Variation Law And Engineering Application Of Protected Range For Deeply Buried Upper-protective Layer Mining In Different Inclination Of Coal Seams

The depth of coal mining is becoming more and more deeper,and the gas disaster of the deeply buried coal mine is becoming more and more serious.In the deeply buried coal seams mining,the protective layer mining is the first measures to control coal mine gas.Affected by high ground stress,high gas pressure and low permeability of coal seam,The variation law of the pressure-relief range of the protected coal seam of the deeply buried coal seams is more complicated than that of the shallow buried coal seam.In order to study the variation law and its determine method of the protected scope of the deeply buried coal seams.Under the conditions of high ground stress,high gas pressure and low permeability of coal(rock)and based on the theoretical analysis,laboratory experiment,physical similarity simulation and field investigation test,the law of the protected range varation of the protective layer on the deeply buried coal seams under the conditions of creep failure of coal and rock is studied.Combined with the field test of YanShitai coal mine in Nantong mining area,the comparative analysis of the method of delineation of protective layer in deeply buried coal seams is carried out.This paper comes to main conclusions as below:(1)In order to describe the influence of the low permeability of deeply buried coal seams and the temperature change of coal gas in the process of gas desorption,the THM coupling model considering the threshold pressure gradient(TPG)of gas flow is established.In order to reduce the unknown quantity of the model equation,the theoretical expression of the desorption differential thermal energy term of the temperature field control equation in the process of gas emission is deduced based on the binary-state theory of thermodynamics,and the model is validated.The model laid a theoretical foundation for the gas interlayer flow caused by mining of protective coal seams.The results show that the TPG has a great influence on the gas flow from the upper-protective layer.(2)In order to describe accurately the damage fracture,collapse and creep behavior of coal strata in the deep protective layer,a constitutive model considering the generalized Kelvin rheological properties and based on the Hoek-Brown yield criterion is established.Three-dimensional stress updating algorithm of the constitutive model under complex stress condition are deduced and redeveloped.The constitutive model of elastic-viscoplastic damage was experimentally verified by Brazilian split,uniaxial compression experiment and physical similarity simulation experiment in this study.And the elastic-viscoplastic model is verified by comparison between the numerical solution of a single element and the theoretical results of one-dimensional model.The correctness of the constitutive model and the redevelopment code is verified experimentally and theoretically.Based on the verification of the model,the constitutive model is applied to the numerical calculation and analysis of the damage,rupture,fracture and collapse of coal and rock after protective coal seam workface be mined out.The results show that under the influence of the rheological properties of coal and rock mass,the protected range determined by the pressure relief angle is increasing with the increasing of time elapsed after extraction of protective coal seam.(3)The variation law of the protected range of the upper-protective layer is studied at different inclination by using the elasto-viscoplastic damage constitutive model and the multi-field coupling model of coal seam gas flow.Combining the constitutive model of elasto-viscoplastic viscous damage with coal seam gas flow multi-field theory,the gas distribution in coal seam and rock formation is obtained coupling the rheology,fracture,collapse with coal seam gas fields.The results of the studing show that the surrounding rock in the stope is more developed in the upper,lower and middle of the roof along inclination.The fissure from the bottom to the middle of the goaf along inclination is gradually declined.(4)The influence of different coal seam inclination on the protection range is studied by using physical similarity simulation test,and the relationship between the protection range and the inclination of coal seam is obtained.The results show that with the increase of the inclination of the coal seam,the pressure relief angle on the upper working face of the upper-protective layer decreases,and is increased and then decreased on the lower boundary of working face along dip direction,and the variation amplitude of the pressure relief angle is larger.The position of the protection range of the inclined and steeply inclined protective layer is shifted to the direction of the downward direction,and the magnitude of the offset increases first and then decreases with the increase of the inclination of the coal seam.With the increase of the inclination angle of the coal seam,the upper pressure relief angle of the lower protective layer increases first and then decreases.(5)The protected range of extraction of protective layer is measured by field test at YanShitai coal mine,and the results of the field test are compared with numerical simulation and physical similar simulation.The correctness of multi-field coupled numerical simulation and physical similarity simulation is verified relatively.Because of the many factors that affect the protected range of deeply buried protective coal seam,the proposed two methods to determine the scope of the protective coal seam have some errors,but still complement each other.