In recent years, safety issues caused by the instability of the remaining coal pillars have increasingly received attention [1
]. For a long time, there have been many small coal mines in western China, usually using the room-and-pillar mining method to mine coal resources [3
]. To obtain the maximum economic profit, only better coal seams are mined. This practice has resulted in many goafs and coal pillars in the coalfield, thereby leaving severe safety hazards for large-scale coal mining today. With the integration of coal enterprises and the upgrading of mining technology, the scale of mining is steadily increased. Under the disturbance of mining activities, these pillars are prone to instability and failure, which in turn lead to mine disasters. In recent years, the instability of coal pillars caused by mining disturbance has emerged consecutively, and the treatment of the empty areas of coal pillars has become an unresolved problem [4
Owing to the support of the remaining coal pillars, the roof strata form a stable state [7
]. With the second mining of the coal seams in this area, the coal pillars have entered the stage of deformation and failure expansion and eventually become unstable [8
]. The destruction of a single coal pillar will cause the destruction of the coal pillar group and eventually the continuous destruction of the coal pillars in the goaf, thereby leading to a rock burst accident [10
]. The strength of the coal pillar will decline under the influence of various factors, and this fact must be considered in the calculation [11
]. The relationship between the load of the overlying strata and the stability of the remaining coal pillars must be considered, especially the change of the coal pillar stress with the advancement of the working face [12
]. Another important factor is the sustained coal pillar strength in the goaf under the wet–dry cycle environment of the goaf [14
]. When a single coal pillar is destroyed, the load borne by this coal pillar is transferred to the adjacent one, thereby resulting in the instability of the whole coal pillar group [15
At the same time, many scientists have been trying to prevent and control the remaining coal pillar disasters. It has previously been observed that mining activities have an impact on the maximum vertical stress of the coal pillar, and it is possible to maintain the stability of the coal pillar by calculating the maximum stress [18
]. The coal pillars first fail in a certain area, and their failure gradually spreads. By strengthening the area, the stability of the coal pillars during the mining stage can be guaranteed [19
At the same time, research into the backfilling method has been the focus for innovation. Tesarik [20
] monitored the long-term stability of coal pillars and pointed out that the backfilling method is helpful for maintaining the stability of pillars and limiting their deformation. Some experts conducted research on the physical properties of the backfilling body [21
], analyzed the interaction between coal pillars and the backfilling body, and concluded that the physical and mechanical properties of the backfilling body affect the peak strength of the coal pillars as well as the post-peak intensity [22
]. Mo [24
] analyzed the influence of different filling amounts, filling types, and backfilling body sizes on goaf roof and coal pillars and proposed different filling strategies.
Through various studies, the failure characteristics of the remaining coal pillars were evident, and a relatively complete prevention method was formed. However, these studies are based on the coal pillars that are entirely exposed or the goafs that are full. No previous study has investigated the effect of an unfilled zone. When the height of the unfilled zone is large, the coal pillars may remain unstable.
Through various research studies, the damage- and disaster-causing mechanism of the remaining coal pillars have been determined, and a complete prevention method has been formed. However, these studies were based on the complete backfilling of the goaf and did not fully consider the problem of incomplete backfilling under actual conditions. When the backfill body height is small, there is a greater possibility of destabilizing the remaining coal pillars, leading to safety accidents.
The main objective of this paper is to analyze the effect of the strength and height of the backfill body on the coal pillar strength when reinforced by backfilling methods, moreover, to use this as a basis for discussing solid waste material backfilling methods.
3. Influence Factors of Backfill Method on the Stability of Remaining Coal Pillars
3.1. Numerical Simulation Test Scheme
3.1.1. Numerical Model
PFC2D was used to study coal pillars’ strength and failure characteristics under different conditions. The study coal pillar is in coal seam No. 9 with an overlying rock thickness of 166.20 m. The size of the numerical model was 6 m wide and 9 m high. A total of 12,506 round particles of different scales were built. The radius of the smallest particle was 0.04 mm, and the radius of the largest particle was 0.045 m.
Six walls were set up when building the numerical model (Figure 3
). Walls No. 1 and No. 2 were used to simulate the sinking of the roof and floor, and the loading speed of No. 1 was 0.02 m/s. The FISH language was used to test the model strength forces and calculate the numerical magnitude of the stresses on the coal pillars based on the magnitude of the forces and the real-time width of the model. Walls No. 3 and No. 4 were used to simulate the backfilling bodies on both sides, and servo control was adopted in the subsequent calculation process. Walls No. 5 and No. 6 were in the unfilled zone and deleted after the initial balance. In the uniaxial compression test, the walls on both sides (No. 3, No. 4, No. 5, and No. 6) were deleted to simulate the failure characteristics of the coal pillar under uniaxial compression. When it was necessary to impose confining pressure on the coal pillar, walls No. 5 and No. 6 were deleted, and walls No. 3 and No. 4 were set on both sides of the coal pillar as servo walls.
3.1.2. Parameter Calibration
To obtain suitable numerical model parameters, the strength of the coal sample was tested. The coal sample was derived from the Yuanbaowan coal mine. A large piece of uncracked coal was selected and transported to the laboratory to be processed into a standard specimen. Uniaxial compression tests were conducted on the MTS C64.106 rock mechanics test system (MTS Systems Corporation, Eden Prairie, MN, USA).
shows the uniaxial compressive strength (UCS) and elastic modulus of the coal samples. The UCS ranges from 9.77 MPa to 11.44 MPa, and the elastic modulus ranges from 0.76 GPa to 0.95 GPa. The average values of the UCS and elastic modulus are 10.03 MPa and 20.81 GPa, respectively.
To reflect the mechanical strength of the coal pillars in the Yuanbaowan coal mine accurately, the parameters of the mechanical model were adjusted to be close to the strength of the mechanics test (Figure 4
). The contact model adopted a linear parallel bond model. The UCS of the numerical model was 9.79 MPa, and the elastic modulus was 0.82 GPa. The trial-and-error method was used to adjust the numerical simulation parameters so that the numerical simulation parameters were similar to the rock test parameters. The micromechanical parameters of the coal are listed in Table 3
3.1.3. Experimental Scheme
The simulations were analyzed in terms of both backfill body height and backfill binding force. The backfill height of 1–9 m was simulated at 1 m intervals for different backfill conditions. Considering the depth of the coal seam is about 160 m when the horizontal and vertical stress is 1:1, the maximum horizontal stress of the coal seam is about 3.8 MPa (rock capacity is 2400 KN/m3
). The surrounding pressure was also simulated at 1 MPa intervals for four scenarios ranging from 0 MPa (no filling) to 3 MPa. The test scheme is shown in Table 4
, with a total of 28 sets of numerical simulations.
3.2. Influence of the Unfilled Zone Height on the Strength of Coal Pillar
In order to analyze the peak coal pillar strength under different scenarios, the maximum strength of the coal pillar is monitored in each calculation. The variation pattern of peak coal pillar strength under different conditions is shown in Figure 5
According to the results of Figure 5
, as the backfill body height increases, the strength of the coal pillar increases gradually. When the backfill body height is less than 5 m, the coal column strength is close to the strength in UCS. When the backfill body height is 7 m and 8 m, the coal pillar strength increases to more than 12 MPa. When the backfill body height is 9 m, the coal column strength is the largest currently.
At the same time, the strength of the coal pillars increases as the confining pressure increases. When the backfill body height is less than 5 m, the strength of the coal pillars under different confining pressures is the same. When the backfill body height is 6 m, the strength of the coal pillars begins to differentiate. Under a confining pressure of 3 MPa, the strength of the coal pillar is 11.81 MPa. Under a confining pressure of 1 MPa, the strength of the coal pillar is 11.28 MPa. The strength appears to increase when the backfill body height is 7 m and 8 m. Under the confining pressure of 3 MPa, the strength of the coal pillar is 14.44 MPa and 15.38 MPa, respectively. Under the confining pressure of 1 MPa, the coal pillar strengths are 12.28 MPa and 12.47 MPa.
This result implies that the strength of the coal pillar is affected by both the confining pressure and backfill body height. When the backfill body height is large, the strength of the coal pillar is affected by the confining pressure. When the backfill body height is larger than 7 m, the strength of the coal pillar changes most obviously.
3.3. Correlation Analysis of Factors Influencing the Stability of Remaining Coal Pillars
To investigate the effect of the backfill body height and the confining pressure on coal pillar strength, we used correlation analysis to the strength and direction of the statistical correlation between backfill body height and confining pressure on coal pillar strength. The correlation of variables uses the Pearson correlation coefficient and Spearman’s rank correlation coefficient. The values range from −1 to +1, with 0 indicating no correlation between the two variables, positive values indicating a positive correlation, and negative values indicating a negative correlation, with larger values indicating a stronger correlation.
Spearman’s rank correlation coefficient is suitable for the detection of variables with monotonic relationships. The Pearson correlation coefficient is suitable for the detection of normally distributed variables. The normal distribution test was performed on the backfill body height, the confining pressure, and the coal pillar strength (Table 5
). According to the calculated results, the asymptotic significance was 0.200, 0.004, and 0.002, respectively. The significant coefficients of the confining pressure and coal pillar strength were less than 0.05 and did not obey the normal distribution. Therefore, we used Spearman’s rank correlation coefficient.
As shown in Table 6
, the correlation coefficient between backfill body height and coal pillar strength is 0.806, which is a strong correlation. The correlation coefficient between the confining pressure and the coal pillar strength is 0.161, which is a poor correlation. This result indicates that the backfill body height has the most significant influence on the stability of the coal pillar. Controlling the backfill body height could improve the strength of the coal pillar and reduce the risk of coal pillar instability.
5. Field Application Results
After backfilling, the borehole TV was suspended along the grouting borehole into the extraction zone to observe the backfilling condition (Figure 12
). The depth below the drill TV is recorded using the grout hole opening as the starting point for observation (0 m). When the borehole TV is lowered to the bottom of the hole, record the depth of the borehole TV. The depth at the bottom of the hole is used as the elevation of the top interface of the goaf. As there is a small amount of water in the goaf, the water surface is used as the top interface of the backfilling. When there is an unfilled zone, the backfill body height can be calculated from these data.
When the goaf is filled with backfill material, the borehole television cannot be lowered. Therefore, when the water surface was observed, the goaf was considered to be filled. Due to some of the boreholes being misaligned or collapsed, only four boreholes were probed (No. 1, No. 5, No. 6, and No. 13), and Table 9
shows peephole results.
Based on this result, coal mining can be carried out generally in two cases, when the goaf is filled, or the unfilled zone height is less than 2 m. The floor collapses when the unfilled zone height is larger (No. 13), which seriously affects the safety of coal mining activities.