Study On The Law Of Movement And Failure Of Overlying Rock In Shallow Seam Mining Under Thick Clay Layers

In this paper,the law of the appearance of strata pressure,overlying strata movement and crack distribution in shallow seam mining under thick clay layers are studied in details with four methods as following:field testing,simulating model test,numerical simulation and theoretical analysis.Proceeding with the engineering geological character of the laterite and loess,a suit of simulating bracket with constant resistance mechanics character is devised and two models of equivalent material are established,thereby the relationship between the support and the surrounding rock is efficiently analyzed. Through analyzing the supporting resistance,influence of laterite on the main roof structure and falling height of overlying rock,it is revealed that the huge thick clay layers is key affecting strata which control the occurrence of intensive appearance of strata pressure accompanying the overlying strata full-thickeness cutting-off along the seam wall,weaken the failure of overlying rock in a certain extent and restrain the evolution of the water transmitting fractured zone. The instability process of the combinatorial key strata in shallow coal is described with the help of the method of catastrophe theory at the first time. Through establishing the mechanical model,both sufficient condition and necessary condition of the system losing its stability of the combinatorial key strata are obtained,the step subsiding gap and the safety distance(L) between working face and gob are quantitatively gained at the same time. Applying the numerical simulations program NCAP-2D to analyze the law of strata pressure appearance under the mining condition of thick clay in shallow seam,stress varying tends to be more complicated with the advancing of working face and about the seam wall engender local stress focus. Since the restriction of bed rock mass and absorption of thick clay layer,the stress is weakened onthe interface between clay and rock. At a certain advancing distance,the full thickness cutting-off of overlying rock mass will not occur from the seam wall to the surface,so the results of simulation test are proved to be correct.