Evolution of cohesion and friction angle during microfracture accumulation in rock

Abstract

The creation of microfractures within rock is commonly observed as rock is strained. The presence of these microfractures constitutes damage to the rock, and this damage can reduce the rock’s strength. This paper explores the evolution of rock strength as microfractures within a rock accumulate. Two approaches involving different laboratory tests are used to study how cohesion and internal friction evolve during progressive damage to rock. The mobilized cohesion and friction angle are measured for intact and damaged rock specimens. Intact rock specimens tested under compression were used to determine the peak values of cohesion and friction angle for two types of rock. Specimens of rock with varying amounts of accumulated microfracture damage were tested under direct shear or multi-stage triaxial compression to measure the Coulomb strength parameters for damaged rock. The laboratory testing shows that cohesion decreases with strain as the rock accumulates internal damage caused by microfracturing before the peak strength. The frictional component of the rock strength starts to be mobilized as strain causes internal microfractures. The mobilized internal friction angle increases up to and slightly beyond the peak strength. A small amount of post-peak strain is required to initiate macroscopic slip surfaces, and until these are created, high frictional resistance is mobilized between the many interacting and interlocked pieces of rock in the test specimen. With further post-peak strain, the friction angle decreases as the macroscopic slip surfaces in the rock become well established.