Mechanism Of Synergistic Evolution Of Mining-induced Stress Within Gob And Rock Mass For Subcritical Panels

The most commonly used longwall mining methods in this country are conventional longwall top coal caving(CLTCC),high seam longwall mining(HSLM),multi-slice longwall mining(MLM)and longwall mining with split-level gateroads(LMSG).So far,the number of studies on ground pressure behavior of working faces employing these methods has been very large and a large number of results have been obtained.However,studies and understanding of mining-induced stress of gob and surrounding rock mass,stress distribution around the gob edge in particular are very insufficient.However,this is very important for gateroad layouts and designs especially for gob-side gateroads and gateroads in the gob that are gaining more and more popularity.With the increase of cover depth and intensity of extraction,the support and maintenance of gateroads are more difficult due to the increased ground pressure,induced disturbance and more adverse geological conditions such as inclination,etc.As a result,large deformation,roof fall,coal and rock burst,repeated downtimes and overhauls,etc.play havoc on safe production for coal mines.Therefore,mining-induced stress of gob and surrounding rock mass,stress distribution around gob in particular is of great theoretical significance and practical value.Synergistic evolution mechanism of mining-induced pressure for subcritical panels was studied against Zhenchengdi and Tangshan coal mines through theoretical analysis,physical,numerical modelling and field observation.Main contents are:(1)Based on existing ground control theory and mining practice,following conclusions were drawn by building and studying strata movement model,theoretical stress distribution model: Equilibrium of mining-induced stress is the consequence of interaction,synactic contribution and coordination of gob and solid rock mass.Change of stress distribution of either side will cause changes of stress distribution of the other meaning that the mining-induced stress of the gob and rock mass has a synergistic characteristic;Gob behavior has a significant influence on stress distribution and failure of solid rock mass.When stress within the gob especially close to the edge of the gob increases,the abutment stress decreases,peak point is closer to the edge,and vice versa.(2)Different panel layouts lead to different panel rurrounding rock mass configurations;For flat coal seams,CLTCC,HSLM and MLM panels have symmetrical panel geometries leading to symmetrical gob development,strata movement and mining-induced stress distribution.LMSG panel,however,has an asymmetrical panel geometry which results in asymmetrical corresponding results.It has larger gob stress and smaller abutment stress on elevating section side;Gob-side gateroad of a successive LMSG panel under the gob edge(GGUGE)is located within the stress free zone which is independent of front and side abutment pressure;When a slice of multiple longwall slices of a coal seam is being mined,the mining height is reduced leading to the increase of gob pressure and smaller abutment pressure;The stress on the elevating section of HSLM are higher than that in CLTCC but samaller than that in MLM.(3)The system of gob-key main roof blocks-pillar dictates the rock mass structures and ground pressure behavior.GGUGE is right under the masonry protective beam,caved-rock cushion on the top of the gateroad has stress and energy dissipation effect that avoids dynamic influence for the entry and stress is transferred to solid rock mass ahead of the face and the gob.Within a limit,subsidence and rotation of the key block is small,then the gob pressure is large and abutment pressure is small;The method for calculating subsidence and rotation of the key block is obtained.(4)Physical modelling and mining practice indicate that collapse and movement of overlying strata generate caving line and angle of break;For a panel with a certain width,the larger the break angle,the more easily a stratum will fracture,thus the larger that gob pressure and smaller abutment pressure and vice versa,the influence of angle of break is tremendous;A LMSG panel overlapping adjacent one has a longer face thus leading to larger gob pressure and smaller abutment pressure;The expression of panel width L,the span of the uppermost key stratum L1 and angle of break ? is obtained.(5)Base on limit equilibrium theory,limit equilibrium equation was build,width of limit equilibrium zone of coal mass(pillar)was derived;When a slice of multiple longwall slices of a coal seam is being mined,the mining height is reduced leading to the reduction of width of limit equilibrium zone,and so are HSLM and LMSG due to their elevating sections;Width of limit equilibrium zone of coal pillar is not a definite value,numerical modelling shows,rather,it is different for different heights within the coal seam(pillar),the value is smallest on the top of the coal pillar,the it increases towards the bottom the pillar,and this is believed to related to angle of break.(6)Numerical modelling demonstrates that: Stress concentration factor and failure zone are larger and elevation of failure zone is lower without taking the influence of gob into account;For flat coal seams,considering gob behavior,vertical bars of stress concentration zone are developed within the rock strata above the coal seam rather than within the coal seam and the most stress-concentrated part is not within the coal seam either which is contrary to the most numerical modelling result so far;when one slice is being extracted in multi-slice longwall mining,the gob pressure is larger than any other methods,stress concentration zone and failure zone are smaller;Decrease of mining height results in decrease of vertical bar of stress concentration zone;No matter what method it is,stress in the gob edge is the smallest in the entire panel system;The opposite shear on two sides of the caving line causes “/ \” shape extension of failure in the direction approximately parallel to the caving line,the stress transfer and flow caused by this leads to smaller yielded zone of surrounding rock mass of GGUGE.(7)It is concluded that strata movement and gob development as well as mining-induced stress distribution are asymmetrical;physical modelling indicates that the abrupt change of the lower end of the gob of CLTCC panel leads to vanish or break(zero)of stress chain resulting in larger abutment pressure;Stress concentrated area within the rock mass at the lower end of CLTCC panel is farther from the coal seam while that at the upper end it is within the coal seam;LMSG panel has a smooth transitional section at the lower end with larger bear load area leading to continuous stress transfer and smaller abutment pressure,which shows that avoiding drastic change of underground space configuration can effectively avoid drastic change of stress;stress around GGUGE is much smaller than pre-mining pressure and surrounding rock mass is more intact,driving gateroads there has little effect on damage and disturbance of surrounding rock mass.(8)Gob pressure within the upper part of the gob is near zero while stress at the lower part of the gob is larger,elevation of the gob of one LMSG panel is higher leading to the reduction of the deflection of the key stratum meaning that the overlying strata is more likely to stay intact;Gob and strata structural models were built,and Macaulay function was used to derive the deflection of the key stratum subjected to action of gob pressure;CLTCC panels cannot reach critical width of extraction because of the isolation of gateroad pillars especially in mining at depth,stress within the gob is small with stress concentration on the pillars and this gives rise to wavy deformation on the ground that is detrimental for structure protection;While multiple LMSG panels form continuous gob leading to higher gob pressure and smaller abutment pressure,and critical or super-critical width of extraction can be reached more readily leading to uniform subsidence which benefits structure protection on the surface;The concentrated stress that the gateroad of successive CLTCC panel close to the gob subjected to is the reason for deformation,slabbing and dynamic disasters within the gateroad;(9)Case studies and field observation demonstrate the coevolution mechanism of mining-induced stress;For inclined coal seams,LMSG is an effective approach to solve the problems including equipment topple down and slide,disconnection of the conveyor and the stage loader,failure of cut by a shearer of coal located in the disconnection;stress within surrounding rock mass of GGUGE and its deformation do not change evidently with the distance to the faceline with significant gentle stress and small stable deformation which is also independent of cover depth,field support and deformation control result is good,and recovery ratio increases considerably with a better profit;LMSG integrates longwall top coal caving and MLM into one panel system,but increases tremendously the practical significance by drawing localized operation of MLM,and pillarless longwall mining is achieved.