Brittle rock fracture and progressive damage in uniaxial compression

Abstract

High compressive stress near a tunnel face significantly contributes to the loss of strength, and eventual failure of the rock, through stress-induced brittle fracturing. These processes are commonly observed around excavations in highly stressed massive brittle rock in forms ranging from minor spalling to violent rockbursts. The research presented in this thesis was undertaken to investigate the mechanisms responsible for these failures, i.e., stress-induced brittle fracturing and the progressive degradation of rock strength. Through the combined use of laboratory strain gauge and acoustic emission techniques, rigorous methodologies were developed to aid in the identification and characterization of the brittle fracture process. Uniaxial compression testing of pink Lac du Bonnet granite from AECL's URL revealed that several stages of crack development could be resolved. These include: crack closure (ócc), crack initiation (óci), secondary cracking (óci2), crack coalescence (ócs), crack damage (ócd), and peak strength (óUCS). Elements of numerical modelling were further used to aid in the conceptualization of the internal mechanisms acting during microfracturing processes. The versatility and full potential of the laboratory methodologies developed for this thesis study was further established through tests involving rock types of varying grain size, mineralogy, sampling disturbance and rheological behaviour. Through these tests, it was found that the mineralogy of the sample had the greatest influence on the initiation of cracking. Increasing grain size and sampling disturbance was found to provide longer paths of weakness for growing cracks to propagate along resulting in lower strengths due to the coalescence and unstable propagation of cracks at lower stresses. Brittle fracture processes were also observed and quantified for Saskatchewan potash and Berea sandstone. Insights into the processes and mechanisms relating to brittle fracture were further utilized to derive empirical relationships describing the progressive accumulation of stress-induced fracture damage. Results from monotonic loading tests were used to quantify the state of microfracturing damage with respect to stress, strain, acoustic velocity and acoustic emission. Cyclic loading techniques were used in a series of damage-controlled tests to investigate the effects of load path and time-dependency on the accumulation of microfracturing damage.