Applications Of Large Scale Direct Shear Testing

Direct shear testing is a common and well-established method for determining the strength of geomaterials. In particular, it is ideally suited to measuring the strength of interfaces between geomaterials and structures, e.g. at the rock-concrete interfaces of concrete dams and concrete piles drilled into rock; and within geomaterials, e.g. natural rock joints.

One area that has been of significant interest to researchers at Monash University since about 1980 has been the behaviour of the interface formed between a concrete pile and the surrounding rock. The performance (capacity and displacement response) of the pile is dominated by the behaviour of this interface. Axial loading of the pile produces slip displacement at the rock-concrete interface, in much the same way as shearing of a rough rock joint. In order to investigate the behaviour of this interface, a large direct shear testing apparatus was designed and constructed. Design criteria for the shear apparatus and associated split shear box were:

• To have the capability of testing large samples so that scale effects could be investigated
• To be as rigid as possible to prevent sample rotation and minimise compliance while keeping friction losses to a minimum
• To be able to replicate in-situ boundary conditions such as normal stress and stiffness as accurately as possible
• To be able to apply any loading pattern (stress and displacement control) so that both cyclic and monotonic loading conditions could be investigated

The construction of the shear rig was completed in 1991 and has since been used almost continuously on a number of research and consulting projects. A photograph of the shear rig is shown in Figure 1. These projects have included the testing of rock-concrete interfaces and rock-rock interfaces, jointed rock masses, base coarse and rock fill material and the dynamic testing of pile-soil interfaces. The rocks used to date have varied in uniaxial strength from 1 MPa to 200 MPa and included siltstone, Johnstone (a synthetic siltstone), sandstone, calcarenite, basalt and granite.

A short description of the capabilities of the direct shear rig and some of the applications for which it has been used are presented in this article.