Experimental and field measurements have shown that the time-dependent behaviour of a jointed rock mass is a function of the rheological behaviour of both the intact rock and its discontinuities. Although numerous numerical techniques exist for modelling the creep behaviour of the intact rock, little attention has been given to creep along the discontinuitities.
In this paper an extension of the Displacement Discontinuity method, based on the correspondence principle of linear viscoelasticity, is used to study the combined effects of these two creep mechanisms. The method requires that the intact rock be modelled as a linear viscoelastic material, while, in general the nonlinear time-dependent response of the joints can be accounted for. A number of practical examples are presented to demonstrate the efficiency and simplicity of this approach. By varying the viscoelastic properties of the rock and its discontinuities, the relative importance of the two creep mechanisms is investigated.
It is generally appreciated that it is the discontinuities such as sheared zones, faults and joints, that govern the behaviour and stability of a rock mass. Although considerable research has been undertaken to model the creep behaviour of intact rock, little attention has been given to the effect of creep along discontinuities. To date, most joint creep studies have been related to an investigation of earthquake mechanisms(Dieterich, 1978). As a result, most tests have been conducted at stress levels greatly exceeding those applicable to rock engineering structures. An ongoing research programme at the University of Toronto has shown that creep deformations along joints may be appreciable, especially at loads near the peak shear strength.
In this paper, the creep behaviour of a jointed rock mass is modelled using an extension of the Displacement Discontinuity (D.D.) method of Crouch (1976). Essentially a boundary integral equation technique, the D.D. method is based on a fundamental solution which expresses the displacements and stresses resulting from a constant displacement discontinuity over a finite line segment in an elastic body. In the numerical method, displacement discontinuities of unknown magnitude are placed along the boundary of the region being analysed. Due to the interaction between the discontinuities, a system of coupled linear algebraic equations result, the solution of which gives the discontinuity displacements that are consistent with the prescribed boundary traction and displacements. The stresses and displacements at any point in the body can be evaluated by summing the effects of the individual displacement discontinuities. Since for two dimensional problems. e.g. plane strain, a rock joint may be considered a one dimensional feature, the D.D. method is well suited to modelling rock discontinuities.
To model the time-dependent behaviour of the intact rock, a fundamental solution which expresses the displacements and stresses resulting from a constant displacement discontinuity over a finite line segment in a linear viscoelastic body is developed. This solution is determined by using the D.D. elastic solution in conjunction with the correspondence principle of viscoelasticity (Flügge, 1975). The rheological response of the joints is modelled in terms of normal
