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Application of the Dynamic Cone Penetrometer (DCP) for determination of the engineering parameters of sandy soils
Engineering Geology 101 (2008) 195–203
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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o
Application of the Dynamic Cone Penetrometer (DCP) for determination of the engineering parameters of sandy soils S.D. Mohammadi a,⁎, M.R. Nikoudel a, H. Rahimi b, M. Khamehchiyan a a b
Department of Engineering Geology, Tarbiat Modares University, Tehran, Iran Department of Irrigation Engineering, Tehran University, Tehran, Iran
A R T I C L E
I N F O
Article history: Received 20 November 2007 Received in revised form 26 April 2008 Accepted 22 May 2008 Available online 4 June 2008 Keywords: Dynamic Cone Penetrometer (DCP) Poorly graded sandy soil (SP) Dynamic Penetration Index (DPI) Index Parameters Coefficient of variations (Cv)
A B S T R A C T Determination of the in situ engineering properties of foundation materials has always been a challenge for geotechnical 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 DCP device was developed and used for evaluation of the engineering properties of sandy soils in laboratory conditions. 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 direct shear 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 in a mould with 700 mm diameter and 700 mm height. Based on the results of the experiments, the relationships between Dynamic Penetration Index (DPI), relative density (Dr), modulus of elasticity (E), shear modulus (G), modulus of subgrade reaction (KS), and the friction angle of the soil were obtained with a high coefficient of determination (N 90%). The repeatability of the test results was also evaluated by calculating the coefficient 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 soil which its in place structure has not been changed in any way. Such samples are desirable for those laboratory tests which are dependent on the structure of soil, such as shear strength. Unfortunately, several issues make it almost impossible to obtain a truly undisturbed sample, specially in non-cohesive soils. Regarding to these issues, variety of techniques have been developed to perform in situ tests such as dynamic probing. Dynamic probing is a continuous soil investigation technique and is assumed as one of the simplest soil penetration tests. It basically consists of repeatedly driving a metal tipped probe into the ground using a drop weight of fixed mass and travel. Testing is carried out continuously from the ground level to the final penetration depth. The continuous sounding profiles enable easy recognition of dissimilar layers and even thin strata by the observed variation in penetration resistance. The Dynamic Cone Penetrometer (DCP) is a lightweight dynamic penetrometer which is considerably faster and cheaper tool than boring, particularly when the depth of exploration is low and the soils being investigated are not coarse gravel (Sawangsuriya and Edil, 2005). Scala (1959) originally developed the DCP in Australia. Since then, it has been used for site characterization of pavement layers and ⁎ Corresponding author. E-mail address: [email protected] (S.D. Mohammadi). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.05.006
subgrades in South Africa, the United Kingdom, Australia, New Zealand, and several states in the United States, such as California, Florida, Illinois, Minnesota, Kansas, Mississippi, and Texas (AbuFarsakh et al., 2004). Some relationships have been developed between 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 of the DCP to study the inplace engineering properties of sandy soils. 2. Materials and methods In order to achieve the appropriate correlations between the DCP test results and engineering parameters of sandy soils, it was necessary to select suitable sample. The appropriate sampling area was selected based on previous experiences and sampling was performed according to standard methods. Then, the selected samples were shipped to the laboratory and was prepared for the tests as explained in the later sections. 2.1. Geology of the sampling area The sampling area contains four types of lithology belonging to Late Eocene and Quaternary deposits (Fig. 1). The Late Eocene rock covers almost 5% of the land surface and it comprises one type of
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Fig. 1. Geological map of sampling location, modified from Geological Survey of Iran (1998).
lithology which is categorised as Trachyte to Trachyandesite. The Quaternary deposits cover almost 95% of the land surface and it comprises three types of lithology which are categorised as conglomerates, old terrace deposits and young terrace deposits. In the present research, the sampling was performed on the young terrace deposits. Geologically, the young deposits comprise subrounded sand grains containing 5% gravel (Fig. 2). The X-ray analysis has shown that the sandy samples are made of quartz, feldspar, pyroxene and calcite. 2.2. Sample preparation The data used in this paper were obtained from laboratory tests undertaken by the authors at the Geotechnical Engineering Laboratory of Tarbiat Modares University, Tehran, Iran. To prepare the soil samples for testing, alluvial deposits were oven dried and passed through sieve No. 4. Fig. 3 shows the gradiation curves of the original soil and the sample after passing sieve No. 4
Fig. 2. Grains of the used soil in prepared thin section (under PPL, 6× magnification).
which is classified as poorly graded sand (SP) according to the Unified Soil Classification System. The index properties of the soil are shown on Table 1. To achieve a uniform compaction, the sample in the testing mould was compacted in seven 100 mm thick lifts. The soil was dried and compaction effort was applied using a 300 mm vibrating plate in a way that the required density to be achieved. The in place density for each case was controlled using the sand cone method. Details of the tests 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. To achieve a uniform compaction in the circular shear mould of the direct shear machine, tamping by a small circular steel plate with 60 mm diameter was used. To eliminate the effects of pore pressure, all direct shear tests were carried out in dry condition.
Fig. 3. Gradiation curves of alluvial deposited and used soil.
S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203 Table 1 The index properties of used soil Parameter
Value
emax (−) emin (−) Gs (−) γd(max)(KN/m3) γd(min)(KN/m3) Cu (−) Cc (−) value of clay (%) value of silt (%) USCS soil classification
0.97 0.46 2.66 17.85 13.24 1.16 1 0 2 SP
197
steel rod to which the cone is attached has a smaller diameter than the cone (16 mm) to minimize the effect of skin friction. Depth of investigation of DCP is 1 m to 2 m. The number of blows during operation is recorded with depth of penetration. The slope of the curve defining the relationship between number of blows and depth of penetration (in millimeters per blow) at a given linear depth segment is recorded as the DCP penetration index (DPI). DPI for each depth can also be calculated by Eq. (1) (Embacher, 2005): DPI ¼
Piþ1 −Pi Biþ1 −Bi
ð1Þ
Where: Table 2 Testing program for laboratory investigations and different densities for tested soil Dr(%)⁎
Mean of water content (%)
Dry unit weight (gr/cm3)
DCP (number of test)
PLT⁎⁎ (number of test)
Direct shear (number of test)
25 35 50 60 75
0.4 0.4 0.4 0.4 0.4
1.44 1.48 1.55 1.60 1.67
3 3 3 3 3
3 3 3 3 3
7 7 7 7 7
⁎ Relative density ⁎⁎ Plate Load Test
DPI P B
DCP Penetration Index (mm/blow) Penetration at i or i + 1 hammer drops (mm); and blow count at i or i + 1 hammer drops
Analysis of the DCP data must be interpreted, following a standard procedure, to generate a representative value of penetration per blow for the material being tested. This representative value can be obtained by averaging the DPI across the entire penetration depth at each test location. For calculating the representative DPI value for a given penetration depth, two methods are available: (i) arithmetic average; and (ii) weighted average. The arithmetic average can be obtained from Eq. (2) (Edil and Benson, 2005):
2.3. Testing procedures Several tests including Dynamic Cone Penetration (DCP), Plate Load (PLT) and direct shear tests were undertaken on the compacted materials as described in the following sections: 2.3.1. Dynamic Cone Penetration tests The Dynamic Cone Penetrometer (DCP) has been described by ASTM 6951-03 (2003). The typical DCP consists of an 8-kg hammer that drops over a height of 575 mm, which yields a theoretical driving energy of 45 J or 14.3 J/cm2, and drives a 60o cone tip with 20 mm base diameter vertically into the pavement or subgrade layer (Fig. 4). The
DPIavg ¼
∑Ni ðDPIÞ N
ð2Þ
where N is the total number of DPI recorded in a given penetration depth of interest. In the weighted average technique, Eq. (3) can be used (Edil and Benson, 2005): DPIwt:avg ¼
1 N ∑ ½ðDPIÞi ð Z Þi H i
ð3Þ
Where Z is the penetration distance per blow set and H is the overall penetration depth of interest.
Fig. 4. Dynamic Cone Penetrometer (DCP) (Edil and Benson, 2005).
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Fig. 7. Correlation between DPI and mould diameter.
Fig. 5. A schematic diagram of DCP test in testing mould (a) side view (b) plan view.
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 a mould with a diameter and height of 700 mm, respectively (Fig. 5). To have more uniform results, readings were taken around the center of the test mould. The results of DCP tests on samples of different relative densities are presented in Fig. 6. To overcome the effects of the mould side walls, the minimum distance between cone and edge of the testing mould was taken as 225 mm (Abu-Farsakh et al., 2004). To investigate the effects of the mould size on the results, several DCP tests were conducted on the
Fig. 6. Average of DPI versus depth for studied soil at test mould.
moulds of different diameters of 300 mm, 500 mm and 700 mm. Variation of the average DPI values versus different mould diameters are presented in Fig. 7. The results show that with increase of relative density (Dr), the effect of the side wall is more pronounced. This effect is fully negligible for moulds with a diameter grater than 500 mm. On the other hand, a distance of 250 mm between the cone and the edge of the test mould can fully eliminate the mould size effects. Thus, in the present research, all DCP tests were carried out in a mould with a diameter of 700 mm. The repeatability of the DCP test results is an important consideration. To evaluate the repeatability, several tests were carried out. Each testing series included three DCP tests. Fig. 8 shows the results of the three series of tests undertaken for different relative densities (Dr). In this figure, DPI values are converted to NDCP , where NDCP is the number of blow for 100 mm penetration. In order to study the repeatability of the results, it was important to choose a suitable parameter that represents the repeatability. For this purpose, percent of the coefficient of variation (Cv) was employed as an indicator for repeatability. Table 3 shows some soil properties, determined by various standard tests, along with their coefficients of variation reported by various researchers (Lee et al., 1983). The sources of variability in soil properties differ, and accordingly the coefficients of variation differ for different properties (Fakher et al., 2006). The coefficient of variation, Cv, for the results of Standard Penetration Test (N), which is basically a super heavy dynamic probe test, is reported to be between 27% and 85% with a
Fig. 8. Example of the results of tests repeated at mould test (a) for Dr = 25% (b) for Dr = 50% (c) for Dr = 75%.
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Table 3 Coefficient variation for soil engineering tests (Lee et al., 1983) Test
Reported Cv (%)
Recommended standard
Angle of friction (sands) CBR Undrained cohesion (clays) Standard penetration test (SPT) Unconfined compressive strength (clays)
5–15 17–58 20–50 27–85 6–100
10 25 30 30 40
recommended value of 30%, (Lee et al., 1983). The repeatability of SPT test results could be used as a measure of the repeatability of DCP results by comparing Cv values of the two methods. In the present research, the values of Cv have been determined for each depth in each series of the tests. The average value of Cv is 5.6% and its standard deviation is 9.51. In more than 68.7% of the tests, the value of Cv is 0% and in 12.5% of the tests, this value is 20.28%. In the tests undertaken, the values of Cv varied between 0 and 28.3% and for all cases it was less than 30%. Therefore, the results of DCP tests for the three relative densities (Dr) can be considered as 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 European countries for many years. Currently, it is used for evaluation of both rigid and flexible pavements (Abu-Farsakh et al., 2004). The PLT in full or small scale, is sometimes considered as the best means of determining deformation characteristics of the soils, but is only used in exceptional cases due to the costs involved (Bowles, 1997). In the present research, a round plate with 230 mm diameter was used for conducting plate load tests. The PLT was used as a reference test to obtain the strength parameters of the soil under investigation. A loading frame was designed to fit the mould and its support. To perform the test, the bearing plate and hydraulic jack were carefully
Fig. 10. Definition of modulus from PLT (Abu-Farsakh et al., 2004).
placed at the center of the sample under the loading frame (Fig. 9). The hydraulic jack and the supporting frame were able to apply a 60 tons load. For measurement of deformations, dial gauges that are capable of recording a maximum deformation of 25.4 mm (1 in) with an accuracy 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 important deformability parameter for geomaterials. As in the case for other stress–strain tests, different elasticity moduli can be obtained from the PLT. Soil elasticity moduli can be defined as: (1) the initial tangent modulus; (2) the tangent modulus at a given stress level; (3) reloading and unloading modulus; and (4) the secant modulus at a given stress level (Abu-Farsakh et al., 2004). In this study, since the stress–strain
Fig. 9. A schematic diagram of Plate Load Test (PLT) set up (a) side view (b) plan view.
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S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203 Table 4 The proposed classification for estimating of Dr by DPI DPI (mm/blow)
Dr(%)
Description
N42 42–23 23–12 12–5 b5
b25 25–35 35–50 50–75 N75
Very loose Loose Medium Dense Very dense
3.1. DPI versus Dr(%) The relative density is a useful parameter to describe the consistency of sands (Coduto, 2001). Kulhawy and Mayne (1990) suggested an empirical correlation between SPT results and Dr as follows (Eq. (6)). Dr ðkÞ ¼
Fig. 11. Correlation between DPI and Dr(%).
curves had a clear peak point, the initial tangent modulus was determined for all plate load test results. To determine the initial modulus (EPLT(i)), a line was drawn tangent to the initial segment of the stress–strain curve, then an arbitrary point was chosen on the line and the stress and deflection corresponding to this point were determined for calculation of the initial modulus. Fig. 10 describes the deformations and stresses used for determining EPLT(i). A reloading stiffness modulus called EPLT(R2), was also determined for each stress–strain curve. The second parameter which can be calculated from PLT results, is shear modulus (G). Shear modulus is defined as the ratio of shear stress to shear strain (Bowles, 1997) and is calculated from Eq. (4) (Timoshenko and Goodier, 1970): GPLT ¼
qD π ð1−vÞ ρ 8
ð4Þ
where: q= D= ρ= υ=
bearing pressure diameter of the loading plate settlement Poisson's ratio
Since the non-rigid methods consider the effect of local mat deformations on distribution of bearing pressure, it is needed to define the relationship between settlement and bearing pressure. This is usually done using the coefficient of subgrade reaction (Ks). Eq. (5) is used to determine Ks from PLT results (Coudoto, 2004): KS ¼ ΔP=ΔS
ð5Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðN1 Þ60 100 Cp CA COCR
ð6Þ
Where (N1)60 is SPT's N value corrected for field procedure and overburden stress, and Cp, CA and COCR are grain size, aging correction and overconsolidation factors, respectively. In this study, the correlation between average DPI and Dr was investigated. Eq. (7) and Fig.11 suggest a good correlation between these two parameters. The determination coefficient (R2) of Eq. (7) is 0.98. Table 4 shows the proposed classification for estimating of Dr(%) by DPI. Dr ðkÞ ¼ 189:93=ðDPIÞ0:53
R2 ¼ 0:98
3.2. DPI versus modulus of elasticity (E) For the data obtained in this study, the best correlations between average 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 the average DCP penetration rate and PLT reloading modulus (EPLT(R2)) compared to the correlation with EPLT(i). EPLTðiÞ ðMPaÞ ¼ 55:033=ðDPIÞ0:54 R2 ¼ 0:83
ð8Þ
EPLTðR2 Þ ðMPaÞ ¼ 53:73=ðDPIÞ0:74 R2 ¼ 0:94
ð9Þ
Fig. 14 and Eq. (10) show the correlation between EPLT(i) and EPLT(R2) which has a power trend. 1:49 2 ðR ¼ 0:94Þ EPLTðR2 Þ ðMPaÞ ¼ 0:16 EPLTðiÞ
modulus of subgrade reaction applied pressure measured settlement
2.3.3. Direct shear test In 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 ASTMD 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 their correlations with important engineering parameters of the studied soil samples are discussed.
ð10Þ
Fig. 15 shows the proposed correlations found in the present study and some correlations between average DPI versus elastic modulus
where: Ks = ΔP = ΔS =
ð7Þ
Fig. 12. Correlation between DPI and EPLT(i).
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suggested by other authors (e.g. Pen, 1990; DeBeer, 1990; Konard and Lanchance, 2000). Due to the smaller grain size, the values of elasticity moduli from correlations suggested in this study are smaller than those presented by others. 3.3. DPI versus shear modulus (G) Several methods are available to evaluate the shear modulus of coarse-grained and fine grained soils, such as geophysical methods, Plate Load Test (PLT) etc., which are all costly. In the present research,
Fig. 16. Correlation between DPI and GPLT.
Fig. 13. Correlation between DPI and EPLT(R2).
Fig. 17. Correlation between DPI and KS.
Fig. 14. Correlation between EPLT(i) and EPLT(R2).
Fig. 18. Correlation between Dr and friction angle.
several correlations between average DPI versus PLT shear moduli (GPLT) were investigated. The best correlation between the average DPI and (GPLT) is presented in Fig. 16 and Eq. (11). The results show that the Table 5 The proposed classification for estimating of (ϕ′) by DPI
Fig. 15. Correlations between DPI and E from other authors.
DPI (mm/blow)
ϕ′
Description
N45 45–25 25–15 15–5 b5
b30 30–34 34–36 36–42 N42
Very loose Loose Medium Dense Very dense
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shear modulus decreases with increasing values of DPI. This correlation is exponential with a determination coefficient of 0.93.
Table 6 Summary of developed e-questions in this paper
GPLT ðMPaÞ ¼ 75:74=ðDPIÞ0:99 R2 ¼ 0:93
Parameters
Equations
Type correlation
Determination coefficient (R2)
Dr–DPI EPLT(i)–DPI EPLT(R2)–DPI EPLT(i)–EPLT(R2) GPLT–DPI Ks–DPI ϕ′–Dr ϕ′–DPI
Dr(%) = 189.93/(DPI)0.53 EPLT(i) = (MPa) = 55.033/(DPI)0.54 EPLT(R2) = (MPa) = 53.73/(DPI)0.74 EPLT(R2)(MPa) = 0.16(EPLT(i))1.49 GPLT(MPa) = 75.74/(DPI)0.99 KS(MN/m3) = 898.36/(DPI)0.9 ϕ′ = 26.31 + 0.21(Dr) ϕ′ = (Deg) = 52.16/(DPI)0.13
Expotential Expotential Expotential Power Expotential Expotential Linear Expotential
0.98 0.83 0.94 0.94 0.93 0.95 0.90 0.90
ð11Þ
3.4. DPI versus modulus of subgrade reaction (KS) The best correlation between average DPI and KS is presented in Fig. 17 and Eq. (12). KS MN=m3 ¼ 898:36=ðDPIÞ0:9 R2 ¼ 0:95
ð12Þ
3.5. DPI versus shear strength Several correlations between relative density (Dr) and friction angle (ϕ) have been suggested by different authors including Meyerhof (1959). He has suggested Eq. (13) for a normally consolidated sand. / ¼ 28 þ 0:15ðDr Þ
ð13Þ
For the results obtained in the present research, the correlation between average effective friction angle (ϕ′) and relative density Dr(%) is presented in Fig. 18 and Eq. (14), / V¼ 26:31 þ 0:21ðDr 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′) have been suggested (e.g. DeMello, 1971). Table 5 presents the proposed classification for estimating of (ϕ′) from DPI, using the results obtained in the present research. The correlation between average DPI and effective friction angle (ϕ′) is presented in Fig. 19 and Eq. (15). / VðDegÞ ¼ 52:16=ðDPIÞ0:13 R2 ¼ 0:90
ð15Þ
4. Summary and conclusions The DCP is a lightweight device, which can be conveniently used for soil investigation up to a depth of 2 m. Therefore, it can easily be used in difficult terrains with poor access. The results of DCP testing can be used rapidly to assess variability of soil conditions, allowing different layers to be identified. Based on the results of the present research, correlations can be established between DPI and engineering parameters of sandy soils. Statistical approach has been applied to find the best correlations of the results with a high coefficient of determination (R2). For the results obtained, the determination
Fig. 19. Correlation between DPI and friction angle.
coefficients (R2) between DPI and engineering parameters were mostly greater than 0.90. Table 6 shows summary of the equations obtained in this study. To control the repeatability of the results of DCP tests, values of coefficient of variation (Cv) were calculated. This coefficient varied between 0 and 28.3%. Therefore, it can be concluded that the results of DCP tests for three relative densities (Dr) can be considered as repeatable when compared with the values presented by Lee et al. (1983). Acknowledgments The authors wish to express their deepest gratitude to the authorities of Tarbiat Modares University for their financial support of the research and to Mr. M. Zarrabi Rad for his close cooperation in performing the experimental part of the work. References Abu-farsakh, M., Khalid Alshibi, P.E., Nazzal, M., Seyman, E., 2004. Assessment of in-situ test technology for construction control of base courses and embankments. Report No, FHWA/LA.04/385. Louisiana Transportation Research Center. American Society of Testing Materials, 2003. Standard Test Method for use of the Dynamic Cone Penetrometer in Shallow Pavement Applications (D 6951-03). ASTM International, West Conshohocken, PA. American Society of Testing and Materials, 1998. Standard test method for repetitive static plate load tests of soils and flexible pavement components, for use in evaluation and design of airport and highway pavements (D1195-93). Annual Book of ASTM Standards 04.08, 110–113. American Society of Testing and Materials, 2000. Standard test method for direct shear test of under drained conditions (D3080-98). Annual Book of ASTM Standards 04.08, 894–904. Bowles, J.E., 1997. Foundation Analysis and Design, 5th edition. McGraw-Hill International Editions. Chen, D.H., Wang, J.N., Bilyeu, J., 2001. Application of the DCP in Evaluation of Base and Subgrade Layers, 80th Annual Meeting of Transportation Research Board, Washington, D.C. Coduto, P.D., 2001. Foundation Design, Principal and Practices, Second Edition. Prentice Hall, New Jersey. De Beer, M., 1990. In: Blight, et al. (Ed.), Use of Dynamic Cone Penetrometer (DCP) in the Design of Road Structures. Geotechnics in African Environment, Balkema, Rotterdam. De Mello, V., 1971. The standard penetration test—a state of the art report. Proceedings of the Fourth Pan-American Conference on Soil Mechanics and Foundation Engineering, San Juan, Puerto Rico, pp. 1–86. Edill, T.B., Benson, C.H., 2005. Investigation of the DCP and SSG as Alternative Methods to Determine Subgrade Stability. Department of Civil and Environmental Engineering, University of Wisconsin-Madison. Embacher, R.A., 2005. Duration of Spring-thaw Recovery for Aggregate-surfaced Roads, TRB Annual Meeting. American Engineering Testing, Inc. Fakher, A., Khodaparast, M., Jones, C.J.F.P., 2006. The use of the Mackintosh Probe for site investigation in soft soils. Quarterly Journal of Engineering Geology and Hydrogeology 39, 189–196. Geological Survey of Iran, 1998. Quadrant Map of Tehran, Scale 1:100000, Tehran, Iran. Karunaprema, K.A.K., Edirisinghe, A.G.H.J., 2002. A laboratory study to establish some useful relationships for the use of Dynamic Cone Penetrometer. EJEG paper 0228. Konard, J.-M., Lachance, D., 2000. Mechanical properties of unbound aggregates from DCP and Plate Load Tests. Proceedings of the Fifth International Conference on unbound aggregate in roads, Nottingham, United Kingdom. Kulhawy, F.H., Mayne, P.W., 1990. Manual on Estimating Soil Properties for Foundation Design. Electric Power Institute, Palo Alto, California. Lee, I.K., White, W., Ingles, O.G., 1983. Geotechnical Engineering. Copp Clark Pitman, Inc., pp. 57–89.
S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203 Meyerhof, G.G., 1959. Compaction of sands and the bearing capacity of piles. JSMFD, ASCE 85, 1–29 SM6. Mohammadi, S.D., Nikudel, M.R., Khamehchiyan, M., 2007. The use of Dynamic Cone Penetrometer (DCP) to determine some useful relationships for sandy and clayey soils. Proceeding of the First Sri Lankan Geotechnical Society (SLGS), International Conference on Soil and Rock Engineering, Colombo. Nazzal, M.D., 2002. Field evaluation of in-situ test technology for Qc/Qa during construction of pavement layers and embankments, MSc thesis, Birzeit University, Birzeit, West Bank. Pen, C.K., 1990. An assessment of the available methods of analysis for estimating the elastic moduli of road pavements. Proceeding of Third Int. Conf. on Bearing Capacity of Roads and Airfields, Trondheim. Rahim, A.M., George, K.P., 2004. Dynamic cone penetrometer to estimate subgrade resilient modulus for low volume roads design. Proc. 2nd Int. Conf. Geotechnical and Geophysical Site Characterization; Porto, pp. 367–371.
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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o
Application of the Dynamic Cone Penetrometer (DCP) for determination of the engineering parameters of sandy soils S.D. Mohammadi a,⁎, M.R. Nikoudel a, H. Rahimi b, M. Khamehchiyan a a b
Department of Engineering Geology, Tarbiat Modares University, Tehran, Iran Department of Irrigation Engineering, Tehran University, Tehran, Iran
A R T I C L E
I N F O
Article history: Received 20 November 2007 Received in revised form 26 April 2008 Accepted 22 May 2008 Available online 4 June 2008 Keywords: Dynamic Cone Penetrometer (DCP) Poorly graded sandy soil (SP) Dynamic Penetration Index (DPI) Index Parameters Coefficient of variations (Cv)
A B S T R A C T Determination of the in situ engineering properties of foundation materials has always been a challenge for geotechnical 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 DCP device was developed and used for evaluation of the engineering properties of sandy soils in laboratory conditions. 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 direct shear 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 in a mould with 700 mm diameter and 700 mm height. Based on the results of the experiments, the relationships between Dynamic Penetration Index (DPI), relative density (Dr), modulus of elasticity (E), shear modulus (G), modulus of subgrade reaction (KS), and the friction angle of the soil were obtained with a high coefficient of determination (N 90%). The repeatability of the test results was also evaluated by calculating the coefficient 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 soil which its in place structure has not been changed in any way. Such samples are desirable for those laboratory tests which are dependent on the structure of soil, such as shear strength. Unfortunately, several issues make it almost impossible to obtain a truly undisturbed sample, specially in non-cohesive soils. Regarding to these issues, variety of techniques have been developed to perform in situ tests such as dynamic probing. Dynamic probing is a continuous soil investigation technique and is assumed as one of the simplest soil penetration tests. It basically consists of repeatedly driving a metal tipped probe into the ground using a drop weight of fixed mass and travel. Testing is carried out continuously from the ground level to the final penetration depth. The continuous sounding profiles enable easy recognition of dissimilar layers and even thin strata by the observed variation in penetration resistance. The Dynamic Cone Penetrometer (DCP) is a lightweight dynamic penetrometer which is considerably faster and cheaper tool than boring, particularly when the depth of exploration is low and the soils being investigated are not coarse gravel (Sawangsuriya and Edil, 2005). Scala (1959) originally developed the DCP in Australia. Since then, it has been used for site characterization of pavement layers and ⁎ Corresponding author. E-mail address: [email protected] (S.D. Mohammadi). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.05.006
subgrades in South Africa, the United Kingdom, Australia, New Zealand, and several states in the United States, such as California, Florida, Illinois, Minnesota, Kansas, Mississippi, and Texas (AbuFarsakh et al., 2004). Some relationships have been developed between 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 of the DCP to study the inplace engineering properties of sandy soils. 2. Materials and methods In order to achieve the appropriate correlations between the DCP test results and engineering parameters of sandy soils, it was necessary to select suitable sample. The appropriate sampling area was selected based on previous experiences and sampling was performed according to standard methods. Then, the selected samples were shipped to the laboratory and was prepared for the tests as explained in the later sections. 2.1. Geology of the sampling area The sampling area contains four types of lithology belonging to Late Eocene and Quaternary deposits (Fig. 1). The Late Eocene rock covers almost 5% of the land surface and it comprises one type of
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Fig. 1. Geological map of sampling location, modified from Geological Survey of Iran (1998).
lithology which is categorised as Trachyte to Trachyandesite. The Quaternary deposits cover almost 95% of the land surface and it comprises three types of lithology which are categorised as conglomerates, old terrace deposits and young terrace deposits. In the present research, the sampling was performed on the young terrace deposits. Geologically, the young deposits comprise subrounded sand grains containing 5% gravel (Fig. 2). The X-ray analysis has shown that the sandy samples are made of quartz, feldspar, pyroxene and calcite. 2.2. Sample preparation The data used in this paper were obtained from laboratory tests undertaken by the authors at the Geotechnical Engineering Laboratory of Tarbiat Modares University, Tehran, Iran. To prepare the soil samples for testing, alluvial deposits were oven dried and passed through sieve No. 4. Fig. 3 shows the gradiation curves of the original soil and the sample after passing sieve No. 4
Fig. 2. Grains of the used soil in prepared thin section (under PPL, 6× magnification).
which is classified as poorly graded sand (SP) according to the Unified Soil Classification System. The index properties of the soil are shown on Table 1. To achieve a uniform compaction, the sample in the testing mould was compacted in seven 100 mm thick lifts. The soil was dried and compaction effort was applied using a 300 mm vibrating plate in a way that the required density to be achieved. The in place density for each case was controlled using the sand cone method. Details of the tests 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. To achieve a uniform compaction in the circular shear mould of the direct shear machine, tamping by a small circular steel plate with 60 mm diameter was used. To eliminate the effects of pore pressure, all direct shear tests were carried out in dry condition.
Fig. 3. Gradiation curves of alluvial deposited and used soil.
S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203 Table 1 The index properties of used soil Parameter
Value
emax (−) emin (−) Gs (−) γd(max)(KN/m3) γd(min)(KN/m3) Cu (−) Cc (−) value of clay (%) value of silt (%) USCS soil classification
0.97 0.46 2.66 17.85 13.24 1.16 1 0 2 SP
197
steel rod to which the cone is attached has a smaller diameter than the cone (16 mm) to minimize the effect of skin friction. Depth of investigation of DCP is 1 m to 2 m. The number of blows during operation is recorded with depth of penetration. The slope of the curve defining the relationship between number of blows and depth of penetration (in millimeters per blow) at a given linear depth segment is recorded as the DCP penetration index (DPI). DPI for each depth can also be calculated by Eq. (1) (Embacher, 2005): DPI ¼
Piþ1 −Pi Biþ1 −Bi
ð1Þ
Where: Table 2 Testing program for laboratory investigations and different densities for tested soil Dr(%)⁎
Mean of water content (%)
Dry unit weight (gr/cm3)
DCP (number of test)
PLT⁎⁎ (number of test)
Direct shear (number of test)
25 35 50 60 75
0.4 0.4 0.4 0.4 0.4
1.44 1.48 1.55 1.60 1.67
3 3 3 3 3
3 3 3 3 3
7 7 7 7 7
⁎ Relative density ⁎⁎ Plate Load Test
DPI P B
DCP Penetration Index (mm/blow) Penetration at i or i + 1 hammer drops (mm); and blow count at i or i + 1 hammer drops
Analysis of the DCP data must be interpreted, following a standard procedure, to generate a representative value of penetration per blow for the material being tested. This representative value can be obtained by averaging the DPI across the entire penetration depth at each test location. For calculating the representative DPI value for a given penetration depth, two methods are available: (i) arithmetic average; and (ii) weighted average. The arithmetic average can be obtained from Eq. (2) (Edil and Benson, 2005):
2.3. Testing procedures Several tests including Dynamic Cone Penetration (DCP), Plate Load (PLT) and direct shear tests were undertaken on the compacted materials as described in the following sections: 2.3.1. Dynamic Cone Penetration tests The Dynamic Cone Penetrometer (DCP) has been described by ASTM 6951-03 (2003). The typical DCP consists of an 8-kg hammer that drops over a height of 575 mm, which yields a theoretical driving energy of 45 J or 14.3 J/cm2, and drives a 60o cone tip with 20 mm base diameter vertically into the pavement or subgrade layer (Fig. 4). The
DPIavg ¼
∑Ni ðDPIÞ N
ð2Þ
where N is the total number of DPI recorded in a given penetration depth of interest. In the weighted average technique, Eq. (3) can be used (Edil and Benson, 2005): DPIwt:avg ¼
1 N ∑ ½ðDPIÞi ð Z Þi H i
ð3Þ
Where Z is the penetration distance per blow set and H is the overall penetration depth of interest.
Fig. 4. Dynamic Cone Penetrometer (DCP) (Edil and Benson, 2005).
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Fig. 7. Correlation between DPI and mould diameter.
Fig. 5. A schematic diagram of DCP test in testing mould (a) side view (b) plan view.
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 a mould with a diameter and height of 700 mm, respectively (Fig. 5). To have more uniform results, readings were taken around the center of the test mould. The results of DCP tests on samples of different relative densities are presented in Fig. 6. To overcome the effects of the mould side walls, the minimum distance between cone and edge of the testing mould was taken as 225 mm (Abu-Farsakh et al., 2004). To investigate the effects of the mould size on the results, several DCP tests were conducted on the
Fig. 6. Average of DPI versus depth for studied soil at test mould.
moulds of different diameters of 300 mm, 500 mm and 700 mm. Variation of the average DPI values versus different mould diameters are presented in Fig. 7. The results show that with increase of relative density (Dr), the effect of the side wall is more pronounced. This effect is fully negligible for moulds with a diameter grater than 500 mm. On the other hand, a distance of 250 mm between the cone and the edge of the test mould can fully eliminate the mould size effects. Thus, in the present research, all DCP tests were carried out in a mould with a diameter of 700 mm. The repeatability of the DCP test results is an important consideration. To evaluate the repeatability, several tests were carried out. Each testing series included three DCP tests. Fig. 8 shows the results of the three series of tests undertaken for different relative densities (Dr). In this figure, DPI values are converted to NDCP , where NDCP is the number of blow for 100 mm penetration. In order to study the repeatability of the results, it was important to choose a suitable parameter that represents the repeatability. For this purpose, percent of the coefficient of variation (Cv) was employed as an indicator for repeatability. Table 3 shows some soil properties, determined by various standard tests, along with their coefficients of variation reported by various researchers (Lee et al., 1983). The sources of variability in soil properties differ, and accordingly the coefficients of variation differ for different properties (Fakher et al., 2006). The coefficient of variation, Cv, for the results of Standard Penetration Test (N), which is basically a super heavy dynamic probe test, is reported to be between 27% and 85% with a
Fig. 8. Example of the results of tests repeated at mould test (a) for Dr = 25% (b) for Dr = 50% (c) for Dr = 75%.
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Table 3 Coefficient variation for soil engineering tests (Lee et al., 1983) Test
Reported Cv (%)
Recommended standard
Angle of friction (sands) CBR Undrained cohesion (clays) Standard penetration test (SPT) Unconfined compressive strength (clays)
5–15 17–58 20–50 27–85 6–100
10 25 30 30 40
recommended value of 30%, (Lee et al., 1983). The repeatability of SPT test results could be used as a measure of the repeatability of DCP results by comparing Cv values of the two methods. In the present research, the values of Cv have been determined for each depth in each series of the tests. The average value of Cv is 5.6% and its standard deviation is 9.51. In more than 68.7% of the tests, the value of Cv is 0% and in 12.5% of the tests, this value is 20.28%. In the tests undertaken, the values of Cv varied between 0 and 28.3% and for all cases it was less than 30%. Therefore, the results of DCP tests for the three relative densities (Dr) can be considered as 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 European countries for many years. Currently, it is used for evaluation of both rigid and flexible pavements (Abu-Farsakh et al., 2004). The PLT in full or small scale, is sometimes considered as the best means of determining deformation characteristics of the soils, but is only used in exceptional cases due to the costs involved (Bowles, 1997). In the present research, a round plate with 230 mm diameter was used for conducting plate load tests. The PLT was used as a reference test to obtain the strength parameters of the soil under investigation. A loading frame was designed to fit the mould and its support. To perform the test, the bearing plate and hydraulic jack were carefully
Fig. 10. Definition of modulus from PLT (Abu-Farsakh et al., 2004).
placed at the center of the sample under the loading frame (Fig. 9). The hydraulic jack and the supporting frame were able to apply a 60 tons load. For measurement of deformations, dial gauges that are capable of recording a maximum deformation of 25.4 mm (1 in) with an accuracy 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 important deformability parameter for geomaterials. As in the case for other stress–strain tests, different elasticity moduli can be obtained from the PLT. Soil elasticity moduli can be defined as: (1) the initial tangent modulus; (2) the tangent modulus at a given stress level; (3) reloading and unloading modulus; and (4) the secant modulus at a given stress level (Abu-Farsakh et al., 2004). In this study, since the stress–strain
Fig. 9. A schematic diagram of Plate Load Test (PLT) set up (a) side view (b) plan view.
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S.D. Mohammadi et al. / Engineering Geology 101 (2008) 195–203 Table 4 The proposed classification for estimating of Dr by DPI DPI (mm/blow)
Dr(%)
Description
N42 42–23 23–12 12–5 b5
b25 25–35 35–50 50–75 N75
Very loose Loose Medium Dense Very dense
3.1. DPI versus Dr(%) The relative density is a useful parameter to describe the consistency of sands (Coduto, 2001). Kulhawy and Mayne (1990) suggested an empirical correlation between SPT results and Dr as follows (Eq. (6)). Dr ðkÞ ¼
Fig. 11. Correlation between DPI and Dr(%).
curves had a clear peak point, the initial tangent modulus was determined for all plate load test results. To determine the initial modulus (EPLT(i)), a line was drawn tangent to the initial segment of the stress–strain curve, then an arbitrary point was chosen on the line and the stress and deflection corresponding to this point were determined for calculation of the initial modulus. Fig. 10 describes the deformations and stresses used for determining EPLT(i). A reloading stiffness modulus called EPLT(R2), was also determined for each stress–strain curve. The second parameter which can be calculated from PLT results, is shear modulus (G). Shear modulus is defined as the ratio of shear stress to shear strain (Bowles, 1997) and is calculated from Eq. (4) (Timoshenko and Goodier, 1970): GPLT ¼
qD π ð1−vÞ ρ 8
ð4Þ
where: q= D= ρ= υ=
bearing pressure diameter of the loading plate settlement Poisson's ratio
Since the non-rigid methods consider the effect of local mat deformations on distribution of bearing pressure, it is needed to define the relationship between settlement and bearing pressure. This is usually done using the coefficient of subgrade reaction (Ks). Eq. (5) is used to determine Ks from PLT results (Coudoto, 2004): KS ¼ ΔP=ΔS
ð5Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðN1 Þ60 100 Cp CA COCR
ð6Þ
Where (N1)60 is SPT's N value corrected for field procedure and overburden stress, and Cp, CA and COCR are grain size, aging correction and overconsolidation factors, respectively. In this study, the correlation between average DPI and Dr was investigated. Eq. (7) and Fig.11 suggest a good correlation between these two parameters. The determination coefficient (R2) of Eq. (7) is 0.98. Table 4 shows the proposed classification for estimating of Dr(%) by DPI. Dr ðkÞ ¼ 189:93=ðDPIÞ0:53
R2 ¼ 0:98
3.2. DPI versus modulus of elasticity (E) For the data obtained in this study, the best correlations between average 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 the average DCP penetration rate and PLT reloading modulus (EPLT(R2)) compared to the correlation with EPLT(i). EPLTðiÞ ðMPaÞ ¼ 55:033=ðDPIÞ0:54 R2 ¼ 0:83
ð8Þ
EPLTðR2 Þ ðMPaÞ ¼ 53:73=ðDPIÞ0:74 R2 ¼ 0:94
ð9Þ
Fig. 14 and Eq. (10) show the correlation between EPLT(i) and EPLT(R2) which has a power trend. 1:49 2 ðR ¼ 0:94Þ EPLTðR2 Þ ðMPaÞ ¼ 0:16 EPLTðiÞ
modulus of subgrade reaction applied pressure measured settlement
2.3.3. Direct shear test In 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 ASTMD 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 their correlations with important engineering parameters of the studied soil samples are discussed.
ð10Þ
Fig. 15 shows the proposed correlations found in the present study and some correlations between average DPI versus elastic modulus
where: Ks = ΔP = ΔS =
ð7Þ
Fig. 12. Correlation between DPI and EPLT(i).
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suggested by other authors (e.g. Pen, 1990; DeBeer, 1990; Konard and Lanchance, 2000). Due to the smaller grain size, the values of elasticity moduli from correlations suggested in this study are smaller than those presented by others. 3.3. DPI versus shear modulus (G) Several methods are available to evaluate the shear modulus of coarse-grained and fine grained soils, such as geophysical methods, Plate Load Test (PLT) etc., which are all costly. In the present research,
Fig. 16. Correlation between DPI and GPLT.
Fig. 13. Correlation between DPI and EPLT(R2).
Fig. 17. Correlation between DPI and KS.
Fig. 14. Correlation between EPLT(i) and EPLT(R2).
Fig. 18. Correlation between Dr and friction angle.
several correlations between average DPI versus PLT shear moduli (GPLT) were investigated. The best correlation between the average DPI and (GPLT) is presented in Fig. 16 and Eq. (11). The results show that the Table 5 The proposed classification for estimating of (ϕ′) by DPI
Fig. 15. Correlations between DPI and E from other authors.
DPI (mm/blow)
ϕ′
Description
N45 45–25 25–15 15–5 b5
b30 30–34 34–36 36–42 N42
Very loose Loose Medium Dense Very dense
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shear modulus decreases with increasing values of DPI. This correlation is exponential with a determination coefficient of 0.93.
Table 6 Summary of developed e-questions in this paper
GPLT ðMPaÞ ¼ 75:74=ðDPIÞ0:99 R2 ¼ 0:93
Parameters
Equations
Type correlation
Determination coefficient (R2)
Dr–DPI EPLT(i)–DPI EPLT(R2)–DPI EPLT(i)–EPLT(R2) GPLT–DPI Ks–DPI ϕ′–Dr ϕ′–DPI
Dr(%) = 189.93/(DPI)0.53 EPLT(i) = (MPa) = 55.033/(DPI)0.54 EPLT(R2) = (MPa) = 53.73/(DPI)0.74 EPLT(R2)(MPa) = 0.16(EPLT(i))1.49 GPLT(MPa) = 75.74/(DPI)0.99 KS(MN/m3) = 898.36/(DPI)0.9 ϕ′ = 26.31 + 0.21(Dr) ϕ′ = (Deg) = 52.16/(DPI)0.13
Expotential Expotential Expotential Power Expotential Expotential Linear Expotential
0.98 0.83 0.94 0.94 0.93 0.95 0.90 0.90
ð11Þ
3.4. DPI versus modulus of subgrade reaction (KS) The best correlation between average DPI and KS is presented in Fig. 17 and Eq. (12). KS MN=m3 ¼ 898:36=ðDPIÞ0:9 R2 ¼ 0:95
ð12Þ
3.5. DPI versus shear strength Several correlations between relative density (Dr) and friction angle (ϕ) have been suggested by different authors including Meyerhof (1959). He has suggested Eq. (13) for a normally consolidated sand. / ¼ 28 þ 0:15ðDr Þ
ð13Þ
For the results obtained in the present research, the correlation between average effective friction angle (ϕ′) and relative density Dr(%) is presented in Fig. 18 and Eq. (14), / V¼ 26:31 þ 0:21ðDr 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′) have been suggested (e.g. DeMello, 1971). Table 5 presents the proposed classification for estimating of (ϕ′) from DPI, using the results obtained in the present research. The correlation between average DPI and effective friction angle (ϕ′) is presented in Fig. 19 and Eq. (15). / VðDegÞ ¼ 52:16=ðDPIÞ0:13 R2 ¼ 0:90
ð15Þ
4. Summary and conclusions The DCP is a lightweight device, which can be conveniently used for soil investigation up to a depth of 2 m. Therefore, it can easily be used in difficult terrains with poor access. The results of DCP testing can be used rapidly to assess variability of soil conditions, allowing different layers to be identified. Based on the results of the present research, correlations can be established between DPI and engineering parameters of sandy soils. Statistical approach has been applied to find the best correlations of the results with a high coefficient of determination (R2). For the results obtained, the determination
Fig. 19. Correlation between DPI and friction angle.
coefficients (R2) between DPI and engineering parameters were mostly greater than 0.90. Table 6 shows summary of the equations obtained in this study. To control the repeatability of the results of DCP tests, values of coefficient of variation (Cv) were calculated. This coefficient varied between 0 and 28.3%. Therefore, it can be concluded that the results of DCP tests for three relative densities (Dr) can be considered as repeatable when compared with the values presented by Lee et al. (1983). Acknowledgments The authors wish to express their deepest gratitude to the authorities of Tarbiat Modares University for their financial support of the research and to Mr. M. Zarrabi Rad for his close cooperation in performing the experimental part of the work. References Abu-farsakh, M., Khalid Alshibi, P.E., Nazzal, M., Seyman, E., 2004. Assessment of in-situ test technology for construction control of base courses and embankments. Report No, FHWA/LA.04/385. Louisiana Transportation Research Center. American Society of Testing Materials, 2003. Standard Test Method for use of the Dynamic Cone Penetrometer in Shallow Pavement Applications (D 6951-03). ASTM International, West Conshohocken, PA. American Society of Testing and Materials, 1998. Standard test method for repetitive static plate load tests of soils and flexible pavement components, for use in evaluation and design of airport and highway pavements (D1195-93). Annual Book of ASTM Standards 04.08, 110–113. American Society of Testing and Materials, 2000. Standard test method for direct shear test of under drained conditions (D3080-98). Annual Book of ASTM Standards 04.08, 894–904. Bowles, J.E., 1997. Foundation Analysis and Design, 5th edition. McGraw-Hill International Editions. Chen, D.H., Wang, J.N., Bilyeu, J., 2001. Application of the DCP in Evaluation of Base and Subgrade Layers, 80th Annual Meeting of Transportation Research Board, Washington, D.C. Coduto, P.D., 2001. Foundation Design, Principal and Practices, Second Edition. Prentice Hall, New Jersey. De Beer, M., 1990. In: Blight, et al. (Ed.), Use of Dynamic Cone Penetrometer (DCP) in the Design of Road Structures. Geotechnics in African Environment, Balkema, Rotterdam. De Mello, V., 1971. The standard penetration test—a state of the art report. Proceedings of the Fourth Pan-American Conference on Soil Mechanics and Foundation Engineering, San Juan, Puerto Rico, pp. 1–86. Edill, T.B., Benson, C.H., 2005. Investigation of the DCP and SSG as Alternative Methods to Determine Subgrade Stability. Department of Civil and Environmental Engineering, University of Wisconsin-Madison. Embacher, R.A., 2005. Duration of Spring-thaw Recovery for Aggregate-surfaced Roads, TRB Annual Meeting. American Engineering Testing, Inc. Fakher, A., Khodaparast, M., Jones, C.J.F.P., 2006. The use of the Mackintosh Probe for site investigation in soft soils. Quarterly Journal of Engineering Geology and Hydrogeology 39, 189–196. Geological Survey of Iran, 1998. Quadrant Map of Tehran, Scale 1:100000, Tehran, Iran. Karunaprema, K.A.K., Edirisinghe, A.G.H.J., 2002. A laboratory study to establish some useful relationships for the use of Dynamic Cone Penetrometer. EJEG paper 0228. Konard, J.-M., Lachance, D., 2000. Mechanical properties of unbound aggregates from DCP and Plate Load Tests. Proceedings of the Fifth International Conference on unbound aggregate in roads, Nottingham, United Kingdom. Kulhawy, F.H., Mayne, P.W., 1990. Manual on Estimating Soil Properties for Foundation Design. Electric Power Institute, Palo Alto, California. Lee, I.K., White, W., Ingles, O.G., 1983. Geotechnical Engineering. Copp Clark Pitman, Inc., pp. 57–89.
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