The influence of rockmass properties on the seismic response to miming in a deep level gild mine

Abstract

Keywords:

Rockbursts, associated with mining induced seismic events, is the biggest risk in ultra-deep level tabular hard rock mines. The high seismic response to production in these mines is a function of the high stress levels owing to the great depth below surface and the rockmass properties. Some rock types are more prone to seismic damage than other rock types. Current risk mitigation strategies are focussed on managing the stress redistribution on stope faces and mining abutments by partial extraction and controlling the maximum mining spans, placing mine-wide backfill and managing the overall mining face shapes and inter-panel lead-lag distances. Whilst these strategies can be accommodated in mine design and implementation, information regarding the rockmass properties is generally sparse, especially in terms of local variations in strength. Typically, the strata in which mining activities occur are composed of successive layers of different rock types with varying strength properties as well as geological structures. The seismic response can be expected to differ according to these different rock types and structural domains. Mponeng Mine is the deepest mine in the world. The deepest stopes are on 127 Level at a depth of approximately 3700 m below surface. For the Ventersdorp Contact Reef (VCR) at the mine, two distinct areas based on footwall lithology can be identified. In the eastern side of the mine the footwall is shale and on the western side the footwall is quartzite. A simplified model of the shale illustrates that this layer dips at a shallow angle underneath the quartzite towards the west. The quartzite footwall is absent or starts off thin in the east and gradually increases in thickness towards the west.

In this dissertation, the seismic response to mining on the eastern side of the mine where the footwall is a siltstone (metamorphosed shale rock that is generally referred to as shale on the mine and in the rest of this document) is quantified and compared to the seismic response on the western side of the mine where the footwall is a strong quartzite. Careful data selection was done to investigate the effect of the footwall rock type on the measured seismic response to mining. The objective was that all other factors that may influence the seismic response to mining varied as little as possible between the two areas of comparison. It is shown that the seismic response to mining in the area with a shale footwall is different to the seismic response in an area with a quartzite footwall. The number of large events is higher on the shale footwall compared to the quartzite footwall while the number of smaller events is higher on the quartzite footwall compared to the shale footwall.

By comparing the normalised seismic response to mining for the shale footwall area with the quartzite footwall area in terms of cumulative annual potency for the annual production (potency per m2 mined), it is shown that the normalised seismic response is higher on the shale footwall than on the quartzite footwall.

Comparing the seismic hazard in terms of the b-slope of the Gutenberg-Richter (GR) graph confirms that the seismic hazard associated with mining on the shale footwall is higher than for mining on the quartzite footwall as more events in the large magnitude range can be expected in the shale footwall area. A comparison of the seismic hazard in terms of the MMax from the GR graphs further supports the interpretation that the seismic hazard associated with mining on the shale footwall is higher than for mining on the quartzite footwall.

Map3D numerical modelling was used to quantify the modelled closure volume for a conceptual mining area at a depth of 3500 m below surface for two different rock strengths. Young’s modulus is the elastic parameter used in Map3D to control the elastic response of the host rock. Map3D does not support the modelling of different layers of rock with different material properties and a single approximation of Young’s Modulus must be used. For this preliminary modelling, the laboratory-determined Young’s modulus of the quartzite footwall (77 GPa) and for the shale footwall (63 GPa) was used as input in two models where all other parameters and the mining geometry were kept constant. As expected, the simulated closure volume for the shale footwall is higher than for the quartzite footwall for the same mining geometry, providing a possible explanation for the observed seismic response. The higher seismic response associated with mining on the shale footwall is associated with the higher closure volume compared to mining on the quartzite footwall. This conclusion is supported by the traditional ERR design criterion. The computed ERR value increases for an increase in closure and this has been shown to indicate a higher risk of rockbursts. The predicted increase in closure for the shale footwall should nevertheless be confirmed by underground measurements in future research.

Moment-tensor analyses are often used to interpret the possible mechanism of a seismic event. The isotropic component can be associated with implosion or volumetric deformation in the rockmass towards the source of the event. This is often interpreted as bursting at the skin of a mining excavation. The Compensated Linear Vector Dipole component can be associated with uniaxial deformation at the source, potentially associated with pillar failure and the Double Couple component potentially describes a planar slip mechanism where the orientation of the slip plane is also interpreted. The location of an event and the moment tensor decomposition can be used to identify the most likely source of a seismic event.

In this study it was found that the majority of the large magnitude events recorded at Mponeng Mine were face related (based on the moment-tensor analysis). This indicates shear failure through intact rock ahead of the mining front when the shear stress exceeds the shear strength of the rock. One of the factors to be considered is the fact that the virgin stress orientation (with σ1 orthogonal to the reef plane, σ2 in the direction of dip and σ3 in the direction of strike) is such that it promotes fracture orientation in the direction of dip (σ1-σ2 plane). The orientation of the overall mining fronts (overhand and underhand) are approximately 15° from this orientation. It is possible that the similarity in the overall face orientation and the most likely fracture orientation (based on the virgin stress orientation) promotes face related seismicity. Alternative mining configurations can be considered where the overall face orientation is not similar to the orientation of the most likely fracture orientation associated with the orientation of the virgin stress field.