Tensile strength and abutment relaxation as failure control mechanisms in underground excavations

Classical assessment of instability potential in underground excavations are normally based on yield and rupture criteria for stress driven failure and on limit equilibrium analysis of structurally controlled failure. While it is true that ultimate failure and falls of ground can be an eventual consequence of stress fracturing and unfavourable structure within the rock mass, the timing of such failure is often controlled by the presence of residual tensile capacity, in the form of rock bridges separating joint segments and fractures and by the mechanisms of clamping and relaxation. Using crack and rock-bridge analogues in conjunction with an updated voussoir beam model, this paper explores the influence of residual tensile strength and boundary parallel relaxation on the failure process. The impact on support design is also examined. In underground hard rock mines with complex geometries and interacting openings, relaxation is identified as a key controlling factor in groundfall occurrence. Empirical stability assessment techniques for underground tunnels and for mining stopes are updated to account for relaxation.