Mechanism Of Overburden Load Transmission And Strata Control In Crossing-Pillar Mining

China has abundant coal resources and most coal fields have multiple coal seams.During crossing-pillar mining,the high abutment stress induced by overburden transfer on the coal pillars is transmitted to the floor stratum,which brings many difficulties to the surrounding rock control in the lower coal seam working face,such as roof falling,supports damage,roadway damage and rock burst,etc.The research methods used in this paper include theoretical analysis,numerical simulation,three-dimensional physical simulation.This paper deeply investigates the mechanism of dynamic overburden load transfer,mechanism of disaster induced by strata instability and strata control during crossing-pillar mining.The main content and results of the paper are as follows:(1)Based on the layered elastic theory,a double-layer floor structure model named damage-intact model considering the influence of upper-coal mining is established.The damage layer(soft layer)caused by upper-coal mining aggravates the stress concentration(stress concentration effect)of the floor under the pillar,which is not conducive to the stability control of the mining roadway under pillar.While the hard layer can spread the load laterally(stress diffusion effect),thereby reducing the stress concentration of the floor rock under the pillar,which is beneficial to the stability control of the mining roadway under the coal pillar;The greater the difference between the equivalent elastic modulus of the damaged and the intact layer,the more obvious the stress concentration and diffusion effects.(2)The mechanism of interface slip between the floor layers under the concentrated load of the coal pillar is revealed.The floor is divided into three slip zones according to the sliding probability of the interface from high to low: the first slip zone,the second slip zone and the third slip zone.As the interface number increases,the interface strength decreases,and the coal pillar load increases,the floor slip degree and the stress concentration effect increase.This result is unfavorable to the stability control of the mining roadway under the coal pillar.(3)The dynamic transmission mechanism of the overburden load between the goaf and the coal pillar and in the pillar during crossing-pillar mining was revealed.When the face exits from the isolated pillar and gob-solid boundary,the pillar lagging a certain distance from the face is disturbed by roof caving.This part of the load will transmit to the complete pillar in front causing load accumulation.When the face enters the gob-solid boundary,the overburden load will be continuously transmitted to the front pillar,and the load accumulation will not form.On the basis,the dynamic transfer model of the overburden load was deduced,realizing the dynamic prediction of overburden load distribution.(4)Through a three-dimensional physical simulation for crossing isolated pillar,the overburden movement and load transfer mechanism are verified.After the upper coal seam was mined,the primary key stratum fractured to form a bridged structure,and the coal pillar's influence angle in the floor strata was about 39°.When the lower face exits from the pillar,the pillar affected by the roof caving yielded and its load was transmitted to the complete coal pillar in front causing load accumulation.Coupled with the instability of the key strata structure,the roof of the lower face has been cut and destroyed in a wide range,which is consistent with the ground behaviour on-site.(5)Based on the theory of elastic foundation beams,mechanical models of roof stability considering entering pillar,exiting pillar and crossing angle are established.The greater the peak load of the coal pillar and the closer it is to the boundary of the coal pillar,the lower the stiffness of the lower goaf,and the thicker(the harder)the basic roof,which is unfavorable for roof stability.When the crossing angle is 0°,it is most unfavorable to the roof control.With the increase of the crossing angle,especially in the range of 0?15°,the stability of the roof improves.When the crossing angle exceeds 30°,it has no obvious influence on the stability of the roof.(6)Using numerical simulation methods,the law of mining-induced stress response under three crossing-pillar mining modes is analyzed.When the working face enters the gob-solid boundary,there are only stress superimposed areas and significant influence areas.While the working face crossed isolated pillar and exits the gob-solid boundary,there are not only stress superimposed areas and significant impact areas,as well as disaster-prone area near the boundary of the coal pillar which is a key area for strata control.(7)An index system(including geological factors and strata control technology)that affects the strata stability during crossing-pillar mining is established.Based on the probability and the consequences of strata instability,a method for evaluating the strata stability in crossing-pillar mining is proposed.The strata stability is divided into four categories: extremely unstable,unstable,moderately stable and stable.Finally,the research results have been successfully applied to the practice of crossing-pillar mining in the 10-103 working face of Mugua Coal Mine.The application includes the stability control of the surrounding rock of roadway under overlying isolated pillar and the strata control during exiting the gob-solid boundary,which has achieved good economic and social benefits.