In the paper the external collapse angle is introduced. It is the angle between the breaking plane and the vertical plane of the roof. To use this angle can calculate the roof apparent pressure and the roof equilibrium condition. This will much simplify the design of roof support without routine tests in the laboratory. The mining practice in undersea mining roadway No. H2101, Shandong Province proved its usefulness.
The conception of the external collapse angle is proposed in the mining practice of the first under-sea coal mine in China. It is a useful means to calculate and predict the roof pressure and equilibrium condition in mining roadways. In the paper, we defined the external collapse angle and then using it, calculated the roof pressure and the roof equilibrium condition of undersea mining roadway No. H2101, Shandong Province.
When under-sea coal mining comes to a certain extent, the rock mass above the gob area tends to swell, subsidence and fracturing. The angle between the breaking plane OB and the horizontal plane is the collapse angle of the rock mass, β and the angle between the breaking plane and the vertical plane is the external collapse angle α (Fig. 1). In other words, the complementary angle of the collapse angle is the external collapse angle of the rock mass. When using a large-scale-collapse mining method, the rock mass above the gob area will generally break up and subside, having α =90° −β. But when using a small-scale-collapse mining method, the rock mass will not break or subside, having α< 90° −β. Since the break and subsidence of the rock mass above the gob area occur usually later than moving of surrounding strata, the self-weight of the overlying rock mass, σ 1 (primitive stress), will not act on the underground space, but in the form of the normal stress σ on the breaking plane OB will be transferred to the underground space.
