Three-dimensional Finite Element Solution for Excavation and Creep in Rock Engineering

The unified model suited the evaluation of excavation and creep for rock engineering is suggested by virtue of the generalized Kelvin model （HK）. The tridimen-sional finite element nonlinear computer programme CRR4 based on this model is described and shown in this paper. The CRR4 code （FORTRAN Ⅳ） has a new calculating capability of elasticity and visco-elasticity for analyzing the stability of the surrounding rock in the sequential excavation. It is also shown that the method of "local stress smoothing" is an effective approach which generates the smooth stresses of the isoparametric elements （ZIB8-element）, especially at the nodes.At first, the undisturbed initial stress field of the rock structure is established. Especially, the tectonic forces presented in the ground can be simulated by means of loading the different distributed pressures on the two or more than two boundary surfaces. This is the evident advantages of three-dimensional simulation than bi-dimensio-nalones.On modelling the procedure of sequential excavation, the perturbation stresses arecaused by the release forces acting around the opening. The excavation load is an induced load come from the engineering disturbance effects. The former stage of excavation has the effects on the next stage, Then the comprehensive resulting state of stress in the rock structure calculated for the given stage is considered as the so-called initial state of stress before the next stage.Furthermore, the creep behaviour of rock must be considered. The time-dependent mechanical properties of rock may be described by means of a generalized Kelvin substance. This model provides an instantaneous elastic deformation followed by the time dependent curve. The instantaneous elastic deformation can be taken to represent the deformation of excavation. Subsequently the time dependent curve is adapted to simulate the transient and steady state phases of creep deformation for the surrounding rock after each stage of excavation.A well known "initial strain" method suited to solve the creep problem is adopted in the CRR4 code. An increment of creep strain may be generated for each interval. Subsequently the creep equivalent nodal force can be obtained. By solving the new governing equation, the nodal displacements at this interval can be obtained. It is interesting that the process curves of displacement-time （u,v,w-t） can be drawn automatically by the computer.Using this programe to the evaluation of room-pillar mining, we can obtain the initial stress field before excavation and the stresses redistribution and displacements caused by mining, then the evaluation of creep for mine pillar and roof and baseplate can be accomplished by means of the method of "initial strain". The rheologic behaviour of heterogeneous rocks formed by three viscoelastic components is described according to the corresponding generalized Kelvin substance.