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INTRODUCTION ABSTRACT: A displacement discontinuity model is described for simulating time-dependent failure in a random assembly of crack elements. It is shown that the slip relaxation parameter must be scaled in proportion to the crack length if the relaxation time function is to be self-similar for all element sizes. Equivalence between the assumed slip rate law, in which the slip rate is proportional to the driving shear stress, and an exponential decay of cohesion is shown to be possible for particular choices of the loading stress. Numerical experiments of the dead weight loading of a random assembly of cracks are described to illustrate the behaviour of the model. It is found that the time series of the energy release and the length of active fractures are highly nonlinear. Attempts to identify precursory information to estimate the time to failure from the cumulative energy release rate series proved to be unsuccessful. In particular, no evidence could be found for any form of organized oscillatory behaviour in the time series prior to the time of failure. The sudden failure of the rock mass close to deep level mine excavations presents an ongoing hazard in the successful exploitation of these mineral resources. In order to establish a mechanistic link between seismic activity and mining operations, it is necessary to understand both the physics of the failure processes and the inherent uncertainty in quantifying the time of failure. These demands are analogous to the persistent problem of the prediction of seismic recurrence cycles in earthquake mechanics. It appears that an essential step towards quantifying the behaviour of large scale deforming rock masses is to describe the statistical or chaotic nature of these systems and, if possible, to determine the circumstances in which the time of significant instabilities can be predicted. This paper outlines a numerical approach which is able to simulate failure in rock using a random grid of cracks which are represented by interacting displacement discontinuity boundary elements. This type of model is closely linked to numerous lattice models of crack formation that have been proposed recently. (Some examples are reported by Lockner & Madden 1991, Schlangcn & Van Mier 1995 and van den Burg & van der Giessen 1994). The model presented in this paper has been shown to be able to reproduce effects such as the primary, secondary and tertiary creep in a simulated static fatigue test (Napier & Malan 1997). Some further results are now presented which relate to the scaling of the relaxation parameter used in the creep model. Numerical simulations of a dead weight loaded sample are presented to explore the question of whether the incipient onset of instability can be inferred from the time series of energy release increments which are observed prior to the time of failure.
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Materials > Metals & Mining (0.86)
- Energy > Oil & Gas > Upstream (0.50)
INTRODUCTION ABSTRACT: Measurements in the deep tabular excavations of the South African gold mines indicate significant time-dependent deformation. Although the host rock consists of hard brittle material, creep-like movements of the order of 0.6 mm/h have been observed in certain areas. This paper describes measurements of time-dependent deformation in three different geotechnical environments. The time-dependent behaviour is the result of the theology of the mining-induced fracture zone surrounding the excavations at these depths, while pre-existing discontinuities such as bedding planes also play a prominent role. It appears that detailed monitoring of stope closure profiles can provide a useful signature of the geotechnical response of the rock mass to the mining process. The closure profiles are simulated using a displacement discontinuity framework that allows for the time-dependent failure processes. Preliminary studies indicated that the initial jump in stope closure after blasting is correlated with the stope face stress. The design of deep level mine layout configurations and the implementation of effective strata control strategies is often reliant on direct experience of mining conditions in different geotechnical environments. If a basic understanding of the underlying failure processes and mechanisms is available, it is possible that these strategies can be refined to reduce the risk of rockburst and rockfall incidents. Considerable research effort has beon directed over many years towards accomplishing this understanding by detailed observations of the fracture zone surrounding deep level stopes (Leeman 1960, Adams & Jager 1981, Bmmmer & Rorke 1984, Legge 1984) and by the development of theoretical models of stress redistribution in the rock mass (Salamon 1974, Cundall et al. 1994). More recently, researchas shown that deep stopes in hard rock undergo significant time-dependent deformation (Malan 1998). The improved understanding of the time-dependent rock response provides a firm foundation for the assessment of different mining rates or multiple shift operations (Napier & Malan 1997). In this paper, continuous closure data collected in various geotechnical areas of the South African gold mining industry are presented to illustrate the characteristic response of the rock mass in each area. The time-dependent failure processes are modelled by means of a random discontinuity assembly in a viscoplastic displacement discontinuty framework. The simulated behaviour of a small span stope is described to illustrate the correspondence between the closure in the model and the field data.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics (0.87)
- Geology > Mineral > Native Element Mineral > Gold (0.69)
- Oceania > Papua New Guinea > Papuan Peninsula > Central Province > National Capital District > Petroleum Retention License 15 > P’nyang Field (0.98)
- Oceania > Papua New Guinea > Papuan Peninsula > Central Province > National Capital District > Petroleum Retention License 15 > Elk-Antelope Field (0.98)
- Oceania > Papua New Guinea > Papuan Peninsula > Central Province > National Capital District > Petroleum Retention License 15 > Angore Field (0.98)
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