Surface Subsidence Control during Deep Backfill Coal Mining: A Case Study

Deep resource exploitation is imperative, but it is facing with more complicated mining environment andmore dangerous mining disturbances to induce the potential catastrophe process. Solid backfill technology, which can control the strata movement and prevent potential hazards, has been used as the primary method in deep mining for surface subsidence control and ecosystem protection. In this study, taking backfill mining area no. 930 in the Tangkou coal mine as background, the probability integral model was adopted to predict the surface subsidence at different mining depths and filling ratios..e filling ratio was designed for deep mining based on the regression analysis of the predicted surface subsidence results..e study shows that the backfilling ratio at the Tangkou deep coal mining area should be controlled at a level greater than 82.5%, and the mining damage to the surface under this condition was analyzed. Furthermore, control strategies for deep backfill mining are proposed in which the backfill density can be enhanced by optimizing the tamping machine, material composition, and tamping process. Finally, the measurement of the backfill mass and surface subsidence showed that the actual filling ratio was controlled at 82.57%, which ensures adequate protection of the surface buildings during the mining process.


Introduction
With the continuous high-intensity extraction from shallow coal mines and gradual exhaustion of coal resources in eastern China, exploitation of coal resources drives deeper underground at a rate of 10-25 m/a [1]; thus, deep mining is gradually becoming a common practice in the coal industry and in natural resource extraction [2,3]. e increasing depth of resource extraction, in particular, for coal mining at a depth of 1000-2000 m may potentially lead to more severe mining hazards including higher frequency and stronger intensity of rock burst, more dangerous coal and gas outburst, increasing mining pressure, higher risks of water invasion, and increasing difficulty in predicting surface subsidence. ese risks pose significant challenges to deep coal mining and mining hazards prevention practices [4][5][6].
Solid backfill mining is a particularly promising technology used in coal mining for controlling stratum movement, preventing potential disaster, treating solid waste (coal gangue), controlling surface subsidence, protecting ecosystems, and improving coal extraction rate [7,8]. As one of the core techniques used for eco-friendly coal mining, backfilling the vacant space after mine extraction with solid waste such as coal gangue has been used as a primary technique for resolving issues in deep underground natural resource extraction. However, the deep mining occurs in a very different environment from shallow mining as the high ground stress induces different stratum movement mechanisms during the deep backfill mining. In addition, the control of surface subsidence in deep backfill mining differs from that in shallow backfill mining considerably. Most previous studies have examined the surface subsidence in deep mining. Fan et al. [9][10][11] proposed an approach combining differential interferometric synthetic aperture radar (D-InSAR) results and probability integral model (PIM) results, to generate the entire mining subsidence basin. Jiřina andŠperl [12] studied the possibility of filling empty underground spaces, which exist due to the extraction of mineral raw materials, with fly ash and cement fly ash mixes for the purpose of reducing the impact of deep mining on the surface. Chang et al. [13] analyzed the surface dynamic subsidence characteristics under deep mining subsidence and the maximum subsidence velocity based on the monitoring data of surface subsidence in 230 mining areas of the Tangkou coal mine. Chen et al. [14] introduced the theories and application principles of five types of integrated surface subsidence-reducing technologies. Guo et al. [15] and Zhang [16] proposed a probability integral model for surface subsidence prediction based on the equivalent mining height (EMH) theory and described the basic control principle for surface subsidence. Most of these studies only targeted deep mining or backfill mining separately. To date, exploration of surface subsidence control in deep backfill mining is still lacking.
Taking backfill mining area no. 930 in the Tangkou coal mine as background, the probability integral model was adopted to analyze the predicted surface subsidence at different mining depths and filling ratios. We established an engineering design method of the filling ratio for deep mining based on the regression analysis of the predicted surface subsidence results. Also, based on the security standard for surface buildings, we propose control methods of the backfill body's density for mining area no. 930 in the Tangkou coal mine. e actual backfill density and surface subsidence were measured to verify the accuracy of the result. e findings from this study are of great significance for realizing safe, efficient, and eco-friendly coal extraction from deep underground areas.

Mining Geological Condition.
e Tangkou coal mine, belonging to the Shandong Energy Zibo Mining Group Co., Ltd., is located in Jining City, Shandong Province, China, and the longwall mining method is mainly used in this mine. However, with the increase of production life, the mining depth has extended to 1098-1220 m. Currently, the mining area is the no. 930 area, which is located at about 670 m south of the industrial square, north of the Beijing-Hangzhou grand canal, east to Xingfugou, and the western boundary is located 96 m west of the Limiao village. e upper part of the mining area is almost covered by the Limiao village and Fengtai village, and the ruins of the Phoenix Palace are located in the middle of the mining area. e mining area covers around 0.85 km 2 area with a total coal reserve of 4.067 million tons. e immediate roof and the immediate floor of the coal seam are mudstone, with an average thickness of 3.77 m and 1.50 m, respectively. e compressive strength is about 30 MPa, which is unstable. In order to protect the landmark buildings from being damaged, coal mining technique with gangue filling is adopted to mine coal resources. A total of 9 working faces are arranged, with the surface length of 60 m∼80 m, as shown in Figure 1.

Overview of Surface Buildings.
Five villages (Fengtai, Sunjia, Limiao, Wujia, and Huangjingcun) and the Tangkou coal industrial plaza are located within the zone of influence of mining activity at mining area 930. ese villages, covering an area of 2.241 × 10 5 m 2 , have a total of 1545 residents, a population of 5300, and a total of 2248 buildings. In addition, Fenghuangtai Ruin, a provincial key cultural relic protected unit, is located at the Fengtai village. e Tangkou coal industrial plaza, covering a total area of 1.62 × 10 4 m 2 , holds a miners' canteen, dormitory buildings, and an electrical substation. Most of the buildings within the zone of mining influence are either constructed with a masonry structure or multifloor buildings with a reinforced concrete frame structure. Summary of all the buildings and its satellite images are shown in Table 1, respectively.

The Characteristics of Surface Deformation
Induced by Deep Backfill Mining e main difference in surface subsidence between deep backfill mining and shallow backfill mining is illustrated in the following aspects. First, the filling material deformation and mining pressure in deep backfill mining are much larger due to the presence of high ground stress. With increasing mining depth, the maximum surface deformation is reduced, but the zone of influence keeps increasing in diameter and is accompanied by an increment in the total influencing area from the surface subsidence [17]. In addition, the critical mining area required for reaching full subsidence at the surface also increases. Meanwhile, the overlying strata and the upper, lower, and isolating coal columns in the mining area are found to have a more significant impact on controlling the surface subsidence.

Methodology.
e probability integral model (PIM) is a prediction theory of mining subsidence that has been widely used in China [18,19]. e PIM was still adopted in the prediction of backfill mining subsidence. According to the principle of the probability integral method, the subsidence of any surface point (x, y) caused by backfill mining can be expressed as [20][21][22] w(x, y) � M e qB where q is the subsidence coefficient; r is the main influence radius, r � (H 0 /tan β); H 0 is the average mining depth; H i is the depth of mining unit i; θ is the mining influence angle; tan β is the tangent of the main influence angle; (x i , y i ) is the planar coordinate of mining unit i; (x, y) is the coordinate of any surface point; and M e is the equivalent mining height of solid backfill mining. e horizontal displacement, inclination, curvature, and horizontal deformation can be obtained through the integration calculation.
In order to use this model to obtain the full subsidence basin, it is necessary to confirm the prediction parameters M e , q, tan β, θ, s, and b. Here, b is the parameter of horizontal movement, and it was usually assumed to be 0.35 by trial and error; s is offset of inflection points. However, the prediction parameters associated with the traditional caving method cannot be adopted directly as parameters in the prediction model for surface subsidence in backfill coal mining. For solid backfill mining, the subsidence is caused by mining coal with an equivalent height that is the mining height of filling working face minus the height of filling material after compaction [17,23], and the equation for calculating the equivalent mining height is shown in the following formula: where h z is the unfilled height in goaf; M is the mining height, 3.31 m; k is the initial porosity of the filling material measured to be 1.15 from the experiments; and k ′ is the residual porosity after compaction measured to be 1.05 from the experiments. e subsidence coefficient of solid backfill mining based on the equivalent mining height can be expressed as where q c is the subsidence coefficient of caving coal mining, which is the ratio of the maximum subsidence with mining height under fully extraction; H 1 , H 2 , and H 3 are heights of collapse, fracture, and sagging zones, respectively; H 2 ′ and H 3 ′ are the heights of the fracture zone and sagging zone by solid backfill mining; and k 1 , k 2 , and k 3 are bulking factors of collapse, fracture, and sagging zones correspondingly. tan β, θ, s, and b share similar values with those used in the fully caving coal mining method. e measurements of surface movement at extraction areas 130, 230, and 430 in the Tangkou coal mine revealed a gradual and continuous movement of the earth's surface induced by the fully caving coal mining operation at the working area in the third coal seam layer.
e measurement results were also found to correspond with the distribution calculated from the   Advances in Civil Engineering 3 probability integral method. e prediction parameters used in the probability integral method were obtained by the inverse genetic algorithm [24] as shown in Table 2.
In summary, based on the inverse calculation of the parameters for surface movement induced by fully caving coal mining, the prediction parameters for surface subsidence at backfill mining area no. 930 in the Tangkou coal mine can be determined as q � 0.82, b � 0.35, tanβ � 2.15, s � 0.036H, and θ � 90°− 0.46 α (α is the inclination angle of the coal seam).

Surface Subsidence under Different Backfill Mining
Conditions.
e ground surface deformation pattern was explored under different backfill mining depth conditions. Specifically, the authors analyzed the surface deformation characteristics with a depth of 300 m, 500 m, 700 m, 900 m, and 1100 m. For all cases, the backfill body's filling ratio was fixed at 80%, which is equivalent to a mining thickness of 0.93 m. Table 3 and Figure 2 summarize the prediction for surface subsidence under different conditions. (Number is measuring points which were arranged twenty meters apart passing through the middle of the work surface.) With a fixed filling ratio, a similar trend was observed between solid backfill mining and fully caving mining where the maximum ground surface subsidence decreases logarithmically with increasing mining depth, y � −337.2 ln(H) + 2780. In addition, the horizontal movement decreases exponentially with increasing mining depth with a relatively small change compared to the surface subsidence. Furthermore, a greater mining depth also resulted in a reduced level of mining completeness. is effect, acting directly on the overlying stratum, is attenuated after each layer which eventually leads to a much smaller impact on the ground surface. e zone of influence increases exponentially with increasing mining depth. is is because the effect of mining on the overlying stratum is amplified after each layer which ultimately resulted in a much stronger impact on the ground surface.

Surface Subsidence Prediction with Different Filling Ratios.
e ground surface deformation pattern was explored with different filling ratios. e backfilling depth was fixed at 1100 m, and the filling ratio was varied between 0%, 60%, 70%, 80%, and 90%, which corresponded to an equivalent mining height of 3.13 m, 1.52 m, 1.23 m, 0.93 m, and 0.63 m, respectively. Table 4 and Figure 3 summarize the prediction values of the ground surface subsidence.
As shown in the result, the ground surface subsidence and horizontal movement decrease exponentially with increasing filling ratio, and the fitting formulas are y � 2662.8e −2.209 φ and y � 1811.1e −2.931 φ, respectively. Compared with fully caving mining methods, the solid backfill mining method yields a much smaller maximum moving and deforming magnitude. erefore, the solid backfill mining induces minor impact on ground surface which effectively secures the safety of surface buildings.

Design of Filling Ratio in Deep Mining at Tangkou Coal
Mine. Using SPSS fitting, the authors obtained the regression equation (see equation (4)) with surface subsidence as a dependent variable and mining depth and filling ratio as independent variables. e change in surface subsidence as a function of mining depth and filling ratio is shown in Figure 4: where φ is the filling rate. According to the regulation in "Coal Pillar and Coal Mining under Building, Water Body, Railway, and Main Roadway Standards" [25], the ground surface building at the Tangkou coal mine belongs to Class I protection building where the maximum allowable surface deformation values are 400 mm subsidence, 2.5 mm/m inclination, 1.5 mm/m horizontal deformation, and 20/km curvature. As shown in Tables 3 and 4, the inclination, horizontal deformation, and curvature fully satisfied the requirement when the filling ratio is greater than 70%. If the surface subsidence was set as the indicator, a backfilling depth of 1098∼1220 m requires a filling ratio of 82.5%∼79.7% to ensure safety. erefore, the filling ratio at backfill mining area 930 in the Tangkou coal mine should be designed with a value greater than 82.5%. Table 5 shows the surface subsidence value predicted using a filling ratio of 82.5%. Figure 5 shows an isocontour map of surface subsidence. Based on the damage rating for masonry buildings, the building damage associated with the maximum surface subsidence of 394 mm falls in the minor level I damage range. erefore, the maximum surface subsidence predicted in the study was controlled in idea scope below the critical deformation value required for inducing permanent damage to the masonry buildings. Hence, backfill mining will not cause any significant risk to the safety and normal operation of surface buildings.

Method of Guaranteeing Filling Ratio.
In this paper, the filling rate was guaranteed by controlling the gradation of filling materials, optimizing the tamping angle and times of the tamping institution, and improving the filling process [16,26]. e specific implementation methods are as follows.

Optimizing Filling Material Composition.
e compaction characteristics of coal gangue used for backfilling at mining area no. 930 at the Tangkou coal mine were analyzed using an electro-hydraulic servo universal testing machine. e compaction-deformation curves of coal gangue with six different maximum diameters (0-10 mm, 0-20 mm, 0-30 mm, 0-40 mm, 0-50 mm, and 0-80 mm) were obtained and are shown in Figure 6. As shown in the result, the compaction deformation increases with increasing coal gangue diameter. A higher rising rate in the deformation was observed when the size of coal gangue increased from 0-50 mm to 0-80 mm. e optimum coal gangue size range was determined to be 0-50 mm, and the natural testing angle was measured to be 37.5° [27] (Figure 7).

Optimizing Tamping
Machine. e hinge height of the tamping machine was increased in the optimization. e new height of the hinge located at the base of the machine was designed to be 900 mm for ensuring adequate storage space. e maximum swing angle of the tamping machine was designed to be 37°, close to the natural resting angle of the filling material. ese design improvements resolve the following three issues: (1) small height of the hinge point connecting tamping machine and the base; (2) lack of storage space in the bottom area; (3) incapability of compressing the coal gangue. By adopting a powerless two-stage tamping structure, the maximum and minimum length of        the tamping machine can now reach 3971 mm and 1948 mm, respectively. is design addresses the issue of filling material shortage at the front end of the tamping board when the tamping machine is too long. Figure 8 shows the optimized tamping machine.

Optimizing Tamping
Process. e backfilling process follows directly after the mining process. A multihole bottom-unloading conveyor is placed at the rear part of the rear top beam on the supporting stand. When one unloading hole reaches a predetermined height, the second unloading  Advances in Civil Engineering hole is opened immediately, followed by tamping on the filling material unloaded from the first unloading hole. is process is repeated until all the filling materials are tamped properly. After the first backfilling cycle is finished, a new multihole bottom-unloading conveyor is then transported to the rear part of the rear top beam on the supporting stand followed by the second cycle of refilling. e tamping process using a single supporting stand proceeds, repeating material unloading and tamping cycles in which the filling materials are immediate tamped after unloading.

Surface Subsidence Prevention Design Process.
e process of designing an operation for deep backfill coal mining under buildings is established as shown in Figure 9. First, according to the subsidence monitoring data and deformation resistance of the buildings, the initial filling ratio is determined on the basis of the surface movement and deformation. Second, based on the prediction model of PIM and inverted predicted parameters, comparing the predicted value with the fortification indicator, thus the permissible filling ratio is determined. Finally, the engineering parameters are confirmed based on the grade of mining damage, and measurements of mine pressure and surface subsidence provide feedback information, thereby ensuring the safety of surface buildings through the adjustment of the filling ratio.

Measurement of Refilling Mass and Surface
Subsidence at Tangkou Coal Mine

Monitor Refilling
Mass. e GPD30 filling body stress monitor (Figure 10(a)) and GUD500 roof dynamic monitors (( Figure 10(b)) were installed to monitor the refilling mass, and the measurement results are shown in Figure 11. e roof deformation is consistent with the variation trend of the pressure bear by the backfill. As the load of the backfill increases, the deformation of the roof increases. e changing process can be divided into three stages: a rapid growth zone, a slow growth zone, and a smooth zone, which reflected the process of backfill gradually compressed under the roof pressure. e results indicated that the impact of mining was gradually weakened as the backfill was compacted, and the movement of roof tended to be stable. As the refilling proceeds to 250 m and the filling ratio controlled above 82.57%, the movement of the roof in the rear of the goaf tends to be stable, effectively ensuring the safe operation of the stope. village and Wujia village, 35 measuring points were installed facing measuring lines T1-T35. Total station lead measurement was taken as the standard mode [26]. e measurement results are shown in Figure 12. Surface buildings were still well preserved until the extraction of working areas no. 9301 and no. 9302. It showed a relatively high surface subsidence at measuring points Q30-Q60 and Z30-Z50, located above the mined work surface, with a maximum surface subsidence of 16 mm. At the locations between measuring points J17 and J20, the surface subsidence is less than 10 mm, which indicates negligible impact on the buildings at Fenghuangtai Ruin.

Conclusion
(1) In this study, based on the theory of the probability integral method, the surface deformation characteristics were analyzed through theoretical calculation and numerical simulation with different depths and filling ratios. e maximum surface movement decreases logarithmically with increasing mining depth and decreases exponentially with increasing filling ratio. Regression analysis shows the relationship between surface subsidence amount and depth and filling rate: w � 3.61e − 0.47φ − 0.31 ln(H) + 0.12.
(2) Based on the deformation resistance of the structures, the analysis suggests that the filling ratio should be maintained at no less than 82.5% for mining activity at mining area no. 930 in the Tangkou coal mine. Further damage analysis on the surface buildings confirmed that the control threshold of filling ratio designed in this study can effectively mitigate damage to surface buildings. (3) An adequate backfill material density was ensured by optimizing the filling material composition, optimizing the tamping machine, controlling the presubsidence of the roof top, controlling the number of tamping processes, and controlling the height of filling material. e monitor result at mining area no. 930 in the Tangkou coal mine showed that the final filling ratio was controlled at 82.57%, which exceeds the designed value of the refill body's filling ratio. During the backfilling process, no fracturing phenomenon or significant deformation of surface buildings was observed. ese findings proved that the proposed strategy can effectively protect surface buildings such as Fenghuangtai Ruin during mining activity and ultimately realize safe and eco-friendly extraction of deep coal resources.

Data Availability
e data used to support the findings of this study are included within the article.

Disclosure
Wenyue Qi should be regarded as the co-first author.
Advances in Civil Engineering 11