Research on ground control based on arch structure in unconsolidated layers theory

Aiming at the problem of support crushing,water inrush and surface collapse during longwall mining under thick unconsolidated layers widely distributed in some mine area of eastern and northern China,the arch structure in unconsolidated layers (ASUL) was established based on systematic summary of the relevant academic progress of load-bearing structure theory and the previous research results.By means of theoretical analysis,experimental simulation,field detection and engineering application,the influence mechanisms of ASUL on load transfer effect,strata movement and surface collapse,fracture and failure of overburden were revealed.The results show that the shape of ASUL is an ellipse whose major axis is located in the strike direction of the longwall face.The ASUL can form in the overlying strata when the thickness of the unconsolidated layer is larger than the sum of thickness and height of ASUL.During longwall mining,the ASUL prevents the overlying strata from subsiding and deforming and transfers the loading stress of the overlying strata to its abutment,which reduces the stress of the lower strata and the mine pressure.Given the effects of ASUL,the distribution of the loading can be divided into a reduced stress zone above the gob area,an increased stress zone under both sides of the base of ASUL,and an original zone outside ASUL.The formula of load reduction factor of unconsolidated layers considering the load transfer effect of ASUL is proposed.The calculation method of load reduction factor of unconsolidated layer is modified when the key strata was determined.The movement of the overlying strata is controlled by ASUL during excavation.In other words,if ASUL break,the strata controlled by ASUL will also break simultaneously.The value of the subsidence velocity of the overburden strata controlled by ASUL and ground surface is maximum when the ASUL fail.When ASUL can be formed in the thick unconsolidated layers and the key blocks can maintain self-stability,the overburden strata will not be slide and the surface will not collapse when mining under the geological conditions of thick unconsolidated layers.Accordingly,the critical width and height of longwall face are proposed to avoid surface collapse for longwall mining under thick unconsolidated layers.The first and periodic fractural interval of the key stratum will increase.However,the sliding and rotational failures of the voussoir beam structure will not occur under ASUL.The reasonable support resistance when mining under thick unconsolidated layers was put forward.The results of the theoretical calculations were verified through field observation performed in longwall face No.6304 in Shandong Province,China,a total of 350 000 tons of coal resources have been mined,and the safe mining of working face under unconsolidated layers has been realized.