Instability of hard rockmasses: The role of tensile damage and relaxation.

摘要

In low stress environments, stability of rockmasses around underground excavations is typically controlled by rockmass structure. This study focuses primarily on stability issues related to underground excavations in “hard” rockmasses, rockmasses that are sparsely to moderately jointed with rock of high strength, high stiffness, and brittle response.; This thesis is divided into two parts. In Part I, the author examines structurally dominated failure modes, investigating the role of residual tensile rockmass strength and relaxation on the stability of tunnels and mining stopes. Because of the demonstrated significance of these mechanisms, the results of this study are used to modify empirical design tools for stability assessment.; In Part II, the author investigates the importance of microscopic and mesoscopic tensile crack growth on compression-induced damage and near-excavation fracturing of heterogeneous rockmasses at depth. TV sensitivity of in situ rockmass compressive strength to low near-excavation confining stress is examined. The findings of this work explain the success of empirical strength criteria and damage prediction tools, improving confidence for future design applications.; Two important aspects of this process, investigated in this thesis, are the tensile nature of damage mechanisms and the consequent sensitivity to low confinement conditions. Mechanisms of extension crack initiation, accumulation and interaction and the impact of low effective confining stress are explored through a review of the nature of solid bonding and damage, a study of granite behaviour in laboratory strength tests, and through the use of a bonded contact analogue.; While such accumulation in a heterogeneous solid shows a first order sensitivity to confinement, it is then demonstrated that increased sensitivity, to low confinement, of actual rockmass yield strength is related to the mechanics of crack extension.; Finally, it is shown that many of these mechanisms act together in situ, reducing the yield strength of rockmasses to the crack initiation threshold. These findings improve our understanding of the rockmass failure process and explain the success, in near-excavation conditions, of the empirical damage threshold described above. (Abstract shortened by UMI.)