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Physical simulation study on grouting water plugging of flexible isolation layer in coal seam mining

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Analysis of the roof failure characteristics of coal seam

Before mining, fracturing was conducted on a portion of gritstone in the lower section of the Naoro Formation and then entered the mining stage. Figure 9 shows the influence law of coal roof rupture under different periodic pressures. With mining of the #2 coal seam working face, the direct roof of the coal seam partially broke and collapsed, forming gangue in the goaf. There is a clear separation between the direct and basic roof. When the working face advanced to 228.2 mm, the old roof ruptured, and the working face started to enter the periodic pressure-bearing stage. As the working face advanced to 592.9 mm, the roof exhibited the fourth periodic pressure. The overlying layer roof in the excavation area was affected by the upper bearing arch pressure, leading to the collapsed rock to not completely contact the upper roof. With the increasing distance of coal seam mining, the roof developed significant subsidence, and the influence range of the bedrock boundary caused by the mining was still in the isolation layer fracturing zone. The bedrock influence boundary angle reached 73.57°, and the rock fracture angle was 56.95°. When the working face advanced to 726.5 mm, the fifth periodic pressure on the roof occurred. The bedrock layer in the upper right of the workings was near the right boundary of the first isolated coal seam rupture. Then, coal mining was suspended, and a second isolated seam fracturing process was conducted. The bedrock influence boundary angle reached 73.57°, and the rock rupture angle was 56.95°.

Figure 9

Influence law of coal roof rupture during different periodic pressure.

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When the processing was advanced to 798.4 mm, the bedrock layer in the upper right of the processed area became close to the right boundary of the second isolated seam fracturing. After the third isolated layer fracturing process, the rock impact boundary angle reached 75.33°, and the rock fracture angle was 50.39°. Proceeding to 1031.6 mm, eighth periodic pressure was generated on the roof. The falling gangue in the mined-out area was in contact with the roof, with the bedrock impact boundary angle reaching 74.77° and the rock fracture angle reaching 57.06°. Thereafter, the bedrock layer of the roof gradually entered the full-scale mining stage. As the working face continues to advance, the bedrock impact boundary caused by coal seam mining should be in isolated coal seam fractures. When the bedrock layer at the working face is close to the right boundary of the isolation layer fracturing, the next isolation layer fracturing should be performed.

Analysis of roof stress evolution law

Figure 10 illustrates the change law of the roof support pressure when mining of the working face, in which the roof support pressure curve is the stress change minus the initial value of the sensor before mining. After the excavation of the working face, the surrounding rock will exhibit stress redistribution. The increase in tangential stress in front of the working face or on both sides is called the support pressure. The peak value of the support pressure generally occurs on the front of the working face. As the working face advanced to 228.2 mm, the direct roof gradually broke and collapsed with mining. Due to the redistribution of surrounding rock stress, the stress fluctuation at the open cut was clear. In front of the working face, the overlying rock stress was redistributed due to mining, and the vertical pressure peak area appeared, with a stress increment of 0.03 MPa. When the working face advanced to 360.8 mm, the first cycle pressure on the roof occurred. The falling gangue in the mine-out area gradually approached its upper strata, and the peak support pressure increments reached 0.05 MPa. During the advancement of the working face to 592.9 mm, the direct roof continued to collapse. The gangue at the cuttings was gradually compacted with the roof, and the stresses gradually restored to stability. Coal seam mining led to the decompression of the floor, and the vertical stress maximum reduction at the working face was 0.045 MPa. The peak vertical pressure in front of the working face shifted to the right as mining progressed. When degradation reached 726.5 mm, the fifth periodic pressure on the roof occurred. Figure 10b shows that the fracture of the isolation layer had no apparent effect on the change in roof stress. Within 560 mm from the open excavation, the mine-out area gangue gradually compacted with the roof. Vertical pressure changes between the fourth and fifth periodic pressures are slight and practically nonsignificant.

Figure 10

Vertical pressure variation law with coal mining. (a) First pressure and First periodic pressure difference. (b) Fourth and First periodic pressure difference. (c) Eighth and Ninth periodic pressure difference. (d) Eleventh and Twelfth periodic pressure difference. (e) Variation laws of vertical pressure with mining.

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When the mining reached 1031.6 mm, the directly caving gangue completely filled the goaf and was compacted with the roof. The upper roof of the caving rock was supported again, and the compaction range of the mining area extended to 821 mm. As the working face advanced to 1338.9 mm, the peak vertical pressure appeared at 1400 mm, with a maximum increment of 0.375 MPa. The compaction range of the mining area extends to 1200 mm. Then, the fractured isolation layer can be grouted. The subsequent working face advances until the end of mining, and the rock movement above the mine-out zone will exhibit a periodic “falling-filling-cutting-compaction” process. Fracture grouting of the flexible isolation layer has no significant effect on the vertical stress changes, and the stress unloading area and the peak vertical pressure will continue to change with mining. Nevertheless, consideration needs to be given to the adequacy of the gangue falling from the roof for isolation layer grouting.

Roof displacement and development pattern of water-conducting fracture zone

Figure 11 shows the development law of the roof water-conducting fissures in the roof of the coal seam during different pressure periods, where the illustration shows the von Mises equivalent strain. Figure 12 shows the development trend of the water-conducting fracture zone height. From the whole observation, although the isolation layer is treated by fracturing before back mining, it has less influence on the displacement and deformation of the overlying rock layer because it is restricted by the surrounding rock of the model. When the working face was mined to 228.2 mm, the upper roof of the mining face collapsed, and the first periodic pressure occurred on the roof. The roof displacement reached the Yan’an Group mudstone layer, and the roof collapse height was only 104.3 mm. As the mining advanced, the roof fractures in the mining-out area continued to develop upwards. When the working face was mined to 360.8 mm, the first cycle pressure on the roof occurred, and the roof collapse height extended upwards to the siltstone of the Yan’an Formation, with a collapse range of 117.6 mm. At this point, only a small displacement change occurred around the direct roof, and the flexible isolation layer was basically not affected by any impact.

Figure 11

Development regularity of roof water-conducting fissures during different period pressure.

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Figure 12

Development height curve of water-conducting fracture zone.

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From the second cycle pressure onwards, the development trend accelerated significantly, and the collapsed height rose rapidly to 210.9 mm. When the working face advanced to 537.1 mm, the third cycle pressure occurred on the roof. The collapsed Yan’an Formation mudstone layer was further pressurized by its upper layers and collapsed to a height of 344.7 mm. The roof displacement had spread to the coarse sandstone of the Naoro Formation, but the height of the water-conducting fracture zone had not reached the bottom of the isolation layer. When the workings reached 592.9 mm, the roof collapsed again, showing the fourth periodic pressure. The water-conducting fissure zone continues to develop upwards to 355.3 mm, which passes through the fissure isolation layer and reaches the gritstone at the top of the isolation layer. The fractured isolation layer is in an “activated” state.

When the working face reached 1031.6 mm, fallen gangue completely filled the mining-out area and compacted with the roof, and eighth periodic pressure occurred on the roof. The height of the water-conducting fracture zone developed to 496.8 mm, which was lower than the height of the water-conducting fracture zone of 565.8 mm at the seventh periodic pressure. After that, the old roof collapsed as a cantilevered beam. The development height of the water-conducting fracture zone was allegedly less than 565.8 mm. Afterwards, the roof fracturing direction was consistent with the direction of working face advancement, from left to right. Displacement and fracture of the overlying rock layer were mainly caused by the overall downwards sliding of the upper rock seam due to the collapse of the bottom rock seam. At different heights of the coal seam roof, the degree of displacement damage decreased with increasing height.

When the working face reached 1178.7 mm, the roof covering the open cut stabilized. The fractured isolation layers in the 1st ~ 13th groups were grouted, and then the coal was mined only after the slurry had completely solidified and reached a certain strength. The eleventh periodic pressure occurred on the roof, with a water-conducting fracture height of 367.6 mm at this time. When the working face was advanced to 1471.9 mm and 1645.2 mm, the roof had twelfth and fourteenth periodic pressures, and the heights of the water-conducting fracture zone were 332.0 mm and 416.0 mm, respectively. Then, the 14th ~ 15th and 16th ~ 17th group isolation layers of the upper coal seam were grouted while fracturing the right isolation layer. However, the disruption of displacement towards the extent of the development had a relatively small impact, mainly on the roof rock layer above the mining face. Table 2 indicates the development height of the water-conducting fracture zone and the fracture and grouting sequence of the isolated layer.

Table 2 Development pattern of water-conducting fracture zone and fracture and grouting sequence of isolated layer.
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During the mining process, damage to the water-conducting fissure zone was always a major factor in the displacement of the roof slab. Nonetheless, after fracturing and grouting measures, the effects of the damage were significantly reduced such that the damage to the roof rock was contained within the flexible isolation layer. After grouting, the enhanced strength of the isolation layer ensured that mining was carried out normally. During the mining period, four grouting reforms were made, and the isolation layer was fractured six times, with the maximum development height of the water-conducting fracture zone located at the seventh periodic pressure, reaching 565.8 mm.

Analysis of water flow evolution law of overburden roof

To analyse the seepage law of the overburden roof, seven water flow monitoring lines were arranged from the top of the flexible isolation layer to the direct roof of the coal seam. The No. 1 water flow monitoring line was placed in the position of the third group of the isolation layer, which is initially located outside the deformation range of bedrock disturbed by mining and outside the stop line. The flow line was mainly used to monitor the influence of the rock disturbance boundary above the open cut on isolated seam fracturing and grouting. No 2–3 water flow monitoring lines were placed at the isolation layer positions of Group 12 and Group 14, which were initially located near the maximum height of the water-conducting fracture zone and were mainly used to monitor the change laws of the water-conducting fracture zone with mining impact. Monitoring Lines 4–6 were placed in isolation layers No. 17, No. 22 and No. 26 to study the impact of water flow changes with mining disturbance and the advanced influence scope. Water flow monitoring line No. 7 was placed in the thirtieth group of isolated layers, which was originally outside the cut-off line. As shown in Fig. 13, white arrows are water flow vectors in mL/min. Fracturing the 1–18 isolation layers before mining, the water tank hot water was injected into the flexible isolation layer such that the iodized salt in the flexible isolation layer was completely dissolved, and the infrared monitor showed the yellow area in the image. At this point, the water flow monitoring Lines 1–3 and 5–7 show yellow status, indicating that after the fracturing of the isolation layer, the aquifer water flows downwards along the fracture. The lower part of monitoring Line 4 was compacted at the top of the coal seam, indicating that the cracks between the roof and the aquifer had not been communicated. Therefore, the water flow rate was 0 mL/min until the sixth periodic pressure. Mining was then undertaken on the working face. The No. 1 monitoring line was therefore less affected by mining due to its layout outside the stop line, and there was no significant change in water flow before the first grouting.

Figure 13

Water flow evolution of the overburden roof with coal mining.

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As shown in Fig. 13, when the working face progressed to second periodic pressure, with the collapse of the coal seam, the stress of the surrounding rock was redistributed, the height of the water flowing fractured zone of the roof increased, and the water flow of the No. 2 monitoring line increased from the initial 9.1 mL/min to 14.0 mL/min. As the working face was advanced above the No. 2 monitoring line, the fifth periodic of pressure were generated in the roof. The development height of the roof water flowing fractured zone reached 504.4 mm. The roof was separated and collapsed, the cracks in the monitoring line communicated with each other, and the rock stress was released. The water flow in the No. 2 monitoring line increased significantly. Monitoring line No. 3 was affected by advanced mining, resulting in the coal seam roof’s increased rock fissures, the water flow path and resistance were reduced, and the water flow reached 48.3 mL/min. At the same time, the influence range of working face bedrock was close to the boundary of the first fracturing of the flexible isolation layer, and Groups 20–22 of isolation lays had been fractured.

When mining started at the sixth periodic pressure, the roof water-conducting fracture zone gradually reached the maximum height and penetrated the fractured isolation layer, and the fracture of the roof rock increased. Lines No. 2 and No. 3 reached 44.4 mL/min and 85.6 mL/min, respectively. In fact, the encounter may indicate that the confined water of the gritstone aquifer was released, and the water flow of the working face increased. Then, the working face progressed, and the collapsed gangue above the mining-out area was compacted into the bedrock roof. The stress in the goaf did not change significantly, and the cracks in the strata decreased. The No. 2 and No. 3 water flows of the monitoring line gradually dropped. During this period, the change law of monitoring Lines 4–7 was similar to that of No. 2 and No. 3. During coal seam mining, the roof underwent a process of fracture, collapse, compaction and full mining, and the water flow monitoring line also went through a process of rising and then falling.

When the working face was advanced to the eleventh periodic pressure, the grouting transformation of isolation layers 1–12 was conducted. The slurry was injected into the flexible isolation layer by hand pressure pump along the grouting pipe. After the slurry solidified, the colour of the No. 1 and No. 2 monitoring lines gradually became shallow, and the water flow gradually decreased under infrared observation. As the extraction of the coal seam progressed and the flexible insulation layer was broken and grouted, the colour of observation Lines 1–4 turned black in the infrared observation until the fourth grouting of insulation layer 18–19, and the water flow rate all showed 0 mL/min. However, the lower strata of the flexible isolation layer were not yet stabilized, so monitoring Lines 5–7 did not undergo any grouting transformation and still had a large water flow until the end of mining. Flow metre and infrared observations show that the destruction and grouting of the flexible isolation layer had a noticeable effect on the seepage characteristics of the overburden. In particular, after the grouting of the isolation layer, the slurry filled and solidified rapidly, the water flow decreased rapidly, and the water plugging effect of flexible isolation layer grouting was remarkable.

Discussion and analysis

During coal seam mining, the fracturing of the flexible isolation layer should be based on the premining overtopping influence range; that is, when the boundary line of bedrock influence extends to the range of the flexible isolation layer reached by the fracturing area of the flexible isolation layer, the next fracturing should continue. The average boundary angle range of the bedrock was 76.7°, and the field angle should not be less than 73.57°. The grouting of the flexible isolation layer considers the full mining degree of the coal seam. When there is no significant change in stress in the mined area, grouting of the flexible isolation layer at the top of the goaf is conducted. According to the simulation experiment in this paper, the full mining distance of the working face is 1338.9 mm, and the actual distance on site is 187.446 m. It is calculated that the distance between the fracture of the flexible isolation layer should be no less than 854.8 mm away from the working face, and the actual distance on site is 119.672 m. After the working face enters full mining, the shortest distance between the fracturing grouting range of the flexible isolation layer and the working face is not less than 242.6 mm, and the actual distance on site is 33.964 m.

As seen from the previous analysis, with the advancement of the working face, the bedrock influence boundary angle of the coal seam does not change significantly, which only plays a guiding role in the fracturing sequence of the flexible isolation layer. The fracturing of the flexible isolation layer had an clear influence on the seepage of water-rich bedrock at the bottom of the Zhiluo Formation. The water-flowing fractured zone formed in the process of coal seam mining promoted the release of fractured water in the water-rich bedrock at the bottom of the Zhiluo Formation. The higher the height of the water-flowing fractured zone is, the greater the seepage of the water-rich bedrock. Coal seam mining had little effect on the seepage characteristics of the water-rich bedrock layer at the bottom of the Zhiluo Formation in the range of not disturbed by mining and advanced influence.

In accordance with the stress sensor data, when the working face passed a certain distance, the bottom plate of the extraction area was compacted by the falling gangue, and the sensor pressure data did not change with the mining face. At this time, the grouting of the fracturing area of the flexible isolation layer corresponding to the above goaf was not affected by the mining face. For example, the stress in the goaf of 1200 mm had no clear change. Therefore, the first grouting was conducted in the fracturing area. After the solidification of the grouting slurry, the water flow of monitoring lines No. 1 and No. 2 decreased significantly. This minimized the impact on the original geological environment and at the same time reduced the goaf water drainage of the working face. The sealing effect of the isolation layer has an important influence on promoting water-retaining coal mining.

The experimental application of the flexible isolation layer has realized its feasibility from the physical simulation test method in this paper. The realization of a flexible isolation layer requires premining fracturing and postmining isolation grouting. At present, premining fracturing can be achieved by directional drilling technology. There are also examples of roof separation grouting for postmining flexible isolation layer grouting28,29. Therefore, there is no technical bottleneck in field applications. Moreover, there is still a certain distance from the specific engineering application. According to the results of this study, it is predicted that the implementation of a flexible isolation layer will have great significance for water conservation coal mining in western China, which can reduce soil erosion and protect surface ecology.


Source: Ecology - nature.com

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