Mechanism and surrounding rock control of gob-side entry formation passing through normal fault - A case study

doi:10.21203/rs.3.rs-2820859/v1

Geological faults are widely distributed in stratum and have an important influence on underground mining. In order to increase the mining rate of coal resources and reduce the occurrence of accidents such as rock bursts, the technology of gob-side entry formed by roof cutting and pressure release (GEFRCPR) was investigated in fault-affected areas. The mechanism of gob-side entry formation passing through a normal fault was studied by theoretical analysis and numerical simulation, and control technology for entry formation was proposed. The results indicate that during the process of passing through a normal fault from hanging wall to foot wall, the vertical stress of the solid coal bank at the working face increased first, then decreased, rose and eventually reached a stable state. For the fault affected area, a combined support of " door type supports + cutting guard stents " was proposed as a temporary support for the formed entry and was tested on site. On-site monitoring showed that the degree of final deformation of surrounding rock of the formed entry was like: at the fault > before the fault > after the fault. By applying the proposed support technology, the deformation of the surrounding rock in the fault-affected area was effectively controlled so that the formed roadway appeared thereafter can be reused.

fault

surrounding rock control

fault-affected area

gob-side entry formation

Coal resources are non-renewable energy. Due to the significantly increasing demands of energy and the amount of coal mined each year, non-renewable energy resource has decreased dramatically (Zhou et al. 2020;He et al. 2017;Zhang et al. 2015). The situation has been further compounded by the even more complex and unstable geological mining conditions (Zhang et al. 2019;Wang et al. 2022a). To address this problem, a new type of mining method—non-pillar long-wall mining method (Zhou et al. 2019;Wang et al. 2020)—has been developed based on traditional long-wall mining with the help of the development of entry support materials and new construction technology (Yan et al. 2019;Wu et al. 2019;Cheng et al. 2022). The most representative method among this type of methods is non-pillar mining with gob-side entry retaining (He et al. 2017;Meng et al. 2021). This method, however, is not applicable under deteriorating geological conditions of coal mining and when the mining process encounters frequent discontinuities or faults (Lin 2020;Li et al. 2021). As the working face comes closer to the fault, the impact of the mining stress transfer tends to be significant since the fault cuts off the integrity of the intact rock strata (Wang et al. 2022b). Mine pressure behavior appears as the working face pass through the fault, which accompanied by a significant increase in the frequency and intensity of rock burst. Since the pillar mining (Wang et al. 2020;Shan et al. 2021) and non-pillar mining with gob-side entry retaining (Kong et al. 2021) are not able to substantially change the stress transfer mode, the stress on the entry retaining and artificial wall gradually increases as the working face approaches the fault. When the stress exceeds the rock bearing capacity limit, it may cause rock burst resulted in huge financial burdens and economic loss (Shukla et al. 2021;Suchowerska et al. 2013;Wang et al. 2018).

To solve these problems, He et al. innovatively proposed a novel coal mining technology named gob-side entry formation by roof cutting and pressure release (GEFRCPR) (He et al. 2015). This technology successfully retains the roadway through the use of roof directional cutting as well as constant resistance and large deformation anchor cable support. The retained roadway would be reused for the next working face. This technology does not only cancel the use of coal pillar, but also artificial wall filling, thus fundamentally changed stress transfer mode of roadway roof. This method opened up a new direction for the non-pillar mining technology and promoted the application of non-pillar mining technology under complex geological conditions. This new technology saves coal resources, reduces the roadway excavation and disasters occurrence in mining under complex geological conditions (Wang et al. 2018;Yang et al. 2019). Currently, technology of GEFRCPR has been extensively used under varied geological conditions in many coal mines across China and achieved impressive economic and social benefits. For example, this technology has been successfully applied to the high gas mine, the Baijiao Coal Mine of the Sichuan Coal Group, the hard roof of the Tangshangou Coal Mine of the China National Coal Group, the compound roof of the Halagou Coal Mine of the Shenhua Group, the large embedded depth of the Chengjiao Coal Mine of the Yongmei Coal Group and the thick coal seam of the Ningtiaota Coal Mine of the Shaanxi Coal Group (Zhen et al. 2019;Gao et al. 2019;Huang et al. 2023;Wang et al. 2020;Liu et al. 2021). Many scholars at home and abroad have carried out a lot of research on this technology. He et al established structural and mechanical models in exploring the mechanism of the novel no coal pillar mining technology and carried out field tests, and finally achieved good application results (Zhang et al. 2020). Wang et al established a mechanical model of short cantilever beam and solved it by using energy theory and displacement variational method. On the basis of comprehensive theoretical analysis and calculation results, some ideas and countermeasures are put forward to control the deformation of short cantilever roof (Wang et al. 2018). Gao et al studied the novel no coal pillar mining technology by means of theoretical analysis, numerical simulation and field tests. The research findings show that the stress condition of the entry was significantly improved and the average roof pressure decreased by appropriately 60.33% after adopting the new technique. This work provides an economical and effective approach to achieve sustainable exploitation of underground coal resources (Gao et al. 2017). However, the application of technology of GEFRCPR under complex geological conditions, such as in the presence of faults, has not been reported yet.

In this study, field tests were carried out on the 9101 working face passing through the fault in Xiashanmao coal. During the test, the numerical model corresponding to the actual situation of the project was established to study the whole process of the working face passing through the fault, especially the mine pressure distribution of the surrounding rock during the cross-fault entry retaining process. At the same time, the stress distribution of surrounding rock in the process of working face approaching the fault was analyzed by establishing a mechanical model. Based on the theoretical analysis and numerical simulation results, the control technology of surrounding rock within the range of fault-affected area is proposed. This work provides a reference for the control of entry retaining under the influence of fault and enriches the technical system of GEFRCPR.

Two roadways need to be excavated in advance to mine a working face when applying the traditional coal mining method. When mining the next face, a coal pillar is left in the middle and is unable to be mined, resulting in the waste of coal resources. The roadway layout in the traditional coal mining method is shown in Fig. 1 (a). The GEFRCPR is able to cut off the connection between the gob roof and the roadway roof by roof directional blasting slitting technology. Therefore, the roadway roof turned from overhung wall beams to short-wall beams without the need of artificial support wall constructed at the gob side. This roadway is automatically formed by special processes using the fracture dilatancy properties of gangues in the gob, which continues to be utilized for the next working face. To work on the next working face, there was no need to retain any pillars between the two working faces, and only a new roadway would be sufficient (He et al. 2021). The roadway layout and construction process are shown in Fig.1 (b).

Fig.1.

The key technical processes of GEFRCPR includes pull solidly, cut openly, full collapse, and protected well.

(I) Pull solidly. The constant resistance anchor cable was developed by Academician He Manchao and his research team (He 2014;He et al. 2014). The anchor cable is mainly composed of two parts: the constant resistance equipment and the steel strand. The constant resistance equipment has a fixed constant resistance value. When the external force is greater than the constant resistance value, the anchor cable can absorb a large amount of deformation energy without damage by using the constant resistance equipment. In practical engineering applications, the constant-resistance anchor cable can better adapt to the large deformation of surrounding rock masses under external forces. Therefore, before working on the working face, in order to ensure the stability of the roof of the entry retaining and to reduce the influence of directional cutting technology on the roof, the constant resistance anchor cable (Zhang et al. 2020) was used to provide support for the roof before other processes. The anchor cable pulls the roof solidly. The anchor cable structure and support condition as is shown in Fig.1 (c).

(II) Cut openly. The two-way energy-accumulating tension blasting technology, which is based on the characteristics of rock high compressive strength and low tensile strength, uses directional blasting technology to achieve directional cutting of rock mass. The key to this technology is the application of bilateral energy-gathering device. It is designed into a tube shape with a row of energy gathering holes on both sides. After loading explosives for blasting, the blasting energy will be accumulated along the drilling direction to form stress concentration, so as to realize the tensile cutting effect on the rock along the energy gathering holes (Zhang et al. 2020;Zhang et al. 2020). During the field test of a certain project, the rock borehole was drilled 6m deep, diameter 50mm, and spacing 600mm. The test used two-stage emulsion explosives and non-electric millisecond detonators to detonate at the same time. The test result showed that the blasting energy cut the rock along the preset direction, and no cracks were found in other directions. After the roof was pulled by the constant resistant anchor cable, the stress transfer path between the roadway roof and the gob roof was cut off by directional roof cutting technology to reduce the support resistance in the roadway, as is shown in Fig.1(d).

(III) Full collapse. The pressure of the overlying strata and the mining pressure caused by the working face were fully used to let the gob roof collapse along the slit, then the fracture dilatancy properties of gangues were used to provide support on the gob-side roof, as is shown in Fig.1 (e).

(IV) Protected well. After the working face was excavated, the roof collapses along the slit side. To prevent the collapsed gangues from falling into the roadway, which may impact and injure the support structure and workers, the U-shaped steel, metal mesh and roof cutting guard stent were used to provide gangue support at the gob side. Therefore, the roadway was able to be retained and was available for reuse in the next working face, as is shown in Fig.1 (f).

The most significant difference between GEFRCPR and the conventional mining method with coal pillars is that the new method does not use the conventional protective pillar between working faces, which saves coal resources and extends the service life of coal mines. When mining the next face, coal miner need only excavate one roadway, which not only reduces the amount of excavation of roadways, but also improves mining efficiency. The other notable difference between the two mining methods is that the GEFRCPR did not require a filling structure and a masonry structure as the gob-side support structure, but rather made full use of the mining structure to make the gob roof collapse along the slit side. And the fracture dilatancy properties of the rock can be used to provide support for the gob-side roof. GEFRCPR can be used to successfully solve the problems arising from mining with coal pillar and conventional gob-side entry retaining under the complex geological conditions of a fault, and would be of great significance for coal mining under complex geological conditions.

The working face often encounters faults as the coal mining process moves to complex geological regions. In this case, when using conventional mining methods with coal pillar and the gob-side entry retaining, the stress on the protective pillar and the roadside filling near the fault increased because the roof transfer path was not changed and the pressure on the roof was not relieved. If the bearing capacity limit of the protective pillar and the roadside filling is exceeded, rock burst or other disasters may occur. However, GEFRCPR was able to successfully avoid these disasters under complex geological conditions owing to its none coal pillar and pressure release characteristics. The diagram in Fig. 2 shows the principle of GEFRCPR passing through the fault.

Figure 2.

Based on a number of academic researches on normal fault (Wang et al. 2014), it is found that the stress increase was most prominent as the working face of the hanging wall is moving to normal fault. As a result, the mining process from the working face of the hanging wall to the normal fault was taken as the key area of our study, and the mechanical model was constructed as is shown in Fig. 3. When the working face moved from the hanging wall to the footwall, the basic roof in the upper gob side was broken to form the hinge structure, as is shown in Fig. 3. At this point, the load on the basic roof was shared by the footwall of the fault, the pillars in the fault, and the gob-side gangues. Half of the load from rotating broken blocks on the basic roof above the gob side was shared by the gob-side gangues, and the other half was transferred to the pillars in the fault and the footwall of the fault by squeezing broken blocks.

Figure 3.

At the fault, the fault surface transferred the load above the pillars in the fault and the load on the basic roof above the gob side to the footwall by a certain pressure transfer factor.

The load above the pillars in the fault is qL

The load on the basic roof above the gob side is qL

The mining load that was transferred through the fault to the footwall can be expressed as

$$Q{\text{=}}K\left( {qL+\frac{{{Q_{\text{1}}}}}{{\text{2}}}} \right){\text{=}}K\left( {qL+\frac{{qL}}{2}} \right)=\frac{{3KqL}}{2}$$

1

where Q in MN/m stands for the load transferred from hanging wall to footwall of normal fault; Q₁ in MN/m represents the load on the basic roof above the gob side and Q₁ = qL; K is the load transfer factor from hanging wall to footwall of normal fault and 0 ≤ K ≤ 1; q in MN/m² is the load on broken pieces of the basic roof above the pillar per unit area and q = γh; γ in N/m³ is the average volume weight of the layer under load and the current layer of the basic roof above the pillar; h in m stands for the total thickness of strata above the coal seam; L in m is the length of the rotating broken pieces of the gob-side basic roof transferred to the basic broken pieces of the basic roof above the pillar and is calculated by periodic weighting pace.

The load that was not transferred to the footwall of the fault was borne by the pillars on the working face, which mainly included the load on the broken pieces of the basic roof above the pillars in the fault, the load on the broken pieces of the roof above the gob side that were not transferred to the footwall of the fault, and the load on strata of the basic roof above the pillar in the fault.

The load that is not transferred to the footwall of the fault is $\left( {{\text{1-}}K} \right)\left( {{\text{qL+}}\frac{{{\text{q}}L}}{{\text{2}}}} \right)=\frac{{3\left( {1 - K} \right)qL}}{2}$

The load on strata of the basic roof above the pillar in the fault is ${\gamma _z}{h_z}\left( {\frac{{{h_z}\tan \alpha }}{2}+B+D} \right)$

The load on the pillars in the fault is

$${Q_d}=\frac{{3\left( {1 - K} \right)qL}}{2}+{\gamma _z}{h_z}\left( {\frac{{{h_z}\tan \alpha }}{2}+B+D} \right)$$

2

The load intensity on the pillars in the fault is obtained from Eq. (2):

$${q_d}=\frac{{3\left( {1 - K} \right)qL+{\gamma _z}{h_z}^{2}\tan \alpha +2{\gamma _z}{h_z}D}}{{2B}}+{\gamma _z}{h_z}+\gamma h$$

3

where Q_d in MN/m is the load on the pillars in the fault; q_d in MN/m stands for the intensity of load on the pillars in the fault; γ_z in N/m³ is the volume weight of strata on the immediate roof; h_z in m is the thickness of strata on the immediate roof; α in ° is the dip angle of fault; B in m is the width of the pillars in the fault; D in m is the distance from the coal seam on the working face to the gob.

According to the Eq. (3), when the working face gradually moved to the normal fault, the pillars in the fault decreased, namely B in Eq. (3) decreased, while other factors remained the same, then the load intensity q_d above the pillars in the fault became larger. When the working face came closer to the normal fault, the cutting effect of the fault made the overlying strata on the working face of the hanging wall on the pillars in the fault an inverted wedge rock. As the working face approached the normal fault, the effect arising from the inverted wedge which overlaid the strata tended to be more prominent and led to greater stress on the pillars in the fault. Under the combined above effects, the closer the working face was to the normal fault, the greater the stress on the pillars in the fault of the working face would be. When reaching the bearing capacity limit, the pillars on the working face were subjected to plastic failure, the stress on the pillars in the fault was transferred to two ends of the working face until it reached the solid coal at both sides of the working face. As a result, as the working face came closer to the normal fault, the stress on the solid coal side at both sides of the working face gradually increased. In other words, when the working face gradually came closer to the normal fault, as the stress on the roof and at the solid coal side was greater, reinforced support should be provided to ensure the effect and stability of the entry retaining.

4.1. Overview of the test face

Xiashanmao Coal Mine is located in Liulin County, Shanxi, China. The test face of GEFRCPR — 9101 working face— is located in #9 coal. This working face is the first working face of coal seam #9 with an overburden of 200 m, an average thickness of 3 m, a dip angle of 2 ~ 9°, a strike length of 480 m, the width of working face of 150 m, and is a fully mechanized mining face. The entry retaining is 9101 ventilation roadway with a width of 4 m and a height of 3.2 m. When mining the 9102 working face, the entry retaining can be reused. The location of the coal mine and the lithology of the working face are shown in Fig. 4. The entry retaining encounters a normal fault F5 which has a height difference of the fault of 6.5 and a dip angle of 70°, the layout of the working face and the location of the fault are shown in Fig. 5.

Figure 4.

Figure 5.

4.2. Numerical calculation model and method

In order to examine the strata behavior law and the stability of surrounding rock of entry retaining during the process of GEFRCPR passing through the normal fault, the FLAC3D numerical calculation model was established according to the engineering conditions of the Xiashanmao Coal Mine. The mechanical parameters of each rock layer were obtained by using laboratory tests. This numerical model simulates the fault by treating the fault zone as a softening zone. For numerical simulation faults, fault parameters are crucial to the correctness of numerical simulation results. Therefore, in order to better simulate the actual situation and fault effect of the site, the monitoring data of numerical simulation were compared with that of on-site surrounding rock deformation to determine the fault parameters. In other word, we selected a monitoring point at the same location in the field engineering and numerical simulation model and then verified the fault parameters in numerical simulation based on the field measured data of that monitoring point. When the monitoring data of the point in numerical simulation was coincided with the field measured data, the fault parameters in numerical simulation could be determined.

In the field engineering and numerical simulation model, a measuring point which is 60m from the fault was selected as a typical monitoring point. Since the actual deformation of the two sides of the roadway and the floor was minor in the test process, the vertical displacement of the roof was selected as the monitoring object, and the verification results are shown in Fig. 6. It can be seen that with the further mining of the working face, the simulation results of the monitoring object had basically the same trend as the field measurement results, and the displacement value of them were very close. Therefore, it is considered that the selected fault parameters met the reliability requirements of the simulation. The mechanical parameters of faults verified by this method and the mechanical parameters of each rock layer obtained by laboratory tests are shown in Table 1.

Table 1. Mechanical parameters for roof layers used in the simulation

Lithology	Thickness /m	Density /(kg·m^− 3)	Bulk modulus /GPa	Shear modulus /GPa	Cohesion /MPa	Tensile strength/MPa	friction angle/(°)
Medium sandstone	13.34	2520	9.3	7.5	4.2	4.58	35
Siltite	11.76	2470	7.3	6.7	3.3	3.74	32
Medium sandstone	7.53	2520	9.3	7.5	4.2	4.58	35
Limestone	11.68	2300	6.1	5.24	2.2	3.13	37
8#mine	2.70	1350	4.91	2.5	1.2	2.43	26
Sandy mudstone	8.13	2240	5.89	3.32	1.4	2.9	31.3
mudstone	6.04	2600	6.73	3.43	1.6	3.1	32
9#mine	3.00	1350	4.91	2.5	1.2	2.43	26
Post stone	5.31	2310	5.24	3.12	1.3	2.75	30.2
Mud stone	10.51	2600	6.73	3.43	1.6	3.1	32
Fault	3	1600	0.33	0.16	0.083	0.08	18

Figure 6.

Table 1.

The numerical model is shown in Fig. 7. The model dimensions are 350m (L) × 210m (W) × 80m (H), the buried depth of the simulated coal seam was 200m, the vertical stress of 5MPa was applied on the upper boundary, and the trapezoidal distributed load was applied horizontally. The vertical direction of the lower boundary was fixed, the horizontal direction of the front, rear, left, and right boundaries were also fixed. The normal fault was simulated with a 3m wide weak zone, a height difference of 6.5m. The width of the working face was set to 150 m, the thickness of the coal seam was 3m, the roadway dimension 4m (W) × 3m (H), the height of cutting slit 8m, the width 0.3m, and the cutting angle was set vertical to the roof. The cutting slit was prepared through excavation and the detail of the slit is shown in Fig. 8. Before mining on the working face, two roadways were excavated in advance until the model reached an equilibrium state. The simulated mining process started from the hanging wall, went through the fault and finally reached the footwall. The first mining length was set to 50m and then 10m each time. The schematic diagram of the simulation process is shown in Fig. 9.

Figure 7.

Figure 8.

Figure 9.

4.3. Evolutionary characteristics of vertical stress

As the working face was mined from the hanging wall to the normal fault and passed through the fault until reached the footwall, the vertical stress distribution nephograms at -30m (mining length: 90m), -20m (mining length: 100m), -10m (mining length: 110m), 0m (mining length: 120m), + 10m (mining length: 130), + 20m (mining length: 140) away from the working face were also obtained, as is shown in Fig. 10. The simulation result showed that the vertical stress in the fault zone was much lower than that in the original rock stress and that the stress was blocked by faults (Fig. 10). The pressure on solid coal of the entry retaining was fully released because the stress transfer path between the roadway roof and the gob roof was cut off. To visualize the evolution law of front stress and the surrounding rock of the entry retaining on the working face of GEFRCPR, the measuring line 1 was placed in the stress concentration zone of the front working face, and the measuring line 2 was placed in the stress concentration zone at the entry retaining solid coal side. Each time the working face advanced 10 m, the values indicated the two measuring lines were recorded respectively.

Figure 10.

The working face was mined from the hanging wall to the fault. The vertical distribution nephograms at -30 m (mining length: 90 m), -20 m (mining length: 100 m), -10 m (mining length: 110 m) away from the fault are shown in Fig. 10(a) (b) (c). The stress curves of line 1 and 2 placed from − 70 m (mining length: 50 m) to 0 m (mining length: 120m) away from the fault are shown in Fig. 11 (a,b). The stress nephograms and the vertical stress curves shown in Fig. 11 revealed that the effect of pressure relief at the front section of entry retaining under GEFRCPR was prominent. When the working face was gradually mined from the hanging wall to the normal fault, the overlying strata on the working face of the hanging wall was an inverted wedge, causing the stress in the front stress concentration zone of the working face to increase gradually. When the working face almost reached the fault, the stress concentration range was still in the hanging wall of the fault and the stress peak appeared to be abnormal due to the fault barrier effect. When the working face reached the fault, the overlying inverted wedge strata reached the minimum, and the stress value in the stress concentration zone was the lowest. In the working face at the solid coal side of entry retaining (at 0 m in the rear of the working face), the stress in the stress concentration zone gradually increased as the working face came closer to the fault. When the working face was about to reach the fault, the stress barrier effect of the fault was prominent and the stress value in the working face at the solid coal side of entry retaining (at 0m of the working face) decreased. However, as the distance in the lagging of the working face increased, the stress value in the stress concentration zone of the lagging working face surged significantly, until it reached a maximum value.

Figure 11.

The working face was mined from the fault to the footwall. The vertical distribution nephograms at 0 m (mining length: 120 m), 10 m (mining length: 130 m), 20 m (mining length: 140 m) away from the fault are shown in Fig. 10(d, e, and f). The stress curves of line 1 and 2 placed from 0 m (mining length: 120 m) to 70 m (mining length: 190m) away from the fault are shown in Fig. 12 (a and b). As is shown in the stress nephograms and the vertical stress curves in Fig. 12, the effect of pressure relief at the front section of entry retaining under GEFRCPR was prominent. When the working face advanced to the footwall, the overlying strata on the working face of the footwall was a wedge. When the working face start to be mined on the footwall, the overlying wedge strata reached the minimum value. The stress in the front stress concentration zone on the working face gradually increased as the working face was mined away from the fault. The stress reached a stable state until the working face left the fault impact zone. In the working face at the solid coal side of entry retaining (at 0m in rear of the working face), the stress value in the stress concentration zone reached the minimum value when the working face started to be mined on the footwall, and as the working face worked away from the fault, the stress value gradually increased. The stress reached a stable state until the working face left the fault impact zone. At the same time, the stress barrier effect and stress surging were clearly observed at the fault position.

Figure 12.

From Fig. 13, which combined the results illustrated in Figs. 10, 11 and 12, we obtained the trend of the stress variation of the mean value of measuring line 1 and the stress value of the coal side of the roadway retaining entity (0m behind the working face) at the working face during the whole process of the working face passing through the fault. The changing trend of the two sets of value is roughly the same throughout the whole process except at the fault.

Figure 13.

When the working face gradually came closer to the fault, the mean value of stress concentration value in the advanced working face and the stress value of the working face at the solid coal side of entry retaining (at 0 m in rear of the working face) kept increasing, but when the working face was within 10m of the fault, the mean value of stress concentration value in the advanced working face suddenly dropped by 12.1MPa, from 19.7MPa to 7.6MPa; the stress of the working face at the solid coal side of entry retaining (at 0 m in rear of the working face) decreased by 1.3Mpa, from 17.9Mpa to 16.6 MPa. This is due to the plastic failure and the bearing capacity of the pillars between the working face and the fault reduced, which led to decreased stress concentration in front of the working face. A part of the stress was transferred to the footwall through the fault. The remaining of the stress would be substantially transferred to the solid coal side of entry retaining through main roof, so that only a small amount of the stress concentration value of entry retaining decreased. After the working face passed through the fault, the stress on the overlying strata and the stress transmitted by the hanging wall of the fault were jointly applied on the working face of the footwall. Since the stress transferred from the hanging wall to the footwall was smaller and the overlying strata was wedge-shaped, the stress decreased significantly, which caused the vertical stress at the working face and the working face on the solid coal side of entry retaining (at 0 m behind the working face) to decrease. As the working face of the footwall moved away from the fault, the range of the overlying wedge-shaped rock gradually increased. From now on, both value gradually increased and shared a same trend of variation, and reached a stable state until the working face gets far enough from the fault impact zone.

Through the analysis of the above two stress evolution processes, it can be seen that the value of two stress increased when the working face approached the fault from the hanging wall. Therefore, the range of 60 ~ 70m from the hanging wall in front of the fault was the stress rise zone. The fault fracture impact zone was prone to cave, slip and activate. Therefore, the range of 60 ~ 70m at the back of the fault was the fracture impact zone.

5.1. Control technology of the surrounding rock in the roadway

Through theoretical analysis and numerical simulation, the impact range of the fault was identified. Reinforced support was provided in the impact range of the fault as is shown in Fig. 5 in pink. Two columns of constant-resistance large-deformation anchor cables were provided on the roof in the reinforced support section, door type supports + cutting guard stents were jointly provided as the temporary support, two rows of ordinary anchor cables were provided at the solid coal, the U-shaped steel, metal mesh and roof cutting guard stent were used to provide gangue backfilling support at the gob side.

The constant-resistance cables (Φ21.8×9300 mm) were arranged at a row spacing of 1 ~ 2 m in two columns on the roof and each column of constant-resistance anchor cables were connected by W steel bands. The solid coal was provided with two rows of ordinary anchor cables (φ15.24*6500mm). The first and second row was 800mm and 1700mm away from the roof, respectively, and the row spacing was 1800mm. Two rows of anchor cables were connected by steel bands. The dimension of the door type support in the entry was 2200 ~ 4100mm (L)×300mm (W) ×2200 ~ 4100mm (H) and the weight of the support was 1.8 tons. The central row spacing of the door type supports was 1500mm, with one end placed at the cutting guard stent side, and the other end near the solid coal. It is at an angle of about 45° with direction of the roadway. The section view, the top view of supports in the fault impact zone and blasting effect are shown in Fig. 14.

Figure 14.

5.2. Surrounding rock deformation

The field monitoring data for roof sinking at #6 (in front of the fault), #8 (at the fault), #10 (behind the fault) monitoring stations and the monitoring data at the solid coal side were selected, #6 station was 60m in front of the fault, #8 station was at the fault, #10 station was 50m behind the fault. The station position is shown in Fig. 15. The specific monitoring results are shown in Fig. 16.

Figure 15.

Figure 16.

The results of the monitoring data of the 3 monitoring stations showed that the deformation on the roof and at the solid coal side was the largest at # 8 station (at the fault). In this monitoring station, the deformation on the roof reached 587.9mm and the deformation at the solid coal side was 339.4mm. The surrounding rock in 6# station (in front of the fault) and 8# station (at the fault) was deformed at a faster rate during the process of passing through the normal fault. This phenomenon was a result of fault activation caused by mining stress. According to the monitoring data of #10 station (behind the fault), when the working face was mined toward the fault on the hanging wall, the surrounding rock deformation of #10 station was not obvious, and when the working face was mined to the footwall, the surrounding rock deformation of #10 station began to change greatly, indicating that the fault has an obvious barrier effect on the advanced mining stress. At about 140m behind the working face, the surrounding rock in these three stations tended to be stable.

In this study, a new technology of gob-side entry formed by roof cutting and pressure release was carried out in field test with faults. Compared to the previous technology, the advantages of this technology are reflected in its fundamental change of the stress transfer mode of roadway roof. The technology would also be helpful in saving coal resources, reducing roadway excavation and disaster occurrence under complex geological conditions of faults.

A geological dinite difference model was established to further explore mine pressure distribution. we discovered an increase in the surrounding rock stress of the roadway and an enlarged deformation when the working face gradually approached the normal fault. At the same time, the numerical simulation results show that when working face is mined from the hanging wall to the footwall, the pilot support stress of the working face and the vertical stress at the entry retaining solid coal side of the working face increased first, and then decreased and increased and eventually reached a stable state, which in turn verified the correctness of the theoretical analysis. Based on theoretical analysis and numerical simulation, the affected area of F5 fault in 9101 working face of Shanximao Coal Mine was determined to be 60 ~ 70m away from either side of the fault.

Finally, a field test was performed. The joint support of the " door type supports + cutting guard stents " was proposed as a temporary support in the roadway. The monitoring results and the field application showed that the joint support method facilitates the effective control of the surrounding rock deformation in the affected area. The effectiveness of the new technology of gob-side entry formed by roof cutting and pressure release under the influence of fault is verified. There are still some room for further research in the strata behavior of the formed entry surroundings passing through fault, and the author would commit to in-depth study on this part in the future.

Acknowledgement

The authors wish to acknowledge the funding support from Fund Project: Project supported by the National Natural Science Foundation of China (Grant No. 52074298), China Postdoctoral Science Foundation (Grant No. 2020T130702), National Key Research and Development Program (Grant Nos. 2018YFC0603705), Yue Qi outstanding scholar award program of China University of Mining & Technology, Beijing (Grant No.800015Z1171), which are gratefully acknowledged.

Data Availability

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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No competing interests reported.

highlight.doc

Mechanism and surrounding rock control of gob-side entry formation passing through normal fault - A case study

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Technical Principle And Construction Technology Of Gefrcpr

3. Principles And Mechanics Of Gefrcpr Passing Through Normal Fault

4. Numerical Analysis

4.1. Overview of the test face

4.2. Numerical calculation model and method

4.3. Evolutionary characteristics of vertical stress

5. Field Engineering Tests

5.1. Control technology of the surrounding rock in the roadway

5.2. Surrounding rock deformation

6. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1