Abstract：In order to study the crack evolution and gas seepage law with a lower protective layer under a medium-long distance, a numerical simulation study on crack evolution and gas seepage law with a lower protective layer under a medium-long distance as the research object of the Guozhuang mine by using UDEC4.0 numerical simulation software of discrete elements.Field validation was carried on the results of numerical simulation.The development situation and degree of the overlying strata crack was well reproduced in the numerical simulation result during lower protective layer mining under a medium-long distance, and the gas pressure and gas flux protected regular variation of change regulation were obtained.The test results show that gas pressure was reduced by 0.5 MPa, gas content was reduced by 5.25 m3/t, and permeability coefficient increased to 600 times of the original.

Keyword：protected seam mining;crack evolution;gas flux;UDEC;medium-long distance;