Study On Overlying Strata Structure Along Goaf And Coal Pillar Stability In Fully Mechanized Top Coal Caving Mining In Extra Thick Coal Seam

In recent years,the thickness of coal seam in fully mechanized top coal caving mining is increasing.The 2016 edition of?safety regulations in coal mine?clearly provides that extra thick coal seam mining ratio can be greater than 1:3 after qualified by industry experts.This will create a condition for 20m extra thick coal seam mining of fully mechanized caving mining.At present,the study on gob-side overlying strata structure is mainly based on the background of thin,medium thick and thick coal seam slicing mining.The structural characteristics of the overlying strata along goaf in fully mechanized top coal caving mining in extra thick coal seam are not clear.Roadway layout along goaf in fully mechanized top coal caving mining in extra thick coal seam lack theoretical guidance with less research results.In this paper,in the background of 25m thick coal seam mining,the research on lateral overlying rock structure and stability of coal pillar in fully mechanized top coal caving mining in extra thick coal seam are carried out.The characteristics of gob-side overlying strata of fully mechanized top coal caving mining in extra thick coal seam is studied by the means of similar simulation and numerical simulation method.Based on the characteristics of gob-side overlying strata,stress field calculation model is established,gob-side entry overburden load transfer mechanism is revealled,main control factors of internal and external stress field are analyzed.Roadway layout selection basis is put forward,engineering practice of coal pillar is carried out in 3107 extra thick coal seam.Electromagnetic wave CT and borehole stress meter are used to measure abutment pressure at different mining stages.The main conclusions are as follows:(1)Gob-side overlying strata will form a stable structure under the condition of different mining thickness after mining.With the increase of mining depth,the fracture height of overlying strata increases gradually.When mining thickness is 5m,gob-side overlying strata form hinged beam structure,and the upper part is curve subsidence zone.When mining thickness is 10m,fractured rock within caving zone form cantilever structure,fractured rock within cracking zone form hinged beam structure,and the upper part is curve subsidence zone.When mining thickness is 15m,fractured rock within caving zone form cantilever structure,fractured rock within lower cracking zone form hinged beam structure,fractured rock within upper cracking zone form masonry structure,and the upper part is curve subsidence zone.When mining thickness is 20 to 25m range,fractured rock within caving zone form cantilever structure,fractured rock within lower cracking zone form hinged beam structure and masonry structure,fractured rock within upper cracking zone form cantilever structure,and the upper part is curve subsidence zone.(2)Compared with medium thick and thick coal seam mining,the range of overlying strata movement,rock breaking angle,influence scope of abutment pressure,abutment pressure peak,high stress area be monitored increase.Because strata movement angle is small in medium thick and thick coal seam mining,the influence of abutment pressure range enlarge,the influence degree is low,a small area of high stress can be monitored.Compared with medium thick and thick coal seam mining,strata movement angle is bigger,the influence of abutment pressure range enlarge,stress amplitude increases,larger area of high stress can be monitored in extra think coal seam mining.(3)Based on the structure characteristics of gob-side overlying strata in fully mechanized top coal caving mining in extra thick coal seam,abutment pressure calculation model and calculation formula are constructed,transfer mechanism of gob-side overburden load is revealed.Hanging part weight of Cantilever block is the main cause of lateral abutment pressure.Low cantilever block is the main cause of lateral abutment pressure in lateral shallow coal.How cantilever block is the main cause of lateral abutment pressure in lateral deep coal.The greater the scope of overlying strata activity,the additional stress in the lateral coal spread farther.Influence degree of various factors on the lateral abutment pressure are:strata fracture angle>cantilever block thickness>strata movement angle.(4)Based on abutment pressure calculation model and calculation formula,abutment pressure is calculated,abutment pressure characteristics is analyzed.This paper provides a basis for the selection of gob-side roadway layout in fully mechanized caving face in extra thick coal seam and determination of reasonable width of coal pillar.There are internal and external stress fields in abutment pressure of fully mechanized top coal caving mining in extra thick coal seam.The range of overburden strata movement enlarge in extra think coal seam mining,the range and stress amplitude of internal stress field enlarge,concentration area appear in internal stress field,therefore,the distribution characteristics of internal stress field also become an important basis for the selection of roadway layout.Abutment pressure has the typical division characteristics and can be divided into five regions,followed by the internal stress stability zone,the stress concentration zone,stress stability zone,stress concentration zone,the original rock stress zone.The gob-side entry should be arranged in the internal stress stability zone or the external stress stability zone.(5)There is a good correlation between attenuation coefficient of electromagnetic wave and coal seam fracture.Field test results show that the attenuation coefficient of electromagnetic wave increases with the increase of crack,and decreases with the increase of crack closure.(6)Electromagnetic wave attenuation coefficient of the stress concentration zone and the stress relaxation zone is relatively larger,higher than the background field in gob-side.The electromagnetic wave attenuation coefficient in the stress concentration zone is larger than that in the stress relaxation zone.It shows that electromagnetic wave attenuation intensity cuased by reduction of coal resistivity in the stress concentration zone is larger than that in the stress relaxation zone cuased by coal crushing.The electromagnetic wave attenuation coefficient in original rock stress zone is smaller than that in the stress relaxation zone and stress concentration zone.Coal stress level is low in the plastic softening zone,and micro fissures exist.Without coal yield and fracture occurring,the electromagnetic wave attenuation coefficient in plastic softening zone is smaller that in the stress relaxation zone and larger than that in the original stress zone.(7)Rapid monitoring of abutment pressure of different mining stage is accomplished by the electromagnetic wave CT detection system.Combined with borehole stress meter measurement,basis for the stability evaluation of coal pillar and pillar width optimization is provided.