Analysis on the Breaking Law of Soft and Thick Roof of Fully Mechanized Top-Coal Caving Face

The study of roof breaking law is very important for the behavior of the underground pressure in fully mechanized top-coal caving face under the condition of weak roof. In this paper, the research is to study the soft and thick roof breaking law in the fully mechanized top-coal caving face 2–100 in coal seam 2#. The theoretical analysis, numerical simulation and field measurement methods are used to analyze the breaking law of the weak thick layer roof in the fully mechanized top-coal caving face. First, the mining conditions of the working face are introduced systematically, the mechanical properties of the weak roof are analyzed, the mechanical model of the immediate roof caving structure and the mechanical model of the main roof fracture structure are established, and the instability characteristics of the immediate roof and the main roof are analyzed. Through UDEC numerical simulation, the failure structure of the soft and thick roof in fully mechanized caving face, the plastic zone and stress distribution of the roof under different propulsive lengths, and the roof subsidence of the basic roof are obtained. It is found that the immediate roof caving will form an “arch” structure, and the main roof fracture will form a “three hinge arches” structure. Under this mine condition, the initial roof caving step is 17.8 m, and the main roof initial fracture step is 41.3 m, the periodic fracture step distance is 16.7 m. Under the condition of instability of immediate roof “arch” structure, when the displacement distance of the vault reaches 5.1 m, the “arch” structure will be unstable. Conditions of sliding instability of main roof “three hinge arches” structure: the ratio of coal seam thickness to the span of main roof is less than 0.2, and when the back angle is less than 8°, it is not easy to slip and lose stability. Through the observation of the field measurement results of fully mechanized top-coal caving face 2–100, the breaking rule of the soft and thick roof in fully mechanized top-coal caving face is verified.