| The fully mechanized top coal caving mining(FMTCCM)in extra thick coal seams is widely applied in the main coal mine such as CHN Energy Investment Group,Jinneng Holding Group,China Coal Energy Group and China Energy Group,because of its advantages of super high output and benefits at present.However,the mining space is huge,and the overlying strata is moving fiercely.The ground pressure behavior degree of a working face can be weakened by stopping caving during the period of terminal mining,but the law of the overlying key stratum movement and the stability transformation of the surrounding rock are mutated.Accidents of spalling rib,roof falling(leakage)and pressing frame occur occasionally in the retracement channel,which affects the efficiency of equipment retracement seriously.The typical fully mechanized top coal caving face of extra thick coal seam with no pre-excavation retracement channel technology in Tongxin coal mine is taken as the research object,and the research methods such as laboratory test,theoretical calculation,simulation comparison and test evaluation are used.Firstly,the structural properties of the overlying strata and the change characteristics of surrounding rock plastic failure before and after the stopping caving are analyzed.And then,the movement characteristics and stability mutation law of the overlying strata during the period of terminal mining,the change law of surrounding rock failure during the period of terminal mining,"roofcoal rib-hydraulic support" stability linkage response and process evolution law during the period of terminal mining,and the reasonable parameters of caving and support scheme of retracement channel during the period of terminal mining are systematically studied.The main conclusions are as follows:(1)According to the analysis of typical working face,it is obtained that: the ground pressure of stope during the terminal mining period with stopping caving in the working face is weaker than that during the normal mining period,and the peak value and influence range of abutment pressure are less than those during the period of normal mining.The ground pressure behavior of the working face can be effectively weakened by stopping caving in the period of terminal mining.The stope surrounding rock is the most stable when there is no periodic pressure during the period of the terminal mining.(2)Based on the characteristics of strong and weak periodic pressure in the working face,the stability mechanical model of high and low trapezoidal key block is established.The analysis shows that: as the working face is closer to the fracture line of the low key stratum,the sliding instability coefficient of the low trapezoidal key block decreases gradually,and the rotation deformation instability coefficient remains unchanged;During the period of normal mining with strong periodic pressure,the low trapezoidal key block is stable during normal mining,and the high trapezoidal key block slides instability when the working face is extracted 18 m away from the fracture line;During the normal mining period with weak periodic pressure and the period of terminal mining with stopping caving,the high trapezoidal key block does not break,and the low key block is stable.(3)The stability transformation law and movement characteristics of trapezoidal key block during the working face from the period of normal mining to terminal mining is explored.From the normal mining period to the terminal mining period,the variation law of the sliding instability coefficient of low trapezoidal key block is "reduce-suddenly increase-reduce-slowly increase",and the rotation deformation instability coefficient does not change.The low trapezoid key block is not subject to sliding instability and rotary deformation instability;The effects of main roof thickness,hydraulic support working resistance,distance between high and low key stratum,and caving height on the stability of low trapezoidal key block during the period of terminal mining with stopping caving are analyzed.(4)The model of coal rib plastic zone range is established,which is based on coal softening characteristics,and the range of coal rib plastic softening zone and plastic residual zone in front of the working face during the period of terminal mining and normal mining are analyzed.The range of coal rib plastic zone during the period of terminal mining is smaller than that during the period of normal mining.(5)The mechanics model of roof beam structural with the elastic-plastic foundation and the mechanical model of coal rib stability in working face based on finite deformation theory are established.The stability linkage law of "roof-coal rib-hydraulic support" is analyzed.The quantitative relationship between the surrounding rock deformation and the plastic support characteristics of coal rib is explored.With the increase of the support peak value of the plastic foundation,the decrease of the width of the plastic zone,the decrease of the tip-to-face distance,and the increase of the support force of the hydraulic support,the bending moment and deflection decrease;and with the decrease of the stope hydraulic supports working resistance,the width of the plastic zone of the coal rib increases.(6)The stability evolution law of "roof-coal rib-hydraulic support" in the process of terminal mining is analyzed by analogy simulation,and the stability of surrounding rock of working face with different overlying strata form is analyzed,compared and sorted:the high and low key stratum are not broken during the period of terminal mining,the low key stratum is broken during the period of terminal mining,the low key stratum is broken during the normal mining,and the high and low key stratum are broken during the normal mining.(7)The numerical model of stability analysis of retracement channel with different stopping caving distance during the terminal mining period is constructed.With the increase of stopping caving distance,the deformation,distortion energy and distortion degree of roof and rib of retracement channel decrease,and the failure degree of surrounding rock decreases.When the stopping caving distance is 30 m,the change of the surrounding rock deformation,distortion energy and distortion degree of the retracement channel tend to be stable,and the reasonable stopping caving distance is determined as 30 m.(8)The numerical model of stability analysis of retracement channel with different caving height during the terminal mining period is constructed.With the decrease of caving height,the deformation,distortion energy and distortion degree of roof and rib of retracement channel decrease,and the failure degree of surrounding rock decreases.Combined with the characteristics of hydraulic supports,it is determined that the reasonable caving height during the terminal mining period is 0 m.(9)The numerical model of stability analysis of retracement channel with different positions of the key block fracture line of the main roof during the terminal mining period is constructed.The stability of the retracement channel is studied with the different relative positions between retracement channel and the key block fracture line of the main roof.The order of the retracement channel stability from good to bad is: the fracture line is in the gob,the fracture line is above the hydraulic support,the fracture line is above the retracement channel and the fracture line is above the solid coal.When the main roof fracture line is in the gob,it is the most reasonable time to arrange the retracement channel.(10)Based on the theory of breaking characteristics of trapezoidal key block,the deformation law of coupling between surrounding rock and supports,and the position and shape transformation law of high deviatoric stress invariants area of surrounding rock during the terminal mining period,and so on,the method of coordinated control of “first regulation and then support” is proposed.The surrounding rock support space of the retracement channel is divided into "three support areas and two strengthening support areas" in accordance with the control area and strength.The novel high strength polyester fiber flexible mesh has been used.The efficient dividing area support technology with the combined action of multiple support structures has been formed.The field tests show that the stopping caving distance during the terminal mining period has been greatly shortened,and the retracement channel is accurately arranged according to the design;the excavation time of the retracement channel is shortened by 12 days.More than 250 sets of bolts are saved and the drilling length of more than 600 m is reduced.During the use of the retracement channel,the deformation of surrounding rock,the stress of bolts and anchors,and the working conditions of hydraulic support are within a reasonable range,which meets the requirements of safe and smooth equipment retracement,and has significant safety and economic benefits. |
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