Predicting the behaviour of underground openings in rock

Abstract

Rock masses range from intact homogeneous media, through regularly jointed assemblies of blocks, to heavily sheared and crushed claybearing fault zones. A brief review of the behaviour of openings created in these media, such as boreholes, tunnels and large mine openings, indicate some consistent trends. The fundamental failure mechanisms and requirements for support are believed to be strongly influenced in ali cases by the volume changes accompanying potential failure. Failure, even in unjointed media, eventually occurs on failure surfaces such as extension fractures or shear fractures. A"plastic" zone may not strictly exist since the rock between the failure surfaces can be intact and relatively less highly stressed. Since the volume changes accompanying failure are to a great extent determined by the dilation (or contraction) along these failure surfaces or pre-existing discontinuities, an understanding of the latter is fundamental to the prediction of underground opening behaviour.

Careful measurements of model jointed media are used to demonstrate some of the volume changes that can be expected when rock masses are subjected to increased shear stress. The components of joint deformation such as closure and shear-induced dilation determine the type of response of individual rock masses. Attempts are made to provide a unifying description of rock mass shear strenght based on discontinuum behaviour. Both the Q-system and the JRC/JCS index characterization of discontinuities are utilized here. The proposed criterion contrasts with the Hoek and Brown criterion, which primarily describes the strength of intact rock, with adjustments for jointed or crushed media.

Examples of discrete modelling of excavations in jointed media are given, using both physical and numerical models as examples. Particular attention is focused on the discrepancy between the behaviour of jointed media and the attempts to simulate these by means of continuum models. Tensile opening of joints and hysteresis on unloading due to shear are cited as reasons for the discrepancy. Examples of recent discrete element modelling using UDEC method are described, to illustrate the prediction of permeability changes in the disturbed zone around tunnels.