureroom will increase the accuracy of the readings.

(3) Calibration, The psychrometer are calibratedby placing approximately 50 millilitres of the salt solu-tions of known molality (table B-1) in the metal con-tainers and following the procedure in b(2) above to de-termine the microvolt output. Equilibration time maybe reduced to 2 or 3 days by surrounding the psy-chrometer with filter paper soaked with solution. Thesuctions given for the known modalities are plottedversus the microvolt output for a temperature of 25degrees Centigrade. The calibration curves of 12 com-mercial psychrometer using the equipment of figureB-1 were within 5 percent and could be expressed bythe equation

To (B-5)

foot. The calibration curves using other equipmentmay be somewhat different.

c. Filter paper technique. This method involves en- —closing filter paper with a soil specimen in an airtightcontainer until complete moisture equilibrium is

reached. The water content in percent of the dryweight is subsequently determined, and the soil suc-tion is found from a calibration curve.

(1) Apparatus. Materials consist of 2-inch-dia-meter filter paper, 2-inch-diameter tares, and a gravi-metric scale accurate to 0.001 g. A filter paper is en-closed in an airtight container with the soil specimen.

(2) Procedure.(a) The filter paper disc is pretreated with 3 per-

cent reagent grade pentachlorophenol in ethanol (to in-hibit bacteria and deterioration) and allowed to airdry. Reagent grade pentachlorophenol is required be-cause impurities in the treatment solution influencethe calibration curve. Care is required to keep the fil-ter paper from becoming contaminated with soil fromthe specimen, free water, or other contaminant (e.g.,the filter paper should not touch the soil specimen,particularly wetted specimens).

(b) Seven days are required to reach moistureequilibrium in the airtight container. At the end of 7days, the filter paper is transferred to a 2-inch-clia-meter covered tare and weighed immediately on agravimetric scale accurate to 0.001 g. The number of

filter papers and tares weighed at one time should bekept small (nine or less) to minimize error caused bywater evaporating from the filter paper.

(c) The tare is opened and placed in an oven for

5 degrees Centigrade. The ovendry weight of the filterpaper is then determined, and the water content as apercent of the dry weight is compared with a calibra-tion curve to determine the soil suction.

(3) Calibration. The ovendry water content of thefilter paper is dependent on the time lapse followingremoval from the drying oven before weighing.

(a) The calibration curves shown in figure B-3were determined for various elapsed times followingremoval from the oven. The calibrations are given forFisherbrand filter paper, Catalog Number 9-790A, en-closed with salt solutions of various molality for 7days. Calibration curve No. 1 resulted from weighingthe filter paper 5 seconds following removal from theoven. Time lapses of 15 minutes and 4 hours lead to asimilar calibration curve (No. 3) of significantly small-er water contents than the 5-second curve for identi-cal suctions. Calibration curve No. 2 was determined

TM 5-818-7

by removing 12 specimens from the oven, waiting 30 (b) Calibration curves based on the method usedseconds to cool, then weighing as soon as possible and to determine curve No, 3 with a waiting time betweenwithin 15 minutes. 15 and 30 minutes are recommended if the suctions of

100

B-6

TM 5-018-7

if the equilibrium moisture profiles of figure 5-1 (para5-4b) are used.

b. Initial matrix suction. The initial matrix suctionT&without surcharge pressure may be evaluated usingthe soil suction test procedure on undisturbed speci-mens or may be calculated from equation (B-7) andthe natural (initial) water content.

without surcharge pressure may be calculated fromthe assumption

(B-8)

coefficient of effective lateral earth pres-surefinal vertical effective pressure, tons persquare foot or from equation (B-1) settinga

The final vertical effective pressure may be foundfrom

(B-9)

(5-3), (5-4), or (5-5).

d. Compressibility factor. The compressibility fac-tor a is the ratio of the change in volume for a corre-sponding change in water content, i.e., the slope of the

density. The value of a for highly plastic soils is closeto 1, and much less than 1 for sandy and low plasticitysoils. High compressibility y factors can indicate highlyswelling soils; however, soils with all voids filled withwater also have a equal to 1.

(1) Figure B-5 illustrates the compressibility fac-tor calculated from laboratory data for a silty claytaken from a field test section near Clinton, Missis-sippi. Extrapolating the line to zero water content, asshown in the figure, provides an estimate of l/R with

B - 7

TM 5-818-7

heave for this case will be

CT=(0.93) (2.79)

(100) (0,046)= 0.564

vantage of this latter approach is that the equilibriummatrix suction or pore water pressure profile is notknown, except that the final matrix suction will besmall and probably close to the saturated profile (equa-tion (5-3)). The program HEAVE will compute the po-tential heave for this case as well as those shown in fig-ure 5-1.

TM 5-818-7

TM 5-818-7

APPENDIX C

FRAME AND WALL CONSTRUCTION DETAILS

Figures C-1 through C-10 illustrate types of construction for expansive foun-dation soils. These figures were taken from U.S. Army Corps of Engineers Con-struction Engineering and Research Laboratory Technical Report M-81. The fig-ures show practical wall ties to concrete and steel beams, wall connections withcontrol joints, details of interior partitions, bar joist first floor framing with gradebeams, and stiffened mat foundations.

c-s

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C-7

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APPENDIX D

D-1

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D-2

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D-3

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The proponent agency of this publication is the Office of the Chiefof Engineers, United States Army. Users are invited to send com-ments and suggested improvements on DA Form 2028 (Recom-mended Changes to Publications and Blank Forms) direct to HQDA(DAEN-ECE-G), WASH DC 20314.

By Order of the Secretary of the Army:

JOHN A. WICKHAM, JR.General, United States Army

. Official: Chief of StaffROBERT M. JOYCE

Major General, United States ArmyThe Adjutant General

DISTRIBUTION:To be distributed in accordance with DA Form 12-34B, requirements for TM

5-800 Series: Engineering and Design for Real Property Facilities,