Application of the Dynamic Cone Penetrometer (DCP) for determination of theengineering parameters of sandy soils

S.D. Mohammadi a,⁎, M.R. Nikoudel a, H. Rahimi b, M. Khamehchiyan a

a Department of Engineering Geology, Tarbiat Modares University, Tehran, Iranb Department of Irrigation Engineering, Tehran University, Tehran, Iran

A B S T R A C TA R T I C L E I N F O

Article history:Received 20 November 2007Received in revised form 26 April 2008Accepted 22 May 2008Available online 4 June 2008

Keywords:Dynamic Cone Penetrometer (DCP)Poorly graded sandy soil (SP)Dynamic Penetration Index (DPI)Index ParametersCoefficient of variations (Cv)

Determination of the in situ engineering properties of foundation materials has always been a challenge forgeotechnical engineers and, thus, several methods have been developed so far. Dynamic Cone Penetration(DCP) test is one of the most versatile amongst them. In the present research, a light weight simple DCPdevice was developed and used for evaluation of the engineering properties of sandy soils in laboratoryconditions. The device consisted of an 8-kg hammer that drops over a height of 575 mm, and drives a 60°cone tip with 20 mm base diameter into the ground. To control the validation of the results, laboratory directshear and plate load tests were used as reference tests. The soil sample was a poorly graded sandy soil (SP)taken from alluvial deposits of the Tehran plain. All DCP tests and PLTs were undertaken on compacted soil ina mould with 700 mm diameter and 700 mm height. Based on the results of the experiments, therelationships between Dynamic Penetration Index (DPI), relative density (Dr), modulus of elasticity (E), shearmodulus (G), modulus of subgrade reaction (KS), and the friction angle of the soil were obtained with a highcoefficient of determination (N90%). The repeatability of the test results was also evaluated by calculating thecoefficient of variations (Cv), which was less than 30% for all tests.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

A truly undisturbed sample is defined as completely intact soilwhich its in place structure has not been changed in any way. Suchsamples are desirable for those laboratory tests which are dependenton the structure of soil, such as shear strength. Unfortunately, severalissuesmake it almost impossible to obtain a truly undisturbed sample,specially in non-cohesive soils. Regarding to these issues, variety oftechniques have been developed to perform in situ tests such asdynamic probing. Dynamic probing is a continuous soil investigationtechnique and is assumed as one of the simplest soil penetration tests.It basically consists of repeatedly driving a metal tipped probe into theground using a drop weight of fixed mass and travel. Testing is carriedout continuously from the ground level to the final penetration depth.The continuous sounding profiles enable easy recognition of dissim-ilar layers and even thin strata by the observed variation inpenetration resistance. The Dynamic Cone Penetrometer (DCP) is alightweight dynamic penetrometer which is considerably faster andcheaper tool than boring, particularly when the depth of exploration islow and the soils being investigated are not coarse gravel (Sawangsur-iya and Edil, 2005).

Scala (1959) originally developed the DCP in Australia. Since then,it has been used for site characterization of pavement layers and

subgrades in South Africa, the United Kingdom, Australia, NewZealand, and several states in the United States, such as California,Florida, Illinois, Minnesota, Kansas, Mississippi, and Texas (Abu-Farsakh et al., 2004). Some relationships have been developedbetween DCP results and CBR (Abu-Farsakh et al., 2004; Chen et al.,2001; Karunaprema and Edirisinghe, 2002; Rahim and George, 2004;Nazzal, 2002) and elastic modulus (E) (Mohammadi et al., 2007;Webster et al., 1992).

The main objectives of this paper are to describe the capability ofthe DCP to study the inplace engineering properties of sandy soils.

2. Materials and methods

In order to achieve the appropriate correlations between the DCPtest results and engineering parameters of sandy soils, it wasnecessary to select suitable sample. The appropriate sampling areawas selected based on previous experiences and sampling wasperformed according to standardmethods. Then, the selected sampleswere shipped to the laboratory and was prepared for the tests asexplained in the later sections.

2.1. Geology of the sampling area

The sampling area contains four types of lithology belonging toLate Eocene and Quaternary deposits (Fig. 1). The Late Eocene rockcovers almost 5% of the land surface and it comprises one type of

Engineering Geology 101 (2008) 195–203

⁎ Corresponding author.E-mail address: s_d_mohammadi@yahoo.com (S.D. Mohammadi).

0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2008.05.006

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lithology which is categorised as Trachyte to Trachyandesite. TheQuaternary deposits cover almost 95% of the land surface and itcomprises three types of lithology which are categorised as conglom-erates, old terrace deposits and young terrace deposits. In the presentresearch, the sampling was performed on the young terrace deposits.Geologically, the young deposits comprise subrounded sand grainscontaining 5% gravel (Fig. 2). The X-ray analysis has shown that thesandy samples are made of quartz, feldspar, pyroxene and calcite.

2.2. Sample preparation

The data used in this paper were obtained from laboratory testsundertaken by the authors at the Geotechnical Engineering Labora-tory of Tarbiat Modares University, Tehran, Iran.

To prepare the soil samples for testing, alluvial deposits were ovendried and passed through sieve No. 4. Fig. 3 shows the gradiationcurves of the original soil and the sample after passing sieve No. 4

which is classified as poorly graded sand (SP) according to the UnifiedSoil Classification System. The index properties of the soil are shownon Table 1. To achieve a uniform compaction, the sample in the testingmould was compacted in seven 100 mm thick lifts. The soil was driedand compaction effort was applied using a 300mmvibrating plate in away that the required density to be achieved. The in place density foreach case was controlled using the sand cone method. Details of thetests on samples having different densities are indicated in Table 2.

To prepare the soil sample for direct shear test, a circular shear box,having 60 mm internal diameter and 25 mm height was used. Toachieve a uniform compaction in the circular shearmould of the directshear machine, tamping by a small circular steel plate with 60 mmdiameter was used. To eliminate the effects of pore pressure, all directshear tests were carried out in dry condition.

Fig. 1. Geological map of sampling location, modified from Geological Survey of Iran (1998).

Fig. 2. Grains of the used soil in prepared thin section (under PPL, 6× magnification). Fig. 3. Gradiation curves of alluvial deposited and used soil.

196 S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

2.3. Testing procedures

Several tests including Dynamic Cone Penetration (DCP), PlateLoad (PLT) and direct shear tests were undertaken on the compactedmaterials as described in the following sections:

2.3.1. Dynamic Cone Penetration testsThe Dynamic Cone Penetrometer (DCP) has been described by

ASTM 6951-03 (2003). The typical DCP consists of an 8-kg hammerthat drops over a height of 575 mm, which yields a theoretical drivingenergy of 45 J or 14.3 J/cm2, and drives a 60o cone tip with 20mm basediameter vertically into the pavement or subgrade layer (Fig. 4). The

steel rod to which the cone is attached has a smaller diameter than thecone (16 mm) to minimize the effect of skin friction. Depth ofinvestigation of DCP is 1 m to 2 m. The number of blows duringoperation is recorded with depth of penetration. The slope of thecurve defining the relationship between number of blows and depthof penetration (in millimeters per blow) at a given linear depthsegment is recorded as the DCP penetration index (DPI). DPI for eachdepth can also be calculated by Eq. (1) (Embacher, 2005):

DPI ¼ Piþ1−PiBiþ1−Bi

ð1Þ

Where:

DPI DCP Penetration Index (mm/blow)P Penetration at i or i + 1 hammer drops (mm); andB blow count at i or i + 1 hammer drops

Analysis of the DCP data must be interpreted, following a standardprocedure, to generate a representative value of penetration per blowfor the material being tested. This representative value can beobtained by averaging the DPI across the entire penetration depth ateach test location. For calculating the representative DPI value for agiven penetration depth, two methods are available: (i) arithmeticaverage; and (ii) weighted average. The arithmetic average can beobtained from Eq. (2) (Edil and Benson, 2005):

DPIavg ¼∑N

i DPIð ÞN

ð2Þ

where N is the total number of DPI recorded in a given penetrationdepth of interest. In the weighted average technique, Eq. (3) can beused (Edil and Benson, 2005):

DPIwt:avg ¼ 1H

∑Ni ½ DPIð Þi& Zð Þi' ð3Þ

Where Z is the penetration distance per blow set and H is theoverall penetration depth of interest.

Table 1The index properties of used soil

Parameter Value

emax (−) 0.97emin (−) 0.46Gs (−) 2.66γd(max)(KN/m3) 17.85γd(min)(KN/m3) 13.24Cu (−) 1.16Cc (−) 1value of clay (%) 0value of silt (%) 2USCS soil classification SP

Table 2Testing program for laboratory investigations and different densities for tested soil

Dr(%)⁎ Mean ofwater content(%)

Dry unitweight(gr/cm3)

DCP(numberof test)

PLT⁎⁎

(number of test)Direct shear(number of test)

25 0.4 1.44 3 3 735 0.4 1.48 3 3 750 0.4 1.55 3 3 760 0.4 1.60 3 3 775 0.4 1.67 3 3 7

⁎ Relative density⁎⁎ Plate Load Test

Fig. 4. Dynamic Cone Penetrometer (DCP) (Edil and Benson, 2005).

197S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

The main advantages of the DCP include:

• speed of operation;• applicability in difficult terrains where access is poor;• minimal equipment and personnel;• low cost of the equipment;• simplicity of the operation and data recording/analysis.

As previously mentioned, the DCP tests were carried out in amould with a diameter and height of 700 mm, respectively (Fig. 5). Tohave more uniform results, readings were taken around the center ofthe test mould. The results of DCP tests on samples of different relativedensities are presented in Fig. 6.

To overcome the effects of the mould side walls, the minimumdistance between cone and edge of the testing mould was taken as225 mm (Abu-Farsakh et al., 2004). To investigate the effects of themould size on the results, several DCP tests were conducted on the

moulds of different diameters of 300 mm, 500 mm and 700 mm.Variation of the average DPI values versus different mould diametersare presented in Fig. 7. The results show that with increase of relativedensity (Dr), the effect of the side wall is more pronounced. This effectis fully negligible for moulds with a diameter grater than 500 mm. Onthe other hand, a distance of 250 mm between the cone and the edgeof the test mould can fully eliminate the mould size effects. Thus, inthe present research, all DCP tests were carried out in a mould with adiameter of 700 mm.

The repeatability of the DCP test results is an importantconsideration. To evaluate the repeatability, several tests were carriedout. Each testing series included three DCP tests. Fig. 8 shows theresults of the three series of tests undertaken for different relativedensities (Dr). In this figure, DPI values are converted to NDCP, whereNDCP is the number of blow for 100 mm penetration.

In order to study the repeatability of the results, it was importantto choose a suitable parameter that represents the repeatability. Forthis purpose, percent of the coefficient of variation (Cv) was employedas an indicator for repeatability.

Table 3 shows some soil properties, determined by various standardtests, along with their coefficients of variation reported by variousresearchers (Lee et al., 1983). The sources of variability in soil propertiesdiffer, and accordingly the coefficients of variation differ for differentproperties (Fakher et al., 2006). The coefficient of variation, Cv, for theresults of Standard Penetration Test (N), which is basically a super heavydynamic probe test, is reported to be between 27% and 85% with a

Fig. 5. A schematic diagram of DCP test in testing mould (a) side view (b) plan view.

Fig. 6. Average of DPI versus depth for studied soil at test mould.

Fig. 7. Correlation between DPI and mould diameter.

Fig. 8. Example of the results of tests repeated at mould test (a) for Dr=25% (b) forDr=50% (c) for Dr=75%.

198 S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

recommended value of 30%, (Lee et al., 1983). The repeatability of SPTtest results could be used as ameasure of the repeatability of DCP resultsby comparing Cv values of the twomethods. In the present research, thevalues of Cv have been determined for each depth in each series of thetests. The average value of Cv is 5.6% and its standard deviation is 9.51. Inmore than 68.7% of the tests, the value of Cv is 0% and in 12.5% of thetests, this value is 20.28%. In the tests undertaken, the values of Cv variedbetween 0 and 28.3% and for all cases it was less than 30%. Therefore, theresults of DCP tests for the three relative densities (Dr) can be consideredas repeatable when compared with the values presented in Table 3.

2.3.2. Plate Load Test (PLT)The Plate Load Test (PLT) is a useful site investigation tool and has

been used for proof testing of pavement layers in many Europeancountries for many years. Currently, it is used for evaluation of bothrigid and flexible pavements (Abu-Farsakh et al., 2004). The PLT in fullor small scale, is sometimes considered as the best means ofdetermining deformation characteristics of the soils, but is only usedin exceptional cases due to the costs involved (Bowles, 1997). In thepresent research, a round plate with 230 mm diameter was used forconducting plate load tests. The PLT was used as a reference test toobtain the strength parameters of the soil under investigation. Aloading frame was designed to fit the mould and its support. Toperform the test, the bearing plate and hydraulic jack were carefully

placed at the center of the sample under the loading frame (Fig. 9). Thehydraulic jack and the supporting frame were able to apply a 60 tonsload. For measurement of deformations, dial gauges that are capableof recording a maximum deformation of 25.4 mm (1 in) with anaccuracy of 0.001 in., were employed. The ASTM-D 1195-93 (1998)standard method was followed to perform the test.

Elasticity modulus is always considered as a more importantdeformability parameter for geomaterials. As in the case for otherstress–strain tests, different elasticity moduli can be obtained from thePLT. Soil elasticity moduli can be defined as: (1) the initial tangentmodulus; (2) the tangent modulus at a given stress level; (3) reloadingand unloading modulus; and (4) the secant modulus at a given stresslevel (Abu-Farsakh et al., 2004). In this study, since the stress–strain

Table 3Coefficient variation for soil engineering tests (Lee et al., 1983)

Test Reported Cv (%) Recommended standard

Angle of friction (sands) 5–15 10CBR 17–58 25Undrained cohesion (clays) 20–50 30Standard penetration test (SPT) 27–85 30Unconfined compressive strength (clays) 6–100 40

Fig. 9. A schematic diagram of Plate Load Test (PLT) set up (a) side view (b) plan view.

Fig. 10. Definition of modulus from PLT (Abu-Farsakh et al., 2004).

199S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

curves had a clear peak point, the initial tangent modulus wasdetermined for all plate load test results. To determine the initialmodulus (EPLT(i)), a line was drawn tangent to the initial segment of thestress–strain curve, then an arbitrary point was chosen on the line andthe stress and deflection corresponding to this point were determinedfor calculation of the initial modulus. Fig. 10 describes the deformationsand stresses used for determining EPLT(i). A reloading stiffness moduluscalled EPLT(R2), was also determined for each stress–strain curve.

The second parameter which can be calculated from PLT results, isshear modulus (G). Shear modulus is defined as the ratio of shearstress to shear strain (Bowles, 1997) and is calculated from Eq. (4)(Timoshenko and Goodier, 1970):

GPLT ¼ qDρ

π8

1−vð Þ ð4Þ

where:

q = bearing pressureD = diameter of the loading plateρ = settlementυ = Poisson's ratio

Since the non-rigid methods consider the effect of local matdeformations on distribution of bearing pressure, it is needed todefine the relationship between settlement and bearing pressure. Thisis usually done using the coefficient of subgrade reaction (Ks). Eq. (5) isused to determine Ks from PLT results (Coudoto, 2004):

KS ¼ ΔP=ΔS ð5Þ

where:

Ks = modulus of subgrade reactionΔP = applied pressureΔS = measured settlement

2.3.3. Direct shear testIn order to determine the soil friction angle ( ϕ), 35 direct shear tests

(Table 2) were undertaken in a circular shear mould as described earlier.Due to the nature of the soil samples (non-cohesive), cohesion parameter(C) was equal to zero and thus, friction angles were calculated. The ASTM-D 308-90 (2000) standard method was followed to perform the test.

3. Results and discussions

In the following sections, the results of the tests and theircorrelations with important engineering parameters of the studiedsoil samples are discussed.

3.1. DPI versus Dr(%)

The relative density is a useful parameter to describe theconsistency of sands (Coduto, 2001). Kulhawy and Mayne (1990)suggested an empirical correlation between SPT results and Dr asfollows (Eq. (6)).

Dr kð Þ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN1ð Þ60

CpCACOCR( 100

s

ð6Þ

Where (N1)60 is SPT's N value corrected for field procedure andoverburden stress, and Cp, CA and COCR are grain size, aging correctionand overconsolidation factors, respectively.

In this study, the correlation between average DPI and Dr wasinvestigated. Eq. (7) and Fig.11 suggest a good correlation between thesetwo parameters. The determination coefficient (R2) of Eq. (7) is 0.98.Table 4 shows the proposed classification for estimating of Dr(%) by DPI.

Dr kð Þ ¼ 189:93= DPIð Þ0:53 R2 ¼ 0:98" #

ð7Þ

3.2. DPI versus modulus of elasticity (E)

For the data obtained in this study, the best correlations betweenaverage DPI, EPLT(i) and EPLT(R2) are presented in Figs. 12 and 13 (Eqs. (8)and (9)). However, there is a better correlation (Eq. (9)) between theaverage DCP penetration rate and PLT reloading modulus (EPLT(R2))compared to the correlation with EPLT(i).

EPLT ið Þ MPað Þ ¼ 55:033= DPIð Þ0:54 R2 ¼ 0:83" #

ð8Þ

EPLT R2ð Þ MPað Þ ¼ 53:73= DPIð Þ0:74 R2 ¼ 0:94" #

ð9Þ

Fig. 14 and Eq. (10) show the correlation between EPLT(i) and EPLT(R2)

which has a power trend.

EPLTðR2Þ MPað Þ ¼ 0:16 EPLTðiÞ" #1:49 ðR2 ¼ 0:94Þ ð10Þ

Fig. 15 shows the proposed correlations found in the present studyand some correlations between average DPI versus elastic modulus

Fig. 11. Correlation between DPI and Dr(%).

Fig. 12. Correlation between DPI and EPLT(i).

Table 4The proposed classification for estimating of Dr by DPI

DPI (mm/blow) Dr(%) Description

N42 b25 Very loose42–23 25–35 Loose23–12 35–50 Medium12–5 50–75 Denseb5 N75 Very dense

200 S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

suggested by other authors (e.g. Pen, 1990; DeBeer, 1990; Konard andLanchance, 2000). Due to the smaller grain size, the values of elasticitymoduli from correlations suggested in this study are smaller thanthose presented by others.

3.3. DPI versus shear modulus (G)

Several methods are available to evaluate the shear modulus ofcoarse-grained and fine grained soils, such as geophysical methods,Plate Load Test (PLT) etc., which are all costly. In the present research,

several correlations between average DPI versus PLT shear moduli(GPLT) were investigated. The best correlation between the average DPIand (GPLT) is presented in Fig.16 and Eq. (11). The results show that the

Fig. 15. Correlations between DPI and E from other authors.

Fig. 14. Correlation between EPLT(i) and EPLT(R2).

Fig. 13. Correlation between DPI and EPLT(R2).

Fig. 16. Correlation between DPI and GPLT.

Fig. 17. Correlation between DPI and KS.

Fig. 18. Correlation between Dr and friction angle.

Table 5The proposed classification for estimating of (ϕ′) by DPI

DPI (mm/blow) ϕ′ Description

N45 b30 Very loose45–25 30–34 Loose25–15 34–36 Medium15–5 36–42 Denseb5 N42 Very dense

201S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203

shear modulus decreases with increasing values of DPI. This correla-tion is exponential with a determination coefficient of 0.93.

GPLT MPað Þ ¼ 75:74= DPIð Þ0:99 R2 ¼ 0:93" #

ð11Þ

3.4. DPI versus modulus of subgrade reaction (KS)

The best correlation between average DPI and KS is presented inFig. 17 and Eq. (12).

KS MN=m3" #¼ 898:36= DPIð Þ0:9 R2 ¼ 0:95

" #ð12Þ

3.5. DPI versus shear strength

Several correlations between relative density (Dr) and frictionangle (ϕ) have been suggested by different authors includingMeyerhof (1959). He has suggested Eq. (13) for a normally con-solidated sand.

/ ¼ 28þ 0:15 Drð Þ ð13Þ

For the results obtained in the present research, the correlationbetween average effective friction angle (ϕ′) and relative density Dr(%)is presented in Fig. 18 and Eq. (14),

/V¼ 26:31þ 0:21 Drð Þ R2 ¼ 0:90" #

ð14Þ

which is similar to Meyerhof's equation (Fig. 18).Several correlations between Standard Penetration Test (SPT)

results and the effective friction angle of uncemented sand (ϕ′) havebeen suggested (e.g. DeMello, 1971). Table 5 presents the proposedclassification for estimating of (ϕ′) fromDPI, using the results obtainedin the present research. The correlation between average DPI andeffective friction angle (ϕ′) is presented in Fig. 19 and Eq. (15).

/VDegð Þ ¼ 52:16= DPIð Þ0:13 R2 ¼ 0:90" #

ð15Þ

4. Summary and conclusions

The DCP is a lightweight device, which can be conveniently usedfor soil investigation up to a depth of 2 m. Therefore, it can easily beused in difficult terrains with poor access. The results of DCP testingcan be used rapidly to assess variability of soil conditions, allowingdifferent layers to be identified. Based on the results of the presentresearch, correlations can be established between DPI and engineer-ing parameters of sandy soils. Statistical approach has been applied tofind the best correlations of the results with a high coefficient ofdetermination (R2). For the results obtained, the determination

coefficients (R2) between DPI and engineering parameters weremostly greater than 0.90. Table 6 shows summary of the equationsobtained in this study. To control the repeatability of the results of DCPtests, values of coefficient of variation (Cv) were calculated. Thiscoefficient varied between 0 and 28.3%. Therefore, it can be concludedthat the results of DCP tests for three relative densities (Dr) can beconsidered as repeatable when compared with the values presentedby Lee et al. (1983).

Acknowledgments

The authors wish to express their deepest gratitude to theauthorities of Tarbiat Modares University for their financial supportof the research and to Mr. M. Zarrabi Rad for his close cooperation inperforming the experimental part of the work.

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Table 6Summary of developed e-questions in this paper

Parameters Equations Typecorrelation

Determinationcoefficient (R2)

Dr–DPI Dr(%)=189.93/(DPI)0.53 Expotential 0.98EPLT(i)–DPI EPLT(i)= (MPa)=55.033/(DPI)0.54 Expotential 0.83EPLT(R2)–DPI EPLT(R2) = (MPa)=53.73/(DPI)0.74 Expotential 0.94EPLT(i)–EPLT(R2) EPLT(R2)(MPa)=0.16(EPLT(i))1.49 Power 0.94GPLT–DPI GPLT(MPa)=75.74/(DPI)0.99 Expotential 0.93Ks–DPI KS(MN/m3)=898.36/(DPI)0.9 Expotential 0.95ϕ′–Dr ϕ′=26.31+0.21(Dr) Linear 0.90ϕ′–DPI ϕ′=(Deg)=52.16/(DPI)0.13 Expotential 0.90

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