Abstract：The law of bottom failure depth is the key reference to safety decision in mining above confined water. Usually，research on the failure depth of coal seam floor is performed through water injecting into cracks caused by mining，numerical simulations，and similitude material experimentation，etc.. Recently，CT and other physical methods are used to measure the integrality/damage of rock mass，even in some coal mines. Shear wave velocity is an in-situ test method and suitable for carrying out on the surface. The trial is made to gain the bottom failure depth in work face 11703 of Tianzhuang coal mine，which is beneath the coal seam No.17 with a thickness of 1.2 m in average and a distance of 20～25 m to No.14 limestone，and about 50 m away from Ordovician limestone. The test is performed before and after mining in two locations，each lying in the roadway of the workface. Accordingly，a plane of strike direction and two planes of incline direction through the testing locations are ascertained to simulate the mining process；and the distribution of tensile stress，plastic area，and variation of plastic zones are analyzed，from which the failure area of coal seam can be divided into severely destroyed area，obvious disturbance area，and affected area. Simultaneously，the distinct variance points of wave velocities recorded before and after mining are at the depth of 13.27 m and 13.63 m，respectively，with which the simulation results agree well. Then the conclusion can be made that the failure depth is shorter，about 11～12 m，in the middle of workface and longer，about 13.5 m at the edge of working face. The results show that shear wave velocity method is reliable to detect the failure zones in the bottom of coal seams in the working face 11703.
