Seismic and micromechanical studies of rock fracture

Abstract

Abstract. "Earthquakes" occur as the result of stress redistribution on major fractures in the earth's crust and are also observed as scaled phenomena along grain boundaries and microcracks. Earthquake seismology has significantly contributed to our knowledge of fault processes, but our fundamental understanding of how micro-fractures progressively weaken rocks and how this contributes to macro-deformation processes is far from understood. Recent advances in particulate mechanics now mean fracture processes can be modelled dynamically to study the micromechanics of fracturing in rock. In addition, advances in recording and analysing very high frequency acoustic emissions (AE) allow for detailed examination of microcracking. The paper describes how particle models and AE monitoring techniques can be used in conjunction to test specific hypotheses about natural and induced rock fracture processes at the grain scale. Intermediate scale processes (between laboratory and field studies) are also studied by examining rock fracture in an underground research laboratory. Rock Fracture and UnloadingFracturing contributes to seismic behaviour, mechanical properties and rock fluid interactions and is also important at all scales in nature. Many cracks and fractures in nature form as a result of stress unloading. For example, the excavation of a tunnel or drilling of a borehole subjects the surrounding rock to a complex loading/unloading history during the removal of the rock. This process provides a potential physical analogue for unloading structures in nature, such as jointing produced by uplift. However, a large proportion of all experimental rock testing is carried out under compressive regimes.Recent advances in particulate mechanics and experimental methods now allow hypotheses about unloading mechanics and stress history to be tested with laboratory polyaxial stress path experiments and numerical models. Results from these tests can be used to interpret information obtained from in Laboratory StudiesMethod Three-dimensional stresses and stress paths can be applied to a rock sample to induce damage under realistic in situ stress conditions using a true-triaxial loading/unloading system developed at Imperial College, London. The damage in the sample is monitored using a variety of laboratory techniques including triaxial strain measurement and ultrasonic velocity/attenuation determination [King, 1995]. The system has recently been modified to include a twelve-transducer high frequency acoustic emission (AE) array [Petatt et al., 1998]. This is used in conjunction with a sophisticated monitoring system similar to that used in situ to monitor damage around underground excavations at the URL and elsewhere [Falls and Young, 1998, Young and Collins, 1999]. Accurate three dimensional hypocentral locations can then be obtained for AEs in the sample volume. This gives the ability to directly observe the localisation of microcrack damage in the sample.Because full-waveforms are recorded, amplitude data is available for the AE...

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Bonded-particle modeling of thermally fractured granite

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