Study On The Mechanism Of Large Deformation And Split-Level Layout Control Of Gob-Side Entry With Narrow Coal Pillar In Extra-Thick Coal Seam

The gob-side roadway with narrow pillar has been widely applied in mediumthick and thick coal seams due to the advantages of less coal pillar loss,high resource recovery rate,etc.Meanwhile,the gob-side roadway with narrow pillar has broad application prospects in extra-thick coal seams as well.However,large mining space,strong overlying strata movement,wide mining influence range,large mining roadway cross-section and small coal pillar width-height ratio often lead to strong ground pressure such as large coal pillar deformation and strong floor heave when the gob-side roadway is applied during mining in extra-thick coal seam,which affects the safe and efficient production of the mine seriously.Firstly,the instability mechanism of surrounding rock of gob-side entry in extra-thick coal seam has been studied.Secondly,the control measures of split-level gob-side entry has been put forward.Finally,the control mechanism has been studied.The research contents mainly include the following four aspects :(1)Investigate and analyze the characteristics of mine pressure of gob-side roadway with different thickness of coal seams,buried depth and width of coal pillars;(2)Study the fracture position of the basic roof of elastoplastic foundation and analyze the internal relationship between the fracture position and the stability of coal pillar;(3)Study and analyze the deformation mechanism of coal pillar,entity coal side and floor in gob-side roadway with narrow pillar;(4)Put forward the control strategy of s split-level gob-side entry with extra-thick coal seams,study the stability control principle of surrounding rock in split-level gob-side roadway with narrow pillar and split-level gob-side roadway with negative pillar,the stability control principle is also verified by experiments.Through the research,the following conclusions are drawn:(1)The two-side displacement of gob-side roadway with narrow pillar is generally greater than that of roof and floor and the deformation of coal pillar side is the largest.Meanwhile,the anchoring performance of coal pillar side bolt is low and floor heave is dominant in roof and floor movement.The deformation of surrounding rock in gob-side roadway with narrow pillar in extra-thick coal seam is significantly greater than that in medium-thick and thick coal seam.Bottom drum of gob-side roadway presents asymmetric characteristics,and the maximum bottom drum position of narrow coal pillar is opposite to that of wide coal pillar.(2)Based on the characteristics of plastic failure of surrounding coal caused by mining,the fracture mechanical model of basic top beam of elastic-plastic foundation is constructed,and the expressions of basic top deflection and bending moment are obtained.It is obtained that the fracture position of the basic top is beyond the range of the plastic foundation and lies above the elastic foundation.On the premise that the gob-side roadway is arranged in the basic roof fault line,the basic roof fault location shifts slowly to the depth with the increase of coal seam thickness,which leads to a small increase in coal pillar width and a continuous decrease in coal pillar width-to-height ratio(less than 1).The instability risk coefficient increases significantly.(3)The mechanical model of the surrounding rock of the gob-side roadway is established,and it is concluded that the incremental load caused by the excavation of the roadway leads deformation of the surrounding rock.The block B sinking seriously dues to the loss of effective support in solid coal,which leads to a large surrounding rock deformation during mining.After the excavation of the gob-side roadway,the transfer distance of the peak abutment pressure on the solid coal side to the depth is less than the sum of the width of the coal pillar and the roadway,the width of the limit equilibrium zone decreases,though it’s peak value increases.The increase of stress concentration leads to the expansion and deformation of the solid coal side.Based on Rankine pressure theory,the mechanical model of asymmetric bottom drum of roadway along goaf is established,and it is concluded that the asymmetrical vertical stress of two sides is the internal reason for the asymmetry of bottom drum.The influence of high abutment pressure on bottom drum is usually greater than that of horizontal stress.The position of the maximum bottom drum deviates to one side of the relatively low stress side.(4)A similar mining model for split-level roadway has been established,and the fracture characteristics and stress distribution law of overlying strata are compared.The test results showed that the fracture height of overlying strata decreases,and the peak value of solid coal side abutment pressure decreases.Furthermore,the limit equilibrium mechanical model of the face with split-level roadway has been constructed and the expression of the limit equilibrium zone width has been derived.The results showed that the limit equilibrium zone width decreased slightly and the stress concentration of solid coal side decreased,which had the effect of reducing pressure.(5)The lateral limit test of fracture specimen is designed,and the lateral limit effect of trapezoidal coal body is emphatically analyzed.The compressive strength and post peak residual strength of the specimen under the lateral limit constraint are significantly improved,which proves that the lateral limit effect of trapezoidal coal body strengthens the integrity and bearing capacity of narrow coal pillar.The unique layout of the gob-side roadway adapts to the movement of the basic roof through the coal deformation in the degraded area of the upper part of the coal pillar and weakens the influence of the movement of the key blocks of the basic roof on the coal pillar side of the gob-side roadway.The trapezoidal coal lateral limiting action improves the anchoring ability of narrow coal pillar,and a stable anchoring foundation for anchor cable are provided,realizing the bolt cable combined support technology of narrow coal pillar in extra thick coal seam.(6)The numerical model of the influence of slope height on the stability of gob-side roadway is established.The simulation results show that the deformation of surrounding rock of gob-side roadway decreases gradually with the increase of slope height of trapezoidal coal body,but when the height of trapezoidal coal body exceeds half of the thickness of coal seam,the influence of confining action on surrounding rock deformation control gradually decreases.Therefore,it is suggested that the slope height should be designed reasonably according to the resource recovery rate during mining in extra-thick coal seam.(7)The engineering cases of monitoring shows that when negative coal pillars are placed in trapezoidal coal roadway along the goaf,the abutment pressure of both sides is less than the dead weight stress,which realizes the effect of abutment pressure peak transferring to the deep coal body.The pressure relief effect is obvious,which is conducive to the prevention and control of strong bottom drum in the gob-side roadway.