1. Introduction
The aquiclude above the longwall panels blocks the overlying aquifer water from flowing downward into goaf voids [1,2,3]. In coal measure geology, the aquiclude is deformed, fractured and likely to be destabilised by longwall mining, hence becoming less capable of preventing aquifer dewatering [4,5,6]. In northwest China, underground coal mining mainly occurs in Jurassic and Paleogene formations, where the rocks are generally characterised by young sedimentation, low strength, weak cementation, and high proneness of disintegration in hydrous conditions [7]. The arid and semi-arid climate results in a fragile environment in which the mine ecology highly depends on the shallow aquifer. Reportedly, in local areas, the mining activities have caused extensive fracturing and aquiclude destabilisation, followed by surface water leakage, desertification, and vegetation degradation [8,9,10,11]. In this condition, exploiting coal resources with minor impact on the regional water system has become a challenge for the mining industry [12]. The first knowledge to be consolidated is a thorough understanding of the aquiclude stability mechanism in such weakly cemented rock environment. Addressing this problem can form a basis for evaluating the impact of future longwall projects and longwall system design.
Studies have been conducted on the weakly cemented rock physics and mechanics [13,14,15]. Yu et al. analysed the impact of water content and confining pressure on the failure pattern and stress-strain trajectory of weakly cemented mudstones (WCM), indicating that the WCM mechanical responses were correlated to the physical and structural properties; in this study, the impact of rock porosity, mineral composition, and matrix-pore continuity on WCM strength was analysed. Wang and Zhao et al. proposed that the water-rock interaction, clay mineral disintegration, and particle expansion had a major contribution to the WCM failure and strength reduction [16,17]. Wang et al. quantified the correlation of the WCM bulk density and P-wave velocity versus uniaxial compressive strength and elastic modulus through laboratorial tests and theoretical analysis [18]. Li and Zhao et al. studied the WCM stress-strain, acoustic emission, and energy variations in a Brazilian test and discussed their relationship [19,20]. Some scholars also studied the joint strength and the coupled seepage-damage effect of natural fractured rock masses [21,22]. A previous study by the authors investigated the WCM permeability variation in complete stress-strain and post-peak loading and unloading conditions, revealing a WCM permeability property, regarding the high initial permeability, and minor differences between the peak and initial permeabilities [23,24]. Existing studies have obtained the physical and mechanical properties of the WCM aquiclude, but have not focused on the mechanisms of aquiclude deformation, fracturing, and destabilisation caused by longwall mining.
Some studies on the aquiclude stability have focused primarily on the roof claystone aquiclude and floor competent aquiclude [25,26,27]. Huang and Fan et al. identified the good water-resisting ability of red soils and claystones and that the fracture convergence was mainly because of rock matrix bulking in the fluid flow environment [28,29]. Yu and Ma proposed that adopting a roadway backfilling method can help control the deformation and settlement of the red soil aquiclude, through which the natural water-resisting ability can be well maintained [30]. Zhang and Sun et al. analysed the mechanism of claystone aquiclude destabilisation and aquifer dewatering in the backfill mining condition, proposing that the regional water resource can be protected by reconstructing the key aquiclude [31,32]. Feng, Fu, and Wang et al. proposed that the water-resisting property of the floor aquiclude mainly depends on both the structural and permeability stability, based on which the impact of lithology, aquiclude thickness, and water pressure on the floor aquiclude stability was analysed [33,34,35]. Liu et al. identified that the existence of water-conducting collapse pillars can directly decrease the effective water-resisting thickness of the floor aquiclude and thus increase the floor water inrush potentials [36]. These studies identified an ordinary mechanism of the roof claystone aquiclude and floor aquiclude destabilisation due to longwall mining, but have not considered the impact of the aquiclude lithology, such as weakly cemented rocks, on the stability. In order to understand this problem, a physical scaled model was constructed in this paper considering its advantages in explicitly reflecting the overlying strata deformation and fracturing due to panel extraction [37,38,39,40].
This paper took the Jurassic and Paleogene coal measure geology in the Yili No. 4 Coal Mine, Xinjiang Province, China, as a case study, of which the lithology is mainly characterised by weakly cemented rocks. A self-developed three-dimensional simulation system was employed to model the coal measure geology and analyse the WCR aquiclude fracturing and aquifer dewatering during panel extraction. Further, the aquiclude destabilisation mechanism was analysed from the mineral composition and water-rock interaction perspectives. The research results can provide a reference for water-conserved mining practices in the weakly cemented rock environment.
2. Study Site
The Yili No. 4 Coal Mine is located in Yining, Xinjiang Province, northwest China. The region has an arid to semi-arid climate, with annual precipitation ranging between 186.3 and 387.7 mm, resulting in a fragile ecology. The shallow gravel aquifer belongs to Quaternary formations, mainly recharged by precipitation and snowmelt. Under the aquifer is a Paleogene mudstone aquiclude and deeper Jurassic coal measure strata. The study area was the first longwall panel (LW21103) of this mine. LW21103 has a mining height of 3.5 m, a panel width of 115 m, and a depth of cover 115 m, on average, and the coal seam is near horizontal. The shallow aquifer is 10 m thick and 20 m in depth, providing water for the mine ecology. The aquiclude is 5 m thick, on average, with an 85 m distance to the panel roof. Figure 1 exhibits the coal measure lithology and thickness.
3. Physical Simulation
A physical simulation was conducted to capture the WCM aquiclude responses to longwall panel extraction. The physical simulation was configured for LW21103 coal measure lithology using a self-developed device. Longwall retreat mining was simulated, during which the aquiclude displacement and groundwater drawdown were monitored.
3.1. Method
The physical simulation adopted a 3D non-destructive fluid-solid coupling test system, developed by the authors [41], as shown in Figure 2. The non-destructive fluid-solid coupling test system abandoned the traditional data monitoring method by buried depth sensors in physical models, which solved the problems of original rock stress state and surrounding rock integrity damage caused by the buried depth sensors. The system used a parallel electrical system to trace groundwater migration by monitoring anomalies of apparent resistivity. The model of the parallel electrical system was YBD11 with six base stations and 96 excitation channels. The measuring current accuracy was 0.5%, and the minimum sampling interval was 1 ms. The panel extraction was configured by varying the height of hydraulic jacks; through the servo-control system, the mining height can be precisely adjusted with a 0.1 cm gradient. The model was 290 cm in length, 150 cm in width, and 115 cm in height, it was geometrically 1:100, with respect to the in-situ geometry. In addition, the time similarity ratio was 1:10. Table 1 lists the type and quality of various materials applied to the physical simulation.
3.2. Process
Before the panel extraction was performed, the monitoring points and devices for the aquiclude displacement and groundwater migration were installed, coupled with parallel electrical surveying points, as shown in Figure 3.
(1) Aquiclude displacement monitoring
The monitoring points were placed in the middle of the aquiclude, starting from 20 cm to the left edge, with a 25 cm spacing. There were 11 monitoring points in total.
(2) Groundwater flow monitoring
Three piezometers were vertically placed on the upper surface of the model, extending to the aquifer bottom. Piezometer 1# and 3# were 45 cm to the lateral edges, and piezometer 2# was in the middle and 100 cm to piezometers 1# and 3#, respectively, and 50 cm to the frontal surface. The three piezometers were used to inject water into the shallow aquifer before mining and to monitor the water level during mining.
(3) Parallel electrical surveying
Parallel electrical surveying points were placed on the top surface of the model, incorporating three surveying lines with 50 cm distance. Line 2# was the model centreline along the width direction, and line 1# and 3# were, respectively, 25 cm to the frontal and back surface. Each surveying line included 32 points with a 9 cm spacing. The copper electrodes were embedded 1 cm below the top surface.
Before the longwall mining, a transparent plexiglass plate was installed along the frontal surface, replacing the previous metal plate, to assist in capturing the strata deformation and fracturing. In addition, transparent silicone grease and butter materials were applied to each surface around the model to make it well sealed. The simulated longwall distance was 230 cm, leaving a 30 cm wide section unmined on each lateral to eliminate potential impacts of boundaries. This design corresponded to the in-situ 230 m longwall progress. The actual retreat speed of the coalface was 10 m/d. In order to restore the actual mining process of the coalface to the greatest extent in the excavation of the physical model, each mining step of the physical model extracted a 10 cm length with a time interval of 144 min, according to geometric similarity and temporal similarity. The longwall mining process of the physical model begins at the left side, as shown in Figure 3. In addition, the extraction of the coal seam was configured by varying the height of the hydraulic jacks by the servo-control system.
3.3. Results
3.3.1. Aquiclude Fracturing
Figure 4 shows the aquiclude fracturing during longwall mining progress.
Figure 4 shows that the aquiclude had a significant bending deformation as the longwall mining progressed 80 m, accompanied by a small-scale subsidence trough formed above. In this stage, a minor fracture (A) was captured above the set-up position due to the horizontal tension impact on the aquiclude.
When the longwall progressed 100 m, the aquiclude continuously subsided, accompanied by an increase in the fracture (A) aperture. In this condition, the fracture had not extended to the aquiclude bottom. Another tensile fracturing occurred above the current longwall workface position, labelled B in Figure 4.
After a further 40 m longwall progress, the previous tensile fracture (A) started to converge as it gradually filled with disintegrated clay minerals. Another fracture (B) had a similar phenomenon, represented by a pronounced aperture decrease. A new aquiclude fracture (C) was initiated above the current workface position.
After finalising the panel extraction, the aquiclude showed significant deflection above both the set-up and longwall stop positions. In contrast, the middle part mainly characterised a coordinated settlement. In this case, the tensile fractures A and D were closed due to the clay mineral fillings, while fractures B and C also converged due to the full settlement. Figure 5 exhibits the post-mining scenario.
The above analysis indicated that the WCM aquiclude behaved as a good resistance to the plastic deformation, of which the bending deformation dominated. Above the goaf voids, the aquiclude was tensioned and then compressed, thus representing a variation regarding fracture initiation, propagation, and closure. Above the set-up and longwall stop positions, the aquiclude mainly characterised tensile fracturing, also exhibiting the progress of fracture initiation, propagation, and convergence.
3.3.2. Aquiclude Displacement
Figure 6 shows the aquiclude displacement during panel extraction.
As can be seen from Figure 6, the longwall impact on the aquiclude displacement was explicitly observed when the workface marched 70 m. In this condition, the vertical displacement at measuring point 2# was −1.5 m, horizontal displacement 0.088 m, and horizontal strain 3.5 mm/m. With further longwall progress, the displacement variation rate (at measuring point 2#) first increased, and then gradually levelled off to 0. The displacement at measuring point 2# reached the maximum, −3.2 m for vertical displacement and 0.238 m for horizontal displacement, when the longwall progressed 120 m.
Measuring points 3# and 9# had a similar displacement variation as point 2#, as the panel was further extracted. A difference among the various measurements was the displacement direction; the aquiclude displaced towards right at left side, as recorded by measuring point 2#, and displaced towards the left on the right side, by measuring point 10#. By contrast, at the middle measuring points (3# to 9#) the aquiclude moved left at first and then right.
After the panel extraction was finalised, the maximum vertical displacement of the aquiclude was −4.4 m, with a subsidence coefficient of 0.88. The maximum horizontal displacement and strain of the aquiclude were around the set-up and longwall stop positions, respectively, 0.238 m and 9.5 mm/m. The maximum horizontal displacement occurred at the edge of the subsidence trough, where the aquiclude was horizontally stretched and thus had a greater displacement. Compared to this position, the trough centre had a minor horizontal displacement, at almost 0.
3.3.3. Groundwater Flow
(1) Water level variation
Figure 7 shows the water level responses to the continuous panel extraction.
Before the longwall mining, the three piezometers observed that the depth of water level was 3.3, 2.8, and 3.1 m, respectively. After the longwall progressed 70 m, piezometer 1# recorded a water level drawdown of 0.2 m. After a further 10 m longwall progress, the water level declined to 2.4 m, and, correspondingly, an accelerated drawdown of 22.6%. Then, the drawdown became much more gradual and ultimately levelled off, causing the original water level to be declined, overall, by 1.4 m, approximately 42.4% of the original level.
Piezometer 2# recorded a water level drawdown of 0.3 m when the longwall progressed 100 m, which is later than for piezometer 1#. The water level at piezometer 2# reached the minimum value 2.0 m after the longwall progressed 130 m, favouring a maximum drawdown of 0.8 m, 28.6% of the original water level. However, in the case of the 150 m longwall progress, the water level of piezometer 2# increased by 0.1 m, then rose further to 3.0 m, with the longwall progressing 210 m and ultimately maintained. The recovered water level was higher than the original value, by 0.2 m. According to the model, the reason why the water level of piezometer 2# rose was that it was located in the middle of the subsidence basin. In the later stage of the experiment, the groundwater on both sides of the subsidence basin converged to the middle, causing the water level of piezometer 2# to be higher than its initial value.
Piezometer 3# had a similar water level variation to piezometer 1#, recording a maximum drawdown of 1.1 m when the longwall progressed 200 m; 1.1 m smaller than the maximum drawdown recorded by piezometer 1#. The maximum drawdown at piezometer 3# was 35.5% of the original level.
(2) Apparent resistivity
Apparent resistivity can help reflect groundwater storage in localised areas. Considering the electrical conductivity of water, the area where water accumulates should have a low electrical resistivity. A ratio of the post-mining resistivity to pre-mining resistivity was defined to explicitly capture any minor variation, namely apparent resistivity anomaly index (ARAI). Figure 8 shows the ARAI profile after the panel extraction was finalised.
As can be seen in Figure 8, there were two low ARAI areas above the goaf void, indicating a localised groundwater accumulation in both areas. Combining the aquiclude displacement profile indicated that both low ARAI areas were in the middle of the subsidence trough. Simultaneously, the aquiclude and strata below had high ARAIs, implying that the aquiclude still had a satisfactory water-resisting competence and that the overlying groundwater was partially maintained above the aquiclude. The results implied that the aquifer was well protected from extensive dewatering.
4. Discussion
The physical simulation indicated that the WCM aquiclude fractured due to longwall mining, whereas the fracture partially converged in the water seepage environment. This can be considered a self-healing ability of the WCM lithology. To understand the mechanism of WCM aquiclude responses to longwall mining, the paper further analysed the contribution of mineral composition and water-rock interaction to the WCM aquiclude stability from chemical and mechanical perspectives. The test samples were taken from the Paleogene mudstone aquiclude of Yili No. 4 Coal Mine.
4.1. Mineral Composition
The WCM mineral composition was observed using X-ray diffractometer (Type D8 ADVANCE); the results are shown in Figure 9.
Table 2 Summaries the WCM mineral composition obtained from Figure 9.
Table 2 shows that the weakly cemented mudstones were mainly dominated by Quartz, Kaolinite, and Montmorillonite, making up 92.2% of the total content. In addition, there was a small quantity of Illite, Plagioclase, and Potash feldspar, forming 7.8% of the total content, on average. Among these minerals, Kaolinite possessed the highest content, 51.5%; followed by Quartz (27.2%) and Montmorillonite (13.9%). The three clay minerals, Kaolinite, Montmorillonite, and Illite, possessed 69%, two times of other minerals. Considering that clay minerals behave significant volumetric expansion after encountering water, a knowledge widely accepted by academics [42,43,44], the aquiclude fractures thus exhibited a gradual convergence in the water seepage environment, demonstrating a self-healing ability.
4.2. Water-Rock Interaction
The paper further studied the water-rock interaction of weakly cemented mudstone in hydrous conditions. A WCM specimen was submerged into distilled water in a beaker, before which a 7 mm wide fracture was fabricated on the specimen surface to test the fracture convergence in response to clay mineral argillisation and expansion. The use of distilled water could truly reflect the water-rock interaction of weakly cemented aquiclude after encountering water and excluded the influence of chemical ions. Figure 10 exhibits the water-rock interaction effect and fracture aperture variation with time.
Figure 10 shows that the water started to be turbid after 0.5 h, accompanied by evident rock expansion and almost no change in the fracture aperture. The rock expansion and argillisation became significant after 1.0 h, and the prefabricated fracture converged to 5 mm, with the bottom part filled with rock fragments. After 1.5 h, the argillisation was complete, causing the rock to become severely deformed and the water to become further turbid. In this stage, the fracture was entirely closed. The results indicate that the weakly cemented mudstone had pronounced volumetric expansion and argillisation after being submerged in water for 1.5 h, during which the fracture can be filled with rock fragments. In contrast, ordinary rocks such as sandstone, limestone, and granite can remain intact in hydrous conditions for tens or even hundreds of days [45,46]. As analysed, the weakly cemented mudstones had a unique property regarding the volumetric expansion and argillisation when encountering water, thus giving this lithology an ability to heal mining-induced fractures. This property made a positive contribution to the aquiclude stabilisation in longwall mining practice.
5. Field Test
In the case mine, a piezometer was installed on the ground, 1265 m away from the LW21103 set-up position, to record the groundwater level. The piezometer was above the LW21103 belt roadway and drilled to the bottom of the gravel aquifer, as shown in Figure 11. Figure 12 shows the groundwater depressurisation and recovery as the workface passed the monitoring position (labelled 0 x-coordinate). The original water level was marked 0.
Figure 12 shows that the piezometer captured the first groundwater depressurisation, as the workface was 60 m ahead of the monitoring position. This can be attributed to the longwall’s impact on the frontal areas; the water flowed to the subsidence trough above the mined-out area; accordingly, the recorded water level declined. The water level continued to drop with the longwall progress, reaching −3.0 m as the workface was just below the piezometer, and reaching the lowest, at −4.8 m, when the workface was 25 m beyond the piezometer. After 130 m of longwall progress, the water level gradually recovered to the maximum due to water recharges from the river and snowmelt. This variation indicated that the WCM aquiclude remained stable and possessed a good water-resisting functionality during the longwall mining process. It can be considered that LW21103 operated a water-conserved mining, which agreed with the physical simulation results.
6. Conclusions
A solid-fluid coupling simulation was conducted using a self-developed non-destructive testing platform, demonstrating that the WCM aquiclude was dominated by a bending deformation in response to the longwall mining. The aquiclude above the mined-out areas was tensioned and then compressed, accompanied by evident fracture initiation, propagation, and convergence. Above the set-up and longwall stop positions, the main characteristic of the aquiclude was tensile fracturing, experiencing a process of fracture initiation, propagation, and closure. This variation implied that the WCM aquiclude had a significant ability for plastic deformation and self-healing.
The self-healing ability of weakly cemented mudstones was correlated to the rich clay minerals, as well as to the argillisation and disintegration behaviour in the water environment. The volumetric expansion and argillisation proneness of the clay minerals in the hydrous environment allowed the mining-induced fractures to be narrowed. The water-rock interaction caused weakly cemented mudstones to be completely decomposed in 1.5 h, further facilitating the closure of the aquiclude fractures.
In-situ piezometric monitoring showed that the shallow aquifer water experienced a temporary drawdown, due to longwall mining, and then a recovery, due to water recharges from the river and snowmelt. The water level variation indicated that the WCM aquiclude maintained its water-resisting functionality, and that the monitored panel achieved water-conserved mining. This measurement had a good agreement with the physical simulation, implying a satisfactory reliability of the research results.
Conceptualization, S.Z. and G.F.; methodology, D.Z.; validation, T.L. and X.G.; data curation, S.D. and H.C.; writing—original draft preparation, S.Z.; writing—review and editing, G.F. and D.Z. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The authors are grateful to the anonymous reviewers for their helpful comments.
The authors declare no conflict of interest.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Enlarge this image.Figure 4. Variation of aquiclude fracture initiation, propagation, and convergence.
Enlarge this image.Figure 5. Post-mining overburden movement profile in the weakly cemented rock environment.
Enlarge this image.Figure 6. Aquiclude displacement during panel extraction: (a) vertical displacement; (b) horizontal displacement; (c) horizontal strain.
Enlarge this image.Figure 6. Aquiclude displacement during panel extraction: (a) vertical displacement; (b) horizontal displacement; (c) horizontal strain.
Enlarge this image.Figure 9. X-ray diffraction patterns of weakly cemented mudstone: (a) Specimen 1; (b) Specimen 2; (c) Specimen 3.
Enlarge this image.Figure 10. Water-rock interaction effect on the WCM specimen and fracture variation with time.
Type and quality of materials used in physical simulation.
| Lithology | UCS |
Sand |
CaCO3 (Bentonite) |
Gypsum |
Paraffin |
Water |
Notes |
|---|---|---|---|---|---|---|---|
| Silts | 3.0 | - | - | - | - | - | Replaced by soil |
| Gravels | 3.0 | 658.13 | 51.19 | 21.94 | - | 8.00 | Shallow aquifer |
| Mudstone | 10.0 | 325.00 | 36.56 |
4.06 | 12.19 | 4.00 | Aquiclude |
| Sandstone | 15.2 | 3290.63 | 255.94 | 109.69 | 73.13 | 40.00 | |
| Mudstone | 10.0 | 639.84 | 63.98 |
27.42 | 18.28 | 8.00 | |
| Sandstone | 12.4 | 658.13 | 51.19 | 21.94 | 14.63 | 8.00 | |
| Mudstone | 10.0 | 639.84 | 63.98 |
27.42 | 18.28 | 800 | |
| 21-1 coal | 3.0 | - | - | - | - | - | Dropping the jacks |
Note: bentonite (containing clay minerals) was used for mudstones.
Mineral composition of weakly cemented mudstone.
| Specimen | Mineral Composition and Proportion (%) | Clay Minerals (%) | |||||
|---|---|---|---|---|---|---|---|
| Quartz | Kaolinite | Montmorillonite | Illite | Plagioclase | Potash Feldspar | ||
| 1# | 31.0 | 48.2 | 13.8 | 4.8 | 0.0 | 2.2 | 66.8 |
| 2# | 20.6 | 55.8 | 14.6 | 3.5 | 0.0 | 5.5 | 73.9 |
| 3# | 30.1 | 49.3 | 13.2 | 3.7 | 3.7 | 0.0 | 66.2 |
| Average | 27.2 | 51.1 | 13.9 | 4.0 | 1.2 | 2.6 | 69.0 |
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; Fan, Gangwei 2
; Zhang, Dongsheng 2 ; Luo, Tao 2 ; Guo, Xue 2 ; Dun, Siqin 2 ; Chen, Hua 2 1 School of Mines, China University of Mining and Technology, Xuzhou 221116, China; School of Resources and Earth Science, China University of Mining and Technology, Xuzhou 221116, China
2 School of Mines, China University of Mining and Technology, Xuzhou 221116, China