Strata movement and shield pressure analysis at Tongxin longwall top coal caving working face with extra-thick coal seam

Roof strata movement and shield pressure are two important factors that affect the production and efficiency of longwall top coal caving (LTCC) method in extra-thick coal seams. The performances of these two aspects can be very different from one mine to another depending on the roof strata conditions. In this paper, the geological conditions of the LTCC 8202 panel at Tongxin Coal Mine of Datong Coal Field are investigated. Low working face pressure was observed in the 8202 panel when mining the 20.6-m-thick carboniferous coal seam. This paper examines the causes of low mining–induced pressure at the LTCC 8202 working face, with both numerical simulations and in situ measurements. A finite element-discrete element software called CDEM is used for two-dimensional modelling of the #3–5 coal seam extraction at the LTCC 8202 working face. The numerical model simulates progressive collapse of the roof strata and computes shield pressure and convergence of the shield legs as the coal face advances. It is shown that the numerical model reproduces the periodical weighting phenomenon observed at the working face, which is created by the successive formation of composite cantilever beams in the lower hard rock layers, while voussoir beams form in the upper hard rock layers. As the upper roof voussoir beams interlock, they will tend to support part of the overlying strata, which ultimately lessens the pressure on the shield. A simple analytical model is developed to calculate the shield pressure under the composite cantilever beam structure. Finally, comparison of numerical simulation and analytical model results with measured shield pressure data from the field shows good agreement.