Study On The Rupture Mechanism Of Coal Mine Shaft Induced By Multi-Layer Water Level Rise And Fall In Deepalluvium Zones

Mine coal shafts are mostly in deep alluvium zone in east China,the deep topsoil layer has the mechanical characteristics of high compressibility and large deformation,and the occurrence conditions of groundwater are complicated.The mainstream view is that the rise and fall of water level in deep topsoil leads to the increase of vertical additional stress of shaft,which is the main cause of shaft failure,but it’s specific influence is related to the geological conditions and the degree of water level change,so a consensus has not been reached at present.In this paper,the deformation law of soil and shaft under different precipitation conditions is studied by numerical simulation,and the safety performance evaluation of shaft under different precipitation conditions is proposed.The main achievements are as follows:⑴The applicability of five seepage calculation models provided by FLAC^3Dsoftware to shaft calculation is analyzed,four main parameters affecting the calculation results of precipitation were determined,the uncoupled（two-step）solution model for shaft precipitation simulation is obtained.Combined with the monitoring data of shaft water level,the deformation law of shaft with two aquifers and different precipitation sequence is studied.The results of this paper show that the lower silt layer dewatering before the middle coarse sand layer results in the fastest increase of additional shaft stress,the most likely to lead to shaft damage,if the two aquifers dewatering at the same time,the shaft damage is slightly weak,the middle coarse sand layer dewatering before the lower silt layer has the least adverse impact on shaft stability.⑵The fish program was designed to simulate the effect of water injection on controlling shaft damage.The results show that water injection in the lower silty layer can partially recover the deformation of settlement process and effectively slow down the continuous settlement of the stratum,thus reducing the vertical additional stress of the shaft and preventing the failure caused by excessive vertical additional stress of the shaft.With the number of cyclic fluctuation of water level increases,the deformation of the silty layer at the bottom of the shaft accumulates,resulting in the increase of the vertical additional stress of the shaft;when the circulation exceeds a certain number of times,the cyclic rise and fall of water level has almost no effect on the settlement of the lower silty layer and the increase of additional shaft stress.⑶In this paper,the whole strength reduction method and dynamic strength reduction method are used to calculate the safety of the shaft under different precipitation conditions.The simulation results show that the whole strength reduction method reduces both the damaged and undamaged soil,and the calculated safety factor is small;however,the dynamic strength reduction method only reduces the destroyed soil.The deformation of the soil is mainly concentrated in the coarse sand layer in the middle of the shaft and the silty sand layer at the bottom,and drives the whole settlement of the overlying soil,in the meanwhile,besides,the additional stress of the shaft increases continuously during the reduction process,the simulation results show that the failure first occurs in the inner part of the shaft at the depth of-168.5m,then develops to the outer and surrounding parts of the shaft,and finally the whole shaft is integrated,compared with the whole strength reduction method,the calculation results of this method are more reliable.With the continuous decrease of the water level of the lower silty sand,the decreasing rate of the shaft safety factor is gradually accelerated.According to the simulation results,the shaft precipitation depth is 20m,and the safety factor of the shaft is 4.12,which is in a safe state.