Deformation and fracture around cylindrical openings in rock—II. Initiation, growth and interaction of fractures

Abstract

Elastic and inelastic deformation, fracture and failure around underground openings were investigated through experiments on thick-walled hollow cylinders of Berea sandstone and Indiana limestone, incorporating plane strain loading, the application of different stress paths, transference of the external pressure to infinity, and freezing of the fracture geometry under stress through metal saturation. This second of two parts investigates fracture development and failure progression on both an experimental and theoretical level. Failure is oriented and occurs on the two opposite sides of the hole with the highest ratio of tangential stress to tangential strength (determined by rock strength anisotropy). The size of the failed zones may be influenced not only by the final stress state but also by stress path, strain rate and test boundary conditions. Microscopic observation reveals that the fundamental fracture mechanism is the growth of small, opening-mode, splitting cracks oriented parallel to the tangential stress, starting very close to the hole wall and occurring deeper in the rock with increasing stress. Numerical models confirm that splitting cracks preferentially grow close to the surface of a stressed hole. These models also predict a size effect on strength. Around unsupported holes these cracks coalesce to form macroscopic splitting fractures subparallel to the hole wall, and also form en echelon patterns that meet the wall. These two types of features combine to define the new surface of a spalled piece, which moves radially into the hole. Slabs of fairly uniform thickness progressively detach from the surrounding rock, resulting in a triangular failure zone with a pointed tip. Significant dilation occurs in the failing material. With 5-10 MPa support pressure in the hole, the rock is greatly strengthened and stabilized. Macroscopic splitting fractures, and therefore detached spalled pieces, do not form. With high enough stress, microcrack interaction eventually results in the formation of what appear to be bands of shear localization, but the rock still maintains significant cohesion. (Author/TRRL)