Available online at www.sciencedirect.com
Particuology 7 (2009) 83–91
Numerical and experimental direct shear tests for coarse-grained soils Ahad Bagherzadeh-Khalkhali ∗ , Ali Asghar Mirghasemi School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran Received 15 February 2008; accepted 18 November 2008
Abstract The presence of particles larger than the permissible dimensions of conventional laboratory specimens causes difficulty in the determination of shear strength of coarse-grained soils. In this research, the influence of particle size on shear strength of coarse-grained soils was investigated by resorting to experimental tests in different scale and numerical simulations based on discrete element method (DEM). Experimental tests on such soil specimens were based on using the techniques designated as “parallel” and “scalping” to prepare gradation of samples in view of the limitation of laboratory specimen size. As a second approach, the direct shear test was numerically simulated on assemblies of elliptical particles. The behaviors of samples under experimental and numerical tests are presented and compared, indicating that the modification of sample gradation has a significant influence on the mechanical properties of coarse-grained soils. It is noted that the shear strengths of samples produced by the scalping method are higher than samples by the parallel method. The scalping method for preparing specimens for direct shear test is therefore recommended. The micromechanical behavior of assemblies under direct shear test is also discussed and the effects of stress level on sample behavior are investigated. © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Discrete element method; Direct shear test; Micromechanics; Coarse-grained soil; Shear strength
1. Introduction Experimental tests on coarse-grained soils always involve difficulties, and it is often necessary to remove large particles due to dimensional limitation of laboratory specimens. Marsal (1967, 1973), Marachi, Chan, and Bolton (1972) and Varadarajan, Sharma, Venkatachalam, and Gupta (2003) attempted to investigate coarse-grained soil properties by experimental tests on reduced-particle-size samples, and presented a positive relationship between maximum particle size and the mobilized internal friction angle. Marachi et al. (1972) and Charles and Watts (1980), indicated that the influence of maximum particle size is not clearly understood, while Varadarajan et al. (2003), using large-scale triaxial test on rockfill, found that the friction angle increases when the particle size of sample increases. This article investigated the effects of particle size on macro and micro mechanical behavior of coarse-grained soils, using both experimental tests and numerical simulations,
∗ Corresponding author at: School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. Tel.: +98 21 8478 2066/912 102 3519; fax: +98 21 8877 6648. E-mail address:
[email protected] (A. Bagherzadeh-Khalkhali).
on a series of both small- (6 cm × 6 cm × 2 cm) and large(30 cm × 30 cm × 15 cm) scale direct shear tests on selected coarse-grained soils to determine the effect of stress level on the relationship between particle size and friction angle and behavior of samples. Parallel numerical models of the samples as assemblages of distinct particles under direct shear test were formulated using the discrete element method (DEM), to acquire qualitative information on the micro and macroscopic features of the particle assemblies. In an assembly of particles, each particle interacts with its neighbors through particle-toparticle contacts, as was noted by Cundall, Marti, Beresford, Last, and Asgain (1978) in their geotechnical study on the dynamic behavior of rock masses and numerical simulation of granular materials (Cundall & Strack, 1979). These results presented the influence of particle gradation and stress level on shear strength of coarse-grained soils under direct shear test using the program ELLIPSE originally developed by Rothenburg and Bathurst (1992) for assemblies of two-dimensional ellipticalshaped particles (Bagherzadeh-khalkhali & Mirghasemi, 2004). There are four different methods for preparing laboratory specimens; namely, parallel gradation technique (Lowe, 1964), scalping method (Zeller & Wullimann, 1957), quadratic gradation curve method (Fumagalli, 1969) and replacement technique (Frost, 1973). The first two methods, commonly used
1674-2001/$ – see inside back cover © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2008.11.006
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A. Bagherzadeh-Khalkhali, A.A. Mirghasemi / Particuology 7 (2009) 83–91
Fig. 1. Particle size distribution of experimental test samples.
by engineers, were adopted to investigate the effect of particle size on direct shear test. In the parallel gradation technique, the reduced-particle-size laboratory specimens were formed with size distributions parallel to that of the original sampled material. In the scalping method, all particles considered oversize were removed (scalped) from the original material. These techniques were used to determine the gradation of specimens in the tests and the numerical simulations related to the scale of shear boxes. In this way the effect of sample preparation methods in direct shear tests could be investigated. 2. Test programs Five samples with different grain size distributions were used in this research. The original sample was taken from coarsegrained soil of Tehran, the capital of Iran. The first sample (Sample 0) was modeled on the original size distribution of the soil. The remaining four reduced-particle-size specimens were prepared by using both modification techniques, parallel and scalping, on the basis of the dimensions of the shear boxes used. Figs. 1 and 2 show the particle size distributions used for both experimental and numerical tests. To investigate the effect of
stress level on shear behavior of the samples, tests were carried out under the normal stress of 1, 2 and 3 kg/cm2 (98.1, 196.2 and 294.3 kPa; designated as T1 , T2 and T3 , respectively). Table 1 shows the tests with different normal stresses. 2.1. Experimental tests According to two available small- and large-scale shear boxes, scalping and parallel methods were used to modify the gradation of the sample for each box. A shear box with 6 cm × 6 cm area was used for Samples 2 and 4, and a large shear box (30 cm × 30 cm) was selected for Samples 1 and 3. The maximum particle sizes of samples were selected based on the dimension of the boxes according to ASTM-D3080: 4.76 mm (sieve No. 4) for Samples 2 and 4 and 25.4 mm (1 in. sieve) for other two samples. Table 2 presents the properties of the samples to be tested in laboratory and simulated by DEM. Tests were carried out under consolidated drained condition, and the remolded method of Lambe and William (1951) was used for preparing samples, which was claimed to have negligible influence on changing the real shear resistance of coarse-grained soil samples. Relative densities of the remolded samples were over 95% (Table 3); that is, the tested samples were all dense soil. All direct shear tests were carried out in accordance with ASTM-D3080 (1998). 2.2. Numerical simulations In numerical simulation, all samples were prepared to simulate experimental samples. However due to limitation of the Table 1 Normal stresses employed in numerical and experimental direct shear tests.
Fig. 2. Size distribution of numerically simulated samples.
Test number
Applied vertical stress kg/cm2 (kPa)
Test 1 (T1 ) Test 2 (T2 ) Test 3 (T3 )
σ v = 1 (98.1) σ v = 2 (196.2) σ v = 3 (294.3)
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Table 2 Properties of samples. Samples
Modification technique
Numerical simulations Maximum particle size (mm)
Experimental tests Maximum particle size (mm)
Sample 0 Sample 1 Sample 2 Sample 3 Sample 4
– Parallel Parallel Scalping Scalping
38 25 9.5 25 9.5
– 25.4 4.76 25.4 4.76
discrete element method, fine particles
