Morphological Characteristics And Evolution Law Of Stress Arch In Hard Roof Stope

After coal seam mining,stress arch(SA)is formed in the overburden.Researches on SA are important for the stability analysis of surrounding rocks and mine pressure behavior control.Hard roof(HF)has a considerable influence on the SA evolution due to its high strength,considerable thickness,and strong integrity.To study the influence of motion-induced fracture of the HF on morphological characteristics and evolution law of SA,based on SA theory,the evolution law of SA under the influence of HF was obtained and numerical simulation and similar model experiments was used to verify the derived equations.Results indicate that:(1)The horizontal stress of the SA foot is calculated by introducing the coal/rock mass interfacial stress model for theoretical SA model optimization.The dynamic relationship between the motion-induced fracture of the HF and evolutionary expansion of the SA is analyzed.Hence,a recursive algorithm is used to establish the evolution model of stress arch(EMSA)in HF stope and the evolution law of SA in HF stope is obtained.After mining,the SA is formed in the overlying strata,and SA morphology is controlled by the HF and experiences dynamic evolution with the fracturing motion of the HF.Before the SA expands to HF,it presents ?-shaped free expansion.After SA expands to HF,HF has the shielding effect on longitudinal expansion of the SA,thereby SA presents ?-shaped transverse expansion.HF has a constraining effect on the SA and SA goes through longitudinal expansion rapidly after HF is fractured,therefore SA presents calabash-shaped expansion.The SA morphology gradually evolves from ? and ? shapes to calabash shape successively with motion-induced fracture of the HF.The evolution characteristics of SA under the influence of HF are studied by numerical experiments,and the correctness of the EMSA in HF stope is verified.(2)Based on EMSA in HF stope the relationship between the HF and the overlying arched stress concentration zone(ASCZ)(part of the SA)is analyzed.The influences of four variables such as the thickness and cohesion of the HF,distances from HF to coal seam,and side pressure coefficient on the morphological characteristics of the overlying ASCZ are studied.Flac3 D software is used to simulate the influence of four variables on the arch height,arch span,arch width and stress distribution of the ASCZ,and the morphological characteristics of SA under the change of HF parameters are obtained.The mechanical characteristics of the HF have a great influence on the shape of the overlying ASCZ.With the increase of the thicknesses of the HF and distances from HF to coal seam,the shape of the ASCZ is gradually flattened,and the bearing capacity of the HF is enhanced.The change of side pressure coefficient of the HF has little influence on the shape of the ASCZ.With the cohesion of the HF increases,the width of the arch becomes sharp and the internal stress of the HF increases.With the side pressure coefficient increases,the shape of the ASCZ becomes sharp,and the appropriate side pressure is beneficial to form a stable ASCZ.(3)The dynamic relationship between the evolution of mining-induced SA and the strata movement in the arch failure zone in HF stope is obtained by similar model experiments.After mining,the overburden forms arch failure zone,and SA is formed outside the failure zone;the arch failure zone is located in the pressure relief zone under the SA,and its expansion is controlled by the SA.Arch failure zone and SA are subjected to the motion-induced fracture of HF.Before the HF is fractured,the SA and the arch failure zone are freely expanded as “?-shaped”.After the HF is fractured,the “voussoir beam” or “cantilever beam” structures are formed in the arch failure zone to bear the rock loads.Outside the failure zone,the HF changes the SA shape,and the SA presents “calabash-shaped” expansion.The SA and HF bear overlying strata load together.