Failure Mechanism And Control Technique Of Surrounding Rocks On A Coal Roadway With Multiple Coal Seams’ Mining Influence In Jinhuagong Mine

According to multiple coal seams mining characteristics of Jinhuagong Mine, the deformation mechanism of roadway’s surrounding rock under multiple mining influences by means of geologyical investigation, mechanical test of coal and rock, theoretical analysis and numerical simulation were researched, and the corresponding truss anchor cable technology finally was proposed. The main coclusions are showd as follows:(1) The surrounding rock of 11# roadway 5709 subjects to mining influences of multiple coal seams. The mining influences mainly come from overlying 7-4# mining face 8709, adjacent 11# mining face 8707 and its serve m11# mining face 8709, and the corresponding strong mining pressure performance, such as roadway’s deformation and supporting system’s damage, would occure. Besides, through mechanical parameters test in laborary, the main mechanical prameters, for example, compress strength, tensile strength and internal friction angle, were obtained.(2) The maximum damage depth of the upper coal swam’s floor is expressed as hmax= 1.57 γ2 H2 L/ (4R2rmc), and the horizontal distance of maimum damage depth and the end of goaf is expressed as Lp=0.42γ2H2L/(4R2rmc).Putting the parameters of 7-4# coal seam, the coclulsions are:hmax=0.55m, Lp=0.15m. Therefore, we cocluded that mining of 7-4# has little influence of the roof of 11# coal seam.(3) The statically indeterminate cantilever beam mechanical model of key roof strata of roadway 5709 with fractional conditions was built, and also the coal pillars were regarded as middle material of which the performance is between Newtonian Fluid and pure elastomer. The constutive relationship was regardes to satisfy σ(t)-dβε(t)/dtβ.(4) Through mechanical analysis of arch triangle block B, the structural parameters were obtained as follows:cycle fracturing length in advancing direction Lz, fracturing length in lateral direction Li, support force from caving gangue FG, support force from coal pillars FM, vertical force of block A, B FAB, and horizontal force from the block A and C to B TAB、TCB.(5) The tilting force σB produced by arc triangle block B would accelerate the damage of roof 5709. Therefore, the roof of 5709 was analyzed by using simple support beam and the moment equations are obtained as:(6) Distribution feature of deviator stress and its influencing depth under trapezoidal load:①Deviator stress invariants are obviously asymmetric saddle distributed within 0-7m. The values in mining face 8709 side were larger than mining face 8707 side. In the range of 7～10m, the asymmetric feature decades. And, the deviator stress forms the clock shape when the range exceeds 12m.② In the floor depth direction, changing trend of deviator stress is the same:they increase firstly and decrease then. Within the range of (-5<x<25), invariants increase to maximum value and decreases to zero. Within the range of (x>25 or x<-25), the difference value of maximum and minimum values are small. And the increasing and decreasing trends are not obvious.③ The peak value area of deviator stress in different depth are different:Within 0-8m, peak value appears at the right coal pillar side and increase with the depth increasing. Within 9-20m, the peak value point obviously transfers to the center of coal pillar; When the depth exceeds 21m, the point totally are in the center of coal pillar. The calculation result reveals that the maimum influencing depth is 50m.(7) Distribution feature of deviator stress and its influencing depth under 7-4# coal pillars with simulating method:① In the range of 0-7m floor of coal pillar, deviator stress invariants are distributed in shape of asymmetric mountain. In the range of 8-20m of coal pillar, deviator stress invariants are distributed in shape asymmetric saddle. In the range of 21-35m of coal pillar, the asymmetric feature decades with depth increasing, the invariants values are small when the depth exceeds 40m.② In the same horizontal level with different depth in floor, the peak value of invariants of deviator stress appear at coal pillar area and it transfers from the internal side to external side with depth increasing. For instance, the peak value appears in the area which is 0-2m to internal side when the detpth is 0-7m, otherwise it appers in rea which is 0～10m to internal side when the detpth is 8-20m. And it spreads to the goaf when the depth exceeds 20m. The maximum influencing depth is 40m.(8) By establishing the trapezoidal deviator stress mechanical model, the maximu influencing depth of goaf coal pillars is calculated as 50m. And the numerical simulation result illustrates that the influencing depth is 40m which are consistent with mechanical calculation result.(9) The superposition of mining impacts of minnig face 8709 and 8707obviously exceeds single mining impacts of mining face 8707, and advancing influencing distance of mining face 8709 is 20m. In the range of 1.0～7.0m above the roadway roof 5709, the tensile deformation is obvious but it decreases when the range are respectively (1.0～2.5m) and (5.0～7.0m). The peak values of second and third deviator stress invariants assemble in the middle line area (3.0-4.5m) in which the tensile deformation is obvious and the distortion energy is large.(10) Both of coal pillar side and coal side of roadway appear compressing deformation, and the deformation in coal pillar side exceeds coal side. The deformation amounts of the former is 1.2～1.5 times than the later. With mining impacts increasing, the peak value transfers to the deep part. When mining face 8707 is working, the second and third deviator stress invariants J2 (J3) transfer from 2.0m to 2.5m in coal pillar side, and the second and third deviator stress invariants J2(J3)transfer from 2.5m to3.0m in coal side. When mining face 8709 is working, the second and third deviator stress invariants J2(J3) transfer from 3.0m to 3.5m in coal pillar side, and the second and third deviator stress invariants J2(J3) transfer from 3.0m to 4.0m in coal side.(11) Middle part in the range of 3.0-4.5m above the roadway 5709 easily owns tensile deformation in which the distortion energy is large and shearing failure easily occcures. Compression deformation appears at both of coal pillar side and coal side, and the peak value locates in deep part.(12) Coupling mechanism of mining impacts increases the difficulty of surrounding rock control, and thus the high prestress truss anchor cable system was put forward. The system has advantages, for instance, large controlling area, strong anti-shearing performance, and achieves effective control to asymmetric deformation which especially accustoms to the multiplu mining impacts.(13) Based on natural equilibrium arch theory and abutment mining pressure law, the loading situation of roadway roof 5709 is obtained and thus the mechanical of truss anchor cable was built. Then the anchoring force F and preload F’are calculated. Besides, the relationship between F’and tilting angle a, span of truss. Where, F and F’ are expressed as follows:(14) The numerical simulation model of 5709 was built and the support parameters were optimized. The parameters were determined as follows:depth of truss in drilling bole is 5.0m, the distance of drilling holes to roadway side is 1.2m and preload is 120KN.(15) The final support program was determined and described as follows:as to roof, the support is combined with truss cable and single cable. And the layout way is "2-1-2", i.e., there is a single cable between two truss anchor cables. The row space of truss anchor cable is 4000mm. Four anchors are layout in each row and the distance is 1000mm. As to the roadway side, anchors are layout in staggered way of which the distance is 1.2mx1.5m. And, the distance of corner anchor to roof is 0.1m, and the middle anchor on the roadway side to roof is 1.3m.(16) We observed the deformation amuounts of roadway surrounding rock when the roadway adopted truss anchor cable and after the multiple mining impacts of minng face 8709 of 7-4#, mining face 8707 of 11# and mining face 8709 of 11#. The observing result proved the good supporting effort.