The effect of seam dip on the application of the longwall top coal caving method for inclined thick seams

Abstract

This thesis presents the results of research into the potential of underground mining methods applicable to inclined thick seams (thickness greater than 3.5m, and seam dip of 15 - 35 degree) in the Quangninh coalfield, Vietnam. The primary objectives of this research are to investigate the most suitable underground methods applicable to inclined thick seams in the Quangninh coalfield and to improve understanding of the operational and geotechnical issues associated with the application of chosen methods in thick seams with steeply dipping conditions.

From a risk and operational assessment, the Longwall Top Coal Caving (LTCC) method is considered most appropriate for inclined thick seams under the current mining conditions in Vietnam. The LTCC method offers great potential for the efficient extraction of thick seams by caving coal from the upper section during the mining of the lower section. This significantly reduces the development cost per tonne. Compared to High reach Single Pass Longwalling, the LTCC method offers a low extraction height, resulting in smaller and less expensive equipment and better face conditions.

Results from this study identified that for extracting an inclined thick seam, the face retreating along the strike has better operational advantages and better cavability than the face retreating updip or downdip of the seam. The operational issues of the LTCC method when extracting inclined seams are: the stability of the support, transport in the mine, and the difficulties in roof control at the transition between face ends and the gateroads. In terms of geotechnical issues, better cavability of the top coal is anticipated for flat coal seams compared to inclined seams. In addition, the chain pillar for flat coal seams is subjected to a higher vertical stress distribution than that of inclined ones.

An improved cavability assessment method for the categorisation of the cavability of the top coal with four categories, ranging from 1 (excellent cavability) to 4 (very poor cavability), was suggested to assist the feasibility and design stages of the LTCC application. The cavability assessment method was conducted by numerical analysis combined with back analysis from the database obtained from past LTCC practices.