Effect of gradation on shear characteristics of granular materials under one-way repeated direct shear and simple shear loadings

Abstract

The shear strength and stiffness of granular materials are crucial parameters for evaluating both stability and sustainability in geoenvironmental engineering. Two types of shear loading, static and cyclic, may directly influence the mechanical properties of granular materials; however, the investigation of stiffness improvement of granular materials has not yet been sufficiently investigated using a loading method. The one-way repeated loading method described in this study may overcome the drawbacks of classical testing approaches. One-way repeated direct shear (DS) and direct simple shear (DSS) tests were conducted on a series of poorly graded bead gradations to investigate the shear strength and stiffness of granular materials under a drained and strain-controlled condition with a shear velocity of 0.1 mm/min. All samples in the two tests were prepared using the same method and initial geometry. Three types of samples referring to different curvature coefficient (C-u) values of 1.8, 3.3, and 5.8 and normal stresses (& sigma;(n)) of 50, 100, and 150 kPa were applied. The test results show that an increase in C-u results in an increase in the shear stress of the granular materials at the peak and ultimate states for both the DS and DSS tests. The peak shear stress ratios between the two tests (& tau;(DSS)/& tau;(DS)) ranged from 0.75 to 0.95 when C-u and & sigma;(n) were increased. The angles of shearing resistance from the DS test were in a wide range of 25.7 & DEG;-33.7 & DEG; at the peak state and 24.9 & DEG;-31.3 & DEG; at the ultimate state, whereas those found in the DSS test were in narrow ranges of 26.0 & DEG;-28.4 & DEG; and 23.6 & DEG;-27.9 & DEG; at the peak and ultimate states, respectively. It should be noted that an increase in C-u led to the increase in the stiffness ratio in both test methods. The difference in stiffness ratios determined from the DS and DSS tests was approximately 14%. Under one-way repeated shear loading, the DS test is recommended to determine the residual shear strength generated along the shear plane, whereas the DSS test is recommended to determine the maximum shear stress at the weakest shear zone developed horizontally owing to repeated shearing.