ABSTRACT

INTRODUCTION

Deformations and failure of rock masses in slopes, foundations or tunnels can be caused by water pressures. On the other hand, deformations of a rock mass affect the flow of water (and hence water pressures) by changing joint apertures. Including the coupling between flow and deformation effects is therefore important both in assessing the stability and deformation of a rock mass and in determining the amount of seepage taking place through it. The model that will be presented here treats the coupling of the flow and deformation behavior of a rock mass. Deformations are modeled using the Rigid Block Method (Distinct Element Method) (Cundall, 1974), that is by assuming that the rock mass is composed of rigid blocks with deformable contact points. Flow is modeled by recognizing its discontinuous features, as occuring in a system of interconnected conduits formed by the joints. The program can also deal with problems involving free-surface flow. Example problems involving representations of rock masses are presented.

Coupled flow modeling of interconnected conduits should be contrasted to equivalent-continuum models on the one hand and to pipe network models on the other hand. Continuous models substitute an anisotropically permeable continuum for the discrete flow that occurs in rock joints. Pipe or parallel plates represent the other extreme of existing models; they account for the discontinuous character of the flow problem, but cannot model rock mass deformation and failure. The coupled rigid block-flow model can eliminate most of these restrictions. The coupling between water flow and deformation is most pronounced in rock masses which, because of the arrangement of their Joints, consist of a number of discrete rock blocks resting upon each-other and separated by Joints. Under such conditions, the Rigid Block Method is quite appropriate for modeling deformations especially Just prior to failure, and is ideally suited for being coupled with a flow model that considers flow as taking place within a network of intersecting conduits.

THE COUPLED RIGID BLOCK-FLOW MODEL

The Rigid Block or Distinct Element method was first proposed and developed by Cundall (1971). Subsequent work by Cundall and other extended its applicability (Voegele, 1982) and also led to hybrid models (Lorig, 1982, Dowding, 1983). Problems of progressive failure were treated by Gencer (1982). In recent publications inclusion of flow is mentioned (Cundall 1982) as well as work on a 3-dimensional procedure (Fairhurst, 1983). The model described here essentially consists of a deformation model and a flow model where coupling is handled through an iteration of "Joint apertures obtained from deformation model-flow and pressure in Joints-forces on blocks-apertures".

The Flow Model

Flow is modeled as taking place within a network of conduits corresponding to block sides (which can be thought as pipes) meeting at a number of 'nodes' corresponding to block corners. In modeling flow the changes in the joint apertures that are produced by the deformation model are taken into account. However, except for changes in apertures, the flow algorithm assumes that the geometry does not change during deformation. This is equivalent to assuming that flow takes place in a network of conduits of varying cross-sections but of constant lengths and positions in space.

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