TY - JOUR
AU1 - Guo,, Zhibiao
AU2 - Wang,, Qiong
AU3 - Li,, Zhaohua
AU4 - He,, Manchao
AU5 - Ma,, Zhibo
AU6 - Zhong,, Feixiang
AU7 - Hu,, Jie
AB - Abstract In case of deep mining engineering, the conventional gob-side entry retaining (GER) method encounters inevitable difficulties, due to high stress, high dynamic disturbance and large deformation issues. Recently, an innovative no-pillar mining approach of GER is proposed and applied in several realistic cases. According to the innovative approach, two novel techniques, the pre-fracturing on the roof before mining activities, and a novel support system with a good energy-absorbing capacity, make it possible to effectively restrain the large deformation in the deep surrounding rock. In this study, the innovative mining approach with the corresponding energy-absorbing support systems in deep mining is systematically presented. The pre-fracturing effect, taking into account the novel support system, is demonstrated in a numerical approach, and the numerical results are compared with those of the conventional method. Finally, some real monitoring data from the Chengjiao coal mine are given to further prove the advantages of the innovative mining method with energy-absorbing support systems. This work provides an effective and economical method to non-pillar mining in deep coal mine. 1 INTRODUCTION Since the 1950s, gob-side entry retaining (GER) is a standard non-pillar mining technique, which is widely used worldwide [1, 2]. According to this technique, the entry of current mining panels is retained as the tailgate for the next mining panel by building an artificial wall with pigsties, crush rocks, concrete blocks and other materials. The technical principle of GER is shown in Figure 1a [3–5]. Besides reducing the entry drive ratios, its application also improves the mining rate of coal resources and reduces the geologic hazards in coal mining. This makes GER a quite lucrative technique for sustainable coal resources [6, 7]. In recent years, multiple theoretical and experimental studies on GER, such as deformation control of entry [8–11], improvement of artificial backfill materials [12–14] and new supporting technologies [15–17], were performed. Figure 1. View largeDownload slide The principles of (a) conventional GER method and (b) RFMEAS approach. Figure 1. View largeDownload slide The principles of (a) conventional GER method and (b) RFMEAS approach. The application of GER in shallowly buried depth coal mines has become a ‘must have’ feature. However, with the rapid consumption of the shallow coal resources, the deep mining becomes more topical. In particular, deep mining in China is generally applied to the buried depth exceeding 700 m [18]. For the deep mining, various disasters occur continually. Moreover, the initial rock and mining stresses are superposed, which increases the deformation and maintenance cost of the retained entry [19]. When conventional GER is applied to a deep coal mine, the strength and energy-absorbing capacity of the conventional supporting system cannot adapt to the large deformation of the entry, which raises serious instability issues [20]. To solve the problem of large deformation of entry in the deep coal mines, two measures can be considered: 1) to decrease of the stress level on the entry roof and 2) to allow the large deformation of surrounding rock in certain extent, so that the energy can be sufficiently released and the surrounding rock can remain stable. Recently, an innovative approach of the GER was proposed, combining both the two abovementioned measures, and could be called as the Roof-Fracturing Method with Energy-Absorbing Supporting (RFMEAS). On one hand, the pre-fracturing along the gob side is performed on the entry roof, before mining activity, which cuts off the stress transmission along the rock beam, and thus lowers the stress level on the roof; on the other hand, the entry roof is reinforced by using the Constant Resistance Large Deformation (CRLD) cables [21] and the gangue rib is supported by a novel retractable gangue prevention structure, which can absorb the deformation energy of surrounding rock and provide a constant resistance during a large deformation, due to their special structure. Hence, an energy-absorbing support system can be formed and the deformation of the surrounding rock surrounding rock can be flexibly restrained. Additionally, the broken expand gangues, formed by the gob roof, can serve to hold the weight of the roof strata and segregate the gob area. The principle of RFMEAS is illustrated in Figure 1b. As compared to the conventional GER approaches, the RFMEAS one is more productive and cost-effective. In this article, based on a typical deep coal mine, namely the Chengjiao coal mine in Henan province of China, the deformation control mechanism of the entry with RFMEAS approach was theoretically substantiated. Then, the roof pre-fracturing and energy-absorbing support systems were numerically simulated to assess the stress distribution and deformation control of the entry. Finally, the effect of RFMEAS approach application to 21 304 panel of Chengjiao coal mine was analyzed. 2 ENGINEERING STATUS OF THE DEEP ENTRY 2.1 Engineering geological conditions The Chengjiao coal mine is located in Yongcheng City, Henan Province, China. The 21 304 panel of Chengjiao coal mine is buried at a depth of 835–915 m, with an average thickness of 3.1 m. The entry is 4.2 m wide and 2.8 m high. Rock strata above the coal seam are composed of mudstone (2.85 m thick), fine sandstone (3.76 m thick) and siltstone (5.23 m thick) in the ascending order. Meanwhile, the floor strata below the coal seam are composed of sandy mudstone (0.43 m thick), siltstone (1.63 m thick) The roof and floor layers of the extracted coal seam are depicted in Figure 2. The roof of the 21 304 head entry is cracked, and the cracked section accounts for 49% of the whole entry. Besides, twelve faults in total were revealed in the tunneling process, which implies a poor integrity of the roof plate. Initially, the combined support with anchor-net-cable was applied to driving gateways and the density of the original support was high. Figure 2. View largeDownload slide Location and lithology of the study site. Figure 2. View largeDownload slide Location and lithology of the study site. 2.2 Deformation patterns and causes Through field investigation, it was found that the roof of the 21 304 head entry has pronounced subsidence with original cable support, the maximum subsidence displacement reached 120 mm and the original cable has been loosened or broken, as shown in Figure 3. Figure 3. View largeDownload slide Large deformation with the original support in the Chengjiao coal mine: (a) significant roof subsidence and (b) tensile failure of traditional cables. Figure 3. View largeDownload slide Large deformation with the original support in the Chengjiao coal mine: (a) significant roof subsidence and (b) tensile failure of traditional cables. According to the actual situation of 21 304 head entry, the reasons for the large deformation of surrounding rock and the failure of supporting structure are analyzed as follows. High stress of deep entry. High ground stress is the direct factor that causes the deformation and failure of the surrounding rock in the entry. With an increase in the mining depth, vertical and horizontal stresses are augmented. Since the Chengjiao coal mine entry depth varies from 835 to 915 m, the vertical stress value induced only by the weight of the overlying strata is as high as 20–23 MPa. The impact of dynamic loads such as rock burst and mining disturbances increases the stress of the entry-surrounding rock by several times, as compared to the initial rock stress value. Besides, the high flaking ratio and faults are also the causes of instability of 21 304 head entry. The conventional support structure is difficult to adapt to the large deformation of a deep coal mine. The loading and deformation characteristics of the entry in deep mining are entirely different from those of a shallow entry. Generally, the shallow entry does not allow the surrounding rock to reach the plastic state. However, the tremendous deformation energy in the deep entry must be released and allow the surrounding rock to reach the plastic state. This requires that the support system should have high energy-absorbing and stress-release characteristics. For 21 304 head entry of the Chengjiao coal mine, the traditional cables, whose ductility is limited, are initially used to support the surrounding rock. Once the entry deformation is large, they are prone to fail. 3 AN INNOVATIVE APPROACH FOR GER 3.1 The implementation procedure of RFMEAS The implementation procedure of the RFMEAS approach can be subdivided into four steps, which are shown in Figure 4. The first step is to strengthen the entry roof with the CRLD cables (see Figure 4a). At this step, the entry extraction activities will cause the stress concentration in the surrounding rock and the CRLD cable is used to control the entry roof deformation during the initial period. The next step envisages the roof pre-fracturing along the gob side (Figure 4b). Since, without roof pre-fracturing, the entry roof and gob roof are connected to each other and the caving activities on the gob roof will affect the entry roof stability. Meanwhile, the entry roof pre-fracturing at the gob side, will drastically weaken this effect and improve the entry stability. Then, as the coal seam is mined out progressively, the gob roof is caved along the roof pre-fracturing line, with the formation of caved rocks. To prevent these caved rocks from packing into the entry and hold the pressure caused by overlying strata, the gangue prevention support and the temporary support in the entry should be carried out immediately (Figure 4c). Finally, when the caved rocks are compacted under the pressure of the roof strata, they can support and control the strains induced by the overlying strata pressure. Under the joint action of the coal rib and gangue rib (composed of caved rocks), the surrounding rocks of the entry reach a new equilibrium state, whereas the temporary entry in the support can be removed (Figure 4d). Figure 4. View largeDownload slide The implementation procedure of the RFMEAS approach. Figure 4. View largeDownload slide The implementation procedure of the RFMEAS approach. 3.2 Formatting of mathematical components To elucidate the mechanism of deformation control in the entry of RFMEAS approach, a mechanical model was elaborated as shown in Figure 5. With assumption that the entry support can be negligible, the roof rock beam can be considered as an elastic cantilever beam with one fixed side and one free side after the working face is pushed over. According to the theory of elasticity [22], the deflection equation of the entry roof can be written as: EId2wdx2=MD+q(L−x)22, (1) where q is the uniform load due to the gravity of the overlying strata; MD is the bending moment on the pre-fracturing surface (which is considered equal to ultimate bending moment of roof rock in the fracturing moment of rock beam); E is the elastic modulus of the rock beam; I is the inertia moment on the cross-section of the rock beam; L is the length of the rock beam. Figure 5. View largeDownload slide A mechanical model of the RFMEAS approach. Figure 5. View largeDownload slide A mechanical model of the RFMEAS approach. According to the theory of elasticity, the velocity and deflection of fixed point (x = 0) are both zero. Therefore, the boundary conditions can be introduced as follows: w′|x=0=0,w|x=0=0, (2) According to Equations (1) and (2), the deflection equation of entry roof for the rock beam fracture condition can be simplified as follows: w=MDx22EI+qx424EI(x2−4Lx+6L2), (3) In case of x = L, the maximum deflection of the entry roof can be obtained as wmax=MDx22EI+qx424EI(x2−4Lx+6L2), (4) wmax=qL68EI (5) According to Equation (4), the maximum deflection of the entry roof is related to the rock lithology, bending moment of the fractured surface and cantilever length of the roof rock beam. According to the RFMEAS approach, the CLRD cables are used to reinforce the entry roof and the stiffness and self-bearing capacity of the entry roof rock are thus improved. In addition, the entry roof in gob side is pre-fractured by directional roof pre-fracturing technique, which can weaken or avoid the bending moment, derived from the gob roof to the entry roof. This also reduces the cantilever length of the roof rock beam. Using the above two measures, the entry roof deformation in RFMEAS approach can be decreased drastically. If the immediate roof is fractured completely, according to the RFMEAS approach in coal mining, the maximum deflection of the entry roof can be estimated according to Equation (5), with the equivalent elastic modulus of the entry roof rock, taking into account the CRLD cable. 4 KEY TECHNIQUES OF RFMEAS APPROACH IN DEEP MINING 4.1 Energy-absorbing supporting system 4.1.1 The CRLD cable with excellent deformability To control the rotation and subsidence of entry roof in RFMEAS approach, the CRLD cable is used to strengthen the entry roof before the coal seam is mined out. The CRLD cable is a new supporting structure, which is characterized by excellent deformability and high constant resistance [23, 24]. It is composed of a constant-resistant device, face pallet and steel strand, as shown in Figure 6. The constant-resistant device provides a considerable elongation with constant resistance for CRLD cable that would mitigate the deformation of deep mining and release some deformation energy of the surrounding rock. Figure 6. View largeDownload slide The structural diagram of CRLD cable. Figure 6. View largeDownload slide The structural diagram of CRLD cable. Experiments and field applications [23] have proved that the constant-resistant value provided by the CRLD cable with a diameter of 21.8 mm is ~300 kN, while the ultimate elongation can reach 1000 mm. The working process of the CRLD cable is shown in Figure 7a. In the static tensile numerical experiment, the axial force-axial deformation curves of the traditional cable and the CRLD one was constructed, as shown in Figure 7b. A better energy-absorbing capacity of the CRLD cable can be observed [25] Figure 7. View largeDownload slide (a) Working principle of CRLD cable and (b) the results in the static tensile numerical experiment of CRLD and traditional cables [25]. Figure 7. View largeDownload slide (a) Working principle of CRLD cable and (b) the results in the static tensile numerical experiment of CRLD and traditional cables [25]. 4.1.2 A NRGP structure To prevent the crushed rocks into the entry, a NRGP structure and high-strength wire net are adopted as gangue side supports in deep mining. The NRGP structure consists of two sections of U-type steel fixed by block cable. It’s structural diagram is shown in Figure 8. It not only exhibits stronger deformation resistance for lateral pressure caused by gangues squeezing, but also can slide to provide movement when subject to axial roof pressure, which can cooperatively conform to the deformation of roof. Figure 8. View largeDownload slide The structural diagram of NPGR structure. Figure 8. View largeDownload slide The structural diagram of NPGR structure. In engineering application, the appropriate torque value of block cable is highly related to energy-absorbing capacity of NPGP structure. If the torque value of block cable is too small, the structure is to be unstable. On the contrary, if the torque value is too large, the structure is difficult to realize pressure-releasing. Therefore, an axial compression experiment was carried out to study the release-pressure and energy–absorbing ability of NPGP structure. The detailed experiment process was presented in Figure 9 and the axial force-axial deformation curves of NPGP structure with different torque value (100, 200, 300, 400 and 450 N m) were obtained, as shown in Figure 10. From Figure 10, it is known that as the torque value of block cable increases, the structural constant resistance also increases. A better energy-absorbing capacity of NPGP structure can be observed when the torque value of block cable is between 200 and 300 N m. Figure 9. View largeDownload slide The axial compression experiment process of the novel gangue prevention structure. Figure 9. View largeDownload slide The axial compression experiment process of the novel gangue prevention structure. Figure 10. View largeDownload slide (a) The results in axial compression experiment of novel gangue prevention structure with different torque values. Figure 10. View largeDownload slide (a) The results in axial compression experiment of novel gangue prevention structure with different torque values. 4.1.3 The hydraulic supports with high resistance In deep mining, the traditional single props often lose efficacy when the roof pressure is too large (see Figure 11a). So, the temporary support of hydraulic supports with high resistance were used to help stabilize the entry Based on mine pressure theory, the maximum abutment pressure for periodic weighting can be estimated by: σ=nγ∑1nhi=nγH, (6) where σ is the peak abutment pressure for periodic weighting of roof, γ is the average volume weight of the rock layer, hi is the thickness of the ith rock layer of caving zone, H is the total thickness of the rock layer of caving zone and n is the ratio of pressure during periodic weighting to normal pressure, with the value <2.0. Figure 11. View largeDownload slide (a) The bend failure of single props in deep mining and (b) the high-resistant hydraulic supports used in 21 304 head entry. Figure 11. View largeDownload slide (a) The bend failure of single props in deep mining and (b) the high-resistant hydraulic supports used in 21 304 head entry. The total thickness of the rock layer of caving zone H can be calculated by H=∑1nhi=MK−1, (7) where M is the mining height and K is the bulking factor. Substituting Equation (7) into Equation (6) to obtain the maximum abutment pressure for periodic weighting of roof: σ=nγ∑1nhi=nγMK−1. (8) Thus, the working resistance of hydraulic supports σr can be calculated as follows σr≥σN (9) N is the quantity of hydraulic supports with high resistance per meter along the length of the entry. 4.2 Directional roof pre-fracturing technique with low disturbance In RFMEAS approach, the roof pre-fracturing plays a significant role in improving the state of stress in surrounding rock. If the immediate roof can cave smoothly along the pre-fracturing line, the dynamic disturbance caused by roof caving activities will be weakened. However, whether the roof can cave smoothly is impacted by cracks connection of roof pre-fracturing plane. In this approach, the directional roof pre-fracturing technique not only makes the roof caving smoothly in a controlled direction, but also minimizes the damage for surrounding rock caused by blasting. In line with this technique, an energy-accumulated device was developed. The device is an energy-accumulated tube with two grooves at 180°angles from each other at the surface, which can control the direction of cracks extension. Before roof pre-fracturing, the explosives and energy-accumulated device are installed in blasting hole. After the explosives are detonated, the enormous energy generated by blasting act on the grooves, making the cracks between two blasting holes to extend in the controlled direction, the principle of the technique is shown in Figure 12. Figure 12. View largeDownload slide The working principle of directional roof pre-fracturing technique. Figure 12. View largeDownload slide The working principle of directional roof pre-fracturing technique. In deep mining the conventional adjacent hole blasting method [26] used in shallow mining will lead to cable failures and entry instability. Therefore, the directional roof pre-fracturing technique with low disturbance was used in 21 304 head entry. It adopted nonadjacent hole blasting method to reduce the influences from dynamic blasting loading to cables and entry. Based on the rules of detonation stress attenuation [27], the radial explosion pressure σr caused by the blasting stress wave at any point in the surrounding rock can be estimated by: σr=Pcrc−α, (10) Where Pc is the impact load of detonation gas act on the wall of the blasting hole, rc is the ratio of the distance of a certain point from the blasting center to the blasting hole radius and α is the attenuation index of blasting stress wave, which is determined by the frequency of the blasting stress wave (α > 1). Based on formula (10), the nonadjacent hole blasting method decreases the explosion pressure by (1/2)α times, as compared to adjacent hole blasting method, which greatly reduces the impact of blasting load on the cables and surrounding rock of entry. 5 CASE STUDY 5.1 Numerical simulation schemes A 3D construction of the mining excavation model of 21 304 panel in Chengjiao coal mine is performed as shown in Figure 13a, by using the finite difference software FLAC3D, with a length of 300 m, a width of 150 m and a height of 45 m. The perpendicular velocities are constrained along each lateral boundary surface, and all the four lateral surfaces are free-slip. The top surface is completely free and the bottom one is fixed in all directions. As the roadway is located with a depth of ~840 m, the gravity effect is introduced by exerting a constant vertical stress of 20 MPa on the top surface of the model (see Figure 13b). Figure 13. View largeDownload slide Geometrical model and boundary conditions: (a) geometric model in 3D and (b) geological and boundary conditions. Figure 13. View largeDownload slide Geometrical model and boundary conditions: (a) geometric model in 3D and (b) geological and boundary conditions. In the model, the mechanical behavior of rock strata is described by a perfect elastic–plastic model with a Mohr–Coulomb failure criterion and the tensile strength of the rock strata is also taken into account. The physical and mechanical parameters of rock strata are obtained according to laboratory tests, as shown in Table 1. Table 1. Mechanical parameters of the rock strata used in the numerical mode. Lithology Thickness (m) Bulk (109 Pa) Shear (109 Pa) Friction (°) Tension (106 Pa) Density (103 kg/m3) Cohesion (106 Pa) Sandy mudstone 11 9 9 30 3.7 2.4 1.7 Siltstone 5.23 15 17 35 2 2.5 9 Fine sandstone 3.76 13 15 38 4.9 2.4 7.9 Mudstone 2.77 8 10 44 3 2.5 1.7 Coal 3.1 1 2 30 0.7 1.4 1.1 Sandy mudstone 0.43 5 6 44 2 2.5 1.7 Silt mudstone 1.63 15 17 35 3 2.5 9 Fine sandstone 11.22 13 15 38 4.9 2.4 7.9 Lithology Thickness (m) Bulk (109 Pa) Shear (109 Pa) Friction (°) Tension (106 Pa) Density (103 kg/m3) Cohesion (106 Pa) Sandy mudstone 11 9 9 30 3.7 2.4 1.7 Siltstone 5.23 15 17 35 2 2.5 9 Fine sandstone 3.76 13 15 38 4.9 2.4 7.9 Mudstone 2.77 8 10 44 3 2.5 1.7 Coal 3.1 1 2 30 0.7 1.4 1.1 Sandy mudstone 0.43 5 6 44 2 2.5 1.7 Silt mudstone 1.63 15 17 35 3 2.5 9 Fine sandstone 11.22 13 15 38 4.9 2.4 7.9 Table 1. Mechanical parameters of the rock strata used in the numerical mode. Lithology Thickness (m) Bulk (109 Pa) Shear (109 Pa) Friction (°) Tension (106 Pa) Density (103 kg/m3) Cohesion (106 Pa) Sandy mudstone 11 9 9 30 3.7 2.4 1.7 Siltstone 5.23 15 17 35 2 2.5 9 Fine sandstone 3.76 13 15 38 4.9 2.4 7.9 Mudstone 2.77 8 10 44 3 2.5 1.7 Coal 3.1 1 2 30 0.7 1.4 1.1 Sandy mudstone 0.43 5 6 44 2 2.5 1.7 Silt mudstone 1.63 15 17 35 3 2.5 9 Fine sandstone 11.22 13 15 38 4.9 2.4 7.9 Lithology Thickness (m) Bulk (109 Pa) Shear (109 Pa) Friction (°) Tension (106 Pa) Density (103 kg/m3) Cohesion (106 Pa) Sandy mudstone 11 9 9 30 3.7 2.4 1.7 Siltstone 5.23 15 17 35 2 2.5 9 Fine sandstone 3.76 13 15 38 4.9 2.4 7.9 Mudstone 2.77 8 10 44 3 2.5 1.7 Coal 3.1 1 2 30 0.7 1.4 1.1 Sandy mudstone 0.43 5 6 44 2 2.5 1.7 Silt mudstone 1.63 15 17 35 3 2.5 9 Fine sandstone 11.22 13 15 38 4.9 2.4 7.9 In the present study, both the previous and the current support systems (the conventional and energy-absorbing support systems) of the 21 304 head entry are considered. To clarify the effect of the roof pre-fracturing and the advantages of the energy-absorbing support systems in RFMEAS approach, two mining cases will be performed, as follows: Mining excavation without the roof pre-fracturing and reinforced using the traditional support system. The mining excavation is performed without the pre-fracturing, and the entry is supported using traditional cables and hydraulic supports. The distribution of the cables in cross-section of the entry is as shown in Figure 14a. In addition, the high-resistant hydraulic supports are considered in numerical model. They are continuously arranged, and the working resistance chosen for 21 304 head entry was 4000 kN; maximum deformation was 800 mm. Mining excavation with roof pre-fracturing and reinforced using energy-absorbing support systems according to the RFMEAS approach. Figure 14. View largeDownload slide Simulation schemes: (a) previous support, (b) improved support of RFMEAS approach and (c) the arrangement plans of support structure in numerical model. Figure 14. View largeDownload slide Simulation schemes: (a) previous support, (b) improved support of RFMEAS approach and (c) the arrangement plans of support structure in numerical model. The mining excavation is carried out after the pre-fracturing, and the roof is reinforced using the energy-absorbing support systems. In consideration of the condition of 21 304 head entry in Chengjiao Coal Mine, two traditional cables on the right of the roof are substituted by the CRLD cables (see Figure 14b), because the stability of the surrounding rock mass near the gob side is usually an issue, and the ductile and energy-absorbing cables are more effective to solve this issue. The mechanical parameters of the supports are listed in Table 2. Table 2. Parameters of the bolt and cable elements used in the numerical model. Parameters Young´s modulus (GPa) Poisson ratio Tensile capability (kN) Constant resistance (kN) Pretension (kN) Maximum deformation (mm) Traditional cable 205 0.31 500 – 245 – Traditional bolt 205 0.31 240 – – – High- strength bolt 205 0.31 360 – – – CRLD cable 205 0.31 500 295 245 350 Parameters Young´s modulus (GPa) Poisson ratio Tensile capability (kN) Constant resistance (kN) Pretension (kN) Maximum deformation (mm) Traditional cable 205 0.31 500 – 245 – Traditional bolt 205 0.31 240 – – – High- strength bolt 205 0.31 360 – – – CRLD cable 205 0.31 500 295 245 350 Table 2. Parameters of the bolt and cable elements used in the numerical model. Parameters Young´s modulus (GPa) Poisson ratio Tensile capability (kN) Constant resistance (kN) Pretension (kN) Maximum deformation (mm) Traditional cable 205 0.31 500 – 245 – Traditional bolt 205 0.31 240 – – – High- strength bolt 205 0.31 360 – – – CRLD cable 205 0.31 500 295 245 350 Parameters Young´s modulus (GPa) Poisson ratio Tensile capability (kN) Constant resistance (kN) Pretension (kN) Maximum deformation (mm) Traditional cable 205 0.31 500 – 245 – Traditional bolt 205 0.31 240 – – – High- strength bolt 205 0.31 360 – – – CRLD cable 205 0.31 500 295 245 350 The pre-fracturing is carried out before mining along the god-side and with an angle of 75° and a length of 8 m (see Figure 14b). The length of the roof pre-fracturing is related to the degree of the broken bulking factor of the roof strata [28]. In theory, the value of the length, which can be estimated by Equation (11), should be sufficiently large to make the mining room fill with caved rocks and thus the caved rocks can provide supporting force to overlying strata. Hf=(H−H1−H2)/(K−1) (11) where H, H1 and H2 are the height of head entry, the roof subsidence and the floor heave, respectively, and K is the broken bulking factor of the roof strata, with a value of 1.3–1.5. In case of the Chengjiao coal mine, H is 2800 mm and K is 1.35. It is reasonable to consider the vanished values of and, in case of the RFMEAS approach. Hence, can be estimated with a value of 8000 mm, according to Equation (11). In addition, according to the previous contribution [27], the angle of the roof pre-fracturing of 15° can be used. 5.2 Numerical results Figure 15 illustrates the distributions of the vertical stress under the gravity, before mining excavation. In the absence of the pre-fracturing, the maximum vertical stress of 6–16 MPa and that of 5–14 MPa can be observed, in the upper and lateral surrounding rocks of the roadway, respectively (see Figure 15a). In the presence of the pre-fracturing, the maximum vertical stress is decreased to 2–10 MPa, as shown in Figure 15b. In addition, the stress on the top of the roadway is noticeably lowered, along the pre-fracture. As a conclusion, the pre-fracturing can cutoff effectively the force transmission along the immediate roof and improve the stress state in the surrounding rock. Figure 15. View largeDownload slide Vertical stress distribution of entry-surrounding rock surrounding rock before coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 15. View largeDownload slide Vertical stress distribution of entry-surrounding rock surrounding rock before coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. The vertical and maximum shearing stresses after the mining excavation are given in Figures 16 and 17, respectively. According to Figures 16a and 17a, an evident continuity and the high value of the stress can be observed on the top of the roadway, and the roadway can be considered as completely destroyed, with the extremely large deformation. In addition, the large bending of the rock beam is induced by the bending moment from the gob zone and can be explained by Equation (5). According to Figures 16b and 17b, the stress level is largely decreased, due to the pre-fracturing. The transmission of the bending moment is cutoff, and the deflection of the rock beam is noticeably reduced, which is agreed with Equation (5). In addition, the immediate roof collapses along the pre-fracture, which makes the automatic caving possible. During the numerical calculations, the axial forces of the CRLD cable, the traditional cable and the hydraulic support are monitored and Figure 18a and b presents their evolutions before and after the mining excavation. In case of no pre-fracturing, the traditional cable fails suddenly, while the axial force reaches rapidly the tensile strength (see Figure 18a). The hydraulic support exhibits a constant resistance of 4000 KPa and then fails, as the roof continues to collapses. However, the CRLD cable presents a quasi constant resistance and remains available, because the roof deformation is controllable, if the pre-fracturing is performed. Similarly, the working resistance of the hydraulic support is also kept with a value of 4000 kPa and the entire support system presents thus a good performance. Figure 16. View largeDownload slide Vertical stress distribution of entry surrounding rock after coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 16. View largeDownload slide Vertical stress distribution of entry surrounding rock after coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 17. View largeDownload slide Maximum shear stress distribution of entry surrounding rock after coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 17. View largeDownload slide Maximum shear stress distribution of entry surrounding rock after coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 18. View largeDownload slide The stress variation curve of support system caused by coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. Figure 18. View largeDownload slide The stress variation curve of support system caused by coal seam excavation: (a) no roof pre-fracturing line and traditional cable support and (b) roof pre-fracturing line and energy-absorbing support. 5.3 Field-application effect and analysis According to the numerical simulation results, it is reasonable to apply the RFMEAS approach to 21 304 working face of Chengjiao coal mine. The roof pre-fracturing line parameters and support parameters of CRLD cable are the same as those in case II of the above numerical simulation. The entry retaining photos are presented in Figure 19. To verify the deformation control effect of RFMEAS approach with energy-absorbing system in deep mining, the field deformation of the roof and both sides of the entry were monitored continuously for 60 days. A-A, B-B, C-C and D-D entry sections, which correspond to the distances from to cutting hole equal to 50, 100, 150 and 200 m, respectively, were selected. Monitoring points were arranged from both sides of the entry roof, as shown in Figure 19. Figure 19. View largeDownload slide Survey station arrangement of 21 304 head entry: (a) plane graph and (b) section view of A-A cross-section. Figure 19. View largeDownload slide Survey station arrangement of 21 304 head entry: (a) plane graph and (b) section view of A-A cross-section. The monitoring results are shown in Figure 20. It can be seen that after the coal seam is mined out, the deformation of the surrounding rock increases sharply, and the deformation lasts for ~25 days. After 25 days, the surrounding rock reaches the equilibrium state again and the displacement of the measuring point tends to be stable. The final deformation of the gangue rib is 120–160 mm, and the ones of the solid coal are 50–65 mm. The maximum deformations of the roof near to the gangue rib and the solid coal are 210–250 and 100–130 mm, respectively. The above results show that the surrounding rock control effect is significant with the RFMEAS approach. The field effect in the god-side is depicted in Figure 21. Figure 20. View largeDownload slide Monitoring results of the 21 304 head entry deformation: (a) displacement–time curve of the gangue rib, (b) displacement–time curve of the coal rib, (c) displacement–time curve of the roof at the gangue rib and (d) displacement–time curve of the roof at the coal rib. Figure 20. View largeDownload slide Monitoring results of the 21 304 head entry deformation: (a) displacement–time curve of the gangue rib, (b) displacement–time curve of the coal rib, (c) displacement–time curve of the roof at the gangue rib and (d) displacement–time curve of the roof at the coal rib. Figure 21. View largeDownload slide Entry retaining effect of RFMEAS approach: (a) overall effect and (b) molding effect of the gangue rib. Figure 21. View largeDownload slide Entry retaining effect of RFMEAS approach: (a) overall effect and (b) molding effect of the gangue rib. 6 CONCLUSIONS Due to the high stress in deep mining activities, the conventional GER approaches are no longer suitable. In this article, the RFMEAS approach, especially the pre-fracturing by directional blasting, and the CRLD cable are first presented in detail. According to this method, the decrease of the stress level and the energy-absorbing capacity of the support system are highlighted. The pre-fracturing on the roof is performed to cutoff the stress transmission along the rock beam, which noticeably lowers the stress level, and the phenomenon is explained in an analytical approach; the energy-absorbing supporting system is introduced, so that the surrounding rock can be allowed to release the energy and remain stable. Second, a numerical model is established and several cases are calculated. The effect of the pre-fracturing is discussed in detail, and the axial forces of the supports are monitored and compared. Third, the real case in Chengjiao coal mine and the corresponding monitored data are presented. As a conclusion, by performing the pre-fracturing and applying the energy-absorbing support system, the novel mining method redounds to significantly decrease the stress level and reliably control the deformation of the surrounding rock, even in the deep mining. The further investigation and application of the RFMEAS approach are worth being performed. AUTHOR CONTRIBUTIONS Zhibiao Guo, Manchao He and Qiong Wang discussed and conceived the research. Zhaohua Li performed the numerical simulation. Zhibo Ma, Feixiang Zhong and Jie Hu conducted the field test and revised the article. FUNDING This research was supported by the National Natural Science Foundation of China (Grant no. 51479195), the National Key Research and Development Plan of China (2016YFC0600901), the Special Fund of Basic Research and Operating of China University of Mining & Technology, Beijing (Grant no. 2009QL06) and the State Scholarship Fund of China. CONFLICTS OF INTEREST The authors declare no conflict of interest. REFERENCES 1 Yang H , Cao S , Li Y , et al. Soft roof failure mechanism and supporting method for gob-side entry retaining . Minerals 2015 ; 5 : 707 – 22 . Google Scholar Crossref Search ADS 2 Yang H , Cao S , Wang S , et al. Adaptation assessment of gob-side entry retaining based on geological factors . Eng Geol 2016 ; 209 : 143 – 51 . 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TI - Surrounding rock control of an innovative gob-side entry retaining with energy-absorbing supporting in deep mining
JF - International Journal of Low-Carbon Technologies
DO - 10.1093/ijlct/cty054
DA - 2019-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/surrounding-rock-control-of-an-innovative-gob-side-entry-retaining-MTE5W4wAl0
SP - 23
VL - 14
IS - 1
DP - DeepDyve
ER -