Support design of main retracement passage in fully mechanised coal mining face based on numerical simulation

Based on the simulation purpose and model construction, the ground stress conditions were set, and the coal rock surrounding rock conditions were combined to carry out the parameters [4]. Given or limited boundary mechanics conditions and displacement conditions, and completing the initial stress field (unexcavated disturbance) calculation balance, to ensure the authenticity of the calculation results, arrange and excavate the retreating roadway, simulate the surrounding rock deformation and failure process and evolution of the retreating roadway, which then simulates the progressive influence of the working face and studies the supporting design of the main withdrawal passage under dynamic pressure mining, including the process and evolution of surrounding rock deformation and failure.

The pressure step of the main roof is a key parameter to describe the motion state of the rock beam. In the mechanical model of the pressure step of the main roof, the hinged rock beam is composed of fractured rock mass [5]. The composition of the rock mass can be approximated as being under force on a cantilever beam, regardless of the deflection of the beam. Then the structural forces on the rock mass include the dead weight of the rock beam as:

The thrust pressure and the corresponding friction force at the hinge point of the fractured rock mass are:

The couple moment generated by shifting the force at the hinge point to the centre of the rock beam is as:

The support reaction force is not considered before the end of the rock beam is not fractured. If the fractured part of the rock beam is in equilibrium, the couple moment of the rock beam at the point is zero:

In the limit condition, the formula can be substituted into:

When the working face comes under periodic pressure, the mechanical conditions of rock beam cracking from the end under the action of structural forces are as follows:

The bending moment value at the end of the beam is:

The actual tensile stress is:

According to σ = [σ_t], it can be concluded that the pressing distance in the support design of the main withdrawal passage is as shown in Figure 1:

Fig. 1

Above the retracement passage, when the suspension length of the main roof reaches the period to press the step distance, the main roof may be above the roadway it ruptures and turns down. In this case, the motion state of the rock block has obvious influence on the stability of the surrounding rock of the main retractable passage, and the length of the rock block is the periodic pressure step of the support design of the main retractable passage [6].

L shall be designed to be in the cantilever beam state as:

Numerical simulation analysis of the surrounding rock stress failure under the influence of dynamic pressure is as shown in Figure 2A. With the gradual progress of the working surface, vertical stress concentration of the surrounding rock of withdrawn roadway increases significantly. Stress redistribution of the surrounding rock of multiple roadway is superimposed with the influence of mining dynamic pressure of working face, and supporting load of bearing structure of surrounding rock of main withdrawal passage is severe. Position the face to stop mining line between retracement channels for vertical stress concentration of surrounding rock and dynamic pressure in the working face vertical peak should focus entirely overlay, the retracement help department between surrounding rock of roadway vertical stress concentration peak 16.2 MPa, and in the central word retracement lord bearing stress distribution of surrounding rock is most prominent, surrounding rock bearing burden that lane position is located in the main retracement channel which department of surrounding rock, the location of the main retreat for skiving of surrounding rock and deformation of roof and floor is most severe, so optimisation design, it is necessary to improve the support load of support strength in order to maintain the stability of surrounding rock during the retreat [7] as −9.5247e+006to−9.0000e+006−9.0000e+006to−8.0000e+006−8.0000e+006to−7.0000e+006−7.0000e+006to−6.0000e+006−6.0000e+006to−5.0000e+006−5.0000e+006to−4.0000e+006−4.0000e+006to−3.0000e+006−3.0000e+006to−2.0000e+006−2.0000e+006to−1.0000e+006 \matrix{ { - 9.5247e + 006to - 9.0000e + 006} \hfill \cr { - 9.0000e + 006to - 8.0000e + 006} \hfill \cr { - 8.0000e + 006to - 7.0000e + 006} \hfill \cr { - 7.0000e + 006to - 6.0000e + 006} \hfill \cr { - 6.0000e + 006to - 5.0000e + 006} \hfill \cr { - 5.0000e + 006to - 4.0000e + 006} \hfill \cr { - 4.0000e + 006to - 3.0000e + 006} \hfill \cr { - 3.0000e + 006to - 2.0000e + 006} \hfill \cr { - 2.0000e + 006to - 1.0000e + 006} \hfill \cr } −1.6215e+007to−1.6000e+007−1.6000e+007to−1.4000e+007−1.4000e+007to−1.2000e+007−1.2215e+007to−1.0000e+007−6.0000e+006to−4.0000e+006−5.0000e+006to−4.0000e+006−4.0000e+006to−2.0000e+006−3.0000e+006to−2.0000e+006−2.0000e+006to−2.7734e+005 \matrix{ { - 1.6215e + 007to - 1.6000e + 007} \hfill \cr { - 1.6000e + 007to - 1.4000e + 007} \hfill \cr { - 1.4000e + 007to - 1.2000e + 007} \hfill \cr { - 1.2215e + 007to - 1.0000e + 007} \hfill \cr { - 6.0000e + 006to - 4.0000e + 006} \hfill \cr { - 5.0000e + 006to - 4.0000e + 006} \hfill \cr { - 4.0000e + 006to - 2.0000e + 006} \hfill \cr { - 3.0000e + 006to - 2.0000e + 006} \hfill \cr { - 2.0000e + 006to - 2.7734e + 005} \hfill \cr }

Fig. 2

Dynamic pressure under the influence of roadway surrounding rock stress redistribution and the mining face more pressure superposition effect, the main vertical stress concentration effect of surrounding rock are the most significant retreat channel up to 16.3 MPa, the level of surrounding rock stress concentration up to 9.05 MPa, shows that dynamic pressure under the influence of vertical stress superposition effect on roadway and its corresponding main retracement retracement tunnel surrounding rock of mine pressure appear ‘deepening’ role, and its bearing structure of roadway surrounding rock stability is the key to the retreat channel lane to help the broken ends and the working face of coal wall rock shallow position, the position is at the end of the fully mechanised working face of coal mine mining period for most prominent position and horizontal deformation [8]. In order to maintain the bearing capacity of the surrounding rock in the broken roadway, more attention should be paid to the maintenance of surrounding rock and the improvement of supporting strength of roof and floor. In a retreat on the distribution of damage evolution of roadway surrounding rock, with face advance to stop mining line, retreat of surrounding rock of roadway affected by dynamic pressure under the influence of vertical stress superposition of roadway and its corresponding main retracement retracement tunnel surrounding rock pressure appeared violently, between surrounding rock destruction through most affected. This also causes the retracement bearing structure to be one of the reasons for the instability of surrounding rock of roadway. Combined with numerical value in the face to stop mining line position, the main retracement channel vertical stress concentration with the help of between surrounding rock dynamic pressure of working face of vertical peak should focus entirely on the overlay, surrounding rock damage, since the shear failure of overburden rock roadway surrounding rock damage fracture and retreat, at the same time between retracement coupon lane surround rock bearing structure since central gradual destruction, eventually go against the main retreat channel bearing structure stability of surrounding rock, Figure 2B is the surrounding rock stress distribution and shape distribution [9].

According to the study of regional geological conditions established for the numerical simulation model, studying the retracement channel is affected by face abutment pressure in the model first excavation main retreat channel and retracement auxiliary lane, three-dimensional calculation by pillar between two roadways model size 40 m × 150 m × width direction long direction high direction 50 m model a hexahedral unit divided into 97419 grid nodes. Mohr criterion is used in model calculation. Coal pillars are left on the left side of the model during excavation. When the working face is far from the stop-mining line, each 10-m excavation step is pushed to the retracement passage. Every 5 m of excavation is when the working face is separated from the stop-mining line. When the working face is away from the stop-mining line, each 1 m excavation step will be excavated for a total of 70 m. Based on the above mechanical formulas and data values, the support design optimisation of the numerical three-dimensional model of the main retracement passage is as follows.

When the distance between the working face and the main retracement passage is >30 m, the axial force and borehole stress of the bolt are always at a low level and hardly change with mining of the working face. The roof subsidence of the retracement passage, the subsidence of the column of the stacking support and the wall moving close of the roadway increase, but the change is small. The maximum principal stress, vertical stress, plastic zone and displacement deformation of surrounding rock have little change in the process of pushing and mining in the main retracement channel. This shows that the surrounding rocks of the main withdrawal passage in this range are relatively stable and the deformation is small, and the ore pressure is not obvious, so it can be used normally. When the distance between the fully mechanised coal mining face and the main withdrawal passage is <30 m, the axial force and borehole stress of the bolt increase significantly. As shown in Figure 3, the roof subsidence, the stacking support column subsidence and the change rate of the roadway wall proximity in the main withdrawal passage all increase significantly. Master retreat channel within the maximum principal stress and vertical stress of surrounding rock were significantly increased, in the main between retracement channel and working face has significant effects in the coal pillar stress, high in advance, the Lord retreat channel increase significantly vertical settlement deformation rate and deformation, face the plastic zone and the main retreat channel, forming a continuous plastic damage area, see Table 1.

Fig. 3

Combined with coal mine retracement of fully mechanised working face, the failure characteristics of surrounding rock support design of the numerical simulation analysis, put forward retracement of fully mechanised working face of roadway surrounding rock anchor + double metal net + + anchor plate of shotcrete support improved design scheme: the deputy retreat channel and its retreat coupon lane roof and the two sides adopt phi 000 18 mm × 2 mm metal bolt, row spacing between 1,000 mm × 1,000 mm, and tightening torque should reach 100 n. m. The roof of the main and auxiliary roadway is arranged with a row of anchor cables (three sets), and the two sides and the roof of the 308 retraction-joint roadway are arranged with a row of anchor cables (two sets), which all adopt 15.24 mm × 7600 mm pre-stressed steel hinge line for reinforcement support. At the same time, the top is supported by a steel ladder beam with a diameter of 14 mm, and the top is supported by a steel ladder beam with a diameter of 12 mm for reinforcement. The surrounding rocks of the withdrawal link roadway are all supported by double-layer metal mesh. The specification of the metal mesh is 5,500 mm × 1,200 mm, the mesh is 50 mm × 50 mm, and the spacing is 200 mm. Anchor force of bolt shall be no <50 kN, and torque shall be no <100 N m. The bottom plate is made of C30 concrete, 150 mm thick. In the final mining period, the decision should be made according to the actual situation. If necessary, the single hydraulic prop should be taken to cooperate with the active supporting measures.

Working face area average pressure continuous step distance 4.12 m, area average cycle pressure step distance 13.2 m; The bolt anchorage force in the main retracting passage of working face is insufficient, and the stress of the bolt in some areas has exceeded the design value of the bolt anchorage force in this mine. The ore pressure in the middle area of the main withdrawal passage (50–200 m) is more serious than that at both ends. The ore pressure of auxiliary retracement channel is not obvious, the surrounding rock is relatively stable and the deformation is small, so it can be used normally. When the distance between the fully mechanised mining face and the main withdrawal passage is <30 m, the main withdrawal passage is significantly affected by the mining movement of the working face at the beginning, and the on-site ore pressure is severe. The surrounding rock failure and deformation amount are large, and the deformation rate is significantly increased. The surrounding rock stress of the main withdrawal passage gradually concentrates and reaches its peak value. The vertical settlement deformation rate and deformation amount of the main withdrawal passage increase significantly. The plastic zone of the main withdrawal passage is connected with the plastic zone of the working face to form a continuous plastic failure zone. It shows that the stability of surrounding rock is obviously affected by mining in this range. In coal mine retracement auxiliary channel of fully mechanised working face and its retreat sanlian lane roadway position as an engineering test section, can be seen from Figure 4, tunnelling advance good Lord retreat channel combination supporting scheme and layout, eventually retreat of fully mechanised working face roof and floor of roadway surrounding rock deformation in 150~ 185 mm, the deformation of surrounding rock of two sides in the 160~ 180 mm, The design effect of shotcrete support of anchor cable and bimetal mesh and bolt plate is remarkable.

Fig. 4