Study on　Evolution and Subsidence Fractal Characteristics of Overlying Strata in Thick Loose Layers and Thin Bedrocks

In order to study the evolution and subsidence law of the overlying rock separation fracture under the mining of thick loose bed and thin bedrock, combined with the geological conditions of 1302 face of the first mining face of Shandong Guotun Coal Mine, the fractal characteristics of the overlying rock fracture network and subsidence in the mining process are studied by using the method of similar material simulation test and fractal. The research results show that: the evolution characteristics of overburden fractures have good self-similarity and fractal characteristics; as the working face advances, the fractal dimension of the overburden fracture network increases slowly and steadily during the early and mid-jumping periods. There are four layers in the whole mining process, and the location and size of the layers show a certain regularity; under the action of thick loose layer, the thin bedrock layer can still form a stable and strong bearing structure; the settlement of overburden in goaf and fractal dimension of fracture network show a nonlinear relation of approximately bifold line.