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Guideline for Compaction Quality Control on Low Volume Roads using the Dynamic Cone Penetrometer

Final Draft

J. Hongve & M. Pinard

AFCAP Project Reference No: MAL2007B October 2015

Guideline for Compaction Quality Control on Low Volume Sealed Roads using the DCP

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The views in this document are those of the authors and they do not necessarily reflect the views of the Research for Community Access Partnership (ReCAP), [optional insert name of author’s organisation] or Cardno Emerging Markets (UK) Ltd for whom the document was prepared

ReCAP Project Management Unit Cardno Emerging Market (UK) Ltd Oxford House, Oxford Road Thame OX9 2AH United Kingdom


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RESEACH FOR COMMUNITY ACCESS PARTNERSHIP (ReCAP)

Safe and sustainable transport for rural communities

ReCAP is a research programme, funded by UK Aid, with the aim of promoting safe and sustainable transport for rural communities in Africa and Asia. ReCAP comprises the Africa Community Access Partnership (AfCAP) and the Asia Community Access Partnership (AsCAP). These partnerships support knowledge sharing between participating countries in order to enhance the

uptake of low cost, proven solutions for rural access that maximise the use of local resources. The ReCAP programme is managed by Cardno Emerging

Markets (UK) Ltd.

See www.afcap.org


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Acronyms, Units and Currencies

AFCAP Africa Community Access Partnership ASIST Advisory Support, Information Services and Training BS British Standard CBR California Bearing Ratio DCP Dynamic Cone Penetrometer DN DCP Number (penetration in mm/blow) GM Grading Modulus ILO International Labour Organisation LS Linear Shrinkage LVR Low Volume Road(s) MDD Maximum Dry Density OMC Optimum Moisture Content PI Plasticity Index PM Plasticity Modulus RCCD Rapid Compaction Control Device RECAP Research for Community Access Partnership UK United Kingdom (of Great Britain and Northern Ireland) UKAid United Kingdom Aid (Department for International Development, UK)


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Contents Acronyms, Units and Currencies 4

1 Introduction ............................................................................................................. 6

1.1 Background 6 1.1.1 Importance of compaction 6 1.1.2 Factors affecting compaction 6 1.1.3 Traditional approaches to compaction quality control 7 1.1.4 Preferred method of compaction quality control 8

1.2 Purpose and Scope of the Guideline 9 1.2.1 Purpose 9 1.2.2 Scope 9

2 Establishing a Method for Compaction Quality Control .............................................. 9 2.1 General 9 2.2 Compaction trials 9

2.2.1 Control section 9 2.2.2 Controlling compaction moisture 10 2.2.3 Determination of the optimum number of roller passes 11

3 Compaction Quality Control using the DCP .............................................................. 13 3.1 Compaction control procedure 13 3.2 Evaluation procedure 13

4 Strengths and Limitations of Using the DCP ............................................................. 16 4.1 Strengths 16 4.2 Limitations 17

Bibliography .................................................................................................................. 17 Annex 1: The DCP-DN design method and catalogue ................................................... 18 Annex 2: Materials selection and testing .................................................................... 20 Annex 3: DN Calculations ........................................................................................... 23 Annex 4: DCP Data Sheets .......................................................................................... 25 Annex 5: The DCP....................................................................................................... 27


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1 Introduction

1.1 Background

1.1.1 Importance of compaction

Compaction is arguably the most important aspect of road construction. Although it accounts for a relatively small proportion of total construction costs, it can nonetheless have substantial influence on long-term durability and performance of the pavement structure and hence, on the whole-life cost of the road. The degree of performance and durability of pavement structures depends heavily on the support provided by the subgrade. A poorly compacted subgrade will not be stiff enough to withstand the stress exerted on it without undue deflection or deformation. The subgrade must therefore be compacted to the same level as the upper pavement layers, i.e. to the maximum level that can be achieved with the available compaction plant. Materials typically make up about 70% of the cost of road construction of Low Volum Roads (LVR), which constitute a significant proportion of road networks in many countries. Substantial savings in new construction, upgrading or rehabilitation costs of such roads can be achieved if optimum use can be made of the relatively inexpensive materials that often occur along their alignments. It is therefore essential that the compaction process is carried out as efficiently and effectively as possible.

1.1.2 Factors affecting compaction

In general, the effectiveness of the compaction process depends on a number of inter-related factors, including the following:

soil type

compaction moisture content

compactive effort

number of roller passes

loose layer thickness

other factors (e.g. contact pressure, speed of rolling, soil temperature) During compaction the soil goes through three phases as illustrated in Figure 1:

Figure 1: Compaction to "refusal" and Deflection/Pavement Life relationship

As soil compaction proceeds, the compactive effort initially causes plastic deformation (non recoverable) and an associated increase in strength of the soil mass. Further compactive effort


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results in further densification of the soil mass until it progresses through its elasto-plastic state (partially recoverable deformation) to its elastic state, when it is able to support the imposed load without permanent deformation. The benefits of proper compaction are thus:

Improved strength/bearing capacity and stiffness;

Increased density with reduced susceptibility to deformation/rutting; and

Decreased permeability and susceptibility to moisture ingress and resultant loss of strength

Compacting the soil to its elastic state is termed “compaction to refusal”, i.e. that for a given compactive effort and moisture content, the soil cannot be densified any further and that additional roller passes would be a waste of time and money. It follows from the above that:

With a light roller the material reaches its elastic state (for that compactive effort) at lower density than it would with a heavier roller;

The only way to achieve the required density and stiffness of the layer with a light roller is to reduce the thickness of the fill;

The material has been compacted to refusal when the roller is no longer making an indent and starts to bounce off the surface.

Compaction to refusal with the heaviest available plant is making optimal use of the construction materials and increasing the pavement life.

Although compaction to refusal is generally desirable, certain materials may break down due to excessive crushing of the coarse fractions. Compaction must then be terminated at a lower than desired density and stiffness of the layer. Compaction is equally important for all roads, whether they are surfaced with a gravel wearing course or a bituminous surfacing. However, the consequences of inadequate compaction are less severe for gravel roads where defects can relatively easily be rectified by blading and gravel patching. On roads with bituminous surfacing inadequate compaction will inevitably cause extensive defects (rutting, cracking, potholes) which are costly to repair and which will result in high maintenance costs and reduced pavement life and waste the initial high investment cost for the surfacing.

1.1.3 Traditional approaches to compaction quality control

a) Typical methods During construction, the compaction must be control tested against the specifications in the contract. Traditionally compaction is specified in terms of dry density of the pavement layers, typically 93% of Maximum Dry Density (MDD) for fill and subgrade, 95% for subbase and 98% for base. These specifications do not reflect the more important engineering properties of soils, such as stiffness of the layers, as shown above. Other problems associated with the use of density as a method of compaction control include:

The large ratio between volume of material tested to that compacted (typically 1: 100,000);

Poor correlation between laboratory and field compaction and poor reproducibility of results;


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Not sufficient account is taken in the test methods for the inherent variabiltiy of natural, unprocessed materials.

To measure dry densities the common compaction control methods used are:

• Sand Replacement Method; and/or • Nuclear Method

b) Shortcomings

The Sand Replacement Method is cumbersome and time consuming and results are often not available until the next day. Nuclear devices, if at all available, must be carefully calibrated and are potentially hazardous to use. In addition, the inherent variability of natural materials makes the use of these methods unreliable. The dry density methods for quality control also have other limitations, especially for coarse and fine granular materials in that it is difficult to establish a representative density relationship for the lot constructed with inherently variable, natural materials. Thus, a working lot can seldom be represented by one maximum density value determined in the laboratory. Internationally, a significant amount of reserach has been undertaken into alternative methods for compaction quality control due to inadequesies of the traditional dry density test methods. Most of these methods assess the mechanical properties of the compacted layers related to its in situ strength or stiffness. The California Bearing Ratio (CBR) test and the Plate bearing test have been used for this purpose, but these tests have been generally abandoned for QC testing due to their shortcomings. Other tests have been developed for measuring the stiffness or strength of the pavement, such as the Clegg Hammer test, the Soil Stiffness Gauge test, and the Light Falling-Weight Deflectometer test. Correlations have been established between these tests and the CBR, which is still the most common material strength parameter used for pavement testing, but poor correlations have been found with field dry density and actual pavement performance.

1.1.4 Preferred method of compaction quality control

Other devices that measure the shear strength of the material, include the Rapid Compaction Control Device (RCCD) and the Dynamic Cone Penetrometer (DCP). The RCCD is in principle similar to the DCP, but it uses a spring loaded mechanism instead of the sliding hammer on the DCP to drive the cone into the ground and also uses a different cone than the DCP. International research has established that the DCP test is well correlated with the stiffness of the pavement layers and that it may be a useful supplement to the traditional test methods. Through evaluation of a number of different pavement structures in South Africa, pavement performance was related to the maximum allowed DCP penetration in mm/blow, the DCP Number (DN), for the various layers. This research has culminated with the development of the DCP-DN Design Method for Low Volume Roads (LVR) and associated DCP-DN Design Catalogue (see Annex 1). The strength of the DCP-DN method is that materials are tested using the same method both in the laboratory and in the field without the need to convert the DN values to equivalent CBR values for design purposes. For LVR projects the DCP offers good scope as a standalone means of Compaction Quality Control testing.


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1.2 Purpose and Scope of the Guideline

1.2.1 Purpose

The purpose of the Guideline is to establish a simple and practical method for compaction quality control using the DCP, that:

will give satisfactory assurance that adequate compaction has been achieved;

is quick and easy to carry out;

can be used by Engineers and Contractors without extensive experience and expertise, which is often the case for LVR projects; and

is founded on results of international research, but adapted for realtively easy use by the target user groups of the guideline .

1.2.2 Scope

The method is primarily aimed at projects designed using the DCP-DN method, where the layer specifications are in terms of the maximum DN for each layer. This has the great advantage that the pavement design, laboratory testing of materials and compaction control are all based on the same DCP test method, unlike in the traditional CBR- based method where the dry density in the field is used as a proxy for the CBR strength determined in the laboratory. The principle of the compaction quality control using the DCP can nonetheless also be used for projects designed using the CBR-based method and may be a useful supplement to the traditional density tests to quickly check level and uniformity of compaction. Unlike high volume roads, LVR are often constructed with non-standard materials found in the vicinity of the roads. These materials are inherently variable in properties even within the same borrow pit and often highly plastic and moisture sensitive. To interpret the DCP measurements, the Engineer as well as the Contractor must therefore become intimately familiar with the materials and how the DCP measurements vary with moisture content, density and material properties.

Material selection, testing and processing are therefore also dicussed in the annexes to the guideline.

2 Establishing a Method for Compaction Quality Control

2.1 General

Before compaction quality control during construction can be carried out, the criteria for assessing if satisfactory compaction has been achieved must be established. This is achieved as described below.

2.2 Compaction trials

2.2.1 Control section

Compaction trials on control sections, which should be as short as practically possible, must be carried out at regular intervals or whenever the material properties change. The recommended minimum frequency is to construct a control section for every 2000 m3 of material. For a LVR 6.5 m wide this will equate to one control section for the base approximately every 2 km.


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2.2.2 Controlling compaction moisture

One of the principal factors in the construction process, and which affects the final compaction, is to compact the materials at or close to the Optimum Moisture Content (OMC). In most soils the natural variation in OMC is wider than the limits around OMC permitted for successful compaction, normally +1%/-2%. In addition, the actual process of adding and mixing water to soils often leads to significant variation of the moisture content within the material. Most moisture content determinations are slow (except for nuclear methods, but these are often unreliable for moisture contents of natural gravels) and the results are frequently only available after compaction is completed. For this reason, the manual control of the moisture after laboratory calibration of the “feel” of the material at various moisture contents at and around optimum is considered the most practical and effective solution. Controlling the compaction moisture to be within the accepted range is important to attain the highest possible compaction for the material. The methods used to determine the compaction moisture content must be quick and as accurate as possible, since materials can dry out relatively quickly on hot and windy days. Two simple and practical methods are therefore recommended.

a) The “Hand-squeeze method”

The control of moisture during construction can be carried out visually by squeezing a sample of the material as tightly as possible in the hand. The material should be moist enough to stick together when squeezed without any visible sign of free water on the surface. If the material disintegrates, it is too dry for compaction. If free water is ejected or if the soil sticks to the hand, it is too wet. If the “sausage” formed by squeezing in the hand is squeezed diametrically between the thumb and forefinger, it should break with some crumbling. It should not break by deformation under the finger pressure, nor should there be excessive crumbling.

Figure 2: Example of "Hand squeeze test"

It should be noted that non-cohesive soils behave differently. This test is therefore not suitable for such materials. The above technique is considered most practical and suitable for the purpose of controlling the compaction moisture. If possible, this method should be practised in the laboratory with material at various known moisture contents, and correlated with the laboratory-determined OMC to “get the feel” prior to commencement of compaction. Site supervisors can be “trained” in this during the determination of MDD and OMC in the laboratory (see Annex 2)


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b) The “Frying pan method”

Place a small sample of material on a frying pan or other suitable metal container and weigh it on a scale to an accuracy of +/- 1 gram. Then soak the material with kerosine and light it. This may have to be repeated twice. When the flames are out, weigh the material again and calculate the moisture content. This method should be calibrated against oven drying, since not all the moisture may be removed. A correction factor for the “Frying pan method” can then be established.

2.2.3 Determination of the optimum number of roller passes

The contract documents will normally specify the minimum weight and type of the compaction plant to be used, which can be either a single drum or double drum vibrating roller with a weight of at least 10 tonnes. Smaller compaction plant can also be used successfully, but this will limit the loose layer thickness to be compacted. Modern compaction plant is very efficient and can be set to compact at various modes, normally at three amplitude levels (High, Medium and Low), as well as without vibration. Attaining the required strength and stiffness expressed by the Target DN value is normally not difficult if the moisture content is close to OMC. However, one must often guard against over-compaction and breaking up of the layer as well as excessive crushing of the coarse fractions (for weak aggregates) which will degrade the material properties and reduce the strength. Different materials also require different compaction efforts to attain maximum strength and stiffness. The compaction trials shall therefore be used to determine the correct compaction procedure and effort for the respective material in terms of optimum number of roller passes and compaction amplitude to be used for each successive pass.

A typical compaction sequence, illustrated in Figure 3, would be as follows:

The first passes with maximum amplitude to break down oversize material and compact to the bottom of the layer

Intermediate passes with medium amplitude to densify the middle and top of the layer

Finishing passes at low or no amplitude to achieve an even surface with a tightly knit texture and no surface cracks

Figure 3: Typical compaction sequence


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Compaction with any level of amplitude should be done only in one direction when the roller is moving forward. If compaction with vibration is done in both directions, the layer tends to break up at the surface due to the material being pushed in opposite directions at each pass. Provided that the material is at or close to OMC, a realistic Target DN value for the compaction control using the DCP can be established as shown in the following example:

After the first 2 passes, do three DCP tests about 0.5 m apart through the layer, then three more after each successive pass, all within the same area. Mark each DCP hole with a splash of spray paint to keep track of where the tests have been done. Calculate the average DN after each roller pass after elimination of eventual anomalous values. The average DN will normally decrease as shown in Table 1 and Figure 5:

Table 1: Average DN with increasing no of roller passes

No of roller passes 2 3 4 5 6

Average DN mm/blow 6.4 5 4.4 4.22 4.19

Figure 5: Average DN with increasing no of roller passes

In the above example it can be seen that:

• After 5 roller passes the effect of additional passes is virtually zero. The optimum no of roller passes is therefore five.

• The Target DN for compaction control is 4.2 mm/blow. On projects designed with the DCP-DN method it is not a requirement to determine the actual dry density of the compacted layers. The DN value for the layers when they have cured and

Figure 4: Shear cracks due to overstressing the surface


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“set up” is the only requirement that must be met. The Target DN established through the compaction trial will not be the same as the Design DN, since the QC test is done straight after compaction has been completed and the material has not yet cured or “set up”.

3 Compaction Quality Control using the DCP

3.1 Compaction control procedure

For LVR projects a simple control testing procedure is recommended as described below. With judicious process control during construction, this procedure is deemed to give satisfactory compaction quality assurance.

1. Determine the optimum number of roller passes and Target DN value (which is deemed to be at “compaction to refusal” at or close to OMC (+1%/-2%) as described above;

2. For each lot do a minimum of 10 DCP tests in a staggered pattern illustrated in Figure 6 with 3 tests on each side and 4 tests along the centre line.

Figure 6: Pattern for DCP Compaction Control tests

The offset from CL for the LHS/RHS tests shall be varied and no tests shall be done closer to the start/end and to the outer edge of the lot than 0.2 m.

The pattern shall not be the same for each lot, but follow the general guideline in Figure 6.

3.2 Evaluation procedure

Figure 7 conceptualises the proposed evaluation procedure and criteria for compaction quality control.

Figure 7: Concept for evaluation of DCP QC results

Daily lot, normally 200 – 250 m


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Table 2: Evaluation procedure Step 1

Step 1: Identify outliers

No DN Calculations and evaluation

1 4.6 Standard deviation STDEV = 0.98

2 4.2 Mean = 4.64

3 3.9 Outliers = Mean +/- 2 x STDEV

4 5.4 Outliers for DN > 6.61

5 6.7 Outliers for DN < 2.67

6 4.3

7 3.1 Measurement 5: DN > 6.61

8 4.9 New Measurement 5: DN = 5.8

9 5.2

10 4.1

Table 3: Evaluation procedure Step 2

Step 2: Replace the outlier with the new test result, calculate new STDEV, Mean and 80th percentile and apply acceptance criteria

No DN Calculations and evaluation

1 4.6 Standard deviation STDV = 0.80

2 4.2 Mean = 4.55

3 3.9 80th percentile= 5.24

Fre

qu

ency

DN

QC Weighted Average DNs from lot Normal distibution

Target DN

80th %-ile ≤ Target DN +

10%

Mean ≤ Target DN + 5%

≤10%

Mean+5%

≤ 5%

How to use the Excel formulae for the following calculations:

where V17:V26 is the data range containing the DN values you want to evaluate

Standard deviation STDEV =STDEV(V17:V26)

Mean =AVERAGE(V17:V26)

80th percentile =PERCENTILE(V17:V26,80%)


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4 5.4

5 5.8 Acceptance test:

6 4.3 Mean≤ Target DN + 5%

7 3.1 80th percentile ≤ Target DN + 10%

8 4.9

9 5.2

10 4.1

Table 4: Examples of evaluatrion results

Example 1 (fail) DN mm/blow

Target DN = 4.2

Target DN + 5% = 4.41 Fail


Example 2 (fail) DN mm/blow

Target DN = 4.4

Target DN + 5% = 4.62 Pass


Example 3 (pass) DN mm/blow

Target DN = 4.8



It must be noted that:

An unusually high DN value indicates a weakness in the layer, which may be caused by high moisture content, inadequate compaction and contamination of the material or a combination of all these factors.

An unusually low DN value does not necessarily indicate high strength or adequate compaction at that point. Most probably the low DN value is caused by the DCP cone hitting a stone in the layer. A new test in the immediate vicinity of the first point is therefore likely to give a more representative DN value which can be used for the evaluation of the lot.

The evaluation procedure described below is therefore more concerned with identification of weak areas.

The evaluation procedure shown in the example above is as follows: 1) Evaluate the results as in Step 1; 2) Outliers may be replaced by new test results taken in the immediate vicinity of the outlier

points. Then: a) if the new results are still outside the acceptable range Mean +/- 2 x STDEV, more

tests shall be done in the area around the failed test to determine the extent of the weak area that needs to be rectified. The Engineer shall then instruct the contractor


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to take the appropriate action, depending on what is deemed to be the cause of the failure, to rectify the failed area; Re-test the rectified area (no of tests depending on size of the area) and include the new results in the data set for the lot. Proceed to 3) below;

b) if the new test result proved that the first one was indeed an outlier and not representative for that area, replace the old result with the new one in the data set and proceed to Step 2 for evaluation of the lot;

3) If the new test results are acceptable, the results shall then be re-evaluated as in Step 1; 4) Evaluate the lot by applying the acceptance criteria;

a) If OK, issue acceptance certificate; b) If not OK, issue instruction to rework the lot, re-test the lot and start over from Step

1 with the new data set.

Figure 8: Flowchart for Evaluation Procedure

4 Strengths and Limitations of Using the DCP

4.1 Strengths

Using the DCP for Compaction Quality Control has a number of strengths compared to other methods, viz:

DCP Test results

Step 1

Identify outliers

Outliers?

Do new tests on outlier point(s)

Acceptable?

Rectify and do new tests

Acceptable?

Include new tests in data set

Include new test result(s) in data set

Step 2

Apply acceptance criteria

Accept the lot?

Issue instruction to rework the lot

Issue acceptance certificate

Re-test the lot

No

No

Yes

Yes

No

Yes

No

Yes


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The equipment is inexpensive, commonly available and portable;

Maintenance of the equipment can be done locally by reasonably skilled mechanical workshops;

No calibration of the equipment per se is required;

The test is easy and quick to carry out, hence many more tests can be done for greater statistical validity then with the traditional dry density tests;

Technicians can easily be trained to vary out the test confidently,

The DCP test is more repeatable than the CBR test;

The same test can be used for field investigations, design, laboratory testing and quality control testing without the need for converting the DN values to equivalent CBR values or dry densities.

4.2 Limitations

Some researchers have identified an effect on the DN value due to the vertical confining pressure of the pavement structure, i.e. that the DN value decreases with increased depth. However, this effect is deemed to be negligible, if at all present, for layer thicknesses of less than 200 mm. The DCP test can be affected by stones or coarse gravel in the pavement structure, either by the cone hitting or skirting a stone or by side friction on the rod in coarse material. If the rod tends to bend over to one side, it is a sign that the cone is skirting a stone and the test must be stopped. In such cases it is sometimes sufficient to do a new test, say, 0.50 m away from the first point. However, experienced operators will get a “feel” for when a result can be accepted or not. If the material is too coarse, the DCP may refuse to penetrate, and the test may have to be abandoned. A recent study found consistent DCP results on material screened on the 20 mm sieve for CBR testing as well as on typical 37.5 mm nominal size base material, but highly variable results on 50 mm nominal size subbase material. For a well graded material with maximum size up to 37.5 mm, the DCP can be used with some confidence. With larger stones in the layer, the DCP test must be done with caution and possibly be repeated until reliable results are obtained. When testing a pavement layer, say 150 mm thick, the DCP result is calculated as a Weighted Average for the whole layer. If the cone is temporarily being blocked by a stone for one or two blows, the effect on the DN value for the layer is therefore quite small and reasonable judgement can be made whether or not to accept the result. Lastly, reliable results using the DCP can only be attained if the:

instrument is in good order; and

is used correctly For operation and checking the DCP before use, see Annex 5.

Bibliography Malawi Ministry of Transport and Public Works, 2013. Design Manual for Low Volume Sealed Roads

using the DCP Design Method. International Labour Organisation, 1998. ASIST Information Service Technical Brief No 9 Material

selection and quality assurance for labour based unsealed road projects.


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Kim H., Prezzi M. and Salgado R, 2010. Use of Dynamic Cone penetration and Clegg Hammer tests for

quality control of roadway compaction and construction. Final report FHWA/IN/JTRP-2010/27 Livneh M and Livneh N, 2013. The Use of the Dynamic Cone Penetrometer for Quality Control of Compaction Operations. International Journal of Engineering Research in Africa Vol. 10 (2013) pp 49-

64 Chen et al, 2003. A Correlation Between Dynamic Cone Penetrometer Values and Pavement Layer

Moduli. Geotechnical Testing Journal, Vol. 28, No. 1 Chen D., Wang J. and Bilyeu J. Application of Dynamic Cone Penetrometer in Evaluation of Base and

Subgrade Layers. Transportation Research Record 1764, Paper No. 01-0349 Abu-Farsakh et al, 2003. Assessment of in-situ test technology for construction control of base

courses and embankments. Louisiana Transportation Research Center. Final report FHWA/LA.04/385.

Siekmeier et al, 2009. Using the Dynamic Cone Penetrometer and Light Weight Deflectometer for

Construction Quality Assurance. Office of Materials and Road Research Minnesota Department of Transportation. Final report MN/RC 2009-12

Amini F, 2003. Potential Applications of the Static and Dynamic Cone Penetrometers in MDOT

Pavement Design and Construction. Jackson State University Department of Civil Engineering. Final report FHWA/MS-DOT-RD-03-162

Pinard M, 2001. Impact Compaction as an Effective Means of Ground Improvement. SAICE

Geotechnical Division: Seminar on Ground Improvement Kleyn, E.G., 1975. The use of the dynamic cone penetrometer (DCP). Report 12/74. Transvaal Roads

Department, Pretoria, South Africa. Kleyn, E.G. and Savage P.F., 1982. The application of the pavement DCP to determine the bearing

properties and performance of road pavements. Proceedings of International Symposium on bearing capacity of Roads and Airfields, Trondheim, Norway.

Kleyn, E.G., 1984. Aspects of pavement evaluation and design as determined with the aid of the

Dynamic Cone Penetrometer (in Afrikaans). M Eng Thesis, University of Pretoria. Kleyn E.G. and van Zyl G.D., 1987. Application of the Dynamic Cone Penetrometer (DCP) to light

pavement design. Laboratory Report L4/87. Transvaal Provincial Administration, Pretoria, South Africa.

De Beer M, 1999. Use of the dynamic cone penetrometer (DCP) in the design of road structures.

Research Report DPVT-187. Department of Roads and Transport Technology, CSIR, Pretoria. South Africa.

CSIR Transportek, 2000. DCP analysis and classification of DCP survey data: Windows version. CSIR,

Pretoria, South Africa. Otto A and Tumwesige R., 2015 (unpublished). A methodology of pavement design for upgrading

gravel roads to bituminous standard.

Annex 1: The DCP-DN design method and catalogue Based on extensive research and evaluation of pavement sections, a DCP Design Catalogue for Low Volume Roads was developed in the 1980’s. This catalogue was initially a California Bearing Ratio (CBR) based catalogue where the DN value for each pavement layer was converted to equivalent CBR values. However, due to the inaccuracy of the CBR test and its poor correlation with performance and the comparative advantage of the DCP for gathering


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field data and testing of materials in the laboratory, the catalogue was converted to a DCP-DN catalogue in the recent years through the African Community Access Programme (AFCAP).

Table 5: The DCP-DN Design Catalogue

Traffic Class

E80 x 106

LE 0.01

0.003-0.010

LE 0.03

0.010-0.030

LE 0.1

0.030-0.100

LE 0.3

0.100-0.300

LE 0.7

0.300-0.700

LE 1.0

0.700-1.000

0-150 mm Base 98% BS Heavy

DN≤8 DN≤5.9 DN≤4 DN≤3.2 DN≤2.6 DN≤2.5

150-300 Subbase 95% BS Heavy

DN≤19 DN≤14 DN≤9 DN≤6 DN≤4.6 DN≤4

300-450 mm Subgrade 95% BS Heavy

DN≤33 DN≤25 DN≤19 DN≤12 DN≤8 DN≤6

450-600 mm In situ material

DN≤40 DN≤33 DN≤25 DN≤19 DN≤14 DN≤13

600-800 mm In situ material

DN≤50 DN≤40 DN≤33 DN≤25 DN≤24 DN≤23

DSN800 (blows) ≤39 ≤52 ≤73 ≤100 ≤128 ≤143

The DCP-DN method of pavement design offers a number of advantages over the traditional CBR based method in that it reduces the reliance of traditional materials tests such as the time-consuming and often unreliable laboratory CBR testing. Moreover, the method lends itself ideally to evaluating in situ road conditions and, by integrating the design strength profile optimally with the in situ strength profile, to designing light road pavement structures in a highly cost-effective manner. The DN is a composite measure of the density, grading, plasticity and moisture at the time of the DCP test. With the DCP-DN design method these parameters therefore do not need to be specified separately as is done in the conventional CBR based method. For a full description of the DCP-DN design method, the reader is referred to the “Malawi DCP Design Manual for Low Volume Sealed Roads”.


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Annex 2: Materials selection and testing

Frequency of testing

The frequency of testing of borrow pits needs to strike a balance between cost and time and statistical validity of the results. It is recommended that for LVR projects, the location of borrow materials and borrow-pit testing should be done according to traditional methods.

As a minimum, materials from borrow pits should be tested in accordance with Table 6:

Table 6: Minimum frequency for testing of borrow materials

Intended Use Maximum Volume m3/DN test

Base 5 000 m3

Subbase & Improved Subgrade 10 000 m3

Fill 20 000 m3

Source: Malawi DCP Design Manual for Low Volume Sealed Roads

The frequency of testing will depend on the variability of the material; the more homogeneous the material the less the amount of testing necessary for statistical validity of the results. Unless proper testing of the borrow materials is carried out prior to commencement of the project, it is usually not possible to quantify the variability in advance of construction.

In practice, therefore, new tests will normally be required more frequently than indicated in Table 6.

Soil Classification

Standard soil classification tests should always be carried out, such as:

Grading and determination of Grading Modulus

Determination of Atterberg limits, Linear Shrinkage (LS) and Plasticity Modulus (PM)

Calculations: GM = 300 – (P2.0 + P0.425 + P0.075)/100, where Pxxx is percent passing the sieve; or GM = (P2.0 + P0.425 + P0.075)/100, where Pxxx is percent retained on the sieve PM = P0.425 x PI, where P0.425 is percent passing the 0.425 sieve

Recommended values:

1.0 ≤ GM ≤ 2.25

PI ≤ 18 (material dependent)

The LS test gives an indication of the material’s susceptibility to cracking when drying out. The LS value is normally approximately half of the PI and is therefore a useful test also to evaluate the accuracy of the PI test.

Some materials are susceptible to crushing and loss of strength during compaction. For weaker materials aggregate hardness and durability tests should therefore also be carried out.


Page 21

Determination of the Laboratory DN value1

The DN values in the DCP-DN catalogue are the required DN values for the layers in service at the anticipated long term moisture regime in the pavement. Thus, if the long term equilibrium moisture in the pavement is expected to remain at or slightly lower than Optimum Moisture Content (OMC), the DN value for the material at OMC when evaluated in the laboratory must not exceed the required DN value as per the catalogue (higher DN means weaker material). If, on the other hand, the pavement is expected to get soaked (as can happen for low lying areas subjected to seasonal flooding), the laboratory DN value after soaking must not exceed the required DN value.

The tables below show examples of Laboratory DN values for two different materials.

Table 7: Laboratory DN values for material A

% of BS Heavy Compaction

Soaked OMC Approx 0.75 OMC

98 % 12.19 2.24 1.37 0.75 OMC

95% 20.39 3.91 2.92 0.76 0MC

93% 28.81 5.76 4.65 0.72 OMC

Table 8: Laboratory DN values for material B

% of BS Heavy Compaction

Soaked OMC Approx. 0.75 OMC

98 % 6.02 3.07 1.99 0.75 OMC

95% 10.47 5.00 2.89 0.74 0MC

93% 13.47 6.38 3.28 0.74 OMC

From these tables it can be seen that Material A is very moisture sensitive with drastically decreasing DN values going from soaked condition to OMC and that Material B is much less moisture sensitive than Material A. Material A is weaker than material B in the soaked state (higher DN) due to high PI, but stronger at OMC and drier.

Both materials would qualify for Base at 98% BS Heavy and OMC for Traffic Class LE 0.3 as per the DCP-DN Design Catalogue. However, due to the extreme moisture sensitivity it may be risky to use Material A for base in case of in-service surfacing defects that will allow water to penetrate into the base.

The required DN values in the DCP-DN catalogue are maximum in-service values after curing of the layers, i.e. after dissipation of pore pressure that has built up during compaction and, for some pedogenic materials like laterites and calcretes, possible self-cementing of the material. This is taken into account in the described procedure for the laboratory DN test.

However, during construction it is not possible to wait for the curing of the layers before approval or rejection of the works. The layer must be tested soon after compaction in order to let the contractor proceed with the works or take remedial action if the layer cannot be accepted for whatever reason.

Determination of Maximum Dry Density (MDD) and OMC

The MDD and OMC for the material need to be determined for estimation of the amount of compaction water to add based on the in-situ moisture content of the material. It is recommended that the MDD/OMC test be done in the standard CBR moulds in accordance with the applicable country standards (BS or AASHTO) with BS Heavy or Modified AASHTO (T180) compaction effort respectively.

1 See procedure in the Malawi DCP Design Manual for Low Volume Sealed Roads


Page 22

Five samples should be compacted moisture contents estimated to be slightly below and above OMC. Each sample should be penetrated with the DCP. The result may look as shown in Table 9 and Figure :

Table 9: Example of MDD/OMC test results

Moisture Content % 6.7 7.2 7.7 8.2 8.7

Dry Density kg/m3 1780 1840 1980 1920 1860

DN mm/blow 6.5 5.3 4.2 5.1 6.2

Figure 9: Plot of results from example in Table 6

In the above example the MDD has been determined to 1983 kg/m3 at OMC 7.8% with a corresponding Min DN value of 4.2 mm/blow at MDD and OMC. It must be noted that this DN value has been determined just after compaction without dissipation of pore pressure and curing and will therefore be higher than the DN used for design purpose determined from the Laboratory DN test above. The main purpose of penetrating each mould as described above, is to get an impression of the variation of the DN with density and moisture content just below and above OMC. This will aid in the interpretation of the DN values obtained in the field during the compaction control. Site Supervisors should also calibrate their “feel” for the OMC of the material during this test. An example of how to record the DCP data and calculate the Weighted Average DN is shown in Annex 3.


Page 23

Annex 3: DN Calculations Example of recording the DCP data and calculation of the Weighted Average DN for the in-mould Laboratory DN test:

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

0 219

2 231 12 6.00

3 243 16 5.33

3 252 15 5.00

4 264 17 4.25

4 274 13 3.25

4 282 11 2.75

5 291 12 2.40

5 302 11 2.20

107

4.02

Penetration depth

Weighted Average DN

BS Heavy

As shown above, variable no of blows can be used between each DCP reading. The data can be recorded in a ready-made spreadsheet, the Laboratory DN Workbook shown in Annex 4, which automatically calculates the penetration depth and the Weighted Average DN. The data recorder can monitor the penetration depth after each reading and vary the number of blows such that the last reading is taken just before the point of the cone has reached the bottom of the mould to prevent the tip from being damaged if it were to hit the steel base plate. It can also be seen from the example that there is a density gradient in the sample with the material at the bottom having lower DN values than the material at the top of the mould. This is due to the bottom layers having received a higher compaction effort than the top layers even though each layer has been compacted with the same no of blows. If the Laboratory DN Workbook is not at hand, the Weighted Average DN can be calculated as follows:

𝑊𝑒𝑖𝑔ℎ𝑡𝑒𝑑 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐷𝑁 = ∑(𝐷𝑁 𝑝𝑒𝑟 𝑛 𝑏𝑙𝑜𝑤𝑠 × 𝐴𝑣𝑔 𝐷𝑁 𝑝𝑒𝑟 𝑏𝑙𝑜𝑤)

𝑃𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑒𝑝𝑡ℎ

Example of recording the DCP data and calculation of the Weighted Average DN for the Pavement Layer DN test:


Page 24

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

0 221

2 225 4 2.00

3 238 13 4.33

3 251 13 4.33

3 263 12 4.00

4 283 20 5.00

4 304 21 5.25

4 328 24 6.00

3 347 19 6.33

2 350 3 1.50

2 359 9 4.50

2 371 12 6.00

150

5.10

Chainage: 2+640

Position: LHS

Penetration depth

Weighted Average DN The procedure is exactly the same as for the Laboratory DN test, except here the blows are varied so the last reading is taken when the full layer, in this case 150 mm thick, has been penetrated. The data can be recorded in the ready-made Pavement Layer DN Workbook shown in Annex 4. Note that in the field it is not uncommon to have a density gradient opposite of that in the mould with the denser material towards the top of the layer. However, if correct compaction procedures have been followed and the material was close to OMC at compaction, the Average DN per blow should be more or less constant throughout the whole layer.


Page 25

Annex 4: DCP Data Sheets

Region: Project: B/pit:

Date:

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - -

- - -

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - -

- - -

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - - - - -

- - -

- - -

Tested by:Sample no:

Laboratory DN test sheet

Penetration depth

Weighted Average DN

Weighted Average DN

0.75 OMC

98% 95% 93%

Weighted Average DN

OMC

98% 95% 93%

Penetration depth

4 days soaked

98% 95% 93%

Penetration depth


Page 26

Region:

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- -

- -

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- -

- -

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

No of

blows n

DCP

Reading

DN per n

blows

Avg. DN

per blow

Relative

Density % FMC %

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- - - -

- -

- -

Penetration depth

Weighted Average DN

Layer:

Position:

Penetration depth

Weighted Average DN

Chainage:

Penetration depth

Weighted Average DN

Chainage:

Chainage:

Position:

Chainage:

Position: Position:

Chainage:

Position: Position:

Chainage:

Tested by: Date:

Pavement Layer DCP test sheet

Project: Sheet no of


Page 27

Annex 5: The DCP Many different DCP models have been developed over the years with different hammer weights, hammer dropping heights and cone angles. The model that is currently used in Africa is shown in Figure 10.

Checking the DCP before use

Before use, the DCP must always be checked to be in good working order. Check that: 1) All parts are undamaged, specifically the connections for the

anvil and the cone; 2) The rods are straight and of the correct length; 3) The hammer dropping height is exactly 575 mm and that it

can fall freely without restriction; 4) The cone is not worn round at the tip or the 3 mm shoulder.

Using the DCP

1) Seat the cone by tapping the anvil with the sliding hammer so that the top of the 3 mm shoulder is flush with the ground. Take the “zero blows” reading off the graduated ruler and note it on the DPC Data Sheet.

2) Lift the sliding hammer so that it touches the handle without

knocking it upwards, then release and let the hammer fall free onto the hammer. Take new readings after “n” blows and record on the DCP Data Sheet.

“N” can be a fixed number or variable depending on what the DCP is used for. For site investigations when penetrating down to a depth of 800 mm, it is common to use a fixed number of blows between each reading, but variable number of blows can also be used. For testing layer works or materials in the CBR mould in the laboratory, a variable number of blows is normally used.

3) Carefully extract the DCP from the ground. For site investigations, a purpose made jack should always be used otherwise the DCP easily gets damaged. Even when testing well compacted layer works, it can be difficult to extract the DCP, so a jack should always be at hand.

Do not extract the DCP by knocking the hammer hard upwards onto the handle, as this practice is will render the DCP useless after a short while.

Figure 10: The DCP

Ruler with mm graduation

Anvil

8 kg hammer

575 mm

Cone

3 mm

20 mm

60o


Page 28

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