| Roadway roof fall control problem has been the research hotspots and difficulties in the field of roadway support, and because of its highly concealment, sudden and dangerousness, roof caving accidents bring people huge mental pressure, psychological fear and property losses. The plastic failure of roadway roof is the significant cause of roof caving, and the mastery of the distribution regularity of plastic zone of roadway surrounding rock is necessary for revealing roadway roof caving mechanism. Because the roadway surrounding rock is commonly anisotropic and is characterized by stratification, its stress field is commonly inhomogeneous, and in that circumstance, the plastic failure pattern is bound to have different characters from those of uniform stress filed or homogeneous surrounding rock. Researching on caving mechanism of the roadway layered roof under non-uniform stress field and seeking for pragmatic theory and method to solve the roof caving problems is one of the key issues to be solved. The Baode Coal Mine was taken as engineering background. By integrated use of theoretical analysis, field test, numerical stimulation, laboratory test and field engineering test, based on butterfly plastic zone theory of roadway surrounding rock, the distribution regularity of butterfly leaf plastic zone in layered rock mass of roadway roof, caving mechanism of layered roof, control method of layered roof caving, support technology of layered roof were studied systematically. The main conclusions and innovations are listed as follows:(1)The formation of mechanics mechanism and distribution characteristics of roadway plastic zone under non-uniform stress field were analyzed. When the surrounding stress is greater than the strength of surrounding rock of roadway and the principal stress ratio of roadway surrounding rock is larger, shear failure occurs in the surrounding rock and the butterfly plastic zone is formed. The larger of the principal stress ratio, the extension of the leaf of butterfly plastic zone is more obvious, however, the leaf extension degree is limitedly affected by roadway cross section shape. At the same time, the butterfly plastic zone has directivity, and in both circular roadways and non circular roadways, the leaf direction keeps fixed angle with the maximum( or minimum) principal stress. When the direction of maximum(or minimum) principal stress rotates, the direction of the butterfly plastic zone rotates accordingly. When the butterfly leaf plastic zone is located above the roof, the roadway roof stability is the worst.(2) The environmental characteristics of the mining superimposed stress field of Baode Coal Mine were obtained, and in the mining process, the roadways of both sides are located in regions with high stress ratio. The principal stress ratio is 2~3 within 18~50 meters of the working face while it is 3~5 within 18 meters of the working face. At the same time, the direction of the maximum principal stress of high stress ratio area deflects in different degree, and the principal stress deflection angle can reach 26~54° within 10~50 meters of the working face. Moreover, the closer to the working face, the greater is the principal stress deflection. Under the condition of the mining superimposed stress field, the butterfly plastic zone appears inevitably in the mining roadway surrounding rock, which butterfly leaf towards the roof.(3) Using numerical stimulation and theory analysis, the typical strata roof combination, the distribution regularity of the butterfly leaf plastic zone of rectangular and circular roadway under the condition of butterfly leaf toward the roof were studied systematically. It is found out that butterfly leaf plastic zone of roadway roof has penetrability, the existence of the hard strata can't block the formation of the butterfly leaf plastic zone in weak rock. Butterfly leaf will penetrate the hard rock of the plastic zone and redistribute in the low intensity rock, and the phenomenon of inter layer expansion occurs. Also, the distribution form of butterfly leaf plastic zone in weak interlayer position is hardly affected by the lower strata. The penetration of butterfly leaf plastic zone was mainly affected by butterfly leaf area, direction of the butterfly leaf plastic zone, dip angle of roof strata and so forth. Certain area of butterfly leaf affected zone is precondition of the formation of penetration in butterfly leaf plastic zone. And the increase of butterfly plastic zone deviation angle and roof strata dip angle will cause the loss of penetrability of butterfly leaf plastic zone indirectly.(4)By high intensity roof borehole imaging and multi-points displacement monitoring, the distribution of butterfly leaf plastic zone of the Baode Coal Mine layered roof was detected. The roof of the plastic failure zone has obvious penetrability, and the butterfly leaf plastic zone redistributes in the weaker mudstone after penetrating the harder coal seam. The damage of butterfly leaf plastic zone of the shallow coal seam and the upper mud rock seam is serious, and the deformation can reach more than 75% of the total deformation, but the deformation of complete coal rock area between two butterfly leaf plastic failure area makes up less than 20% of total deformation and the integrity is relatively good. It is mainly due to the extrusion of redistributed butterfly plastic zone to the complete lower coal seam after penetrating in the mud rock seam. Observation and numerical simulation results are basically identical in the same conditions(5) The roof caving mechanism of layered roof roadway was revealed, and with the huge expansion pressure and strong deformation, the roof includes weak strata and the butterfly plastic zone reforms in the weak strata after penetrating the lower hard strata, and this kind of surrounding rock deformation and failure is almost a given in the existing technology conditions, which imposed continued, huge “squeezed” load on the lower hard surrounding rock, which resulted in fracture failure. When the deformation caused by the butterfly leaf plastic zone of this kind of soft rock area reaches a certain degree, the lower hard strata caved of instability completely. If the support can no longer bear the weight of the destructed roof rock, roadway roof caving occurs.(6) The influence factors of layered roadway roof caving were elaborated, and a range of destruction of the plastic zone is the core factor of roof caving. Under the condition of the same rock composition, roadway cross-section, the absolute value of stress, when the butterfly leaf deflects above the roof, the roadway roof breakage depth, deformation is the largest, the roadway roof caving hazard degree is the largest. The height of roof caving hazard depends on the location of the weak strata in butterfly leaf plastic zone, and is hardly relate to the weak lower strata, but only relate to the weak rock itself and its upper strata.(7) Based on field measurement, combining with theoretical analysis, the control effect and function form of supporting resistance to roof deformation and destruction was studied, and the roadway roof caving control method was formed, and the plastic damage of surrounding rock and their deformation under the condition of the existing support is almost a given, and supporting materials should be adopted which can adapt to the large deformation of surrounding rock, sustainably provide high working resistance and with sufficient anchorage range, to contain the malignant expansion of plastic zone of surrounding rock, ensure the stability surrounding rock of plastic zone, and then eliminate the roof caving.(8) On the basis of studying the mechanism of layered roof roadway roof caving, roof risks horizon identification and roof caving height calculation system of layered rock mass roadway in non-uniform stress field was developed, which is convenient to engineering application and can quickly identify dangerous rock roof, determine roof caving height, make anchorage horizon reasonable and so forth.(9) The destruction of instability forms and control key of different areas of layered rock mass of roadway roof was illustrated: roof surface rock is generally more broken, and the limit span of roof surface strata should be greater than bare strata size. Unstable rock mass of shallow roof is easily to fall, and high-density short bolt support should be adopted, and combined with extension anchor and cable support, to prevent unstable roof rock mass falling of shallow strata. The middle layer butterfly plastic zone or thoroughly distributed butterfly plastic zone is the root cause of roof caving in this area. The extension bolt can adapt to the severe deformation caused by plastic damage of surrounding rock and sustainably provide relatively high support resistance and enough anchorage range, and maintain the stability of broken surrounding rock of plastic zone, and avoid vicious expanding of surrounding rock mass. In the process of roadway construction, the collapse hazard of large scale in deep strata roof appears, and long bolt should be adopted to control roof collapse of deep layer in large scale. The scope of the deep strata of roof is generally not more than the span of roadway.(10) The level control system of butterfly leaf plastic zone of layered rock mass of roadway which dominated by extension anchor is formed. The auxiliary supporting material prevent broken rock drop of surface roof within the scope of 0.5 meter. High density ordinary bolt control shallow perilous rock roof fall and local roof leakage within the scope of 1.5~2.0 meter. Overall stability of butterfly leaf plastic zone surrounding rock is maintained by extension bolt within the scope of 3.0~4.0 meter and the malignant expansion of surrounding rock and roof caving in large scale is prevented. The long cable anchor control the wide range collapse of instability of deep strata roof while roadway construction, at the same time, the surrounding rock of butterfly leaf plastic zone reinforced.(11)The calculation system of unstable strata identification and height roof caving hidden trouble in non-uniform stress field was adopted to calculate the related statistics of two air return ways of 81306 working face. The results show that: shallow coal seam of roof and the upper thin mudstone layer of coal seam and sandy mud stone of partial region is unstable strata(the butterfly leaf plastic zone penetrates the distribution area). The largest roof caving height of No.1 return air roadway should be 3.3 meter, and the reasonable anchorage strata should be 3.8 ~ 5.3 meters. The largest roof caving height of No.2 return air roadway should be 3.9 meters, and the reasonable anchorage strata should be 4.5 ~ 6.0 meters. Based on the support basic data above, the support scheme test was designed and the engineering practice was conducted. The engineering test results shows that: under different supporting intensity, roadway roof subsidence changes between 220 ~ 1320 millimeters and roadway roof subsidence and supporting intensity is hardly related, and this change is mainly affected by the heterogeneity of dynamic stress and the spatial difference of surrounding rock. And the support resistance plays limited role in controlling the deformation of this kind of roadways. Taking roadway level control system of butterfly leaf plastic layered rock mass, which dominated by extension long bolt can prevent the roadway collapse effectively. Monitoring results of support force of extension bolt shows that: during the mining influence period, the support force of extension bolt was stable at 79~159 KN, and it was in good working condition and the bolt fracture phenomenon seldom appeared and effectively prevented the broken rock mass caving of the plastic zone which induced by the malignant expansion of the plastic zone. |
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