Application Study On Deformation And Failure And Seepage Evolution Law Of Overlying Soil Layer In Coal Seam Mining In Thin Bedrock Area

Thin bedrock coal seam areas widely exist in many coalfields in China.That is,the fracture zone or even the caving zone develops through the bedrock section and enters the Cenozoic overburden.Then the Cenozoic soil layer overlying the thin bedrock,especially the cohesive soil lyaer,becomes the key structure to block shallow groundwater and surface water.In this dissertation,the characteristics of deformation and failure and discriminant method and seepage law and crack extend-closure characteristics of overlying soil layer in thin bedrock coal seam mining are systematically studied by means of experimental research and theoretical analysis.The main conclusions are as follows,(1)According to the theory of shear strength of soil and in view of the characteristics of the overlying soil layer of thin bedrock which is mainly subjected to self weight load and mainly to vertical deformation,the failure criterion of overlying soil layer expressed by subsidence is deduced.Firstly,the relationship between vertical stress,horizontal stress,shear stress and maximum principal stress and minimum principal stress is deduced.Then,the failure criterion of overlying soil layer in thin bedrock expressed by subsidence is deduced by establishing the relationship between subsidence and vertical stress,horizontal stress and shear stress.The failure criterion is???.According to the cirterion,the destructive subsidence of soil layer can be calculated by means of experiment or numerical analysis,and the destructive subsidence curve can be drawn to distinguish the failure state of soil layer,which provides a new idea and method for the analysis of the destructive state of the overlying soil layer on thin bedrock.(2)On the basis of the extend-closure characteristics of soil crack and the static expressions and dynamic expressions of crack extend-closure in soil layer,the discriminant???of crack extend-closure state in soil layer is deduced.The crack size decreases and soil crack tend to close when???,the crack expands and soil layer tends to failure when???,the crack is stable and soil layer is stable when???,the crack re-closure when Cw(t)=0.Theoretical analysis shows that the extend-closure state of soil crack is determined by the soil swelling rate and the compaction speed under stress and the widening speed of flow scouring and the first derivative of the three factors.(3)The variation law of crack density and height of overlying soil layer is obtained when the thickness of thin bedrock decreases from 70 m to 20 m.It is considered that the shear/tension cracks developed in the near vertical direction are the key factors affecting the failure of soil layer.There is a critical value for the influence of the reduction of the thickness of thin bedrock coal seam on the crack development of overlying soil layer.For the the mining thickness is 6 m and the overburden layer thickness is 180 m,the critical value of bedrock thickness is 30 m.That is,when the bedrock thickness is more than 30 m,the development height of shear/tension cracks in the soil layer is smaller and the density of cracks is 0.04 strip/m.When the bedrock thickness is less than 30 m,the development height of separation cracks reaches the surface and the development height of shear/tension cracks reaches 66%of the total height of the soil layer and the density of shear/tension cracks increases to 0.14 strip/m.(4)Based on the similarity simulation experiment and numerical simulation analysis,it is concluded that the soil layer in the middle area of working face is most likely to form seepage channel and lose water-blocking capacity.The aquifers in this area is most affected by the mining and the vector distribution of the seepage field is the most dense.When the bedrock thickness is less than 30 m(mining thickness is 6 m and overburden layer is 180 m),the mining u=0 Pa isopotential seepage zone begins to form in the soil layer of this area and the seepage zone communicates vertically and downwards to the middle part of the overburden layer.When the bedrock thickness decreases to 20 m,the mining u=0 Pa isopotential seepage zone communicates vertically and downwards to goaf to form seepage channel.(5)The order of influence of different factors on the extend-closure of soil crack is crack width>particle composition>soil layer thickness>soil water content according to the crack extend-closure test,crack width and soil particle composition are the primary factors affecting crack extend-closure and soil layer thickness is the secondary factor,while soil water content has little effect on crack extend-closure.The test results also show that the greater the sand content and the wider the crack width and the smaller the thickness of the soil layer,the easier the crack will expand and cause soil layer failure.The crack will close when the sand content in soil is less than 50%and the crack width is less than or equal to 1.0 mm,the soil layer is prone to further failure when the sand content in soil is more than 50%and the crack width is greater than or equal to 3.0 mm.There are 99 figures and 41 tables and 222 references in this dissertation.