Interpretation of direct shear box testing of sands as quasi-simple shear

Abstract

Interpretation of the direct shear box (DSB) test implicitly assumes a simple shear mode of deformation, but this assumption has not been fully verified, in particular in tests on sands. This paper describes a laboratory investigation, which was commissioned to find an optimum configuration of a DSB apparatus for measuring the strength and dilatancy characteristics in direct simple shear. The examination was made in a newly developed DSB apparatus using dry specimens of two standard sands: Toyoura sand and silver Leighton Buzzard sand. Boundary effects, such as the wall friction, the size of the opening between the two halves of the shear box and the constraint imposed by the loading platen, were each independently examined in pilot tests to determine their influence on the measurements of strength and angle of dilatancy. The optimum configuration was determined through careful examination of the boundary stresses and strains of rectangular specimens subjected to shearing under drained and constant-volume conditions, and also by comparing the results with those of comparative torsional simple shear and plane strain compression tests. On the basis of the test results obtained using an optimum configuration of the DSB device, the stress dilatancy of the sands was characterized by a non-associated rule based on a classical energy theory for the interrelationship between the angle of shearing resistance and the angle of dilatancy.

Viscous Behaviour of Unbound Granular Materials in Direct Shear

“…Numerical studies of the deformation and failure of granular material in DS by FEM or more recently by DEM showed that the principal axes of distributed contact force and initial rupture zone mayˆrst develop diagonally, not horizontally from the specimen edges (Potts et al, 1987;Cui and O'Sullivan, 2006). To minimize the eŠects of these inherent drawbacks and to match as much as closely to a``quasi-simple shear'' mode in the potential horizontal shear zone, attempts have been made to optimize the DSA design by modifying the conventional type (Jewell and Wroth, 1987;Shibuya et al, 1997;Lings and Dietz, 2004). Referring to Fig.…”

“…To alleviate these problems, the top loading platen isˆxed against rotation with types B and C. With respect to the side wall friction, the vertical load W is basically free from these eŠects in type B, whereas, with type C, the vertical load should be measured at the bottom of the lower shear box (Wlower in Fig. 3; Shibuya et al, 1997).…”

“…Therefore, if the two air-cylinders apply the same normal load, the upper shear box should rotate signiˆcantly, associated with a signiˆcantly non-symmetric stress distribution of the vertical stress along the shear zone. The normal stress distribution along the central shear zone can become uniform only by keeping the upper shear box level (Shibuya et al, 1997). 3) A piece of sponge tape (No.…”

Constant-Pressure and Constant-Volume Direct Shear Characteristics of Reinforced Sand

“…They include, for example, the rotation of box, unknown effects of the opening between the top and bottom halves, and effects of side-wall friction. Some features of different types of direct shear device were discussed by Shibuya et al [1]. The shear box that sustains shear load tends to rotate if the shear load develops large moment and no measures are taken to prevent it.…”

Viscous Property of Toyoura Sand Over a Wide Range of Shear Deformation Rate and its Model Simulation

“…The specimen size is 12 cm×12 cm×12 cm. This DS apparatus is characterised by the following features; 1) a feedback control of the vertical normal stress by adjusting the air-pressures applied to two air cylinders so that the upper shear box is always accurately kept level (e.g., Mikasa, 1960;Jewell and Wroth, 1987;Shibuya et al, 1997) while the total normal load is always kept constant; 2) travelling of the lower shear box on a pair of very lowfriction rails; 3) accurate evaluation of the shear and vertical stresses acting in the central shear zone by careful design of the apparatus, in particular by the use of a pair of load cells to evaluate any friction acting on the bottom of the lower shear box, which is essential to ensure that rate-dependent responses in the measured shear stress are due fully to material viscous properties; and 4) direct measurements of shear displacements between the upper and lower boxes with a laser displacement transducer. The importance of these features to obtain reliable data of DS tests on granular materials was demonstrated by Qiu et al (2000) and Wu et al (2008).…”