ABSTRACT:

The paper presents a constitutive relation of rock media and weak surfaces taking strain softening and permeation softening into consideration. It provides a general formulation of the instability theory of rock system due to softening of rock media and weak surfaces. It is applied specifically to the stability analysis in rock engineering.

RESUME:

Get article presente la relation constitutive pour la masse rocheuse ainsique pour les surfaces de faiblesse qui est applicable au cas de l'adoucissement dû au deformations et au cas de l'adoucissement occassione par l'ecoulement de l'eau. On propose une formulation generale de la theorie de la instabilite du systeme rocheux occassionee par l'adoucissement et s'applique au calculs de stabilite dans la genie geotechnique.

ZUSAMMENFASSUNG:

In diesem Artikel werden die konstitutiven Gleichungs-systeme fuer Fels mit weichen Zwischenlagen unter Beruecksichtigung des ‘strain-softening’ Verhaltens und der Sickerwasserströmung aufgestellt. Es wird eine allgemeine Instabilitatstheorie des Felssystems mit weichen Zwischenlagen vorgestellt und Beispiele fuer deren Anwendung bei der Analyse der stabilitat von Projekten des Felshaues gebracht.

1. INTRODUCTION

The strength (or yield limit) of a rock medium and a weak structural surface may decrease with the development of plastic deformation and water permeation. These phenomena are called strain softening and permeation softening respectively. Landslide, collapse and subsidence of rock engineering project due to excavation, water permeation or the action of the earthquake force can be regarded essentially as the loss of stability of rock system due to softening of rock media and weak surfaces. In order to discuss these instability phenomena by means of solid mechanics, this paper presents a constitutive relation for rock medium and weak surface taking strain softening and permeation softening into consideration. It also incorporates the incremental analysis of solid mechanics together with an energy criterion for determining the stability of a rock system. Thus it provides a general formulation of the theory for the instability of rock system. The theory can be used to calculate the critical load (water load or the excavation step at which the project would become unstable) of an engineering project and to study precursories of oncoming instability of the rock system, so that it provides a necessary basis for the design of an engineering project to have sufficient resistance against disaster. The paper also illustrates the implementation of the theory in the finite element program NOLM and its application to the stability analysis of rock slopes and underground openings.

2. A CONSTITUTIVE BELATION OF ROCK MEDIUM AND WEAK SURFACE IN A ROCK MASS

In uniaxial tests or triaxial tests under low confining pressure with an ordinary testing machine, the rock medium fails suddenly in a brittle manner, but with a stiff testing machine one can obtain a continuous stress-strain curve including both the hardening range and the softening range. In plasticity theory, when the yield stress of a medium increases or decreases with the development of a plastic internal variable K, they are called strain hardening or softening respectively. Strain softening is an intrinsic property of the rock medium. On the other hand, most rocks have porosity to a certain degree and pore water has a significant effect on the deformation character and the strength of a rock mass. Permeation of water will decrease the strength (or the yield limit) of rock media (e.g., the cohesion So and the internal friction coefficient μ). This is called permeation softening. On the contrary, drain~ age will result in hardening. The elasto-plastic constitutive relations accounting for strain softening has already been established (Wang and Yin, 1981). In order to extend these relations to the case of permeation softening, we have to make use of the concept of effective stress. It is the stress acting on the solid skeleton of the two phase fluid-solid model. In order to describe the discontinous properties of joints, faults and weak structural surfaces in a rock mass, we introduce the concept of a displacement discontinuity surface. Due to the complexity of its behaviour (e.g., it has both dilation and softening) it is necessary not only to set up the discontinuity conditions for the stress and the displacement but also to establish a constitutive relation which relates the stress vector a acting on the surface with the displacement discontinuity vector (u) across the surface. In doing so, we look upon the discontinuity surface as the limit of a material layer when its width approaches zero (Fig.l). This discontinuity surface is not a simple geometric surface but a surface consisting of material particles and possessing material properties.

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