A Review of Roof Instabilities Associated With Longwall Top Coal Caving

Longwall Top Coal Caving (LTCC) is a thick coal seam mining method which uses the Single Pass Longwall (SPL) method for extracting the lower section while the upper section (top coal) is mined by means of caving. The mining height in LTCC, compared to other methods, may result in roof instabilities such as caving and weighting. The understanding of roof instabilities involved in LTCC, however, is limited and the applicability of conventional rock mass stability assessment systems into LTCC can be questioned. This paper presents a systematic review of the LTCC-associated roof instability mechanisms and the applicability of widely-applied rock mass classification systems into LTCC. The study confirms that the vertical stress redistribution caused by LTCC is in general similar to that caused by SPL; the predominant failure mode in roof rock mass can be either shear or tension while the controlling failure mechanism in top coal is shear; the movement of immediate and main roofs in LTCC is similar to that in SPL; conventional rock mass classifications can be applied in assessing LTCC roof stability however their sensitivity to coal stability is low. The paper's findings can assist engineers in better applying and managing LTCC operation.