Abstract
According to the failure law of the 1070 main roadway, this paper proposes a reasonable position for large section roadways under small coal mines and a design for seven roadway positions. RS2 software was used to establish a numerical model to select a reasonable position for the roadway. The influences of roadway positions on roof separation amount, roof subsidence, lateral displacement on the ribs, vertical stress on ribs and roadway failure areas were revealed. On the basis of influence laws, reasonable positions for the roadway could be determined. In this study, a center distance of 5 m was deemed a reasonable position. Finally, an industrial test was conducted at the original roadway. The experimental results indicate that the deformation of the roadway could be well controlled and the roadway position is reasonable.
1. Introduction
China's current coal production is about over 3.3 billion tons per year. The coal exploited from thick coal seams accounts for more than 45% of the total production (Cheng et al.2010). Due to high-intensity exploitation of thick coal seams, there are high requirements for ventilation and transportation capacity of the main roadway. The roadway section is usually over 15 m2 in thick coal seams (Luan et al.2019). When the coal is of a high strength, a roadway arranged in a coal seam could bring more output for coal mines and also improve the roadway excavation rate (Yan et al.2016). Therefore, a roadway designed and excavated in a coal seal is widely applied when the onsite geological conditions satisfy the safety requirements (Coggan et al.2012).
Due to the sharp increase in energy demand and economic development in the 1990s in China, small coal mines were widespread in ultra-thick coal seams. These mines had poor mining efficiency, severe safety problems, low resource use rates and high environmental pollution (Chen et al.2012). When small coal mines were abandoned, the whole coal seam exploitation faced more challenges in roadway excavation and support (Zhang et al.2018b). For example, many abandoned small mines could be found in 3–5# coal seams in the Tashan Coal Mine (Yu et al.2015). In 3–5# coal seams, the floor fracture area and the stress concentration area of abandoned small mines have great effects on large section roadway failure.
When the roadway is right under the floor of a small coal mine, it is just beneath the stress-decreasing area of the mines and roadway support is easier (Wang et al.2018). However, when the roadway support is under the floor fracture area of a small coal mine, support may become difficult to maintain (Xu et al.2019; Coggan et al.2012). When a roadway is directly below the coal pillar of two abandoned small mines, it is in the stress concentration area where stresses are high and support is difficult. However, if the surrounding rock of the roadway is relatively intact, support could be easier (Zhang et al.2018a). Therefore, to achieve the surrounding rock deformation control of roadways under small coal mines, it is vital to choose a reasonable roadway position to minimise the coupled effects of floor fracture area of the abandoned mines and the stress concentration area of protective coal pillars.
In the mining of the close seam group, scholars conducted studies on the reasonable position of the roadway in a lower coal seam. Zhang et al. (2015) analysed different roadway positions in a lower coal seam. The results indicated that lower roadways should be arranged in the stress relief zone and far away from the upper coal pillar. When roadways were excavated beneath the overlying goaf, roadway deformation was well controlled. Song et al. (2015) gave a new roadway layout in close distance coal seams. The method could not only improve the recovery rate of coal mines, but could also decrease the stresses for a lower roadway. Therefore, this was great for roadway excavation and maintenance. Yan et al. (2017) analysed the reasonable positions of a roadway in the close coal seam group. When the roadway was below the goaf of the upper working faces, it experienced minimum deformation and failure. FLAC3D software was used to optimise the roadway supporting parameters. Huang et al. (2010) put forward two roadway layout schemes of inner stagger and outward stagger in an upper coal seam after analysing the goaf fractured zone and concentrated stress distribution. The proposed roadway layouts could effectively reduce the influences of fractured zones and concentrated stress zones.
These studies mainly focus on the reasonable positions of a roadway in close distance coal seams. However, there has been little research on reasonable positions for roadway in the same coal seam. Such research could determine optimal roadway positions and find how to decrease the influences of the floor fractured zone of a small abandoned mine and the high stress zone of coal pillars.
This paper proposes a reasonable position for a large section roadway and designs for seven optimisation schemes. A numerical model is established to analyse the effects of roadway positions on roof separation amount, roof subsidence, lateral displacement on ribs, vertical and horizontal convergence, vertical stress on ribs and failure areas. Based on the influence laws, the reasonable position of a roadway is determined. The industrial test is described that was conducted at the main roadway in the Tashan Coal Mine.
2. Background
2.1. Engineering background
Tashan Coal Mine is one of the largest in China, producing more than 33 million tons of coal annually. The coal mine location and geological conditions are shown in figure 1. The Carboniferous 3–5# coal seams are currently under exploited. The thickness of the coal seam is 14–25 m, with a buried depth of 330–800 m.
The location and layout of the roadway.
To transport coal and other supplemental materials, there are three main roadways constructed in the Carboniferous 3–5 # coal seam. The protective coal pillar between the three main roadways is 45 m.
During the main roadway construction, two small coal mines with a size of 120 × 30 m were found at the top of 1070 main roadway. The mining thickness of two coal mines was 3.0 m. The distance between the center line of 1070 main roadway and that of the left small coal mine (hereafter referred to as center distance) was 15 m. A borehole was arranged near the 1070 main roadway to obtain a strata histogram. Based on the histogram, the thickness of the 3–5# coal seam was found to be 17.5 m. The immediate roof was medium sandstone with a thickness of 4.3 m and the floor was thick coarse sand with a thickness of 3.0 m.
2.2. Failure characteristics of roadway
As shown in figure 2a, the roof separation amount, roof failure, vertical stress and lateral displacement on ribs were monitored so as to reveal the failure characteristics of the roadway. Figure 2b shows that multi-point extensometers were installed in the roof of 1070 auxiliary transportation roadway to monitor the roof separation at 0–10 m depths. This consists of an internal fixation, wire rope and measuring device. The fixations were put at the depths of 1, 2, 3, 6 and 10 m into the roof. The KDVJ-400 borehole endoscope was set as shown in figure 2c. The borehole diameter was 28 mm, the maximum peering depth was 18 m and the horizontal distance between the borehole and the left rib was 6.0 m. As shown in figure 2d, 10 borehole stress sensors were put in the ribs to monitor the vertical stress of the surrounding rock. The distances between sensors and the ribs were 2, 4, 6, 8 and 10 m, respectively, and the vertical distance into the roof was 2 m. To monitor the lateral displacement on the ribs, 10 rib extensometers were arranged symmetrically on both ribs. As shown in figure 2e, the distances between stress sensors and the right rib were 1, 2, 3, 6 and 9 m, respectively.
Monitoring schemes. (a) The roof separation amount, roof failure, vertical stress and lateral displacement on ribs were monitored so as to reveal the failure characteristics of the roadway. (b) Multi-point extensometers were installed in the roof of 1070 auxiliary transportation roadway to monitor the roof separation at 0–10 m depths. (c) The KDVJ-400 borehole endoscope was set. (d) Ten borehole stress sensors were put in the ribs to monitor the vertical stress of the surrounding rock. (e) The distances between stress sensors and the right rib were 1, 2, 3, 6 and 9 m, respectively.
During the roadway excavating process, the roadway experienced serious deformation and local failure, along with roadway leakage and rib spalling. The roof separation reached 1.3 m at the site 82 m away from the connecting roadway, followed by roof leakage. Serious lateral movement occurred in the right rib at the site 123 m away from the connection roadway and lateral displacement reached 1.1 m.
To further investigate the roadway deformation pattern and failure mechanism, the roof separation amount, roof failure, vertical stress and lateral displacement on ribs were monitored further.
Roof separation amount. As shown in figure 3, the roof separation amount was small and changed slowly when the distance into roof was within 6–10 m. The range of the roof separation amount was 0.10–0.41 m. When the distance into the roof was within 0–6 m, the roof separation amount increased rapidly from 0.41 to 1.30 m. The roadway roof failure was more serious when the distance into the roof was 0–6 m.
Roadway roof failure. In figure 4, many fractures occurred within the roadway roof penetrating 1.9 m into it. At a 3.8 m roof depth, fractures and fissures were observed. At a 7.5 m roof depth, the fractures decreased obviously. At a 8.2 m roof depth, only a few fractures could be observed.
The vertical stress on ribs. To reveal the roadway stress states, the data from borehole stress sensors in ribs were processed, as shown in figure 5.
From figure 5, the variation of the advancing stress on ribs can be seen. The peak value of advancing stress was 12.6 MPa on the left rib and the stress concentration factor was 1.17, while on the right rib, the peak value of advancing stress was 17.7 MPa and the stress concentration factor was 1.64. The advancing stress on the right rib was obviously larger than that on the left rib, because the left rib was near the stress decreasing area of the small coal mine floor, and the right rib was near the stress concentration zone of the coal pillar. The failure depth in the left rib was 2.2 m, while that in the right rib was 6.0 m. Therefore, the surrounding rock failure on the right rib was greater.
The lateral displacement on ribs. The lateral displacement on ribs made the roadway extrude inwardly. To understand extrusion conditions, the lateral displacements on ribs obtained by rib extensometers were processed, as shown in figure 6.
Amount of roadway roof separation.
The borehole imaging of the roadway roof.
The vertical stress in the ribs
From figure 6, the maximum lateral displacements on the left and right rib were 0.420 and 1.101 m, respectively. The lateral displacement on the right rib was about three times that of lateral displacement on the left rib. Right rib deformation was more serious. In addition, the lateral displacement changed significantly when the rock surrounding the roadway was close to the ribs.
The lateral displacement on the ribs.
3. Optimisation schemes for reasonable roadway positions and the numerical model
3.1. The optimisation schemes for reasonable roadway positions
According to roadway deformation characteristics, the 1070 main roadway experienced serious damage and roof leakage. The right rib had relatively greater advancing stress and serious inward extrusion.
When the roadway was arranged below the floor of the small coal mine, it was in the floor stress decreasing area and had little surrounding rock stress. The roadway support became easy to control. However, the roof of roadway could be affected by the floor fracture area and roadway support may become difficult. When the roadway was below the stress concentration area of protective coal pillars, the vertical stress of the surrounding rock was large. Roadway support was difficult to maintain. However, the rock surrounding the roadway was relatively intact and roadway support was easy. Therefore, if the position of the 1070 main roadway could be designed in a reasonable place, the coupling effects of floor fracture area of a small coal mine and stress concentration area of a protective pillar could be alleviated. Furthermore, the failure of the roadway could be controlled effectively.
Theoretically, the greater the vertical distances between the roadway and the floor of small coal mines, the smaller the effects of small abandoned mines on the roadway. Therefore, the 1070 main roadway was excavated along the coal seam floor in all schemes. As shown in figure 7, seven schemes were designed for determining the 1070 main roadway position. The center distances were 0, 5, 10, 20, 25 and 30 m, respectively. The original parameters were adopted in the supporting scheme.
The optimisation schemes for reasonable roadway positions.
3.2. Numerical model
The ground stress measurement was conducted around the 1070 main roadway. Among the in situ stress field, maximum horizontal principle stress, minimum principal horizontal stress and vertical stresses were 12.0, 6.4 and 11.4 MPa, respectively. The maximum horizontal principle stress was parallel to the roadway extension direction.
The model was built following these steps. First, according to engineering background, a two-dimensional model was built at a size of 190 × 68.7 m. The mesh size was set at 0.3–1.2 m (Shabanimashcool & Li 2012; Kong et al.2014). The rock mass followed the Generalised Hoek–Brown criterion (Sofianos 2003; Fraldi & Guarracino 2012). Compared with the Mohr–Coulomb criterion or the strain-softening and double-yield criterion, the Hoek–Brown criterion has an advantage in that it can consider the influence of fracture or joint on rock strength etc. The special parameters for the Generalised Hoek–Brown criterion are shown in table 1. Horizontal displacement was constrained around the model, and the bottom of the numerical model was restrained in the vertical direction. A vertical load of 10.0 MPa was imposed on top of the model to replace the gravity of the overlying 400-m thick strata (Gao et al.2016; Bai et al.2016). After strata collapses in the goaf, the broken rock will become gradually compacted. The double-yield model is used to describe the stress–strain relationship in the process of compaction.
The mechanical parameters of rock mass
| Lithology | Thickness in m | Elastic modulus E/(GPa) | Poisson's ratio λ | σci/(MPa) | GSI | mi | mb | s | a |
|---|---|---|---|---|---|---|---|---|---|
| Sandstone | 5.1 | 41.2 | 0.26 | 38.6 | 87 | 17 | 10.69 | 0.24 | 0.50 |
| Siltstone | 9.4 | 45.1 | 0.27 | 55.5 | 92 | 16 | 12.02 | 0.41 | 0.50 |
| Sandy mudstone | 5.7 | 34.9 | 0.30 | 33.3 | 82 | 14 | 7.36 | 0.14 | 0.50 |
| Conglomerate | 3.2 | 36.2 | 0.31 | 23.6 | 86 | 19 | 11.52 | 0.21 | 0.50 |
| Mudstone | 4.4 | 15.8 | 0.21 | 17.4 | 76 | 10 | 4.24 | 0.07 | 0.50 |
| Medium sandstone | 4.3 | 45.1 | 0.28 | 35.2 | 89 | 24 | 16.20 | 0.30 | 0.50 |
| 3–5#coal seam | 17.5 | 6.8 | 0.21 | 15.9 | 65 | 7 | 2.01 | 0.02 | 0.50 |
| Gritstone | 3.0 | 34.2 | 0.31 | 43.9 | 90 | 23 | 16.09 | 0.30 | 0.50 |
| Medium sandstone | 5.0 | 25.3 | 0.29 | 23.6 | 85 | 14 | 8.19 | 0.19 | 0.50 |
| Fine mudstone | 11.2 | 47.6 | 0.25 | 80.3 | 95 | 25 | 20.91 | 0.57 | 0.50 |
| Lithology | Thickness in m | Elastic modulus E/(GPa) | Poisson's ratio λ | σci/(MPa) | GSI | mi | mb | s | a |
|---|---|---|---|---|---|---|---|---|---|
| Sandstone | 5.1 | 41.2 | 0.26 | 38.6 | 87 | 17 | 10.69 | 0.24 | 0.50 |
| Siltstone | 9.4 | 45.1 | 0.27 | 55.5 | 92 | 16 | 12.02 | 0.41 | 0.50 |
| Sandy mudstone | 5.7 | 34.9 | 0.30 | 33.3 | 82 | 14 | 7.36 | 0.14 | 0.50 |
| Conglomerate | 3.2 | 36.2 | 0.31 | 23.6 | 86 | 19 | 11.52 | 0.21 | 0.50 |
| Mudstone | 4.4 | 15.8 | 0.21 | 17.4 | 76 | 10 | 4.24 | 0.07 | 0.50 |
| Medium sandstone | 4.3 | 45.1 | 0.28 | 35.2 | 89 | 24 | 16.20 | 0.30 | 0.50 |
| 3–5#coal seam | 17.5 | 6.8 | 0.21 | 15.9 | 65 | 7 | 2.01 | 0.02 | 0.50 |
| Gritstone | 3.0 | 34.2 | 0.31 | 43.9 | 90 | 23 | 16.09 | 0.30 | 0.50 |
| Medium sandstone | 5.0 | 25.3 | 0.29 | 23.6 | 85 | 14 | 8.19 | 0.19 | 0.50 |
| Fine mudstone | 11.2 | 47.6 | 0.25 | 80.3 | 95 | 25 | 20.91 | 0.57 | 0.50 |
The mechanical parameters of rock mass
| Lithology | Thickness in m | Elastic modulus E/(GPa) | Poisson's ratio λ | σci/(MPa) | GSI | mi | mb | s | a |
|---|---|---|---|---|---|---|---|---|---|
| Sandstone | 5.1 | 41.2 | 0.26 | 38.6 | 87 | 17 | 10.69 | 0.24 | 0.50 |
| Siltstone | 9.4 | 45.1 | 0.27 | 55.5 | 92 | 16 | 12.02 | 0.41 | 0.50 |
| Sandy mudstone | 5.7 | 34.9 | 0.30 | 33.3 | 82 | 14 | 7.36 | 0.14 | 0.50 |
| Conglomerate | 3.2 | 36.2 | 0.31 | 23.6 | 86 | 19 | 11.52 | 0.21 | 0.50 |
| Mudstone | 4.4 | 15.8 | 0.21 | 17.4 | 76 | 10 | 4.24 | 0.07 | 0.50 |
| Medium sandstone | 4.3 | 45.1 | 0.28 | 35.2 | 89 | 24 | 16.20 | 0.30 | 0.50 |
| 3–5#coal seam | 17.5 | 6.8 | 0.21 | 15.9 | 65 | 7 | 2.01 | 0.02 | 0.50 |
| Gritstone | 3.0 | 34.2 | 0.31 | 43.9 | 90 | 23 | 16.09 | 0.30 | 0.50 |
| Medium sandstone | 5.0 | 25.3 | 0.29 | 23.6 | 85 | 14 | 8.19 | 0.19 | 0.50 |
| Fine mudstone | 11.2 | 47.6 | 0.25 | 80.3 | 95 | 25 | 20.91 | 0.57 | 0.50 |
| Lithology | Thickness in m | Elastic modulus E/(GPa) | Poisson's ratio λ | σci/(MPa) | GSI | mi | mb | s | a |
|---|---|---|---|---|---|---|---|---|---|
| Sandstone | 5.1 | 41.2 | 0.26 | 38.6 | 87 | 17 | 10.69 | 0.24 | 0.50 |
| Siltstone | 9.4 | 45.1 | 0.27 | 55.5 | 92 | 16 | 12.02 | 0.41 | 0.50 |
| Sandy mudstone | 5.7 | 34.9 | 0.30 | 33.3 | 82 | 14 | 7.36 | 0.14 | 0.50 |
| Conglomerate | 3.2 | 36.2 | 0.31 | 23.6 | 86 | 19 | 11.52 | 0.21 | 0.50 |
| Mudstone | 4.4 | 15.8 | 0.21 | 17.4 | 76 | 10 | 4.24 | 0.07 | 0.50 |
| Medium sandstone | 4.3 | 45.1 | 0.28 | 35.2 | 89 | 24 | 16.20 | 0.30 | 0.50 |
| 3–5#coal seam | 17.5 | 6.8 | 0.21 | 15.9 | 65 | 7 | 2.01 | 0.02 | 0.50 |
| Gritstone | 3.0 | 34.2 | 0.31 | 43.9 | 90 | 23 | 16.09 | 0.30 | 0.50 |
| Medium sandstone | 5.0 | 25.3 | 0.29 | 23.6 | 85 | 14 | 8.19 | 0.19 | 0.50 |
| Fine mudstone | 11.2 | 47.6 | 0.25 | 80.3 | 95 | 25 | 20.91 | 0.57 | 0.50 |
To simulate the effects from the two abandoned small coal mines, two small coal mines were set and exploited successively. The left-hand small coal mine was excavated first, and the double-yield model was applied to the mass rock in the caving zone (Zhang et al.2017). The right-hand small coal mine was then excavated similarly. Next, the roadway position was determined according to optimisation schemes, and the bolt, anchor cable and metal mesh were applied.
Taking Scheme 7 as an example (figure 7), a numerical model was built based on the aforementioned procedures, as shown in figure 8.
The numerical model.
4. Roadway deformation and failure characteristics in different positions
To find out the roadway deformation and failure characteristics in different positions and determine a reasonable position for the 1070 auxiliary transportation roadway, it is necessary to analyse the deformation laws of the rock surrounding the roadway, vertical stress on ribs and fractured areas in schemes 1 to 7 (figure 7).
4.1. The roadway deformation
The section size of the roadway is the fundamental parameter to ensure material transportation, ventilation and provision for pedestrians. To determine the section size of the roadway, the roof separation amount, roof surface subsidence, lateral displacement of both ribs, vertical and horizontal convergence are key parameters to take into consideration.
The roadway roof separation amount is one of the main parameters for estimating the damage to the roadway roof. In figure 9, the roof separation amount was extracted in a 10-m depth into the roof from schemes 1 to 7.
Amounts of roof separation.
It can be seen from figure 9 that: (i) with the increase in center distance, the roof separation amount first dropped and then increased. When the center distance was 0–5 m, the roof separation amount decreased rapidly. When the center distance was within 10–20 m, the roof separation amount increased rapidly. When the center distance was within 25–30 m, the roof separation increased slowly. (ii) When the center distances were 0, 5, 10, 15, 20, 25 and 30 m, the corresponding maximum roof separation amounts were 0.90, 0.54, 0.81, 1.18, 1.20, 1.26 and 1.23 m, respectively. (iii) When the center distances were 0, 5, 10 and 15 m, the roof separation amount changed rapidly at range of 0–6 m depth into roof. While the depth increased to from 6 to 10 m, roof separation remained stable. When the center distances were 20, 25 and 30 m, the roof separation amount changed rapidly at range of at the range of 0–3 m depth into roof. From the depth of 3–10 m, the roof separation amount did not change much. (iv) When the center distance was 15 m, the trends and values of the numerical simulation and field measurement were in good agreement, thus providing proof for the simulation reliability.
Based on this summary, when the center distance was 5–10 m, the roadway roof separation amount was relatively small. The roof separation amount could be controlled effectively.
The roadway height is also a key parameter for equipment transportation. It is always affected by roadway roof surface subsidence. Figure 10 shows roof subsidence under different center distances extracted from schemes 1 to 7.
The roof surface subsidence.
From figure 10, it can be summarised that: (i) with the increase in center distance, the roof surface subsidence amount first reduced and then bounced up. When the center distance was 0–5 m, the roof surface subsidence decreased rapidly. When the distance was 10–15 m, the roof surface subsidence increased rapidly. When the center distance was 20–30 m, the roof surface subsidence basically remained unchanged. (ii) When the center distances were 0, 5, 10, 15, 20, 25 and 30 m, the corresponding maximum roof surface subsidence measurements were 0.93, 0.56, 1.22, 1.22, 1.23, 1.27 and 1.24 m, respectively. (iii) The trends and values of numerical simulation and field measurement showed a great agreement. Based on these laws, when the center distance was 5–10 m, the roof surface subsidence was relatively small and could be better controlled.
Under the effects of the horizontal extrusion force, the surrounding rocks on both ribs moved into the roadway, which triggered lateral displacement. Figure 11 shows the lateral displacement on ribs extracted from schemes 1 to 7 with the center distance.
The lateral displacement on ribs.
The following findings were obtained: (i) when the center distances were 0, 5, 10, 15, 20, 25 and 30 m, the corresponding maximum lateral displacements on the left rib were 0.19, 0.19, 0.45, 0.39, 0.58, 0.68 and 0.80 m, respectively. The maximum lateral displacements on the right rib were 0.31, 0.41, 0.76, 1.09, 0.92, 0.91 and 0.69 m, respectively. (ii) When closer to the ribs, the lateral displacement increased greatly. This sudden increase indicated that the surrounding rock near the ribs suffered from serious damage. (iii) The lateral displacement mainly occurred on the left rib within a 5-m depth into it, while the lateral displacement on the right rib mainly occurred within a 10-m depth into it. Therefore, the right rib suffered more damage than the left. (iv) When the center distance was 10–15 m, the lateral displacement on the right rib was relatively larger. (v) The values of the numerical simulation and field measurement coincided with each other, which proved simulation reliability. According to these laws, when the center distance was 0–5 m, the range and values of lateral displacement on the ribs were relatively small.
The vertical and horizontal convergence of roadway are common indices for roadway deformation. Figure 12 shows the roadway vertical and horizontal convergence extracted from schemes 1 to 7. The influence laws of center distance on vertical and horizontal convergence were analysed.
The influences of roadway positions on vertical and horizontal convergence.
From figure 12, it can be seen that: (i) with the increase in center distance, horizontal convergence and vertical convergence experienced low growth, rapid increase and a stable stage with the corresponding center distances of 0–5, 5–15 and 15–30 m, respectively. (ii) When the center distances were 0, 5, 10, 15, 20, 25 and 30 m, the horizontal convergences were 0.50, 0.60, 1.21, 1.48, 1.50, 1.56 and 1.65 m, respectively. The vertical and horizontal convergences were 0.95, 0.56, 1.24, 1.40, 1.44 and 1.45 m, respectively. Based on these laws, when the center distance was 0–10 m, the vertical and horizontal convergences were relatively small. Thus, these convergences could be controlled effectively.
4.2. The vertical stress on the ribs
The vertical stress of the ribs could be regarded as a specific index of roadway stress state. Figure 13 shows the vertical stress on ribs with different center distances.
Vertical stress on the roadway ribs.
It can be seen from figure 13 that: (i) the advancing stress is obvious on both ribs. With the increase of the center distance, the peaks of advancing stress continued to increase on the ribs. (ii) When the center distances were 0, 5, 15, 20, 25 and 30 m, the peak values of advancing stress on the left rib were 6.90, 8.88, 11.28, 11.93, 14.12, 15.9 and 17.23 MPa, respectively. The stress concentration factors were 0.64, 0.82, 1.04, 1.10, 1.31, 1.42 and 1.60, respectively. The peak values of advancing stress on the right rib were 7.04, 12.88, 15.33, 17.96, 18.90, 19.20 and 20.00 MPa, respectively, and the stress concentration factors were 0.65, 1.19, 1.42, 1.66, 1.75, 1.78 and 1.85, respectively. (iii) When the center distances were 0, 5, 15, 20, 25 and 30 m, the failure depths on the left rib were 4.24, 4.24, 3.45, 2.87, 3.45, 4.24 and 4.24 m, respectively. The failure depths on the right rib were 2.66, 6.60, 4.24, 9.75, 6.60, 5.71 and 3.51 m, respectively. (iv) The accuracy and reliability of the numerical simulation were verified through comparison with the field measurement. From these laws, when the center distance was 0–10 m, the ranges and peak values of advancing stress were relatively small.
4.3. The fractured areas of rock surrounding the roadway
Roadway failure occurred under tensile and shearing effects. Figure 14 shows the tensile-shear failure elements of the rock surrounding the roadway extracted from schemes 1 to 7.
Rock failure areas surrounding the roadway.
The failure areas from figure 14 show that: (i) when the center distance was 25–30 m, the failure depth of the roadway roof was large, while the failure depths of both ribs and floor were relatively small. (ii) When the center distance was 20 m, the roadway roof failure area was not connected. Meanwhile, the failure area reduced significantly on the right rib. (iii) When the center distance decreased from 15 to 5 m, the rock failure area surrounding the roadway continued to decrease. When the distance decreased from 5 to 0 m, the rock failure area surrounding the roadway obviously increased.
It can be summarised that, when the center distance was 5–10 m, the failure areas of roadway roof, floor and ribs were relatively small.
4.4. Reasonable position of the roadway
To determine the reasonable positions of the 1070 auxiliary transportation roadway, the roadway roof separation amount, roof surface subsidence, lateral displacement on the ribs, vertical and horizontal convergence, vertical stress on ribs and failure areas of the roadway were taken as the judgment standards. Table 2 shows the reasonable positions.
Reasonable positions for the roadway
| Deformation and failure area | Reasonable position |
|---|---|
| Roof separation amount | 5–10 m |
| Roof surface subsidence | 5–10 m |
| Vertical and horizontal convergence | 5–10 m |
| Lateral displacement on the ribs | 0–5 m |
| Vertical stress on the ribs | 0–10 m |
| Failure area of the surrounding rock | 5–10 m |
| Deformation and failure area | Reasonable position |
|---|---|
| Roof separation amount | 5–10 m |
| Roof surface subsidence | 5–10 m |
| Vertical and horizontal convergence | 5–10 m |
| Lateral displacement on the ribs | 0–5 m |
| Vertical stress on the ribs | 0–10 m |
| Failure area of the surrounding rock | 5–10 m |
Reasonable positions for the roadway
| Deformation and failure area | Reasonable position |
|---|---|
| Roof separation amount | 5–10 m |
| Roof surface subsidence | 5–10 m |
| Vertical and horizontal convergence | 5–10 m |
| Lateral displacement on the ribs | 0–5 m |
| Vertical stress on the ribs | 0–10 m |
| Failure area of the surrounding rock | 5–10 m |
| Deformation and failure area | Reasonable position |
|---|---|
| Roof separation amount | 5–10 m |
| Roof surface subsidence | 5–10 m |
| Vertical and horizontal convergence | 5–10 m |
| Lateral displacement on the ribs | 0–5 m |
| Vertical stress on the ribs | 0–10 m |
| Failure area of the surrounding rock | 5–10 m |
As shown in table 2, when the center distance was 5 m, the roadway deformation, vertical stress and failure were relatively small. Therefore, it is reasonable to choose the site as the roadway position.
5. Engineering application
As shown in figure 15, No.3 and No.4 small coal mines could be found on the left side of No.1 and No.2 small coal mines in the Tashan Coal Mine. The center distance between the roadway and No.3 small coal mine was 5 m.
Position of the auxiliary transportation roadway.
Similar to the roadway deformation scheme in Section 2.2, Figure 16 shows the roadway roof separation amount, vertical stresses on the ribs and lateral displacement on the ribs when center distances were 5 and 15 m. As shown in the figure 16, when the center distance reduced from 15 to 5 m, the maximum roadway roof separation amount decreased from 1.3 to 0.5 m. The peak advancing stress on the left rib reduced from 12.6 to 7.2 MPa, and the advancing stress peak on the right rib reduced from 17.7 to 7.7 MPa. The maximum lateral displacement on the left rib lowered from 0.42 to 0.21 m, and the maximum lateral displacement on the right rib decreased from 1.10 to 0.31 m. Based on these laws, when the center distance was 5 m, the roadway roof separation amount, lateral displacement on the ribs and vertical stresses on the ribs could be controlled effectively.
Field measurement of roadway deformation and stress state.
6. Conclusions
Based on the actual measurement, when the center distance was 15 m, the roadway deformation was large and failure was serious. Roof leakage and lateral extrusion on the right rib could be found. The failure depths of the roof, left rib and right rib were 6.0, 2.0 and 6.0 m, respectively. The stress concentration factors on both ribs were 1.2 and 1.7, respectively.
When the center distance was 5 m, the maximum roadway roof separation amount, maximum roof subsidence, maximum lateral displacement on the left rib, maximum lateral displacement on the right rib, vertical and horizontal convergence were 0.54, 0.56, 0.19, 0.41, 0.60 and 0.56 m, respectively. The peak values of advancing stress on the left and right ribs were 8.88 and 12.88 MPa. The failure depths of left and right ribs were 4.24 and 6.60 m. The failure area was relatively small. Based on these parameters, the reasonable position of the roadway could be determined at the place that was 5 m to the center distance.
Based on field measurements, when the center distance reduced from 15 to 5 m, the maximum roof separation amount decreased from 1.3 to 0.5 m. The maximum lateral displacement on the left rib decreased from 0.42 to 0.21 m and the maximum lateral displacement on the right rib decreased from 1.10 to 0.31 m. The peak value of advancing stress on the left rib decreased from 12.6 to 7.2 MPa, and that on the right rib decreased from 17.7 to 7.7 MPa. Finally, the rock surrounding the roadway deformation and stress states could be controlled effectively.
Acknowledgements
This work was supported by the Chinese Scholarship Council (CSC), State Key Research Development Program of China (grant no. 2018YFC0604500) and LiaoNing Revitalization Talents Program (grant no. XLYC1807219).
Conflict of interest statement. All authors declare that they have no conflict of interest.
