As mining operations go deeper, along with an increase of on-situ stresses, faults may be reactivated and seismic events may be induced. This paper describes several cases of mining induced seismic events and focuses on the strength of faults. From stress analyses it is known that very small amounts of induced stresses are necessary to activate faults as many faults are critically loaded. From back calculation fault friction angles are deduced which are well below published values such as the Byerlee friction angle of 40?. It has been found that friction angles as low as 18? must be present at mining depth. These values are substantiated by laboratory tests on slicken sides.
Several geological and geotechnical properties of the coal seam and of the coal measure rocks are observed when coal mining methods are selected. Seam undulations and seam thickness determine the longwall system. Reserves limit the cost of the mining system and the coal quality determines the price of the product. Roof and floor stability are important for gate development and longwall operations, particularly track and belt operations. Structural characteristics (dip and strike of the seam and faults) determine the blocks of reserve. During exploration focus is placed on estimating fault displacement – if exceeding seam thickness longwall mining may be excluded. In-seam seismic techniques and extensive drilling may allow accounting for the presence of faults. However, as mining operations go deeper, along with an increase of on-situ stresses, faults may be reactivated and seismic events may be induced.
Seismic events are the results of rock/rock mass failures. Failure in rock mechanics is anticipated by comparing stresses acting on and strengths of the material. There exist basically two different types of failure: rock/rock mass failure by formation of new fractures or slip of an existing discontinuity by exceeding its strength. Discontinuities include bedding planes, slicken sides, joints and particularly faults. Especially slip on discontinuities is attributed to noticeable seismic events while mining. Accordingly, realistic assumption of stresses and strength is necessary but difficult to pursue.
Both types of failures may be observed on large scale during mining and allow to draw constraints for rock mass strength and especially for faults.