3D Stress Effects of Longwall Caving as Revealed by Microseismic Monitoring

The use of microseismic monitoring to resolve caving issues and to provide warning of weightings, windblasts and other caving related problems has been demonstrated by a number of ACARP and industry funded projects. In this, the latest project, the direct association between microseismic activity and rock damage is investigated. The objectives of this project were to look into this issue through:

• Determining the nature of microseismic events in terms of actual rock damage/fracture.

• Providing 3D longwall caving mechanistic models and monitoring systems based on the results of microseismic and geotechnical monitoring.
• Laboratory studies of the acoustic emissions from rock samples undergoing destructive testing,
• Detailed assessment of the microseismic results from one or more of CSIRO's monitoring sites,
• Development of 3D numerical models of the longwall caving at that site which were to be combined with the microseismic results and any other geotechnical information available,
• Extension of the findings from these detailed studies to other longwall situations in Australia and overseas.

The site chosen for the detailed study was LW704 at Southern Colliery. This was mined in 1999 and was the subject of a separate microseismic and geotechnical study undertaken by CSIRO (Guo et al., 2000).

Main Findings and Conclusions

Rock failure as expressed by microseismic activity tends to start at stress levels well below the strength of the rock. For intact rock, this activity may commence at stresses of less than about half the rock strength. If fractures, bedding planes or other forms of weakness are present, the failure may be initiated on the defects. In the context of microseismic monitoring at longwall mines, detectable microseismic activity may occur well over 100 m ahead of the face in regions where the abutment stresses have only had a minor effect on the in-situ stress field. The amount of microseismic activity then steadily increases as the face approaches. This pattern of behaviour has been frequently observed through field monitoring and in laboratory acoustic emissions experiments. If the stress build-up temporarily ceases through lulls in the mine production or as a result of cyclic loading, the amount of microseismic activity decreases.

The energy associated with the microseismic events depends on the size and type of rock failure. Shear failure of intact rock produces more energy than bedding plane shear. Tensile failures produce the least energy. Microseismic events are conventionally interpreted in terms of these failure mechanisms. Strong events that have an implosive component to the failure mechanism have also been identified but the significance of these is currently unclear.

Analytical techniques have been developed to allow a detailed assessment of the nature of the microseismic events due to the mining of coal bearing strata.

Potential For Short Term Commercial Applications

This project has been a significant integrated study into longwall geomechanics. Applications exist at the daily operational level and at the level of the understanding of the general geomechanical response at the panel to mine scale.

Operational applications

As demonstrated by the microseismic monitoring under wind blast conditions at Moonee Colliery, microseismics can be used to provide warnings of impending roof falls. Through this project, analysis procedures have been developed to determine crack dimensions. The cumulative crack size indicates the growth of the failure surface. In a situation such as Moonee, roof falls can be expected when the cumulative crack size is about the same size as the panel width. Microseismic monitoring in situations involving massive roofs and potential windblasts should continue. When microseismic methods are used for this type of application, the geophone arrays need to be adequate to determine microseismic source locations. Real time analysis is also required.

Panel/mine scale applications

Through this project and other recent studies into longwall geomechanics, the complexity of the interaction between the geological conditions, the stress field and the operation of the longwall face has been revealed. For example, over the 500 m length of Southern Colliery LW704 that was studied, quite different caving behaviours occur as a result of changes in the thickness of the sandstone in the immediate roof and through the existence of a parting plane 60 m into the roof and joints that facilitate an interaction with strata in the intermediate roof. Longwall operations need to be sufficiently robust to ensure that these and other changes in conditions can be accommodated without compromising production or safety