Forecasting coal burst risks near various types of faults, folds and dykes

Stress state and geotechnical conditions often change significantly near major geological structures (e.g. faults, shear zones, dykes), which is the cause of most major mine instabilities and/or safety hazards including coal burst, roof falls, water inrush, gas outburst. A previous project C26053, for which this one is an extension, investigated stress changes and/or microseismic activities near a major dyke using field monitoring and numerical modelling techniques. This project investigated various types of geological structures to determine their effects on the risk of coal burst via a literature review, case studies, analyses of stresses and microseismic data from previous projects and 3D geotechnical numerical modelling.

Geological structures have been reported as a major factor in association with rock bursts and coal bursts. They could trigger such dangerous dynamic failure through:

• seismic wave induced by the slip of a fault;
• high stress concentration from the stress redistribution caused by a major geological structure;
• lack of roof beam integrity during longwall mining, which leads to high abutment stresses.

Geological structures are the result of past tectonic stress and movements that will have caused stress redistribution and likely to have cause stress concentration in the local areas. For a normal fault or a thrust fault, it is found that stresses at the footwall side of the geological structure are significantly elevated from the far-field stresses and are higher than those at the hanging wall side.

When mining through a fault using longwall mining methods, the mining direction has a major influence on the stress concentration changes near fault. Mining from the hanging wall to the footwall of a fault or dyke can reduce the stress concentration nearby, hence having a lower risk of coal burst or gas outburst than mining in the opposite direction.

This study has found that in-situ horizontal stress magnitudes have a major effect on stress concentration near faults. Higher horizontal stresses lead to higher stress concentration and higher risk of dynamic instabilities.

Only simple gentle folds are investigated in this study. For the limited cases of fold geometries, it is found that the stresses in the seam and roof/floor rocks are not significantly affected by a gentle fold. This conclusion may not apply to cases where severe folded structures are present and uneven stresses could be locked into the rock mass from historical tectonic processes, as suggested by some literature.