Strata Movement of Overburden-Separation Grouting Working Face and Its Influence on Gas Emission during Mining

Abstract

Due to its excellent effect of surface subsidence reduction, the overburden-separation grouting (OSG) technology has been widely applied to green mining. However, OSG changes the mine pressure, and the resultant gas emission in the stope remains unclear. Therefore, with the 22301 working face of Tunlan Coal Mine as an example, the overburden deformation and stress distribution in the absence and presence of OSG were studied through numerical simulation and on-site tests. Furthermore, the gas emission law in the stope was contrastingly analyzed. The following results were obtained. With the rise of grouting pressure (GP), the subsidence of strata above the separation layer decreases while that below the separation layer increases; the heave displacement of floor strata below the coal seam gradually decreases. OSG significantly changes the geometric shape and range of the overburden stress relief zone. As the GP rises, the gas enrichment area moves downward continuously and shrinks spatially. Horizontally, the ranges of the stress relief zone and the free space in the goaf shrink, while the range and compaction degree of the compaction area gradually grow. After OSG, the total amount of gas emission in the working face falls by 52.2%.

1. Introduction

Traditional coal mining modes often cause surface subsidence, destroy surface water bodies or buildings, and seriously damage the ecological environment of the mining area [1,2,3]. In addition, a large amount of gangue produced during coal mining forms huge gangue hills on the ground, which not only occupies land resources but also pollutes the environment [4,5]. For the purpose of protecting villages near the mining area from the impact of mining activities, coal resources buried beneath them are generally abandoned, which seriously affects the recovery rate of resources.

As China’s economy develops quickly, people have increasingly higher requirements for the ecological environment and are trying to discard the traditional destructive coal mining methods. Hence, for the purpose of realizing coordinated development of coal mining and ecological environment [6,7], it is urgent to seek a new ecologically and environmentally friendly coal mining mode that can not only prevents surface subsidence and underground aquifer damage but also disposes of solid wastes such as surface gangue in a large scale.

In order to alleviate the mining-induced surface subsidence, researchers have developed a series of technologies and widely applied them to engineering practice. In China, common surface subsidence reduction technologies include strip mining [8], overburden-separation grouting (OSG), vacant space grounding and backfill mining [9]. Strip mining means that a coal seam is divided into several strips, and mining is conducted by retaining one coal pillar after recovering a strip. The retained coal pillars can support the overburden and control surface subsidence [10,11]. Compared with full caving mining, strip mining can effectively control the movement of rock strata and surface subsidence; hence, it is widely used in mining coal resources buried under villages, buildings and water bodies. However, this technology is of a low coal resource recovery rate (usually about 50%), wasting massive resources. Backfill mining means backfilling specific materials, such as gangue and fly ash, into mined space to control the movement of overburden and surface subsidence [12]. Compared with strip mining, backfill mining has a high recovery rate of coal resources, and can realize a large-scale disposal of solid waste such as gangue while effectively controlling surface subsidence [13]. However, serious mutual interference occurs during backfilling and coal mining, which leads to low mining efficiency. In addition, compared with other technologies for surface subsidence reduction, backfill mining greatly raises the production cost per ton of coal. OSG works as follows: the slurry of fly ash is injected through ground drilling into the separation zone among rock strata under a high pressure to establish a compaction zone under the separation zone. Such a compaction zone serves to support the overburden key strata and prevent them from breaking, thus controlling surface subsidence [14,15,16]. Compared with strip mining and backfill mining, OSG boasts a high recovery rate of coal resources; moreover, it is free from the perplex of mutual interference because the grouting process and the coal production process are spatially isolated from each other, which ensures the coal mining efficiency. However, the precondition of OSG is relatively stricter, usually key strata with large thicknesses and high strengths in the implementation area.

Tunlan Coal Mine is located in Shanxi Province, China. It mainly produces rare resources such as coke and fat coal, with an annual production capacity of 5 million tons. In the scope of the mine field, massive precious resources are buried under tributaries of Fenhe River (i.e., Yuanping River and Tunlan River), Zibei railway and Huancheng expressway. The surface above the 22301 working face of the mine is crossed by a fly ash transport belt corridor of Xingneng Power Plant, and the plant of Xishan Coal Power and Fly Ash Comprehensive Utilization Project is located nearby. To find an effective mining technology for coal resources under rivers and buildings, OSG surface subsidence reduction tests were conducted in the 22301 working face. The implemented project yielded an excellent effect of surface subsidence reduction, and no apparent damage was found in the belt corridor or the nearby plant.

At present, a series of research achievements have been yielded concerning the OSG technology, greatly promoting its engineering practice. During coal seam mining, the overburden often forms horizontal separation layers with a certain opening due to the difference in lithology [17]. Chen and Guo studied the development law of overburden separation layers during working face mining through numerical simulation, and analyzed the influences of position of grouting layer and number of grouting separation layers on the surface subsidence reduction effect [18]. It was found a shorter distance between the grouting layer and the coal seam and a larger number of separation layers corresponded to a better surface subsidence reduction effect. Through field tests, Shen and Poulsen found that under full caving mining, the overburden separation height could reach 1000 mm; the horizontal stress near the surface surged, while OSG could significantly reduce the horizontal stress near the surface and protect the surface rivers from damage [19]. Through field tests, the maximum surface subsidence under OSG was merely 10% of the mining height, 79% lower than that under full caving mining is proposed [20]. The impact of coal gas in long-wall mining is also significant; Russian researchers optimized degasification parameters by changing the distance between ventilation crosscuts to improve methane control [21,22,23]. With the huge thick igneous rock strata of Haizi Coal Mine in China as the research object, Xuan et al. concluded that OSG could greatly relieve the stress concentration of coal pillars, thus alleviating the hazard of coal and gas outbursts [24]. This research result was confirmed by on-site microseismic monitoring results [25]. In addition, scholars explored the characteristics of slurry distribution in strata after OSG through on-site borehole coring, and quantitatively characterized the state of slurry distribution through mathematical modeling [26,27]. In general, previous research on OSG was mainly focused on the evaluation and analysis on the surface subsidence reduction effect, while there is a lack of study on the law of gas enrichment and migration in the stope after OSG [28,29,30,31,32]. Xuan et al. proposed a technology of “one borehole for two purposes” for collaboratively realizing pressure-relieved gas extraction and OSG and established a collaborative model of extraction-grouting to determine the reasonable grouting time [33].

Overburden movement and ground pressure variation inevitably influence the gas enrichment and migration in the stope, thereby changing the law of gas emission and the effect of gas extraction during working face mining [34,35]. However, at present, no relevant study on the law of gas emission in the working face after OSG has been reported, and the mechanism of how ground pressure variation controls gas emission remains unclear, which brings new challenges to gas control during the mining of the 22301 working face in Tunlan Coal Mine.

Considering the above problem, this study took the 22301 OSG working face in Tunlan Coal Mine of Xishan Coal and Electricity Group as the research object. The characteristics of overburden deformation and stress distribution under different grouting pressures (GPs) were analyzed through theoretical analysis, laboratory experiment, numerical simulation and how the range and geometric shape of the stope stress relief zone vary with the GP were discussed. Subsequently, the law of gas extraction and emission variations in the working face before and after OSG was contrastingly analyzed based on on-site test results. Finally, the internal mechanism of gas emission variation in the goaf after OSG was discussed from the perspective of mine pressure variation in the goaf. The research results are expected to provide a reference for gas prevention in OSG working faces under similar stratum conditions.

2. Overview of the Experimental Site and Scheme

2.1. Geological Conditions of the Experimental Site

Tunlan Coal Mine adopts the OSG technology to realize high-pressure grouting and filling for the mining-induced separation space in the overburden for two purposes: (1) mining coal resources under the fly ash transport belt corridor of Xingneng Power Plant and the plant of Xishan Coal Power and Fly Ash Comprehensive Utilization Project and (2) exploring technology for coal mining under rivers and roads in the future. A compaction area of a certain width is created in the middle of the goaf through OSG to form an effective pillar bearing structure of “overburden structure-isolated coal pillars-filling and compaction area”, which can effectively control surface subsidence and prevent the surface buildings from damage.

The OSG tests were conducted in the 22301 working face of the 2# coal seam in Tunlan Coal Mine. The working face is located at the right wing of North Panel 3, it neighbors the return airway of North Panel 3 on the west and boundary pillars of North Panel 1 on the west. Its minimum distance from the 22118 goaf is only 30 m, and its south and north are solid coal. The roof is managed by the complete caving method in the first 1100 m of the working face, and overburden movement in the following 1400 m of the working face is controlled by OSG to prevent the belt corridor and the plant from damage. The distribution of the working face and surrounding facilities is shown in Figure 1.

In the 22301 working face, the 1#, 2# and 3# coal seams, with an average dip angle of 4°, a strike length of 2531 m, a dip length of 210 m and a mining height of 4.65 m, are mined together. The working face presents a synclinal structure as a whole, and the area near the axis is the stress concentration area. The fault is developed; the coal seam is soft and bears many cracks. During working face mining, the amount of gas emission reaches 37 m³/min, that is, the coal seam is prone to outburst. There are three groups of thin coal seams, namely the T3#, T2# and T1# coal seams which are 3.7 m, 13.37 m and 18.31 m away from the coal seam roof, with thicknesses of 0.7 m, 0.22 m and 0.6 m, respectively. The T4# coal seam, which is 0.94 m thick, is located at the lower part of the coal seam floor. These coal seams are the main sources of gas emission in the working face during mining.

2.2. OSG Technology and Process Scheme

According to the key stratum theory, the OSG technology can prevent the main key strata or the target key strata in the overburden from breakage. Its basic procedure is as follows. First, in the initial period of mining, the stability of the main key strata or the target key strata is ensured by designing a reasonable mining width. Second, the overburden of two adjacent working faces is controlled to maintain a state of insufficient mining by reasonably retaining section isolation coal pillars of a certain width. Third, the overburden separation area is filled with high-pressure grouting through surface drilling to form a grouting-filling-compaction bearing area of a certain width in the middle of the goaf. The resultant composite bearing structure of “key overburden stratum structure-compaction bearing stratum in the filling area-section isolated coal pillars” can effectively control surface subsidence [14].

The 2# coal seam has a medium sandstone main key stratum that is 5.85 m thick and 332.48 m away from the coal seam floor. It also has 5 sub-key strata, namely IK5, IK4, IK3, IK2 and IK1, which are fine sandstone, siltstone, medium sandstone, medium sandstone and medium siltstone with a thickness of 6.2 m, 7.9 m, 6.15 m, 9.35 m and 7.1 m, respectively; their distances from the coal seam floor are 299.08 m, 235.23 m, 192.68 m, 105.48 m and 71.93 m, respectively. The grouting layer lies above the water-conducting fracture zone. Theoretical calculation reveals that the grouting target should be more than 93.5 m higher than the coal seam. In the light of the geological histogram of rock strata of the 22301 working face, the separation area under the sub-key stratum IK2 was selected as the grouting point.

A total of 15 grouting boreholes were arranged in the 22301 working face (Figure 1), and 3 inspection boreholes were arranged near the protected buildings to detect the grouting effect. The target positions of the grouting boreholes were below the sub-key stratum IK2. After grouting, the inspection boreholes were drilled, with their target positions being 3 m from the lower boundary of the coal seam and their depths being 390 m. The spacing of grouting boreholes was determined as 150 m, according to the slurry diffusion radius, the calculation results and the actual stratum conditions.

The GP shall not be lower than the natural ground pressure of strata above the grouting filling layer so as to ensure surface stability. In the actual construction process, the surface subsidence can be controlled by adjusting the GP at the orifice of grouting boreholes. Assuming that the specific gravity of slurry is 12–15 kN/m³, then the GP is calculated to be 2.2–2.8 MPa. Taking fly ash as the slurry aggregate, slurry with the density of 1.3 g/cm³ is prepared at the water-cement ratio of 0.82:1. When the working face advances by 4.0 m per day, the grouting consumes 3600 m³ slurry a day and 150 m³ slurry an hour.

3. Numerical Modeling Study of OSG on the Strata Movement and Mine Pressure

3.1. Numerical Model and Verification

To study the law of overburden movement after OSG, a numerical model based on Flac^3D was established on the engineering background of the 22301 working face (Figure 2). The model is 1000 m long, 600 m wide and 486 m high; the simulated working face is 220 m wide and 600 m long along the strike. Mohr–Coulomb yielding criterion was used in the numerical simulation. To facilitate modeling and calculation, some thin strata were merged during modeling. The geological histogram of the model is shown in Figure 2. Its basic input parameters are given in

Table 1. Basic input parameters of the model.

Strata	ρ/kg·m−3	E/GPa	ν	σt/MPa	φ/°	C0/MPa
Loess layer	2520	7.34	0.41	1.5	32	1.88
Medium sandstone	2650	13.00	0.22	2.03	33	2.03
Sandy mudstone	2520	7.34	0.41	1.3	34	1.5
Fine sandstone	2700	15.05	0.31	1.5	29	1.9
Siltstone	2720	11.51	0.31	1.26	30	1.4
Coal seam	1460	7.00	0.26	0.64	25	1.2

. The law of overburden movement under different GPs was explored with this model, providing theoretical support for gas control in the working face under this condition.

To verify the rationality of the model, this study contrastingly analyzed the field-measured data and the model calculation results by taking the surface subsidence after excavation as the research object. Figure 3 shows the orifice pressure data of Wells 3–1 and 4–1 in the actual grouting process. It can be seen that the orifice pressure fluctuates within a large range (0–3 MPa) during grouting, as a result of grouting material supply, grout density variation and equipment operation and maintenance. The variation of GP influences the diffusion effect of slurry in the strata, which further affects the surface subsidence.

A measuring line (Figure 1) was set along the working face strike, on which several measuring points were selected to monitor the variation of surface subsidence of the grouting stratum during mining. Figure 4a shows the variation of surface subsidence at 9 measuring points. Generally, with the passage of time, the surface subsidence intensifies at a rising rate until it finally stabilizes. However, affected by the variation of GP, the stratum experiences different degrees of subsidence at different locations. For example, the maximum subsidence at Point Z18 is nearly 1100 mm, while that at Point Z25 is merely about 300 mm. Considering this factor, this study simulated the surface subsidence under different GPs and obtained the surface subsidence curves (Figure 4b). In general, the subsidence curves match well with the field monitoring data in terms of both variation trend and value, which indicates that the above model can well simulate the law of overburden movement in the OSG working face.

3.1.1. Analysis on the Characteristics of Overburden Subsidence and Deformation

An isolation barrier was constructed in the original separation area through OSG. This barrier can support the overlying rock strata, prevents the overlying key strata from breakage and control surface subsidence. Moreover, it can compact the lower rock strata. The above two effects change the deformation and displacement of strata.

The displacement distribution of strata above the goaf under different GPs is displayed in Figure 5 where the negative values represent downward subsidence of overburden strata while the positive ones represent upward heave of floor strata. When grouting pressure p equals 0 MPa, grouting has not been performed yet, and the roof is managed by the full caving method. For the overburden strata above the separation layer, when the GP rises, the subsidence gradually decreases. For example, when p equals 0 MPa, the rock stratum subsidence in the central area is 1–1.8 m; when p equals 3.5 MPa, it decreases to about 0–0.2 m; when p further rises, the rock stratum even moves upward, which is mainly caused by the supporting effect of slurry on the overburden strata. Obviously, it is crucial to find an optimal GP to control rock strata movement.

For the overburden strata below the separation layer, the subsidence increases gradually with the rise of GP. When p equals 0 MPa, the subsidence is 2–3.4 m; when p equals 4 MPa, it reaches 3.8–4.2 m, which is mainly attributed to the compaction effect of slurry on the lower strata.

For the floor strata of the coal seam, as the GP rises, the upward floor heave displacement gradually decreases. When p equals 0 MPa, the heave of floor strata is 0.2–1 m; when p equals 4 MPa, it decreases to 0.2–0.6 m. The decrease mainly arises from the fact that as the GP rises, the pressure of the overburden transfers to the floor strata and compacts them downward.

In order to quantitatively characterize the characteristics of rock strata migration, two measuring lines were drawn in the middle of the model (AB) and 10 m away from the left excavation boundary (CD). The measuring lines (total length 190 m) started at 10 m below the coal seam floor and measuring points (spacing 10 m) were set on the measuring lines. The displacement distribution on the measuring lines under different GPs is exhibited in Figure 6. The upper displacement distribution on Line AB can be divided into three areas. In Area 1, i.e., the floor strata, the displacement increases with the decrease in distance from the coal seam, and a higher GP corresponds to a smaller displacement. For example, when p equals 0 MPa, the displacement of strata near the lower coal seam boundary is about 1200 mm; in contrast, when p equals 4 MPa, the displacement there is about 850 mm. In Area 2, i.e., the area between the coal seam and the separation layer, the subsidence displacement gradually decreases with the increase in distance, and a higher GP leads to a greater subsidence, which mainly results from the downward compaction effect of slurry. In Area 3, i.e., the upper part of the separation layer, the displacement varies slightly with distance. Under a high GP, the displacement remains almost unchanged with the increase in distance. However, when the GP rises, the subsidence displacement in this area decreases gradually. Just like that on Line AB, the displacement distribution on Line CD also fell into three areas. The differences are as follows: in Area 1, the displacement no longer varies notably with the change of GP; in Area 2, the displacement drops at a decelerating rate with the increase in distance, which differs obviously from the case on Line AB.

3.1.2. Analysis on the Stress Distribution and Evolution Law

The difference in displacement distribution within the overburden is mainly caused by stress distribution. Therefore, the overburden stress distribution under different GPs is further analyzed here, and the results are shown in Figure 7. In the absence of OSG, after the coal seam is mined, the stresses in the upper and lower strata in the goaf are relieved in large quantities. In the presence of OSG, the stresses in the floor strata, the roof strata below the separation layer and the roof strata above the separation layer all increase. In addition, the higher the GP is, the higher the corresponding stope stress is. With the roof below the separation layer as an example, when p equals 0 MPa, the overburden stress is 0–4 MPa. When the GP reaches 4 MPa, the overburden stress rises to 6–10 MPa. This is mainly because the artificial rock stratum formed by grouting can support the overburden strata to a certain extent, which hinders the stress relief of strata. Moreover, the artificial rock strata also transfer the weight of the upper strata downward, thus compacting the lower strata. Therefore, after grouting, the stresses of the upper and lower strata in the goaf both grow; and the higher the GP is, the more notable the stress growth.

Similarly, the stress distributions on Lines AB and CD are extracted (Figure 8). For Line AB, the stress distribution can be generally divided into three areas. In Area 1, i.e., the floor area, the stress decreases slightly with the decrease in distance from the coal seam, and it rises gradually with the increase in GP. With the measuring point near the coal seam floor as an example, when p equals 0 MPa, its corresponding stress is about 3.5 MPa; in contrast, when p equals 4 MPa, the stress rises to 7.2 MPa. In Area 2, i.e., the area between the coal seam roof and the separation layer, a smaller distance from the coal seam or a higher GP corresponds to a higher stress. In Area 3, i.e., the area above the separation layer, the stress varies insignificantly with the increase in distance in the absence of grouting, while it decreases gradually with the increase in distance in the presence of grouting. The stress distribution on Line CD is quite complicated. In Area 1, the stress plummets with the decrease in distance, but it is barely affected by GP. In Area 2, the stress rises first and then falls with the increase in distance, which is caused by stress concentration at the excavation boundary. Moreover, the turning point of stress variation moves upward with the rise of GP. In Area 3, the stress distribution is similar to that on Line AB, but the stress distribution on Line CD is less sensitive to GP variation than that on Line AB.

3.1.3. Analysis on the Geometric Shape and Range of the Overburden Pressure Relief Zone

OSG greatly changes stress distribution in the stope, which then affects the migration and enrichment of relieved gas there. In the hope of revealing the influence of OSG on gas enrichment in the stope, this study extracted the overburden pressure relief zone under different GPs (vertical stress below 4 MPa) (Figure 9). It can be seen that OSG greatly changes the geometric shape and range of the overburden pressure relief zone. In the absence of OSG, the pressure relief zone can develop to all seams below the sub-key seam IK2, and the geometric boundary of the pressure relief zone is not smooth, which mainly results from different lithologies. When the GP rises, the pressure relief zone gradually shrinks both horizontally and vertically. When p equals 0.5 MPa, the pressure relief zone changes little vertically, but the range of some rock strata in the horizontal direction shrinks. When p equals 1 MPa, the pressure relief zone becomes disconnected at a certain middle seam. When p rises to 2 MPa, the pressure relief zone plunges vertically, but changes insignificantly horizontally. Afterwards, as the pressure further rises, the pressure relief zone is divided into upper and lower parts from the middle, and the lower part shrinks continuously with the rise of GP in the vertical direction. This demonstrates that as the GP rises, the range of the gas enrichment area in the overburden moves downward continuously and the spatial range gradually shrinks.

In addition, to explore how grouting influences the stress relief zone in the horizontal direction of overburden (that is, the traditionally so-called abscission circle) [36], this study selected a section at the middle point of the vertical height of the coal seam, and the vertical stress distribution on this section was obtained. The results are given in Figure 10. In the absence of grouting, a large stress relief zone where the stress is close to 0 exists in the goaf; a large free space where the roof and floor of the coal seam are not contacted is formed on the periphery of the goaf; and a compaction area where the corresponding stress is 2.5–5 MPa is formed in the middle of the goaf. As the GP rises, the stress relief zone and the free space in the goaf gradually shrink, while the compaction area gradually expands. When the GP increases to 4 MPa, the corresponding stress in the compaction area increases to 2.5–12.5 MPa. The above results indicate that after the application of OSG, the stress relief zone gradually shrinks and the compaction area gradually expands; meanwhile, the compaction degree is further promoted.

4. Contrastive Analysis on the Law of Gas Emission in the Working Face before and after OSG

OSG dramatically changes the stress distribution and rock movement of overburden in the working face, which inevitably changes the law of gas enrichment and emission in the working face [37,38,39]. This study discussed the influence of OSG on gas emission by contrastingly analyzing the measured data of extracted gas and ventilation air methane (VAM) before and after OSG in the 22301 working face concerning the characteristics of overburden stress distribution and movement. The discussion provides a reference for the optimization of gas control technology under this working condition.

4.1. Gas Control Scheme for the 22301 Working Face

Based on comprehensive analysis on gas extraction conditions and gas sources, it was determined that we would adopt the following schemes to solve gas problems of the 22301 working face during excavation: pre-extraction in the coal seam (Scheme 1), gas extraction in upper and lower adjacent seams (Scheme 2), extraction through buried pipes in the goaf (Scheme 3) and air ventilation (Scheme 4). Figure 11 shows the layout of extraction boreholes in the four schemes.

(1)

Layout of boreholes in Scheme 1: to pre-extract gas in the coal seam, along-measure boreholes (spacing 8 m) were drilled from the belt roadway and the track roadway to the coal seam. The azimuth angle of the boreholes is 90°. The boreholes at the belt roadway and the track roadway are 150 m and 70 m deep, respectively.

(2)

Layout of boreholes in Scheme 2: to intercept relieved gas in the upper adjacent seams, a total of 317 high- and low-level cross-measure boreholes (spacing 8 m) were constructed from the track roadway and the belt roadway to the upper adjacent seams. The high-level boreholes are 100 deep, with 31° dip angle and 30° azimuth. For the low-level boreholes, these three parameters are 100 m, 24° and 25°, respectively.

(3)

Layout of boreholes in Scheme 3: to deal with the threat of gas in the lower adjacent seams to the working face during mining, four along-measure long-borehole drilling fields were arranged along the strike direction in the belt roadway, and a total of 54 boreholes with a depth of 120–800 m were constructed in these drilling fields.

(4)

Layout of boreholes in Scheme 4: to solve the problem of gas emission in the goaf during mining, a total of 57 large-diameter boreholes (spacing 50 m) were arranged in the 22301 gas control roadway.

4.2. Analysis on the Law of Gas Extraction and Emission in the Stope

4.2.1. Gas Extraction Effect in this Coal Seam and Adjacent Seams

In order to simplify the production system, gas in the same roadway from different extraction sources was uniformly measured during the 22301 working face mining. In the main gas extraction pipeline in the belt roadway, gas was mainly extracted from three sources, namely, along-measure boreholes in the coal seam, cross-measure boreholes in the upper adjacent seams and directional boreholes in the lower adjacent seams. In the main gas extraction pipeline in the track roadway, gas was mainly extracted from two sources, namely, along-measure boreholes in the coal seam and cross-measure boreholes in the upper adjacent seams.

Figure 12 shows the variations in gas extraction flow rate in the belt roadway and the track roadway during working face mining, including the conditions before and after OSG. For both roadways, the gas extraction flow rate of boreholes was significantly reduced after OSG. For the track roadway, before OSG, the gas extraction flow rate was 0.17–3.39 m³/min, with an average of 1.49 m³/min; after OSG, these two parameters become 0.18–1.22 m³/min and 0.37 m³/min, respectively. Compared with that before OSG, the average gas extraction flow rate in the track roadway declined by 75.2%. For the along-measure boreholes in the coal seam, the stress field of the coal seam was not disturbed by grouting, so their extraction effect was slightly affected by grouting. Then, the reduction in gas extraction flow rate in the track roadway mainly comes from the reduction in the upper adjacent seams. The upper adjacent seams are of a poor gas extraction effect because grouting reduced their stress relief degree and thus lowered their permeability. In addition, it has been revealed in the above section that the stress relief zone gradually moves downward and shrinks after OSG. Gas in the gas enrichment area fails to be extracted if drilling parameters are designed based on the pre-grouting conditions, which is also an important reason for the poor gas extraction effect in the adjacent seams after OSG.

Before OSG, the gas extraction flow rate in the belt roadway was 0.82–5.22 m³/min, with an average of 2.64 m³/min. After OSG, it became 0.2–1.27 m³/min, with an average of 0.50 m³/min. That is, the average gas extraction flow rate in the belt roadway decreases by 81.1% after OSG. Compared with the value in the track roadway, the gas extraction flow rate in the belt roadway is higher, primarily because this roadway includes directional boreholes in the lower adjacent seams, in addition to boreholes along-measure boreholes in the coal seam and cross-measure boreholes in the upper adjacent seams. For this roadway, the reduction in gas extraction flow rate mainly resulted from the reduction in the upper and lower adjacent seams. Just like the track roadway, the belt roadway experiences a notable reduction in gas extraction flow rate because OSG hinders stress relief of the upper adjacent seams and compacts the lower adjacent seams, resulting in a decrease in permeability enhancement.

4.2.2. Gas Extraction Effect through Buried Pipes in the Goaf

Figure 13 shows the gas extraction effect through buried pipes in the goaf. Before OSG, the gas extraction flow rate in the goaf was 1–4.16 m³/min, with an average of 2.65 m³/min. After OSG, it fell to 0.24–2.33 m³/min, with an average of 1.10 m³/min, 57.0% lower than the value before OSG. The sources of gas extracted from the goaf mainly include relieved gas from the upper and lower adjacent seams and gas released from residual coal in the goaf, of which the latter rarely changed after OSG. Hence, the reduction in gas extraction flow rate in the goaf was mainly caused by the decrease in the amount of relieved gas flowing into the goaf from the upper and lower adjacent seams. According to the previous analysis, first, OSG lowered the stress relief degree of the upper and lower adjacent coal seams and thus reduced the amount of relieved gas there; second, it decreased the number of gas flow channels connecting the coal seam and the adjacent seams, so that relieved gas could hardly enter the goaf, resulting in a reduction in the amount of gas extracted through buried pipes.

4.2.3. Air Ventilation Effect

Figure 14 shows the variation of the amount of VAM during working face mining. Before OSG, the amount of VAM in the working face was 2.04–7.80 m³/min, 4.88 m³/min on average. After OSG, the amount was 0.6–5.2 m³/min, with an average of 2.65 m³/min, 45.7% lower than the value before OSG. The amount of VAM was mainly composed of two parts: gas emitted from coal wall and falling coal and gas emitted from the goaf, of which the former was barely affected by OSG. Thus, the reduction in VAM emission was mainly caused by the reduction in gas emitted from the goaf. Just like gas extracted through buried pipes, gas emitted from the goaf decreased mainly because of the reduction in relieved gas emitted from the adjacent seams to the goaf.

4.2.4. Total Amount of Gas Emission in the Working Face

Figure 15 shows the variation in the total amount of gas emission during working face mining, which was mainly composed of two parts: the amount of extracted gas and the amount of VAM. Before OSG, the total amount of gas emission in the working face was 6.8–18.99 m³/min, 13.03 m³/min on average. After OSG, it fell to 3.3–9.4 m³/min, with an average of 6.23 m³/min, 52.2% lower than the value before OSG. The reduction can be explained as follows: OSG inhibited the stress relief of the upper and lower adjacent seams, hence decreasing the amounts of relieved gas from the adjacent seams into the mining coal seam and extracted gas.

5. Discussion

OSG in the working face has a particularly strong effect on the distribution of overburden stress and its caving characteristics, hence affecting the law of gas enrichment and emission in the stope. This section aims to summarize a general law of the influence of OSG on overburden stress evolution and strata caving and attempts to discuss the influence mechanism of change of rock strata movement characteristics on gas emission in the working face. The results can provide theoretical support for the layout of gas extraction boreholes in the stope under OSG.

After OSG, as a result of an increase in upper stress, the stress relief height of overburden gradually decreased with the rise of GP. In addition, an increase in GP significantly reduced the stress relief degree in areas below the key stratum. After OSG, the compaction area in the middle of the goaf experienced a notable increase in its range and compaction degree. Consequently, the stress relief zone in the goaf shrunk, as did the gas enrichment area (Figure 16).

From the top view of the stress relief zone, the stope space within the goaf can be divided from inside out into the compaction area, the stress relief zone and the free space (i.e., the area on the periphery of the goaf where the roof and floor are not contacted yet due to the support of coal pillars). The latter two jointly constitute the dominant area for gas enrichment and migration. As GP rose, the range of the compaction area gradually expanded. Meanwhile, the internal boundary of the stress relief zone was gradually compressed, while the external boundary expanded outward, leading to a shrinkage of the free space. In general, OSG promotes the range and compaction degree of the compaction area and decreases the range of the dominant area for gas enrichment and migration.

The influence of OSG on the lower adjacent seams was similar to that on the upper seams. As GP rose, the range and compaction degree of the compaction area in the lower adjacent seams both grew, leading to a shrinkage of the stress relief zone.

The above analysis shows that after OSG, the stress relief degrees of the upper and lower adjacent seams in the goaf plunge, and their permeability enhancement weakens dramatically, resulting in a notable reduction in the amount of gas flowing into the goaf from the adjacent seams. This conclusion agrees with the reduction in the total amount of gas emission measured on site. Moreover, after OSG, the range of the overburden gas enrichment area changed considerably. Under this condition, the traditional layout of gas extraction boreholes was no longer able to realize the extraction of high-concentration relieved gas, which is also an important reason for the reduction in gas extraction in Figure 12 and Figure 13.

In order to obtain better effect of gas extraction in gob under OSG conditions, corresponding optimization measures should be taken for borehole layout. For the overburden fracture zone which has undergone a reduction in height and an outward movement of internal boundary, both the elevation angle of the high-level gas extraction boreholes and the azimuth angle towards the interior of the goaf should be reduced. The specific value of reduction shall should determined according to the actual range of the fracture zone measured on site. For relieved gas control in the lower adjacent seams, traditionally, uniformly distributed directional boreholes are constructed from the belt roadway to the lower seams to cover the entire mining disturbance area. By doing so, an excellent extraction effect of relieved gas can be achieved, as the lower adjacent seams correspond to a small compaction area and a low compaction degree. However, after OSG, the range and compaction degree of the compaction area in the lower adjacent seams both surged, and the coal seam in the middle of the working face corresponded to a poor stress relief effect and an unsatisfying gas extraction effect. Based on this, gas extraction in adjacent strata should focus on gas enrichment areas. Specifically, directional boreholes which are drilled from the belt roadway and the track roadway, respectively. These boreholes would be mainly distributed in areas with a high stress relief degree on both sides instead of the compaction area in the middle, hence realizing efficient gas control.

6. Conclusions

The laws of overburden movement and mine pressure variation in the goaf of OSG working faces remain unclear, which hinders gas control in the stope. In view of this problem, this study systematically analyzed the stress distribution and deformation characteristics of the upper and lower strata in the goaf of an OSG face, and discussed the influence mechanism of OSG on gas emission. The main conclusions are as follows:

(1)

For the overburden strata above the separation layer, the subsidence gradually decreased with the rise of GP. For the overburden strata below the separation layer, the subsidence gradually increased with the rise of GP, which was primarily caused by the compaction of slurry on the lower rock strata. For the floor strata below the coal seam, its upward floor heave displacement decreased as the GP rose, mainly because the pressure of the overburden strata transferred to the floor strata and weakens floor heave.

(2)

After OSG, the stresses of strata in different layers all rose, and a higher GP corresponds to a greater stope stress. OSG remarkably changed the geometric shape and range of the overburden stress relief zone. The gas enrichment area in the overburden moves downward continuously and shrinks spatially with the rise of GP. Horizontally, the goaf can be divided from inside out into the compaction area, the stress relief zone and the free space. As the GP rose, the ranges of the stress relief zone and the free space in the goaf gradually shrunk, while the range and compaction degree of the compaction area gradually grew.

(3)

After OSG, the total amount of gas emission in the working face decreased by 52.2%; the flow rates of gas extraction in the track roadway and the belt roadway fell by 75.2% and 81.1%, respectively; the flow rate of gas extraction through buried pipes in the goaf declined by 57%; and the amount of VAM dropped by 45.7%. The reduction in gas emission and extraction in the working face can be explained as follows: after OSG, the range and compaction degree of the compaction area in upper and lower adjacent seams both grew; resultantly, the permeability of these seams lowered, hence reducing the amount of relieved gas flowing into the goaf from the upper and lower adjacent seams. For an OSG working face, the original gas control technology needs to be optimized to achieve efficient control of relieved gas in the working face and ensure safe mining in the working face.