A rock test cell with true triaxial capability

Conventional so-called triaxial test cells apply the axial stress to a cylindrical sample using steel platens, with the confining stress developed via an annulus of hydraulic fluid retained by a liner in a pressure cell. This does not allow differentiation between the two principal stresses around the core and inhibits the realism with which the rocks can be tested, for example in determining the effect of the intermediate principal stress on the strength of the sample. This paper describes the development and application of a new test cell - believed to be the first in the world - which enables truly triaxial stresses to be applied to cylindrical core samples, opening up the possibility to test rocks routinely in a more realistic manner. An array of 24 trapped tubes replace the single annulus which usually generates the uniform radial stress. Selective pressurization of the tubes enables differential radial stresses to be generated, while axial stresses are applied as before through steel platens. The first results of multi-state failure and permeability stress sensitivity of samples tested in the cell are presented. These demonstrate the influence of the intermediate principal stress on measured rock properties and the orientation of induced fracture planes.