Static and Cyclic Laboratory Testing of Brisbane Rocks

The use of underground facilities for purposes such as transportation tunnels, power station caverns, radioactive waste repositories, and water and gas storage is increasing. Fracturing is a failure mechanism of brittle rocks that is of great importance to their performance for underground facilities. A major challenge in rock fracture mechanics research at its current stage of development is the extrapolation of parameter values measured at laboratory-scale on small, intact rock specimens to large-scale jointed rock masses. Understanding crack growth is therefore very important for assessing the stability of rock masses/materials and for the safe design of structures within rock masses. The aim of this thesis is to investigate the tensile strength parameters of rocks, such as the indirect tensile strength and fracture toughness value, under monotonic static and cyclic loading conditions. The indirect tensile strength of rocks was initially investigated using the standard Brazilian test. However, this was found to crush the brittle rocks tested, and steel loading arcs of different angles were assessed to obtain more reliable diametral tensile splitting cracks in disc specimens, as expected theoretically. The maximum recorded ultimate load and most appropriate tensile splitting crack was obtained with a loading arc angle 2α = 30. However, the highest standard deviation of the ultimate load was obtained from the standard Brazilian test results. The most fundamental parameter in fracture mechanics is the fracture toughness, which describes the resistance of a material to crack propagation. Fracture toughness is an important material parameter that corresponds to the critical state of the stress intensity factor required for crack initiation and subsequent propagation. Both the static and cyclic loading tests were carried out on Cracked Chevron Notched Brazilian Disc (CCNBD) rock specimens for fracture toughness investigations. In order to yield consistent fracture toughness values, two test methods were performed: CCNBD and short rod (SR) tests. After a series of experiments, the fracture toughness value obtained by the SR method was found to be higher than the value obtained by the CCNBD method. It was realised that the critical stress intensity factor (SIF) used in the formula for calculating the fracture toughness of the CCNBD specimens is a crucial factor in the method proposed by the ISRM (ISRM, 1995). In the cyclic loading tests, the reduction in fracture toughness under increasing cyclic loading was found to be up to 48% of the static fracture toughness. The experimental results showed continuous irreversible damage accumulation at the tip of the chevron notched crack in the dynamic cyclic loading tests, strongly influenced by the loading amplitude. Contrary to the static tests, the cyclic tests produced much more crushed material in front of the tip of the chevron notched crack. On the other hand, the reduction in the indirect tensile strength (ITS) was found to be 30% of the static ITS with sinusoidal cyclic loading tests, whereas increasing cyclic loading with the same valley values cause a reduction of the ITS by up to 35 % of the static ITS. In order to examine the major characteristics of the fatigue damage process at the microscopic level, scanning electron microscope (SEM) imaging was used. SEM investigations show that particle boundaries play an important role in rock fatigue. When compared with static rupture, the main differences are two-fold: (a) the number of fragments produced is much greater under cyclic loading than under static loading, and (b) intergranular cracks are formed due to grain loosening under cyclic loading. The main characteristic is particle breakage under cyclic loading, which probably starts at contacts between particles and is accompanied by the production of very small fragments, probably due to frictional sliding within the weak matrix. It is believed that point contacts at particle boundaries are regions of stress concentration (i.e. indenters). Transgranular cracks may emanate from these regions, and intergranular cracks sometimes pass through the contact points. This stage can be correlated with a steady progression of damage and produces a general “loosening” of the rock, which is a precursor to the formation of intergranular cracks.