Field survey on the evolution of main ground cracks under condition of forced hard roof caving in mining ultra-thick coal seams under shallow overburden

The Yellow River Basin is an important ecological barrier and also an important coal base concentration area in China. The contradiction between ecological protection and coal mining is prominent. Under the conditions of typical shallow overburden, extra-thick coal seam and hard roof in the Northern Jinbei coal base, the forced roof caving method is generally adopted, but the research on the evolution of surface cracks is rare, which restricts the coordination of ecological protection and coal mining in such mines. This paper presents a field meas-urement research on the evolution of main ground fractures under such conditions. The main conclusions are: ① The cracks formed were mainly at the mining boundaries, which first stretched, then staggered up and down, and the upper and lower parts of the cracks were separated, and cavity appeared below the upper step. ② When the working face became “square”, the development of cracks at the all boundaries was similar. After “square”, the key strata was caved in a three-times distance of the cyclic blasting step (20 m), i.e., every three times of forced caving, 60 m, was the period of cyclic failure and collapse. The main ground fractures above the gate roads on both sides also developed in a “step” manner of 60 m. ③ The cracks right above the working face were smaller than other three sides in terms of crack width and altitude difference, and the closure speed changed once per cycle forced caving distance. ④ The ratio of mining depth to mining height was small and the topsoil was thin leading to the non-self-repairing large cracks at the mining boundary which has the most serious impact on surface. The fracture zone directly reached the surface. Most of the main cracks were inside the edge of the stope and the closure cycle were shorter, approximately 9 days. ⑤ The 110 and N00 method and split-level longwall mining without gate pillars can expand the uniform subsidence area and mitigate the boundary crack damage, which are good for healing from the root and saving investment.