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 T3292 working face, F5001 working face, and F5002 working face. The monitoring data span more than 5 years and so can reflect the long-term surface deformation after solid backfill mining.
4.1 Surface monitoring results in Area T
The T3292 working face was put into production in March 2013 and stopped being used in April 2017. Line A, measuring surface subsidence in the T zone, crosses the middle of the T3292 working face, and the projected positions of the selected measuring points below the working face are located at a distance of 340 m from the open-off cut of the working face. The surface subsidence data of each measuring point on line A were obtained by processing the monitoring data, as shown in Figure 7(a). The surface subsidence data of each time period at the A9 measuring point are shown in Figure 7(b).
As shown in Figure 7(b), the surface subsidence increased slowly from the start of mining on the working face to June 2015, with the subsidence value lower than 25 mm. Between June 2017 and August 2017, after the working face was pushed 90 m beyond measuring point A9, the surface subsidence value increased significantly, with its maximum value reaching 41 mm at measuring point F16. In April 2017, the working face had been mined completely. By December 2019, the T3292 working face had reached full mining impact and its maximum subsidence had increased to 66 mm. From the time of sudden subsidence to full mining impact, the subsidence value increased by approximately 37.8%.
The CORS monitoring system indicated a final maximum surface tilt deformation of 0.76 mm/m, a maximum curvature deformation of 0.064 mm/m2, and a maximum horizontal deformation of 50.6 mm, or 1.53 mm/m, which meet the surface building protection requirements.
4.2 Surface monitoring results in F zone
(1) F5001 working face
The F5001 working face was put into production in October 2016 and stopped being used in September 2017. The line F, measuring surface subsidence in the F zone, crosses the middle of the F5001 working face, and the projected positions of the selected measuring points under the working face are located at 400 m from open-off cut of the working face. The monitoring data were processed to obtain the surface subsidence data of each measuring point on line F, as shown in Figure 8(a). The surface subsidence data of each time period of measuring point F9 are shown in Figure 8(b).
As shown in Figure 8(b), the surface subsidence increased slowly, with subsidence values lower than 10 mm, from the start of the mining on the working face to May 2017. Between June 2017 and July 2017, after the working face had pushed through 66 m from measuring point F9, the surface subsidence increased significantly, with a maximum value of 19 mm at measuring point F16. In November 2017, the working face was mined completely, and by April 2018, the F5001 working face had reached full mining impact and its maximum subsidence had increased to 31 mm. From sudden subsidence to full mining impact, the time elapsed was approximately 8 months and the subsidence value had increased by approximately 38.7%.
The CORS monitoring system indicated a final maximum surface tilt deformation of 1.65 mm/m, a maximum curvature deformation of 0.094 mm/m2, and a maximum horizontal deformation of 16.1 mm, or 1.25 mm/m, which meet the surface building protection requirements.
(2) F5002 working face
The F5002 working face was put into production in November 2017 and stopped being used in December 2018. The line E, measuring surface subsidence in the F zone, crosses the middle of the F5002 working face, and the projected positions of the selected measuring points below the working face are located at 400 m from the open-off cut of the working face. The monitoring data were processed to obtain the surface subsidence data of each measuring point on line E, as shown in Figure 9(a). The surface subsidence data of each time period of measuring point E3 are shown in Figure 9(b).
As shown in Figure 9(b), there was almost no increase in surface subsidence from the start of mining on the working face to September 2018. Mining on the working face was fully stopped in December 2018, and by July 2019, the surface subsidence of the F5002 working face had reached 17 mm. By December 2019, the full mining impact had been reached, and the maximum increase in subsidence was 25 mm. From sudden subsidence to full mining impact, the time elapsed was approximately 5 months and there was an increase in subsidence value of approximately 47.1%.
The CORS monitoring system indicated a final maximum surface tilt deformation of 0.29 mm/m, a maximum curvature deformation of 0.012 mm/m2, and a maximum horizontal deformation of 7.84 mm, or 0.81 mm/m, which meet the surface building protection requirements.
4.3 Comparison of control effects
Observation of the surface movement and deformation caused by mining on the F5009 working face started in February 2018, and a total of 27 observations were made until August 2020. Line n, measuring the surface subsidence in the caving area, crosses the middle of the F5009 working face, and the projected positions of the selected measurement points below the working face are located 400 m from the open-off cut of the working face. The surface subsidence data of each measurement point on line n are shown in Figure 10(a). The surface subsidence data of each time period of measuring point n4 are shown in Figure 10(b).
As shown in Figure 10(b), in comparison with solid backfill mining, the initial value of surface subsidence of the caving method was larger, reaching 60 mm. As mining on the working face continued, the value of surface subsidence increased gradually without any sudden increase. The maximum value of subsidence of measuring point n4 reached 210 mm by August 2020, when all of the mining operations on the working face had been completed.
The CORS monitoring system indicated a final maximum surface tilt deformation of 6.65 mm/m, a maximum curvature deformation of 0.59 mm/m2, and a maximum horizontal deformation of 168.1 mm, or 4.25 mm/m, which do not meet the surface building protection requirements.
A comparison of surface deformation parameters between the solid backfill 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 T3292, 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 backfill mining was significantly affected by monitoring time. Although the surface did not show large subsidence during the mining, surface subsidence increased significantly from 3 months to 1 year after mining. The time of 2 years after backfill 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.