Analysis of the Control Effect and Inuencing Factors of Urban Surface Deformation in Underground Coal Mining With Solid Backlling

To solve the problems of surface deformation and destruction of buildings caused by urban mining and realise coordinated development of mining cities, the solid backlling method was used to extract coal resources beneath the buildings of Tangshan. Based on surface deformation monitoring data of the continuously operating reference station (CORS) system for the past 5 years, the surface deformation process caused by solid backlling was analysed. The nal results revealed a maximum surface subsidence of 66 mm in the T zone coal area and 31 mm in the F zone area. Furthermore, the surface control effects of the caving method and the solid backlling method were compared and analysed, and it was shown that solid backlling could meet the surface building set-up requirements. Moreover, based on the probability integral method, the effects on surface deformation due to the surface length of the F zone, compression ratio, and coal pillar width were analysed, and the effects on the prediction results due to the subsidence factor, tangent of the major effective angle, and offset distance of the inection point were studied. The results showed that the compression ratio is the main factor controlling the surface deformation and that it should be kept above 80% for solid backlling of urban mines. The subsidence factor should be 0.82 and the tangent of the major effective angle should be 2.15 when the surface subsidence of solid backlling is to be predicted. This paper provides a technical reference for realisation of urban mining with solid backlling.


Introduction
In recent years, the rapid development of China's economy has increased the demand for coal resources. In 2019 alone, China's coal production reached 3.85 billion tons, a year-on-year increase of 4% [1,2] . Such large-scale coal resource development has brought exhaustion of conventional coal resources, and some mines have had to explore coal resources beneath buildings, railways, and water bodies, which directly affects and restricts the harmonious development of ecology and society around the mining area. China's production of coal resources from beneath buildings, railways, and water bodies under uni ed allocation alone has reached 140 billion tons, with the amount of coal resources under buildings accounting for 70% of the total amount [3,4] . When such coal resources are mined, the surface deformation causes buildings to be stretched, compressed, and bent, thus causing various degrees of damage and collapse hazards to the buildings. Mining is also accompanied by the discharge of a large amount of solid waste gangue, which pollutes the environment and encroaches on the land. All of these problems cause serious con icts in the development of a mining city.
The coal industry has been actively exploring ways to coordinate the development of coal mining and mining cities and has done considerable research and practical work. Solid back ll mining [5~7] , which uses back ll to control rock movement and surface subsidence, has become the main technology to solve the con icts between the development of the mine and the city. This technology has existed for some time and has been applied in more than 20 mines in China, effectively solving the problem of redundant resource extraction. However, long-term practice has shown that even the solid back ll mining method can cause large-scale movement and deformation of the ground surface. Current research on this problem has focused mainly on how the ll body controls rock movement, monitoring of surface movement and deformation, and analysis of disasters caused by surface deformation.
Many scholars have researched the aforementioned problems. Miao Xiexing [8~10] , Zhang Jixiong [11][12][13] , and Zhang Qiang [14,15] have advanced the equivalent mining height theory, key layer control theory, main roof control theory, and immediate roof control theory. These studies concluded that solid back ll mining can be regarded as extremely thin coal seam mining, allowing the surface subsidence to be controlled. The control targets for solid back lling in the rock are the key layer, the main roof, and the immediate roof, with the rst being the most di cult target and the last being the most precise target. Guo Guangli [16][17][18] , Cha Jianfeng [19,20] , Yao Qiangling [21,22] proposed the use of the probability integral method to describe patterns of surface deformation, established a prediction model parameter system for surface subsidence with solid back lling, and actually measured surface subsidence results using an electronic total station. Jiang Feifei [23] and Peng Fuhua [24] further analysed surface deformation characteristics using the analytic hierarchy process evaluation model and evaluation system.
The methods of surface deformation monitoring used in the aforementioned studies are not continuous and cannot show the surface deformation process caused by solid back ll mining because only the control effect of solid back lling on the overlying rock layer was investigated. In this paper, taking the Tangshan mine as an example, 5 years of continuous surface subsidence data monitored by the continuously operating reference station (CORS) system were used to study the surface subsidence control effect of solid back ll mining. The effects on surface deformation due to surface length, compression ratio, and working surface arrangement were analysed. The effects of subsidence factor, tangent of major effective angle, and offset distance of the in ection point on the prediction results were also studied.
Finally, the ranges of the compression ratio control index and the surface subsidence prediction parameters for solid back ll mining in urban mines were obtained.
2 Test Area

Mining geological conditions
Tangshan Mine, with a eld area of 37.28 km 2 and a mining area of 55 km 2 , is located in Lu'nan District, Tangshan City, Hebei Province. It is the only state-owned mega coal mine in China located in a city centre with convenient transportation access. Tangshan Mine uses the progressive mining area development method of inclined shafts. At present, there are 7 vertical shafts, which divide the mine into 10 production areas. There are no large-scale faults in the Tangshan mine eld, and the main coal seams are the 5th, 8th, 9 th , and 12th seams, with average thicknesses of 2.4, 3.7, 3.5, and 6.4 m, respectively. Among these, the 5th coal seam has an immediate roof consisting of 4.8 m thick ne sandstone and a bottom plate of 0.9 m thick mudstone, whereas the 9th coal seam has an immediate roof consisting of 0.7 m thick mudstone and a bottom plate of 1.0 m thick sandy mudstone. The geographical location of the Tangshan Mine is shown in Figure 1.

Mine and surface buildings
As shown in Figure 2, the production area of Tangshan Mine is located beneath the city. The main coal areas are the T zone and the F zone. The T zone has an area of 2.01 km 2 Figure 2 shows a comparison between the surface and subsurface structures.

Forti cation and control indicators
Inspection results of the surface buildings show that the main structures of the surface buildings above Tangshan mine are brick-concrete, brick-wood, and steel structures, which meet the national requirements for Class II-III protection. When mining the coal resources beneath the city, the buildings must be kept below the Class I damage level after mining, i.e., horizontal deformation less than 2.0 mm/m, curvature less than 0.2 mm/m 2 , and inclination less than 3.0 mm/m.

Solid back lling control method
The solid back ll mining method, which is carried out through integrated mechanised mining operations on working faces, is used for mining under urban buildings. In the solid back ll coal mining system, the gangue is transported from the surface feeding well and underground separation chamber to the working faces, is then lled into the goaf using key equipment such as the multi-hole bottom dump conveyor and hydraulic support system for solid back lling, and is nally compacted by the ramming mechanism behind the hydraulic support system. From 2015 to 2019, the surface feeding well accumulated 604,600 tons of gangue, and the underground separation system accumulated 421,100 tons of gangue. The back lling system, shown in basic concept in Figure 3, is running well.  3 Surface Deformation Monitoring Methods

Main existing monitoring methods
Monitoring the subsidence and deformation of the surface, buildings, and other structures is an important task in the process of mining beneath a city. The conventional monitoring methods used for the Tangshan Mine include mainly theodolite tracking, total station monitoring, and water level monitoring. The main problems faced in the monitoring process are addressed below. The accuracy of the conventional monitoring methods is not very high. Moreover, the detection process is easily affected by rain, snow, and the mutual obstruction of the surface buildings. In addition, the conventional monitoring systems are not capable of continuous monitoring.

Tangshan Mine monitoring method
The CORS system for intelligent monitoring of surface subsidence can carry out continuous and uninterrupted observation and achieve real-time collection, transmission, calculation, and analysis of data within the subsidence range. The CORS system applied in the Tangshan Mine is equipped with China's Beidou Satellite Navigation System, which is compatible with the United States' GPS system and Russia's GLONASS system, to improve the accuracy of surface subsidence deformation measurement. The CORS system basically consists of a data centre, a reference station, a data communication subsystem, and a user application subsystem. It achieves all-weather, fully automatic, and precise positioning through satellite positioning technology, the reference station, and the Internet. The CORS base station and Class I and II measuring points of the Tangshan Mine are shown in Figure 5.

Station layout
The T 3 292 working face has two surface observation lines, i.e., line A and line L (

Analysis Of Monitoring Results
The test used the surface subsidence monitoring data obtained from 1 January 2015 to 1 December 2019 as a reference to analyse the surface subsidence after mining on the T 3 292 working face, F5001 working face, and F5002 working face. The monitoring data span more than 5 years and so can re ect the long-term surface deformation after solid back ll mining. As shown in Figure 8 As shown in Figure 9 As shown in Figure 10 A comparison of surface deformation parameters between the solid back ll mining method and the caving method is shown in Table 1.
In addition, the relationship between surface subsidence values and monitoring time was analysed for T 3 292, F5001, and F5002. The monitoring data were acquired at the time points of the goaf square period, end of mining, 3 months after mining, 1 year after mining, and 2 years after mining. The analysis results showed that the surface subsidence of back ll mining was signi cantly affected by monitoring time. Although the surface did not show large subsidence during the mining, surface subsidence increased signi cantly from 3 months to 1 year after mining. The time of 2 years after back ll mining can be regarded as the time when mining was completed, and the surface subsidence above the working face did not increase further at this time.

Results And Discussion
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Analysis of factors in uencing surface subsidence
Taking the F zone as an example, under the mining geological conditions of the Tangshan Mine, it is considered that the surface subsidence above the F zone was in uenced by the face length, compression ratio of the working face, and width of the coal pillar in the section. To analyse the causes of surface subsidence, 15 sets of test plans were designed, as shown in Table 2. Tests 1-5 analysed the effect of face length on surface deformation parameters, tests 6-10 analysed the effect of compression ratio on surface deformation parameters, and tests 11-15 analysed the effect of coal pillar width on surface deformation parameters.
The surface deformation prediction uses the probability integral method based on equivalent mining height. Due to the presence of a large number of primary ssures, joints, and strata in the subsiding rock, it is feasible to predict surface movement and deformation by the probability integral method of random medium. Its main prediction parameters are shown in Table 3.
Predictions were made using an analysis software for coal mining subsidence prediction, and the results are shown in Table 4.
The surface subsidence, horizontal deformation, curvature, and tilt deformation under different in uencing factors were analysed and compared. The test results obtained are shown in Figure 11.
As shown by the analysis in Figure 11, the subsidence, horizontal deformation, curvature, and tilt deformation increase gradually as the face length increases gradually. The subsidence, horizontal deformation, curvature, and tilt deformation decrease gradually as the compression ratio increases gradually, and the subsidence, horizontal deformation, curvature, and tilt deformation do not change much with increase in coal pillar width.
In particular, when the compression ratio is 0.3, i.e., the caving method is used to deal with the hollow area, the surface deformation exceeds the set damage level; thus, the compression ratio is the main in uencing factor of surface deformation. From the predictive analysis of surface deformation, it is apparent that the compression ratio should be 0.6 or more to ensure the safety of surface buildings. In consideration of the in uence due to early subsidence of the roof of the mining area, the compression ratio should be at least 0.8 in an actual back lling operation.

Factors in uencing the prediction parameters on the surface
In comparison with the caving method, when the probability integral method is used to predict surface subsidence for solid back ll mining, prediction parameters such as the subsidence factor, tangent of major effective angle, and offset distance of the in ection point change greatly. Therefore, these prediction parameters are considered to have a relatively large in uence on the prediction results. Using a compression ratio of 0.8, 15 sets of test plans were designed to analyse the prediction results of surface deformation, and the plans are shown in Table 5. Tests 1-5 analysed the in uence of the subsidence factor on the projected results, tests 6-10 analysed the in uence of the tangent of the major effective angle on the prediction results, and tests 11-15 analysed the in uence of the offset distance of the in ection point on the surface deformation parameters.
Predictions were made using an analysis software for coal mining subsidence prediction, and the results are shown in Table 6.
The surface subsidence, horizontal deformation, curvature, and tilt deformation under different in uencing factors were analysed and compared. The test results obtained are shown in Figure 12.
From the analysis shown in Figure 12, as the subsidence factor increases gradually, the subsidence increases gradually while the horizontal deformation, curvature, and tilt deformation tend to stabilize. As the tangent of the major effective angle increases, the subsidence, horizontal deformation, curvature, and tilt deformation increase gradually. As the offset distance of the in ection point increases, the subsidence, horizontal deformation, curvature, and tilt deformation do not change.
It can be seen that the subsidence factor is the main in uencing factor of the surface subsidence prediction results and that the tangent of the major effective angle is the main factor affecting the prediction results of surface horizontal deformation and tilt.

Calibration of predicted surface parameters for the Tangshan Mine
According to the surface subsidence monitoring results of the Tangshan Mine of previous years, the subsidence factor of the Tangshan Mine with the caving method was determined to be between 0.55 and 0.85, the tangent of the major effective angle was between 1.92 and 2.40, and the offset distance of the in ection point was 0. Through parameter inversion of the surface subsidence in the F zone, the subsidence factor of back ll mining was determined to be 0.028 and the tangent of the major effective angle was 1.20, as shown in Table 7.
As shown in Table 7, the surface subsidence prediction parameters currently obtained from monitoring one to two working faces are not accurate, as the subsidence factor is extremely low. The prediction with these parameters is thus not representative of the surface subsidence after mining in the area. Therefore, by comparing the predicted results of surface subsidence, considering multiple factors in the F zone, the subsidence factor appropriate for the predicted subsidence with solid back ll mining of underground coal in the Tangshan Mine was determined to be 0.82, and the tangent of the major effective angle is 2.15. At this time, the predicted result of surface subsidence is 152 mm, which can be regarded as the nal amount of surface subsidence after the end of mining in the F zone.

Conclusion
(1) The CORS system was used to monitor the surface subsidence of coal mining under Tangshan city, which spanned more than 5 years and covered the T zone and the F zone, corresponding to a mining area of 150,000 m 2 . The monitoring results showed that the surface protection requirements of urban mining were met by solid back ll mining, which had gentler surface deformation than caving mining. The maximum subsidence in the T zone after 32 months of working face mining was 66 mm, and the maximum subsidence in the F zone after 6 months of working face mining was 31 mm.
(2) Through the multi-factor surface subsidence prediction based on the probability integral method, it was concluded that surface subsidence is strongly in uenced by the working face length and the compression ratio. The surface deformation increases gradually as the face length increases and the compression ratio decreases. To ensure the safety of surface buildings, the compression ratio must be at least 0.8.
(3) By comparing the actual measurement data in the eld with the quality simulation data, the subsidence factor suitable for the Tangshan Mine was nally determined to be 0.82 and the tangent of major effective angle was determined to be 2. 15   concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Tangshan mine surface-subsurface comparison.  Layout of working faces.   Surface subsidence values.

Figure 9
Surface subsidence values.

Figure 10
Surface subsidence values.

Figure 11
Experimental results.

Figure 12
Predicted results