Abstract：The deformation and shear strength of anisotropic granular materials are dependent on the shearing direction or the orientation of principal stress relative to the bedding plane. In order to investigate the shear strength properties of any shear plane for anisotropic granular materials，an improved direct shear apparatus was designed，with which the ψ of Fujian standard sand defined as the angle of shear plane with respect to the bedding plane can be changed in the whole range of 0°–180°. The characteristics of the improved direct shear apparatus is that its walls are independent and demountable，thus the apparatus can be assembled according to different needs；and some top and side surfaces can be opened. By using the improved direct shear apparatus，direct shear tests are conducted on Fujian standard sands with the variation of ψ from 0°to 180° at intervals of 15°. The experimental results show that the plane with the minimum peak internal friction angle is achieved at the angle of ψ from 0° to 90°. After that，the peak internal friction angle increases with the increasing ψ. However，the plane with maximum peak internal friction angle is not the plane perpendicular to the depositional plane(ψ = 90°)，but that with ψ ≈105°. The planes related to the maximum and minimum peak internal friction angles are dependent on the anisotropy degree of materials.
[1] ODA M， KOISHIKAWA I. Effect of strength anisotropy on bearing capacity of shallow footing in a dense sand[J]. Soils and Foundations，1979，19(3)：15–28.
[2] TATSUOKA F，NAKAMURA S，HUANG C C，et al. Strength anisotropy and shear band direction in plane strain tests of sand[J]. Soils and Foundations，1990，30(1)：35–54.
[3] AL-KARNI A A，AL-SHAMRANI M A. Study of the effect of soil anisotropy on slope stability using method of slices[J]. Computers and Geotechnics，2000，26(2)：83–103.
[4] GUO P J. Modified direct shear test for anisotropic strength of sand[J]. Journal of Geotechnical and Geoenvironmental Engineering，2008，134(9)：1 311–1 318.
[5] LADE P V，NAM J，HONG W P. Shear banding sand cross- anisotropic behavior observed in laboratory sand tests with stress rotation[J]. Canadian Geotechnical Journal，2008，45(1)：74–84.
[6] 殷宗泽，徐志伟. 土体的各向异性及近似模拟[J]. 岩土工程学报， 2002，24(5)：547–551.(YIN Zongze，XU Zhiwei. Anisotropy of soils and its approximate simulation[J]. Chinese Journal of Geotechnical Engineering，2002，24(5)：547–551.(in Chinese))
[7] 于艺林，张建民，童朝霞，等. 定轴排水剪切试验中各向异性砂土的力学响应[J]. 岩土力学，2011，32(6)：1 637–1 642.(YU Yilin，ZHANG Jianmin，TONG Zhaoxia，et al. Behavior of anisotropic mica sand under fixed principal stress axes drained shear test[J]. Rock and Soil Mechanics，2011，32(6)：1 637–1 642.(in Chinese))
[8] JEWELL R A. Direct shear tests on sand[J]. Geotechnique，1989，39(2)：309–322.
[9] ZHANG L，THORNTON C. A numerical examination of the direct shear test[J]. Geotechnique，2007，57(4)：343–354.
[10] MAHMOOD A，MITCHELL J K. Fabric-property relationships in fine granular materials[J]. Clays and Clay Minerals，1974，22：397–408.
[11] FU P，DAFALIAS Y F. Study of anisotropic shear strength of granular materials using DEM simulation[J]. International Journal for Numerical and Analytical Methods in Geomechanics，2011，35(10)：1 098–1 126.
[12] FU P，DAFALIAS Y F. Fabric evolution within shear bands of granular materials and its relation to critical state theory[J]. International Journal for Numerical and Analytical Methods in Geomechanics，2011，35(18)：1 918–1 948.
[13] CERATO A B，LUTENEGGER A J. Specimen size and scale effects of direct shear box tests of sands[J]. Geotechnical Testing Journal，2006，29(6)：1–10.
2012, Vol. 31 
