Simulating fault slip areas of mining induced seismic tremors using static boundary element numerical modelling

Abstract

Two mining induced tremors that occurred in a South African deep-level gold mine are considered, with the aim to simulate the shear slip area and source mechanism using static boundary element numerical modelling. Large seismic tremors, induced by the extraction of the tabular ore body typically 1 m high over horizontal spans of kilometres, are a major seismic hazard in South African gold mines. For the cases considered here, virgin stress levels are y54 MPa, and seismic related damage is mainly due to geological faults failing in shear under gravitational loading. The tremors occurred y2 km below surface, 700 m apart in space, and in consecutive months. Seismic recordings and underground observations suggested that the tremors, of local magnitudes 3?0 and 4?0 respectively, were associated with normal slip on similar geological faults. A boundary element numerical model was implemented to some degree of accuracy, since the fault geometries could be inferred from extensive mining spans and fault intersections, and an in situ stress state could be estimated from other available information. Furthermore, the rock mass stress changes and associated surface stress changes on faults could be modelled from monthly measured advance of the mining excavations. Simulation of the seismic sources entailed finding appropriate Mohr-Coulomb strength parameters to allow yielding of the faults at the actual mining stage, and quantitatively in agreement with the seismic moment as inferred from seismic waveform recordings. A critical input into the model was the use of non-zero cohesion in addition to friction angle as specified in the Mohr-Coulomb shear failure criterion, allowing the simulation of coseismic slip. Hence, the breaking of fault cohesion and subsequent stress transfer to the surrounding fault area is simulated, albeit statically, conforming to the perceived triggering of mining related seismic tremors by induced stress changes. The back analysis furthermore suggested that coseismic stope closure can comprise a significant part of the total recorded seismic moment, depending on the proximity of the hypocentre to mining excavations. From the back analysis a modelling methodology and input parameters are proposed, aiming at identifying vulnerable mining excavations subject to seismic hazard posed by geological structures, hence providing relevant knowledge towards safer mining.

Dynamic modelling of a mining-induced fault slip

“…As reported by Alber (2013), the magnitudes of strain bursts are generally less than 0; and the magnitudes of pillar bursts usually fall between 1.0 2 and 2.5. In contrast, seismic events induced by shear failure of rockmasses or fault reactivation have magnitudes greater than 2.5; occasionally the magnitude exceeds 4.0 (Alber, 2013;Hofmann & Scheepers, 2011), thus resulting in devastating damage to mine openings in a large area within underground mines.…”

“…Numerical analysis software, which employs finite difference methods (FDM), is used to simulate the behaviour of geological discontinuities (Cappa & Rutqvist, 2010;Hai-min, Yi-ming, & jun-gao, 2011;Zhi-hua, Lin-ming, Cai-ping, Zong-long, & An-ye, 2008). A boundary element method (BEM) is employed to simulate the areas of fault-slip that occurred in an underground mine, South Africa (Hofmann & Scheepers, 2011). Simon (1999) analyzed the behaviour of fault-slip in hard rock mines with BEM while applying a newly developed shear strength model.…”

“…In the field of rock mechanics, the simulation of fault-slip in underground mines has been carried out, aiming at estimating the mechanical properties of faults, evaluating fault-slip potential, and investigating the effect of faults on ambient stress states, albeit the classical Mohr-Coulomb criterion has been mainly used under static conditions (Alber, Fritschen, Bischoff, & Meier, 2009;Hofmann & Scheepers, 2011;Sneilling et al, 2013).…”

“…It is the classical Mohr-Coulomb criterion that is used the most widely for the stability analysis of faults in underground mines (Alber et al, 2009;Hofmann & Scheepers, 2011;Potvin, Jarufe, & Wesseloo, 2010;Sjoberg et al, 2012;Sneilling et al, 2013) because of its simplicity. The failure criterion is expressed as follows:…”

Water Inrush Mechanism Caused by Fault Activation Based on Stress Evolution of Surrounding Rock of Key Aquifer

“…Liang et al used field test method to test the water pressure resistance of the normal fault and put forward the failure capacity index of unit seepage to reflect whether the fault activation occurred [14]. Based on the Mohr-Kulun failure criterion, Hoffman studied the variation law of the slip zone cohesive force when fault activation was unstable and concluded that the change of cohesion was the key to the activation of faults [15]. Zhang et al investigated the process of water inrush from fault caused by coal seam mining above the confined aquifer by physical simulation test and found that mining causes fault permeability enhancement, fault zone seepage flow from small to large which could be divided into three distinct phases, and the final fault generated activation of water inrush [16].…”