The cavity expansion test is frequently used to determine the shear stiffness of rock. When stiffness is derived from an unload/reload event (cycle) then the process is repeatable and will give consistent values that are unaffected by drilling disturbance.
The simplest interpretation of rock stiffness data is to assume the cycle is a linear-elastic event. In practice the response of the rock mass can be complicated by tensile failure and the level of shear stress. If the tensile strength is overcome and failure occurs, fracture growth can give cycles of reducing stiffness, unrepresentative of the rock mass at the insitu state. Material showing shear failure is likely to give a non-linear reduction of stiffness with strain. This includes decomposed and weathered materials. If the material remains largely intact, increasing the applied stress level will often produce a stiffer response with a power law trend.
The cavity expansion test is frequently used as a means of determining the shear stiffness of many different types of ground, including rock. The experimental data are presented as a plot of cavity expansion (displacement or strain) against total pressure. A large volume of material contributes to the result, over 1000 times greater than a standard laboratory sample. Stiffness is derived with minimal assumptions and without empiricism.
Commonly, insitu testing of this kind is undertaken using a pressuremeter or dilatometer, placed in a pre-bored pocket formed by rotary tools. The cavity is completely unloaded prior to the expansion test commencing. All examples in this paper were obtained using a High Pressure Dilatometer (HPD) able to apply up to 20MPa of stress and resolve a movement of 0.3 μm.
The observed response depends on the extent to which the rock mass is an intact material. A competent, largely intact rock mass is unlikely to fail in shear at the pressures that can be applied by commercial equipment. The entire test is an elastic process, but the expansion can be influenced by tensile failure and fracture development. However, a test in a weathered or weak rock is likely to show shear failure and the response can resemble that of a dense sand. Figure 1 is an example of three tests, all conducted with a HPD, in competent mudstone, weathered mudstone and a dense sand.
