Model Test And Numerical Simulation Study On Anti-Permeability Failure Of The Bottom Of Extremely Thick Loose Stratum Under Mining Influence

Yanzhou coalfield is a typical North China type Carboniferous-Permian concealed coalfield,which mainly mines No.3 coal seam of Shanxi Formation.The outcrop area is generally covered by extremely thick loose stratum,and there are weak to medium water rich aquifers（bottom aquifers）at the bottom of the loose stratum.In order to ensure safe mining,the original design generally reserved waterproof and safe coal（rock）pillars,overstocking a large amount of high-quality coal resources.With the increase of mining years,the bottom water level gradually decreases,and its hydrogeological conditions have significantly changed.Many working faces in the thin bedrock area have been successfully mined in accordance with the safety coal（rock）pillars for sand control.However,due to the thinning of the bedrock,the development of fractures in the rock mass affected by wind oxidation zones and structures,and the poor bearing capacity affected by mining,the possibility of water inrush and sand burst disasters caused by the combined effects of ground stress,mining induced secondary stress,and bottom aquifer seepage pressure generated by the overlying strata under the thick and loose stratum load is also increasing.Therefore,based on the background of three coal mine thin bedrock areas in Yangcun,Xinglongzhuang,and Baodian Coal Mines,this thesis systematically studied the anti-permeability failure characteristics of bottom aquifer soil samples through on-site borehole sampling,using microscopic test analysis,indoor testing,and numerical simulation methods.Obtained the following research results:（1）According to the drilling data in the study area,the thickness of each group of stratum in the Quaternary system was analyzed.Overall,the lower group of the Quaternary system gradually thinned from north to south and from east to west.After analyzing the structural characteristics and lithological thickness of the lower group of the Quaternary system,it is found that the clay stratum accounts for a large proportion in each borehole according to the calculation results of on-site pumping test data,the aquifer at the bottom of the lower Quaternary group in the study area has weak water yield and strong permeability.The microscopic testing of the underlying soil samples showed that the chemical elements of the soil samples were basically the same,and the mineral composition was relatively single,mainly consisting of quartz,feldspar,calcite,and kaolinite.（2）According to the samples taken in the study area,16 groups of water inrush and sand burst tests were carried out by grouping design.The test results show that the process of water inrush and sand burst can be divided into two types:rapid outburst and fluctuating outburst.For the rapid outburst type,with the continuous increase of seepage pressure,the internal energy of the soil is accumulated.When the critical water inrush pressure is reached,the water and sand mixture rapidly gushes out from the crack,and the seepage pressure drops sharply in a short time,and the number of fluctuations is small and the amplitude is small.While the fluctuating outburst type is characterized by a large number of fluctuations and a large amplitude in the process of decreasing the osmotic pressure after the start of water inrush.The main reason is that when some particles emerge from the seepage channel,they rub against the surrounding sand particles and block the channel,resulting in a temporary rise in the osmotic pressure.（3）The critical hydraulic gradient of water inrush and sand burst of soil sample is calculated,and it is found that the critical hydraulic gradient decreases with the increase of crack width,but after 20mm,the correlation between the critical hydraulic gradient and the crack width is significantly reduced.Based on the analysis of previous research results,the critical hydraulic gradient of water inrush and sand burst in the loose stratum at the bottom of the study area is generally maintained below 300,and its value is closely related to the content of fine particles in the sample.（4）Based on the results of particle size analysis,a numerical model of triaxial test is established in PFC^3D,and the sensitivity of some mesoscopic parameters is analyzed.Combined with the deviatoric stress-strain curve in the shear process,the changes of peak strength,strain and residual strength of the sample are studied.Combined with the CFD module in PFC^3D,a three-dimensional model is built for fluid-solid coupling simulation,and the process of water inrush and sand burst is studied.The results show that with the increase of fracture width and inclination,the duration of water inrush and sand burst becomes shorter and the severity increases.However,with the decrease of fracture width and inclination,the inconsistency of grain movement direction and fracture inclination direction will easily cause silting of sand burst channel,which can effectively reduce the severity of water inrush and sand burst disaster.