China is a large country of coal production and consumption. The reserves and outputs of the thick coal seam (coal thickness more than 3.5m) both amount to 45% of the total amount, and 95% of the output comes from the underground mines. The longwall top-coal caving technology (LTCT) was generalized in China at the beginning of the 1980's, and now the LTCT has grown up and occupied an important position in thick coal seam mining. In China the soft coal seam accounts for quite a lot of the thick coal seam. The paper mainly studies the working coal face stability of longwall top-coal caving in the soft coal seam by the distinct element numerical simulation software UDEC2D. The coal-cutting height is divided into five cases: 1.8m,2.0m, 2.2m, 2.5m, 2.8m and 3.2m. The results show that the limit coal-cutting height is 2.2m in the soft coal seam. Under this height, the movement behaviours of the overlying rock stratra for the different coal thickness are studied. The research conclusions are important to the practical mine production.

There are a lot of uncertainties in the mining engineering, and they make a notable impact on the engineering. The uncertainties of the physical and mechanical properties of geotechnical mass, the surrounding rock stability, the nature condition, the equipment operation, the personnel and the external environment are proposed and described in this paper. The stochastic analysis should be widely used for considering the impacts of the uncertainty on the mining engineering. The economical risk analysis and the stochastic index system should also be set up in the mining engineering.

Since the middle of 1980's, the Iongwall top-coal caving technique has developed rapidly in China. At present, it is one of the main approaches in the thick coal seam mining. This paper describes some mechanics problems of the caving technique, such as the damage and failure of the top-coal, the strata behaviors in the caving face, and the relation of the support and the surrounding rock. In order to employ the caving technique in a widespread scope, the problems such as the caving technique in the hard coal seam, the moving and running of the loose top-coal, and the upper floating of the gas etc. should be systematically studied.