Abstract
<title>Abstract</title> <p>When mining in shallow buried ultra-large mining height working faces with directly overlying double key strata, the working face is often accompanied by periodic strong dynamic load weighting. During weighting periods, the support has large convergence and high safety valve opening rate, which brings serious hidden dangers to safe and efficient mine production. Taking the 122104 fully mechanized working face with 10 m ultra-large mining height in Caojiatan coal mine of Yushen Mining Area as the engineering background, field observation and physical similarity simulation were adopted to study the overburden structure characteristics of shallow buried ultra-large mining height mining with directly overlying double key strata. A mechanical model of roof control was established, and a theoretical calculation method of support working resistance for ultra-large mining height working face was proposed. The research results show that: (1) Physical simulation shows that when mining in shallow buried ultra-large mining height working face with directly overlying double key strata, the lower key stratum has a large rotation angle, forming a long cantilever structure after initial fracture, and a low-level stepped rock beam structure during periodic fracture. It controls the movement of overlying strata within 22 m, which is the main reason for the small-period weighting of the working face. After the upper key stratum fractures, it forms a voussoir beam structure. Its periodic rotation and subsidence may force the lower key stratum to fracture in advance, and drive the load of the overlying non-key strata to be transmitted to the working face, causing large-period weighting. (2) Mechanical analysis shows that the setting load of supports in the initial mining stage should ensure that the fractured blocks of the lower key stratum do not undergo sliding instability. In the small-period weighting stage dominated by the first key stratum, the working resistance should include two parts: the weight of strata in the caving zone and the balance force used to balance the hinged structure of the second key stratum. During large-period weighting, the first key stratum, the second key stratum and the overlying strata groups controlled by them move synchronously, resulting in that the existing supports of the working face cannot meet the bearing demand in the large-period weighting stage. (3) Physical experiments of roof pre-splitting show that after implementing hard roof pre-splitting, the initial and periodic fracture intervals of the first key stratum are reduced by 57.98% and 52.97% respectively, and those of the second key stratum are reduced by 56.6% and 60.78% respectively. The maximum stress concentration coefficient of the working face is reduced by 19.86%. (4) Engineering verification shows that after hard roof pre-splitting, the periodic weighting interval ranges from 9.71 m to 24.19 m with an average of 16.7 m, which is 37.17% smaller than the physical simulation results, and the field ground pressure behavior is stable.</p>