Investigation Into Face Stability Of A High-seam Longwall Panel And Its Control Techniques

Since the import of high seam longwall mining equipment in 1978,the techniques and development of this mining method and its equipment has been extensively investigated during the past few decades.However,face falls followed by roof falls in the face area remain one of the most difficult and complex ground control problems in mining engineering.As the face width,mining depth,especially the mining height continue to increase,the failure of overlying strata extends upward and periodic roof weighting occurs more fiercely,leading to severe face falls and even casualties in some extreme cases.The mining height has recently increased to 7-8 m with increasing risk of face failure.Face falls threaten the safety of miners,influence the use and maintenance of equipment,and reduce the productivity and economic benefits.Therefore,it is of great importance to investigate the face failure mechanism and influencing factors,to have a better understanding of the mining stope system stiffness,especially support stiffness on face stability,and to develop the face and its surrounding rock control techniques.This dissertation combines theoretical analysis,numerical modeling,mechanical parameter experiment and physical modeling to solve the increasing face fall problems in highly intensive mining longwall faces.The coal wall stability model is first built to analyze the face failure mechanism.With this model,the influence of roof load,roof equivalent concentrated load,roof equivalent moment,face plate load,face plate length,coal cohesion and coal internal frictional angle on face stability are studied.The fractures in coal mass greatly deteriorate the mechanical properties of coal and thus have a negative influence on coal face stability,which is also included in this paper using experiment and numerical modeling.Based on the system stiffness relationship of the mining stope along the face advancing direction,numerical models with varying gob stiffness,a support-face system stiffness mechanical model and 3D face stability physical models are established to learn the influence of mining stope system stiffness,especially support stiffness,coal GSI,mining height,roof concentrated load and roof moment on face stability.Finally,a main roof key block breakage model is built with proposed face failure control techniques.Main conclusions are listed below.(1)The coal face stability model is built by using Ritz method in the energy approach to investigate the mechanism of face failure.By solving the strain energy and potential energy of external force of the coal face,the contours of face displacement and stress ahead of face are obtained.It shows that the maximum face displacement and stress occurred in the upper side of the face,i.e.the most vulnerable position of the face.(2)The Face Stability Factor k is defined based on Mohr-Coulomb failure criterion.The face is in failure state if k is less than 0,in limited equilibrium if k is equal to 0 and in intact state if k is larger than 0.According to the distribution of Face Stability Factor,the most common face failure modes observed in the field are captured,i.e.upper face failure,upper and lower face failure,and entire face failure.Combining the coal face stability model and Face Stability Factor,sensibility analysis are performed on roof load,roof equivalent concentrated load,roof equivalent moment,face plate load,face plate length,mining height,coal internal frictional angle and cohesion.It presents that the roof load,roof equivalent concentrated load,roof equivalent moment and mining height play an important role in face stability.(3)The mechanical properties experiment on manual pre-existing fracture samples is performed.The results show that the cracked samples create secondary wing fractures and are relatively crushed as compared to the intact ones.The fractures in the sample greatly influence the properties of the sample,and the equivalent modulus and strength fall in the range of 0.5-0.85.The damage variable of the coal face largely influences the horizontal displacement of face and face failure area.(4)A face stability numerical model with joints in coal is established using 3DEC to investigate the influence of joint-face orientation,joint spacing and bedding plane on face stability.It reveals that face displacement increases with the increase of joint-face angle,but this effect becomes rather limited when this angle is large.Face displacement decreases if joints in coal have a large spacing,but again coal face with large joint spacing cannot further reduce the face displacement.Bedding planes in coal however play a rather limited role in face stability,though they slightly increase the face displacement.Thus mining direction of the longwall face should be parallel to the strike of joints in coal if possible.(5)The mining stope system stiffness relationship in the face advancing direction has a significant influence on face stability.The numerical models with varying gob stiffness are proposed and reasonably simulate the de-stressed and stable stress areas ahead of and behind the face.The results show that as the face advances,the abutment pressure,extension of yielded elements ahead of face and face displacement first increase and then stabilize,indicating that the numerical model is in stable.(1)Based on the PHASE 2D analysis,increasing the gob stiffness can effectively reduce the front abutment pressure,extension of yielded elements and coal face displacement.With increased GSI of coal,the front abutment pressure is enlarged,but the peak value is brought closer to the coal face.As a result,extension of face failure and face displacement is reduced.Furthermore,with increase of mining height,the peak front abutment pressure is decreased,but the distance between the coal wall and the peak abutment pressure is increased,together with the extension of face failure and coal displacement.(2)According to the FLAC 3D results,the central section of the longwall face has the largest front abutment pressure and the most extension of face failure ahead of face.Furthermore,since the stiffness and loading taken ability of gob is less than the solid coal,the front pressure and yielded elements of the current longwall face is enlarged at the section close to the adjacent mined-out area of the previous longwall.Thus,this region and the central section of the face are the most vulnerable positions where the face failure control should gain more attention.(6)The “support-face” system stiffness mechanical model is established based on the theory of beam on elastic foundation.The deflection curves of support and coal ahead of are obtained.It shows a negative nonlinear relationship between the support stiffness and the vertical displacement of coal ahead of face,deflection of coal wall and the support.Therefore,increasing the support stiffness can reduce the sinkage of coal face and support,and improve the stability of face.However,this control becomes less effective when the support stiffness reaches a particular value.(7)A 3D face stability physical model is performed to analyze the influence of roof concentrated load,roof moment and support stiffness on face stability.The physical model shows the face failure characteristics,deflection of roof and movement of coal wall and reveals the mechanism of support stiffness on face stability.The results show:(1)The maximum roof deflection is about 27 mm under roof concentrated load.Face falls occur after 22 times of loading applied by the roof hydraulic cylinder to the coal.Thus the roof deflection rate is relatively large.The coal face horizontal displacement is low,but the time period of face horizontal displacing is only about 200 s,i.e.the rate of face horizontal displacement is also large.Thus,face fall occurs in a shortly time fashion.Small amount of displacement accumulated by the face can trigger face falls.(2)The maximum roof defection is about 35 mm under roof moment.After 26 times of loading applied by the roof hydraulic cylinder to the coal,face failure occurs.Therefore the roof deflection rate is relatively large.The time duration of face horizontal displacing is about 500 s,so the rate of face horizontal displacement is relatively large.Thus face fall also occurs in a shortly time fashion under roof moment.(3)If the face area is protected by a support with relatively low stiffness,face failure occurs after 81 times of loading applied by the roof hydraulic cylinder.The maximum roof deflection is 47 mm,thus roof deflection rate is small.Most of the roof deflection occurs during the unloading of the support and the deflection of roof shows a staged fashion.The deflection step is about 5 mm.Face horizontal displacement is large and the time duration of face horizontal displacing is 1200 s,indicating that face horizontal displacing rate is relatively low.In other words,longwall face has to accumulate large amount of vertical and horizontal displacement in order to trigger face falls.(4)With increased support stiffness,regional face fall occurs after 86 times of loading applied by the roof hydraulic cylinder.The roof deflection is 35 mm when regional face fall occurs.Therefore,roof deflects in a slow and staged fashion,with deflection step of 3-3.5 mm.Horizontal displacement of face is large,though.The time duration of the face deflecting is 1400 s.That is to say,face horizontal displacement rate is low.To conclude,face condition is better off with increased support stiffness.(5)When the support stiffness is further increased,face stays stable after 87 times of loading applied by the roof hydraulic cylinder.The roof vertical displacement is 25.55 mm in a slow and staged fashion.The step of roof deflection is 2 mm.Small amounts and slow rate of face horizontal displacement are observed.In other words,coal face is unable to accumulate enough displacement to cause face falls if the support has a large value of stiffness.(8)The main roof key block breakage model during periodic roof weighting is built to investigate the effect of roof load,roof concentrated load and roof moment on face stability.It shows that reducing the roof load,decreasing the roof concentrated load and restricting the rotation of main roof are the principles for face fall control.The field observation of Wangzhuang coal mine shows that the working resistance of the support is considerably lower than the designed value because of limited support leg lifting time and insignificant fluid providing.Thus,the face failure control techniques,i.e.increasing support stiffness and early resistance,improving the usage of face plate,face grouting and optimizing mining techniques are proposed to control face falls in this coal mine,and better face control results are obtained.