Experimental investigation of damage evolution during strain burst in brittle rocks for deep mines

Abstract

The increasing demand for resources and depletion of near ground mineral resources caused deeper mining operations under high-stress and high-temperature rock mass conditions. As a results of this, strain burst, which is the sudden and violent release of stored strain energy during dynamic brittle failure of rocks, has become more prevalent and created considerable safety risks damaging underground infrastructures. This research focuses on the development of experimental methodologies to better understand the fundamental knowledge concerning the failure mechanism of strain burst and the influence of thermal damage, high confining pressure and various loading rate on the overall mechanical behaviour of highly brittle granitic rocks leading to strain burst. Strain burst is related to the elastic stored strain energy and how this stored energy is released during the unstable spontaneous failure. Therefore, it is significant to investigate the energy state during strain burst from the viewpoint of energy theory. In this sense, circumferential strain controlled quasi-static tests on Class II rocks over a wide range of confining pressures at different heat-treatment temperatures were conducted to capture the snap-back behaviour and calculate excess strain energy that is responsible for the spontaneous instability. A new energy calculation method associated with acoustic emission (AE) was developed to express the propensity of strain burst and investigate the post-peak energy distribution characteristics for brittle rocks under the coupling influence of confinement and temperature. In order to quantify the micro-crack density and reveal the micro-fracture characteristics of the brittle rocks exposed to various temperatures, scanning electron microscopy (SEM) analysis was also conducted. This is highly relevant to link the excess strain energy and the main failure mechanism triggering strain burst under high-temperature condition. The failure process of strain burst is the outcome of the unstable growth and coalescence of secondary micro-cracks. If the dissipative energy to grow pre-existing cracks and the secondary cracks is smaller than the elastic stored strain energy in rock masses, the residual strain energy will be released suddenly in the form of kinetic energy, resulting in ejecting high-velocity rock fragments. Therefore, understanding the crack initiation and propagation in rocks is of great concern for engineering stability and security. As an intrinsic property of rocks to resist crack initiation and propagation, the rock fracture toughness is the most significant material property in fracture mechanics. In this respect, the three-point bending method was applied using cracked chevron notched semi-circular bend (CCNSCB) granite specimens subjected to different temperatures under a wide range of loading rates in pure mode I. A suitable relation for the dimensionless stress intensity factor (𝑌∗) of SCB with chevron notch samples were presented based on the normalised crack length (𝛼) and half-distance between support rollers (𝑆/2). The minimum dimensionless stress intensity factor (𝑌𝑚𝑖𝑛∗) of CCNSCB specimens were determined using an analytical method, i.e., Bluhm’s slice synthesis method. In this study, the influence of thermal damage and loading rate on the quasi-static mode I fracture toughness and the energy-release rate using CCNSCB method was investigated. In the deep mining process, the rock mass is subjected to a dynamic disturbance caused by blasting, and mechanical drilling resulting in dynamic fractures in the forms of strain burst, slabbing, and spalling. The dynamic rock fracture parameters, including dynamic initiation fracture toughness and fracture energy which are an important manifestation of dynamic rock failure (strain burst) in deep underground engineering and they are of great practical significance to assess the dynamic fracture behaviour of deep rock masses. Since deep rock engineering operations in high temperature and high pressure environment is prone to strain burst, the influence of thermally induced damage on the dynamic failure parameters of granite specimens was investigated. The damage evolution of granitic rocks were studied over a wide range of loading rates to reveal the rate dependency of strain burst. Dynamic fracture toughness tests were carried out on granite under different temperatures and impact loadings using a Split Hopkinson Pressure Bar (SHPB) apparatus at Monash University. With dynamic force balance achieved in the dynamic tests, the stable-unstable transition of the crack propagation crack was observed and the dynamic initiation fracture toughness was calculated from the dynamic peak load. The thermal damage influence on strain burst characteristics of brittle rocks under true-triaxial loading-unloading conditions was investigated using the AE and kinetic energy analyses. A unique strain burst testing system enabling to simulate the creation of excavation at the State Key Laboratory for Geomechanics and Deep Underground Engineering in Beijing (China) was used to replicate strain burst condition. Time-domain and frequency-domain responses AE waves related to strain burst were studied, and the damage evolution was quantified by b-values, cumulative AE energy and events rates that can be used as warning signals to rock failure. The ejection velocities of the rock fragments from the free face of the granite specimens were used to calculate kinetic energies which can be used as an indicator for quantitatively evaluating the intensity of strain burst. Based on the energy evolution characteristics of brittle rocks under uniaxial and triaxial compression, true-triaxial loading-unloading and three-point bending, new strain burst proneness indexes and strain burst criterion were proposed. The effects of temperature, confinement and loading rate on strain burst proneness were discussed. This study aims to advance the understanding on underlying processes that govern the macro-behaviour of brittle rocks during strain burst and make use of this insight to further advance our current predictive capabilities of strain burst with references to large-scale underground mining. Using the developed experimental methodologies in this study, fractures around an excavation to reduce the amount of excess strain energy leading to strain burst can be determined and ultimately incipient strain burst in deep mines can be predicted avoiding potential hazards. Using the methodology for forecasting of strain burst in this research can be used for enhanced understanding of the design of rock support in strain burst-prone areas in deep mining activities. The findings of this study will facilitate achieving a better and comprehensive understanding of the damage process during strain burst in deep mines. This study underpins the development of better and more efficient prediction methods for strain burst which will lead to better planning guidelines and ultimately safer deep underground working conditions.