Material shear strength is among the most important geotechnical parameters used in the design process for engineers. For soils, soft rocks and rockfills the determination of the shear strength properties is often accomplished through the use of the direct shear test. Unfortunately, much controversy exists concerning the stress and strain distributions, strength calculations and failure process within the direct shear test.
The objective of this study was to investigate the effect of material properties on material behavior under non-uniform stress and strain conditions in the direct shear test using the finite difference numerical technique. This research illustrated the effects of progressive failure within elastic-plastic and strain softening materials on shear strength. In particular, this paper describes the numerical simulation of a well documented earth material within the direct shear test. Within an individual material property model, the effects of changes in stiffness, dilatancy, and strain dependent yielding were identified and discussed. The analysis of these tasks allows for greater insight into the behavior of materials in the direct shear test so that more realistic material properties can be determined.
The direct shear test is a procedure in which a specimen is confined within rigid, fixed rotation frames and caused to shear on a plane. The specimen is generally elongated so that the plane of shear is along the long axis. The confinement within the specimen is produced by the application of a force normal to the plane of shearing and by the boundaries of the frame (Figure 1). The shearing process is induced by the displacement of the upper half of the solid frame with respect to the lower half. The normal stress (sn) and horizontal shear stress (Txy) acting along the failure plane are calculated from the forces applied to the direct shear box. This test is intended to force specimens to fail along a predetermined plane, the area of which is constantly reduced as failure progresses. In this paper, the shear strength of a material is defined as the highest average shear stress (T) across the plane of shearing. Stiff, dense or over consolidated material have a distinctive peak shear stress condition which separates elastic from yield behavior. Yield occurs when there is a departure from linearly elastic behavior, i.e. when some of the deformation becomes irrecoverable. Soft, loose, or normally consolidated materials experience a gradual increase in shear stress as deformation occurs until a residual shear strength is achieved. When the residual shear strength is reached, the specimen undergoes ductile deformation. Once the shear strengths are known for a number of different normal stresses, a plot of shear versus normal stress can be made. Drawing a straight line through these points allows for the evaluation of the strength parameters (ö and C) used to define the linear Mohr-Coulomb failure criterion.
The direct shear box was first utilized by Alexandre Collin in the 1840's to aid in the analysis of slope stability problems (Skempton, 1949).