Control effect of coal mining solid-waste backfill for ground surface movement in slice mining: A case study of the Nantun Coal Mine

doi:10.21203/rs.3.rs-1503421/v1

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Control effect of coal mining solid-waste backfill for ground surface movement in slice mining: A case study of the Nantun Coal Mine

https://doi.org/10.21203/rs.3.rs-1503421/v1

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Management of solid waste and protecting the ecological balance of the region are key challenges that the coal mining industry has to face. This study evaluated the effect of solid waste backfilling mining on the overlying strata movement and surface deformation variation pattern in slice mining. The mechanical characteristics of different cemented paste backfills (CPB) were compared. The CPB specimens were made of coal gangue and cement with or without the addition of fly ash. The experiments showed that the mechanical strength of the CPBs made of coal gangue and cement increased dramatically. A numerical simulation was then performed to analyze the variation patterns of the overlying strata displacement and surrounding rock stress distribution before and after filling the 3_lower and 3_upper coal seams with CPB. The CPBs reduced the movement of the surface by 95.1% and 95% during the mining of the 3_lower and 3_upper coal seams, respectively. Finally, we used a mining-induced subsidence prediction and analysis system to predict the influence of the 3_lower and 3_upper coal seams on the ground surface subsidence. It was found that the ground surface subsidence induced by CPB mining was 1/20 that of the cumulative ground surface subsidence caused by caving mining. CPB mining could effectively control the ground surface subsidence caused by multi-slice mining of the thick coal seam, offering protection for buildings above the ground. Our research provides theoretical and technical support for coal mining under buildings subjected to similar conditions.

Environmental protection

Sand-based cemented paste backfill

Backfilling mining

Deformation prediction

China is rich in coal resources but lacking in oil and natural gas resources [1-3]. China's demand for coal resources has grown over the years due to the large economic boom. At present, industrial development in the country is heavily dependent upon coal resources [4-7]. According to statistics, by 2021, China's primary energy consumption totaled 4.33 billion tons of standard coal. China's coal consumption accounted for 56% of the total energy consumption in the country. As shown in Fig. 1, it is estimated that by 2050, China's coal resources consumption will still account for over 50% of the total energy consumption [8-10].

Economic development fueled by large-scale mining and utilization of coal resources usually comes at the expense of the environment. As the depth of mining increases, a larger scope of the ground surface will be affected by coal mining [11-13]. The influence of coal mining increases for buildings and eco-environment above the ground surface. Moreover, China produces a massive amount of solid wastes from coal mining. The cumulative coal mining solid wastes heaped on the ground surface have exceeded 6 billion tons. There are over 1700 gangue mountains in China, occupying over 200 thousand mu of land. The quantity of coal mining solid wastes keeps increasing at a rate of 600 million tons per year [14-16]. High-intensity coal mining may cause severe damage to buildings above the earth's surface. Ground surface subsidence induced by coal mining amounts to over 300 thousand mu, with over 100,000 buildings affected every year [17-20]. Such damage continues to expand as coal mining intensifies [21-22]. So long as coal remains the mainstay in China, protecting the ecological environment and promoting coordinated development are urgent problems that need to be solved.

Cemented paste backfill (CPB) mining technology is a green, safe mining method that is widely used for the mining of coal resources under buildings, railways, and water bodies. CPB mining can achieve a high compaction rate that reaches approximately 95% [23-25]. CPB is primarily made of solid wastes such as gangue and fly ash. CPB is a good method to dispose of the coal mining solid wastes heaped in large quantities above the earth's surface. Besides, CPB can achieve effective control of ground surface deformation due to its high compaction rate, protecting the buildings from damage [26-27]. Many studies have been conducted on the use of CPB to dispose of coal mining solid wastes and control ground surface deformation. Lin et al. [28] discussed the laws that govern overlying strata movement under continuous mining with CPB. This mining method effectively controlled overlying strata movement in the stope, reduced the ground surface subsidence in the ecological park, and increased the recovery rate of coal resources. Lu et al. [29] applied CPB mining to moderately inclined coal seams. CPB improved the mining efficiency, reduced the cost, and effectively controlled ground surface deformation. This method was proven to have a broad engineering application prospect. Fang et al. [30] studied the shear properties at the interface between the rocks and CPB. The stability of CPB was analyzed by considering three influence factors, namely, curing stress, drainage conditions, and filling rate. Their findings provided a technical basis for improving CPB stability. Deng et al. [31] investigated upward slice longwall-roadway CPB technology for mining extra-thick coal seams. The high compaction rate of CPB was utilized, and the coal seam was divided into several slices and mined in sequential order. They further discussed the compressive performance of CPB under different mix ratios and curing time. Helinski et al. [32] carried out finite element modeling for backfilling mining using cemented tailings backfill. They simulated the interactions between the cemented tailings backfill and the stope wall. They studied how the cemented tailings backfill influenced the overall stress distribution of the stope during the backfilling process. Behera et al. [33] presented a review of the cemented tailings backfill made of slag and fly ash. They reviewed the main factors that influence the performance of cemented tailings backfill and how additives influenced the properties of the modified cemented backfill. Many systematic studies have been conducted on the use of CPB in coal mining. Most of these studies are focused the influence of CPB on the overlying strata movement under the excavation of a single coal seam or a thick coal seam, or simply on the properties of CPB itself. However, few studies have been conducted on how CPB mining influences the overlying strata movement laws in the multi-slice mining of thick coal seams. The relationship between overlying strata movement and ground surface deformation by the division of the coal seam into two slices followed by CPB mining is not yet fully understood. It is necessary to discuss the influence of CPB mining on the movement of the overlying strata and ground surface deformation during multi-slice mining of the thick coal seam by combining theoretical analysis with laboratory experiment, numerical simulation, and engineering practice.

The study is concerned with the influence of CPB mining on overlying strata movement and ground surface deformation characteristics during slice mining of the thick coal seam. It discussed, in particular, the control effect of the CPB mining for ground surface deformation under the slice mining of thick coal seams under buildings, railways, and water bodies. This paper combined numerical simulation with theoretical analysis, laboratory experiment, and mining-induced ground surface subsidence prediction to perform a case study of the Nantun Coal Mine. We investigated the influence of CPB mining on overlying strata movement and ground surface deformation in multi-slice mining of the thick coal seam. The deformations of the overlying strata and ground surface were determined for the 3_upper and 3_lower coal seams under backfilling mining. The protection offered by CPB mining for surface buildings was verified. Our research laid down a theoretical basis for coal mining under the same geological conditions.

2.1 Geological conditions for mining

The Nantun Coal Mine belongs to the Yankuang Group. As shown in Fig. 2, the mine is located in the west of Zoucheng City, Shandong Province. The total volume of coal to be excavated from beneath Zoucheng City is 26.69 million tons. As shown in Fig. 3, the 3_upper and 3_lower coal seams are the primary ones to be mined. The thickness of the 3_upper and 3_lower coal seams is 4.73 m and 3.0 m, respectively. Between the two coal seams is an interbedding of siltstone and fine sandstone 2.2 m thick.

At present, the volume of coal to be excavated in the Nantun Coal Mine reaches about 15 million tons. This volume consists of 2.0462 million tons below the industrial square, 655.7 thousand tons below the village of No. 1 mining area, and 2.4226 million tons below the living quarters of Nantun village. As high-quality coal resources are depleted, Nantun Coal Mine has entered the residual mining stage. Therefore, excavating coal resources under the buildings, railways, and water bodies in the mine is an urgent need.

2.2 Buildings on the earth's surface

There are many surface buildings above the unmined coal resources in the Nantun Coal Mine. The distribution of these buildings follows a complex pattern. Backfilling mining is planned for the coal resources under the industrial square where the main and auxiliary shafts are located. As shown in Fig. 4, the buildings in the industrial square mainly include the air draft plant, sedimentation tank, coal preparation workshop, and cooling tower. Most surface buildings have been abandoned to prevent personnel entry. They are also structures under protection.

2.3 Anti-deformation ability indicators

The surface buildings in the Nantun Coal Mine mainly include the air draft plant, sedimentation tank, coal preparation workshop, and cooling tower. The Standard for Coal Pillar Retention and Coal Mining under Buildings, Railways, and Water Bodies and in the Main Roadways has been jointly published by the State Administration for Work Safety, State Administration for Coal Mine Safety, National Energy Administration, and National Railway Administration. According to this standard, the damage classification for brick-and-concrete structures with a length within the deformation seam below 20 m is shown in Table 1. The allowable ground surface deformation values are as follows: horizontal deformation ±2 mm/m, inclination deformation ±3mm/m, curvature deformation ±0.2mm/m².

The surface buildings in Nantun Coal Mine, including the air draft plant, are of good construction quality. The standard for grade Ⅰ mining-induced building damage is chosen for the structures in the design area. The limits for mining-induced ground surface deformation were set up as follows: subsidence ≤300mm. The coal seams excavated by slice mining in the Nantun Coal Mine are filled by CPB. The surface buildings are deemed well-protected if the above-ground surface deformation standards are met.

3.1 Principle of backfilling mining

CPB is usually prepared with coal mining solid wastes, such as gangue and fly ash. CPB is a high-concentration slurry prepared by mixing with fly ash or cement added as a cementing agent. There is no need for dehydration in CPB preparation. The CPB is delivered underground using a backfilling pump via the pipelines as the excavation proceeds. The schematic of backfilling mining is shown in Fig. 5.

According to the figure above, longwall-roadway CPB mining involves using a system that is specific to longwall mining. A fully mechanized coal mining machine or a continuous miner is used instead of a coal cutter. The backfilling roadway is excavated using a fully mechanized coal mining machine or a continuous miner between the transport and the air return roadway. After the excavation of the backfilling roadway is completed, the working face is filled by CPB. Another backfilling roadway will be excavated at the same time. In this way, the backfill mining that involves the cyclic operation of coal mining by tunneling and filling the roadway is realized in the longwall mining working face.

3.2 Properties of CPB

Nantun Coal Mine is located in eastern China, about 6 km from Zoucheng, Shandong Province. The mining license was issued for Nantun Coal Mine in 1988. The excavation of the main mining seam was completed after over 30 years of operation. Now, the mine has entered the stage of freeing coal resources under buildings, railways, and water bodies. However, extensive gangue dumps have accumulated as coal gangue is discharged over long years of coal mining. At present, gangue production in the Nantun Coal Mine keeps increasing at a rate of 650 thousand t/a. Moreover, the long-term accumulation of solid wastes, such as fly ash, produced by the power plant near Nantun Coal Mine, has adversely impacted the surrounding environment. The backfill used for longwall-roadway CPB mining is primarily made of coal gangue, fly ash, and cement as a way of disposing of the accumulated coal gangue.

(1) Coal gangue + fly ash + cement

CPB was made from coal gangue, fly ash, and cement. The grain size of gangue used to prepare CPB was 0-10 mm. In the mix, the gangue accounted for 80%, fly ash 7.5%, and cement 12.5%. The slurry concentration was 82%. The basic mechanical properties of CPB were tested. The results of the compressive strength test of CPB used in the Nantun Coal Mine are shown in Fig. 6.

The compressive strength of CPB was determined at different curing ages. Two specimens were prepared for each curing age, and the average was taken. At the curing age of 1 d, 3 d, 7 d, 14 d, and 28 d, the average compressive strength of CPB was 0.4165 MPa, 0.9735 MPa, 1.315 MPa, 3.05 MPa, and 3.42 MPa, respectively. At the curing age of 28 d, the compressive strength reached 3.42 MPa, which is sufficient to support the overlying strata and prevent its caving.

(2) Coal gangue + cement

The power plant has been shut down by the time the mine entered the stage, and there is an inadequate supply of fly ash. Therefore, we used gangue and cement as the main raw materials to prepare CPB. The doping amount of gangue and cement in the CPB slurry was 80% and 20%, respectively. The slurry concentration was 82%. The doping amount of fly ash could be reduced by increasing the amount of cement added, thereby increasing the strength of the CPB. Fig. 7 shows the mechanical strength of the CPB made of gangue and cement.

It can be seen that the mechanical strength of CPB was greatly improved by increasing the doping amount of cement while reducing the quantity of fly ash. According to Fig. 7, at the curing age of 1d, 3 d, 7 d, 14 d, and 28 d, the average compressive strength of the CPB made of coal gangue and cement was 1.514 MPa, 3.2315 MPa, 4.2 MPa, 8.93 MPa, and 10.3225 MPa, respectively. Fig. 7 (f) compares the mechanical strengths of CPBs made of different raw materials. With the increase in curing age, the mechanical strength of CPBs made of gangue and cement increased significantly. At the curing age of 28 d, the compressive strength of the CPB increased four times. Given the above, coal gangue and cement are sufficient to replace the original three ingredients of CPB used at the earlier stages, namely, coal gangue, fly ash, and cement. CPBs prepared by coal gangue and cement alone can effectively support the overlying strata in the goaf.

3.3 Support design

The width of the roadway tunneled for longwall-roadway CPB mining in the Nantun Coal Mine is 5.5 m. A new roadway is tunneled for every three roadways, for a length of 172 m. The longwall-roadway CPB mining undergoes three stages. As shown in Fig. 8 (a), the two sides of the first roadway being tunneled are solid coal. The solid coal in the roof and on the two sides of the roadway needs support. The support states are shown in Fig. 9 (a). As shown in Fig. 8 (b), when the roadway is tunneled along the CPBs, one side of the roadway is CPB, while the other side is solid coal. CPB can offer support to the roadway. In this context, only solid coal in the roof and on one side of the roadway is supported, as shown in Fig. 9 (b). As shown in Fig. 8 (c), the roadway is tunneled close to CPB on the two sides. There are CPBs on both sides of the roadway, and only the roadway roof needs to be supported, as shown in Fig. 9 (c).

As shown in Fig. 9, the backfilling roadway is supported by anchor bolts and cables. Moreover, the roof is supported using Φ20×2200mm high-strength bolts. An anchor cable beam is deployed in the center. A row of anchor cables is deployed in the roadway center along the longitudinal direction of the roadway. The Φ16×2000mm glass fiber anchor bolts are used to support the ribs. Fig. 9 (d) shows the top view of the roof support for the roadway for cemented paste backfilling mining, with the dimensions and spacing between the bolts.

4.1 Model construction and simulation

We performed a numerical simulation using the FLAC3D software to study the surrounding rock deformation and migration laws in the working face of the longwall-roadway CPB mining in the Nantun Coal Mine. The Mohr-Coulomb model was used for characterizing the failure of the coal-rock masses. According to the actual geological conditions of Nantun Coal Mine, the numerical model illustrated in Fig. 10 had a height of 273. 2 m, a strike length of 300 m, and a dip length of 172 m. First, the numerical model was balanced, with the horizontal displacements of the four vertical planes restrained in the normal direction. The vertical displacement of the model's bottom was set to 0. The ela

Table 1 Physical and mechanical characteristic parameters of the roof, floor, and coal seam

Mechanical properties Lithology	Thickness /m	Bulk modulus /GPa	Shear modulus /GPa	Density /t·m^-3	Tensile strength /MPa	Cohesive force /MPa	Internal friction angle /（°）
Mudstone	16	7.8	2.6	2.2	3	4	26
Coarse sandstone	21	17.5	8.6	2.6	3.2	3.0	26
Sandy mudstone	30	12.8	4.6	2.5	3.2	2.4	27
Coarse sandstone	28	17.5	8.6	2.6	3.2	3.0	26
Medium sandstone	25	23.3	13.2	2.6	3.1	8.3	26
Fine conglomerate	25	18.5	12.6	2.6	2.5	2.26	26
Medium sandstone	28.8	23.3	13.2	2.6	3.1	8.3	26
Fine conglomerate	20	20.5	12.6	2.6	2.5	2.26	35
Siltstone	7.2	20.5	6.2	2.6	1.1	1.26	30
Fine sandstone	7.3	18.5	12.6	2.6	2.5	2.26	26
Mudstone	9.8	17.8	4.6	2.7	3	4	33
Medium sandstone	10.8	23.3	13.2	2.6	3.1	8.3	26
Fine conglomerate	3.05	18.5	12.6	2.6	2.5	2.26	26
Medium sandstone	4.66	23.3	13.2	2.6	3.1	8.3	26
Siltstone	4.05	20.5	1.2	2.6	1.1	1.26	26
3_upper coal seam	4.73	8.6	0.9	1.38	1.8	1.96	26
Interbedding of siltstone and fine sandstone	2.2	18.5	12.6	2.6	2.5	2.26	26
3_lower coal seam	3.0	8.6	0.9	1.38	1.8	1.96	26
Fine sandstone	5.41	18.5	12.6	2.6	2.5	2.26	26
Siltstone	6.15	20.5	1.2	2.6	1.1	1.26	26
Mudstone	6.05	17.8	4.6	2.7	3	4	33

The numerical simulation of the Nantun Coal Mine was divided into four stages: (Ⅰ) calculate the initial stress state caused by the gravity of the overlying strata, (Ⅱ) excavate the 3_uppercoal seam, (Ⅲ) fill the 3_upper coal seam, (Ⅳ) excavate the 3_lower coal seam, (Ⅴ) fill the 3_lower coal seam. We were particularly concerned with the changes in the stress state of the surrounding rocks and the changes in the displacement state of the overlying strata in the working face before and after the filling of the 3_upper and 3_lowercoal seams.

4.2 Numerical simulation results

The 3_upper and 3_lower coal seams were excavated by longwall-roadway CPB mining. A numerical simulation was performed to analyze the variation of overlying strata displacement and surrounding rock stress distribution before and after filling the 3_lower and 3_upper coal seams with CPB.

4.2.1 Comparative analysis of surrounding rock displacement

The variations in overlying strata displacement and ground surface displacement were obtained by numerical simulation under the backfilling mining of the 3_upperand 3_lowercoal seams of the Nantun Coal Mine. Figure 11 shows the nephograms of overlying strata displacement at the end of the excavation of the 3_uppercoal seam, filling of the 3_upper coal seam, excavation of the 3_lowercoal seam, and filling of the 3_lowercoal seam. The displacement changes in the nephograms were analyzed for the 3_upper and 3_lower coal seams of Nantun Coal Mine along the profile at different stages of mining. As shown in Fig. 11, the monitoring lines were arranged in the immediate roof and ground surface after excavating and filling the 3_upper and 3_lower coal seams. The displacement was monitored throughout the entire excavation and filling process. The control effect of the CPB on overlying strata and ground surface movements were analyzed.

As shown in Fig. 12 (a), when the excavation of the 3_upper and 3_lower coal seams was completed, the maximum displacement of the immediate roof was 3.02 m and 3.49 m, respectively. When the filling of the goaf in the 3_upperand 3_lower coal seams was completed, the maximum subsidence of the immediate roof was 0.14 m and 0.24 m, respectively. They were reduced by 95.3% and 93.12% compared with those under the caving mining, respectively. The variation of the vertical displacement of the immediate roof under caving mining could be described using a U-shaped curve, with smaller values at the two ends and larger values in the middle. The maximum displacement occurred in the center of the goaf. As shown in Fig. 12 (b), the ground surface subsidence was generally small. The maximum ground surface subsidence was 1.62 m and 0.08 m after the excavation and filling of the 3_upper coal seam, respectively. After the backfilling mining, the ground surface subsidence decreased by 95.1%. The maximum ground surface subsidence was 240 mm and 120 mm after the excavation and filling of the 3_lower coal seam, respectively, both being lower than 300 mm. After the backfilling mining, the ground surface subsidence decreased by 95%. The nephograms of vertical displacement of the surrounding rocks are shown in Fig. 11. The variation of displacement of the overlying strata in the working face was approximately symmetrical about the midline of the advance distance. The vertical displacement of the overlying strata was the largest when approaching the midpoint of the advance distance of the working face. The vertical displacement of the overlying strata decreased significantly after goaf filling.

CPB was used to fill the space taken up by the excavated coal. Besides, CPB itself has a specific strength and can support the roof load upon roof weighting induced by backfilling mining. Hence, the roof integrity is preserved, and the overlying strata damage is put under proper control. As a result, roof caving was lessened, which prevented the upward propagation of failure in the rock strata, thereby reducing ground surface deformation. Given the above, it is feasible to control ground surface deformation by backfilling mining.

4.2.2 Comparative analysis of surrounding rock stress

We compared the nephograms of overlying strata stress distribution under the excavation and filling of the 3_upper and 3_lower coal seams to investigate the weighting laws in the backfilling mining stope, as shown in Fig. 13. Measuring lines were arranged in the immediate roof and the main roof. The vertical stress curves measured along the horizontal distance are shown in Fig. 14.

The surrounding rock stress was redistributed in the stope, as shown in Fig. 13. There were stress concentration regions in the coal pillars in the cut hole and on the two sides of the stop line. Due to the mining-induced influence, the roof above the working face was considerably unloaded. This area remained within the stress relief zone. Tensile stress occurred in the middle of the goaf. The degree of stress concentration in the coal pillars decreased after the backfill mining of the 3_uppercoal seam was completed. The peak stress in the immediate roof during caving mining was 15.8 MPa, and it decreased to 6.72 MPa after the backfilling. The point with the peak stress made a rightward shift towards the upper margin of the coal pillar. As shown in Fig. 14, the peak stress in the immediate roof was 17.3 MPa, and 7.34 MPa after the caving mining and the backfilling mining in the 3_lower coal seam, respectively. The peak stress in the immediate roof was 15.0 MPa and 5.72 MPa, respectively, indicating that the stress in the immediate roof decreased significantly under backfilling mining than under caving mining. The stress curve of the immediate roof was symmetrical about the midline of the advanced distance. Under caving mining, the stress above the coal pillars on the two sides of the goaf first increased, then decreased, and then increased again, showing minor fluctuations. The stress in the roof above the goaf remained approximately unchanged, consistently being tensile.

Finally, a stress shell was formed in the coal pillars outside the cut hole and the stop line. The stress in the goaf roof finally evolved into tensile stress. The roof was not broken or caved in due to the support of CPB under backfilling mining. The overlying strata tended to move altogether, with a small overall displacement. The rock bursts were inconspicuous in the stope. Therefore, CPB mining could effectively control the stress in overlying strata, reducing the disturbance caused by slice mining of the thick coal seam.

4.3 Prediction of overlying strata deformation

4.3.1 Model for predicting overlying strata deformation

(1) Excavation plan for the coal seam

The ground surface subsidence prediction for the 3_upper and 3_lower coal seams consisted of the prediction of four cumulative ground surface subsidences: ground surface subsidence upon excavation of the 3_upper coal seam, cumulative ground surface subsidence upon backfilling of the 3_upper coal seam, cumulative ground surface subsidence upon excavation of the 3_lower coal seam, and cumulative ground surface subsidence upon backfilling of the 3_lower coal seam. The basic parameters used for predicting ground surface subsidence induced by the excavation of the 3_lower and the 3_upper coal seams are tabulated in Table 2. Table 3.

Table 2 Basic parameters for ground surface subsidence prediction

Name of the working face	Angle of strike	Mining thickness (mm)	Coal seam dip angle (°)	Subsidence coefficient	Tangent of the main influence angle	Horizontal movement coefficient	Propagation angle (°)
Ground surface subsidence induced by caving mining of the 3_lower coal seam
C13 upper 01/02	10	4730	5	0.5	1.4	0.27	88
C13 lower 01/02	10	3000	5	0.5	1.4	0.27	88
Ground surface subsidence induced by the excavation of the 3_lowercoal seam
C13 upper 01/02	10	236.5	5	0.5	1.4	0.27	88
C13 lower 01/02	10	150	5	0.5	1.4	0.27	88

Table 3 Coordinate parameters of working faces

C13_{upper (lower)} 01 working face

3916387.6543

39532404.9715

3916387.3679

39532524.9715

3917012.2816

39532524.9715

3917012.2816

39532805.5888

3917120.6483

39532798.8176

3917122.8205

39532404.9715

350

C13_{upper (lower)} 02 working face

3916387.8696

39532284.9715

3916387.6292

39532400.1427

3917122.8206

39532400.1427

3917122.8206

39532284.9715

350

4.3.2 Comparison of predicted ground surface deformations

(1) Excavation of 3_uppercoal seam

According to the prediction methodology, the subsidence was first predicted for the excavation and backfilling of the 3_upper coal seam. The basic parameters in the different backfilling working faces shown in Tables 2 and 3 provided important data for predicting the ground surface subsidence. Fig. 15 and 16 show the ground surface subsidence curve vs. position of the working face and the 3D diagrams of mining-induced ground surface subsidence under the excavation and backfilling of the 3_uppercoal seam using the two mining methods, respectively.

As shown in Fig. 15 and 16, the red box indicates the scope of the backfilling working face. The prediction of ground surface subsidence directly above the backfilling working face was the priority. The ground surface subsidence curves in Fig. 15 show that the maximum ground surface subsidence was 1620 m when the caving mining of the 3_uppercoal seam was completed. The maximum ground surface subsidence was only 80 mm when backfilling mining was applied to the excavation of the 3_upper coal seam. CPB mining had a good control effect for ground surface deformation under the backfilling mining of the working face. As shown by the 3D diagrams of ground surface subsidence under the excavation of the 3_upper coal seam in Fig. 16, the ground surface subsidence was the largest around the corner of the CT3 _{upper (lower)} 01 backfilling working face. This was primarily because the mining area was larger around the corners of the CT3 _{upper (lower)} 01 backfilling working face and the CT3 _{upper (lower)} 02 backfilling working face. As a result, the ground surface subsidence was substantial.

(2) Excavation of the 3_uppercoal seam and 3_lower coal seam

We then studied the influence of CPB mining on overlying strata movement under multi-slice mining of the thick coal seam. Following the mining of the 3upper coal seam, we investigated the influence of subsequent CPB mining of the 3lower coal seam on the cumulative ground surface subsidence. Fig. 17 and 18 show the ground surface subsidence curves and the 3D diagrams of ground surface subsidence under the caving mining and CPB mining of the 3upper and 3lower coal seams.

As shown in Fig. 17, the cumulative ground surface subsidence under the caving mining of the 3_upper and 3_lower coal seams reached 2400 mm. By contrast, the cumulative ground surface subsidence reached 120 mm when the 3_upper coal seam was first excavated by CPB mining, followed by CPB mining of the 3_lower coal seam. The surface subsidence induced by CPB mining was 1/20 that of the cumulative ground surface subsidence caused by caving mining, showing a 20-fold reduction. CPB mining could effectively control surface subsidence caused by multi-slice mining of the thick coal seam, offering protection for the buildings above the ground.

As shown in Fig. 18, caving mining of two-slice coal and CPB mining had a similar influence on ground surface subsidence. The ground surface subsidence was the largest around the corner of the backfilling working face using either method. According to the 3D diagram of ground surface subsidence induced by backfilling mining of the coal mine below buildings, railways, and water bodies, the variation in ground surface subsidence could be described by an “L-shaped” curve. The maximum ground surface subsidence was found at the corners. This was because the mining area was larger around the corner of the working face, resulting in a larger influence scope and more severe ground surface subsidence. Besides, multi-slice mining resulted in greater overlying strata movement than single-slice mining.

We studied the control effect of CPB mining on overlying strata movement under multi-slice mining of the thick coal seam in the Nantun Coal Mine. Based on the actual situation of the coal mine, we combined laboratory experiments with numerical simulation and theoretical analysis to study the control effect of CPB made of coal gangue, fly ash, and cement or of coal gangue and cement on the overlying strata. We arrived at the following conclusions:

(1) The protection of surface buildings against mining-induced damage was first determined for Nantun Coal Mine based on the actual geological conditions. CPB mining was performed to excavate the coal resources under the factory. We also analyzed the technical principles of longwall-roadway CPB mining and the support method.

(2) The mechanical characteristics of CPBs made of coal gangue, fly ash, and cement or of coal gangue and cement were compared. At the curing age of 28 d, the mechanical strength of CPB made of coal gangue and cement increased significantly. The compressive strength had a four-fold increase, sufficient to support the overlying strata and prevent its caving.

(3) A numerical simulation was performed to analyze the variation patterns of overlying strata displacement and surrounding rock stress distribution before and after filling the 3_lower and 3_upper coal seams with CPB. CPB mining reduced the ground surface movement by 95.1% and 95% after excavating the 3_lower and 3_upper coal seams, respectively. Thus, CPB mining had a good control effect for ground surface deformation in Nantun Coal Mine.

(4) The ground surface subsidence caused by CPB mining was 1/20 that of the cumulative surface subsidence caused by caving mining. CPB mining could effectively control the surface subsidence caused by multi-slice mining of the thick coal seam, offering protection for the buildings above the ground.

Acknowledgments:

This research were funded by the National Natural Science Foundation of China [grant number 51725403].

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Editorial decision: Major Revision
20 Sep, 2022
Reviewers agreed at journal
05 Jun, 2022
Reviews received at journal
17 Apr, 2022
Reviewers invited by journal
17 Apr, 2022
Editor assigned by journal
30 Mar, 2022
First submitted to journal
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Control effect of coal mining solid-waste backfill for ground surface movement in slice mining: A case study of the Nantun Coal Mine

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Abstract

Figures

1. Introduction

2. Background

3. Technology And Method

4 Comparative Analysis Of The Simulation Of The Surrounding Rock Migration Laws

5. Conclusions

Declarations

Acknowledgments:

References

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Version 1