## 4 Discussion  
In this paper, based on field data, a numerical simulation of the PFC was established, and the laws of dynamic evolution of mining overlying strata and discontinuous surface movements in the presence of a thick loess and thin bedrock were studied. The main discussions are as follows:  
(1) With the advance of the working face, the failure of the overlying rock underwent a process of the breaking of the main roof, and its caving and periodic weighting. As the overburden moved forward with the arch structure until a force chain arch developed to the surface, steplike failure of the surface occurred with periodic pressure. Based on the stress state of the main roof, a model of the mechanical structure of the main roof during the initial breaking and periodic weighting was established, and provided the basis for the analysis of the mechanism of overburden-induced movement in the mining process. (2) In the presence of a thick loess and thin bedrock, the process of fracture owing to mining overburden was a surface fracture at the front of the working face after the first breaking of the main roof. With the advance of the working face, the number of mining fractures of the overlying strata increased, and fractures began to appear when the body of the slope was stretched in the process of moving. The working face continued to advance, where the direction of this advance first formed a stretch area; the surface cracked, and its width constantly increased, and the cracks continue to deepen. When periodic weighting occurred, the cracks began to narrow, and those from the ground through to the coal seam formed a more prominent step in the ground. (3) The form of the force chain underwent the dynamic process of formation of the force chain arch, its development, and failure. When developed to the surface, the force chain arch appeared to fracture. As the surface load layer thickened, the force chain arch could not continue to develop to bear the load, became unstable, and failed. This resulted, in the face of the periodic weighting, in subsidence at the surface in steps. According to the law of the evolution of the force chain, the trajectory equation before the instability of the force chain arch was established. The theoretical analysis showed that the force chain arch was the main bearing capacity system of the rock surrounding the stope, and its fracture and instability was the main cause of the discontinuous deformation of the surface. (4) During the advance of the working face, the body of the slope was affected by the slip, which caused the direction of the horizontal movement of the loess layer to be opposite to the overlying rock in the panel. Under the influence of slope slip and discontinuous deformation, the subsidence of the surface was two times that of flat land, and the subsidence curve was a V shape that was not symmetrical at the center of the panel. The curve of horizontal surface movement was also asymmetric, and horizontal movement in the downhill direction was larger than that of in the uphill direction. With the periodic weighting of the working face, the surface subsidence and horizontal movement changed suddenly. The highlights of this study. Surface discontinuous deformation is a dynamic evolution process, and most of the existing research results from surface fracture distribution characteristics through theoretical analysis to describe the causes of surface discontinuous deformation [6; 7; 8; 9], but this paper has innovative points:  
(1) For the first time, the process of fracture development and surface steps formation of mining overburden rock is analyzed by using discrete element PFC software from the view of mining subsidence. The internal cause of discontinuous deformation of mining surfaces under the condition of thick yellow soil and thin bedrock is obtained. (2) The action and evolution processes of force chain underground mining with thick loess layer and thin bedrock are firstly revealed in this study.