An improved direct shear apparatus for sand

Abstract

A conventional direct shear apparatus (DSA) has been modified to improve its articulation by moving the point of shear load application to the sample centre. Jewell's symmetrical arrangement has been adopted for a 100 mm square shearbox, resulting in increased dilation rate and reduced rotation. Testing using coarse Leighton Buzzard sand has enabled the performance of the modified DSA to be optimised by exploring different test configurations. Optimum results were obtained with a substantial initial gap between the shearbox frames of 5D50, the use of thin rubber edging strips to contain the sand, and the omission of grid plates at the sample boundaries. Upper-frame rotations persist during testing, but have almost no effect on measured parameters. A framework of relationships between parameters measured in a direct shear test ((ϕ′ds)p, ψp and (ϕ′ds)Id) and relevant plane strain parameters ((ϕ′ps)p and ϕ′crit) is set out, based on Rowe's flow rule and Davis's (1968) relation. Stroud's simple shear apparatus (SSA) data for coarse Leighton Buzzard sand have been examined and shown to fit Rowe's flow rule. The output from the modified DSA also fits these relationships well, and shows good internal consistency. A simple relationship enables modified DSA peak direct shear friction angles to be converted into plane strain friction angles. Large displacement direct shear friction angles for the modified DSA are related to critical state friction angles by tan(ϕ′ds)Id = sin ϕ′crit.