Characteristics Of Overburden Failure Transfer And Their Criteria Of Critical Mining Due To High-intensity Longwall Mining

High-intensity longwall mining(referred to as HILM)has become the main mining method of coal mines in China due to the improvement of coal mining technology and types of mining equipment.However,strata movement induced by HILM is severe,inducing to the serious destruction of surface and ecology.Control technologies and remediation processes are difficult and the cost is extremely high.Therefore,they become the key technical problem restricting the safe and efficient green intensive coal mining in China.Investigating the characteristics of overburden failure transfer and their criteria of critical mining due to high-intensity longwall mining can provide theoretical supports for solving this key technical problem.In this paper,by the theoretical analysis,numerical simulation and other methods,a calculation method of the overburden failure height(also known as "the height of water-conducting fractured zone"),a judgment method of overburden critical failure and their criteria are researched quantificationally by equations.The main conclusions are as follows:(1)Ten factors affecting the overburden failure of HILM were analyzed by theoretical analysis,statistical analysis,and empirical equations,i.e.,overburden lithology,overburden lithology sequence,occurrence of coal seam(i.e.,mining depth,dip angle,et al.),mining method,roof control method,mining height,penal strike length,penal dip length,penal advanced rate,residual deformation of overburden failure.(2)The process of overburden failure transfer induced by HILM was analyzed,which was divided into the transfer development stage and the transfer termination stage.A “π-shaped” mining overburden model was put forward.The mining overburden was divided into four areas: the overburden failure area,overburden overhanging-bending area,overburden unsupported-bending area and original rock stress area.The characteristics of each area were analyzed in detail.(3)Two criteria of overburden failure were put forward,i.e.,the limited unsupported length and the limited overhanging length.The corresponding mechanical models of overburden failure were established.A new theoretical method for calculating the overburden failure height by HILM based on the overburden failure transfer was presented.It was applied to a HILM panel,and the theoretical calculation result was compared with the results of numerical simulation and field measurement.The rationality of the theoretical calculation method was verified.(4)A theoretical judgment method of overburden critical failure was proposed based on two-dimensional trapezoidal-area and three-dimensional quadrilateral-prism volume.Overburden failure and surface subsidence due to the longwall mining were innovatively simplified as four kinds of mining volumes,i.e.,the cuboid volume of gob,the quadrangle volume of pre-damaged overburden,the volume of damaged overburden and the volume of surface subsidence.The theoretical equation of the maximum overburden failure height of the overburden critical failure and the judgment equation of "two-zone" overburden failure mode was obtained.(5)Based on a high-intensity mining panel,the 3D numerical models considering the maximum horizontal principal stress and minimum horizontal stress and their orientations with the panel advance direction were established.The influence of different factors(mining height,penal strike length,penal dip length,and mining depth)on overburden critical failure was analyzed by the numerical simulation analysis.The results show that: the advanced distance with the overburden critical failure is directly proportional to the mining height and inversely proportional to the dip length of the panel,depth-to-thickness ratio and the face mining rate.The criteria of overburden critical failure in HILM and its applicable conditions were given and applied to some panels for verifying its rationality.The above investigation puts forward a theoretical method for calculating the overburden failure height in HILM.A theoretical judgment method and criterion equation for overburden critical failure in HILM are presented.This research has some theoretical significance for modern mines in the aspects of water conservation mining,"three under" mining and gas control in the mining of liberated strata.