Mechanical and Hydromechanical Behaviour of Hard Rock Joints. A laboratory study

Abstract: As a consequence of rock engineering works, the rock mass will deform due to stress changes. Most deformation occurs in the rock joints, in form of normal and shear strains. These deformations will also change the void geometry and thereby the joint aperture and fluid flow. The purpose of the study was to increase the knowledge and understanding about mechanical and hydro-mechanical behaviour of rock joints and to provide essential parameter values.

A comprehensive laboratory test programme was performed on both granite joints and replicas, followed by an analysis. The replicas consisted of high strength concrete with rock mechanical parameters close to those of the granite samples. 36 identical replicas were cast in a rubber mould, using an impression from one of the 19 granite samples. Roughness measurements were performed with, among other methods, a laser profilometer. Based on measured profile lines, an attempt was made to use fractal dimension (D) as a measure of joint roughness.

Most of the mechanical and hydromechanical laboratory tests were performed in a direct shear box. Before shear testing, normal loading and unloading cycles were performed to consolidate the joints. The shear tests were performed with both constant normal load (CNL) and constant normal stiffness (CNS). To provide relevant rock mass stiffness for the (CNS) shear tests, a model was suggested. A clear difference between CNL and CNS tests occurred in the shear stress paths and in the peak shear stiffness, while the dilation angle and peak friction angle were unaffected. In the hydromechanical shear tests, the water flow through the joint was parallel; the joint opened much more than the hydraulic aperture which certainly depends on gouge production from the degradation of the joint surfaces. A minor modelling exercise was performed to verify some results obtained from the shear tests.