Research On Development Depth Of Floor Fissures In Paste Filling Mining

With the decrease of coal resources in the upper group of Shanxi Formation in the middle and eastern part of China,when mining the lower group of coal downward,because the basement of the lower group of coal is mainly the weathered and denuded surface of the Middle Ordovician with developed karst fissures and high water-rich,if the mining method is incorrect,it is easy to induce water inrush from the floor.Therefore,how to ensure that the effective thickness of key strata of floor water-proof after mining is still able to resist the influence of floor karst water hazard is the main problem faced by mine researchers when extending the mining of lower coal group.At present,the technical measures to control floor water inrush mainly include mining under pressure,floor grouting and filling mining.However,due to the different hydrogeology faced by each mine,floor water inrush often occurs in the process of working face mining,and the key factors of floor water inrush induced by floor cracks are discussed,therefore,it is very important to select a reasonable way to control the goaf to reduce the depth of floor crack expansion and increase the thickness of effective water-proof layer to prevent floor water inrush.Based on the engineering background of No.3 upper coal seam in Daizhuang Coal Mine,This paper takes 6351 working face of No.3 upper coal seam and 8300 belt roadway of No.3 lower coal seam in Daizhuang Coal Mine as engineering background,based on the combination of theoretical analysis,analogous material simulation experiment,numerical simulation experiment and field measurement,the failure depth of floor in caving mining and paste filling mining are compared and analyzed.The failure mechanism of floor in paste filling face and the evolution law of cracks are studied,it provides theoretical guidance for safe mining of coal seam above confined water in the next step of mine and determination of reasonable treatment method of goafAccording to the theory of floor failure depth in caving mining,the plastic zone formed by floor failure in caving mining is divided;The stope structure model of backfilling mining is established,and the formula for calculating the low damage length of paste backfilling floor is put forward:L=ls+l?+nl?;The failure mechanics model of floor in paste backfilling mining is established,and the safety factor for evaluating floor failure is deduced:(?);Geometric model of failure of paste backfilling floor is established,and the formula for calculating the crack propagation depth of floor under paste backfilling mining condition is deduced:h1=2(1+u)(l?+nl?)/E.Similar material simulation experiments show that under the condition of caving and paste filling,the distance between the bottom of crack and 8300 belt roadway is 3.32m and 16.32m,respectively.Under the condition of caving,the deformation of roadway is serious and paste filling roadway is complete;The stress evolution law can be divided into three stages:initial stress stage,stress growth stage and stress recovery stage.The stress change of paste filling mining shows a slow change.There are only fissure zones and bending subsidence zones in overlying strata.The stress change of caving mining shows a sudden change,and caving zones appear in overlying strata.Through numerical simulation experiments:it is known that the failure of surrounding rock is mainly shear and tension,and the failure depth of floor under paste filling and caving mining is 4 m and 18 m respectively;The stress nephogram shows a symmetrical"butterfly" distribution.The peak stress of paste filling mining and caving mining can be divided into 12.83 MPa and 36.97 MPa;Improving the filling rate of paste filling mining can reduce the damage to floor.With the help of borehole television and multi-point displacement meter,field measurement was carried out on 6351 filling working face in upper coal seam 3 and 8300 belt roadway in lower coal seam 3.The results show that the floor failure depth is 3.802m,which is consistent with the theoretical analysis and experimental research results,and the surrounding rock deformation of 8300 belt roadway under the working face is small(roof and floor 78 mm,two sides 58 mm).