1、Numerical and experimental direct shear tests for coarse-grained soilsAhad Bagherzadeh-Khalkhali , ,Ali Asghar MirghasemiSchool of Civil Engineering, College of Engineering, University of Tehran, Tehran, IranReceived 15 February 2008. Accepted 18 November 2008. Available online 15 January 2009.http:
2、/dx.doi.org/10.1016/j.partic.2008.11.006, How to Cite or Link Using DOICited by in Scopus (7)Permissions Direct shear test;Micromechanics;Coarse-grained soil;Shear strength1. IntroductionExperimental tests on coarse-grained soils always involve difficulties, and it is often necessary to remove large
3、 particles due to dimensional limitation of laboratory specimens. Marsal, 1967 and Marsal, 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, a
4、nd 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 triaxi
5、al 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, on a series of both
6、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 di
7、stinct 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-to-particle
8、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 namely, parallel gradation technique (Lowe, 1964), scalping method (Zeller designated as T1, T2 and
9、T3, respectively). Table 1 shows the tests with different normal stresses.Fig. 1. Particle size distribution of experimental test samples.Fig. 2. Size distribution of numerically simulated samples.Table 1. Normal stresses employed in numerical and experimental direct shear tests.Test number Applied
10、vertical stress kg/cm2 (kPa)Test 1 (T1) v = 1 (98.1)Test 2 (T2) v = 2 (196.2)Test 3 (T3) v = 3 (294.3)Full-size table2.1. Experimental testsAccording 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 she
11、ar 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.
12、 sieve) for other two samples. Table 2 presents the properties of the samples to be tested in laboratory and simulated by DEM.Table 2. Properties of samples.Samples Modification techniqueNumerical simulations Experimental testsMaximum particle size (mm)Maximum particle size (mm)Sample 0 38 Sample 1P
13、arallel 25 25.4Sample 2Parallel 9.5 4.76Sample 3Scalping 25 25.4Sample 4Scalping 9.5 4.76Full-size tableTests 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 c
14、hanging 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).Table 3. Densities of numerical and expe
15、rimental samples after compaction.Samples Numerical simulations Experimental testsa(average coordination number)Densitya Relative density (%)Sample 04.75 0.73 Sample 14.74 0.75 94.8Sample 24.92 0.66 96.4Sample 4.78 0.74 95.73Sample 44.95 0.65 97.5aThese parameters are dimensionless.Full-size table2.
16、2. Numerical simulationsIn numerical simulation, all samples were prepared to simulate experimental samples. However due to limitation of the discrete element method, fine particles 5 mm were removed from the simulated assemblies, thus causing difference between experimental and numerical procedures
17、. Five assemblies were simulated according to the above mentioned requirements. The dimensions of simulated shear boxes, determined according to ASTM-D3080, were similar to experimental tests. Table 2 shows the properties of the simulated samples and Fig. 2 shows the particle size distribution of th
18、e samples.Sample 0 is simulated according to the original gradation of sampled soil to compare the behavior of original soil to reduced-particle-size specimens. Because of removal of fine particles from the simulated samples, to avoid a uniform gradation, the maximum particle size of Samples 2 and 4
19、 was increased to 9.5 mm. The same samples used in the experimental tests remained unchanged (4.76 mm). The size of the simulated shear box was also increased accordingly.For the numerical simulations, the program ELLIPSE was adopted and modified in order to simulate the direct shear test (Bagherzad
20、eh-khalkhali (b) compacted assembly; (c) after loading; (d) sheared assembly.The numerical simulations were carried out in three stages. The required boundary forces or displacements or servo controlled boundary conditions are applied on the boundary particles to simulate the test conditions in diff
21、erent stages. In the first stage, the generated loose assembly was compacted hydrostatically. Vertical stress was applied on the assembly in the next stage. Finally the assembly was sheared in the direct shear box under constant vertical stress.At the first stage, a constant compression strain was a
22、pplied on an assembly until a mean confining pressure of 0.5 kg/cm2 (49.05 kPa) was induced within the assembly. Fig. 4 shows the variation of average pressure () mobilized in the assembly as a function of volumetric strain. As expected, bulk module of sample at the first stages of compaction is low
23、 but increases with increasing the compaction of Sample 3 as an example. In Table 3 the density and coordination number of assemblies at the end of first stage are shown.Fig. 4. Compaction on simulated Sample 3.Densities in Table 3 are defined as the ratio of sum of area occupied by all particles to
24、 total area of the assembly, while average coordination number () represents the ratio of the sum of contacts number for all particles in an assembly to the total number of particles. Accordingly, assemblies with the same range of particle sizes (Table 2) have the same density and coordination number at the end of compaction (Table 3). On the
