4.1. Bed Separated Displacement Monitoring of the Backfill
One of critical controlling factors of backfill stability is bed separation displacement. In the study area, mining has proceeded from top to bottom in a series of horizontal four-meter-high drift slices. Once excavation is finished within an access drift, the void is filled using cement paste before the next access drift is excavated. To guarantee output, mining occurs at two sub-levels simultaneously. Backfill in different levels are formed at different times. The bed separation displacement occurred because of the volume shrinkage of the backfill. The bed separation displacements were monitored in sub-level 1100 m, using displacement meters in different depths of backfill.
The monitoring meters layout in sub-level 1100 m along four exploration lines, respectively. The monitoring meters were set in exploration line-12, line-14, line-16, and line-17. Bed separation displacement monitoring work was built together in groups of four meters that extend to the following four depths: 20 m, 25 m, 30 m, and 35 m. The duration of monitoring was three months, and the results of the monitoring are showed in Figure 6
The bed separated displacement monitoring results demonstrated contraction deformation of different parts of the backfill in sub-level 1100 m. All the displacement values were less than 20 mm. Separated displacement of the backfill located exploratory line-12 occurred at the initial stage. However, this displacement tended to reduce as the mining went deeper. The separated deformation was intended to be stable in this part. The trend of bed separated displacement of the backfill located in exploratory line-14 and line-16 was consistent. At the initial stage, the upper displacement (monitored by 20 m deep meter), the middle displacement (monitored by 25 m and 30 m deep meters), and lower displacement (monitored by 35 m deep meter) have the same change. After two months, the separated deformation appeared upper displacement at the upper part of backfill, and the downward displacement appeared in the lower part of the backfill. The bed separated deformation was accelerated at these parts of backfill. Separated displacement of the backfill located at exploratory line-17 was not obvious.
One of factors influencing bed separated displacement is the effect of mining at two levels simultaneously. The monitoring results indicated that separated displacement of the backfill part in exploratory line 14 was larger than in other parts. This characteristic was consistent with the ground movement.
4.2. Stress Distribution of the Backfill in Sublevel-1100 m
Based on the site conditions, monitoring points were set in several of the exploration lines. The backfill of sublevel-1100 m was distributed between exploration line-7 and line-25. The monitoring points were located in exploration line-7, line-8, line-10, line-12, line-14, line-16, line-21, and line-25, respectively. The monitoring duration was three months. The cumulative horizontal and vertical stresses are shown in Figure 7
The monitoring results in Figure 7
demonstrate the stress distribution of different parts of backfill in sub-level 1100 m. For most monitoring points, horizontal stress is larger than vertical stress in all parts of the backfill at same time. This was caused by local tectonic stress in the study area. This level of backfill can divided into two parts—the boundary part and the internal part. The backfill in sub-level 1100 m is stretched from exploration line-7 to line-25, and the line-7 and line-25 parts of backfill presented the boundary part, while the other lines parts presented the internal part. The monitoring results showed that the horizontal stress of the backfill part in line-7 is 117.62 MPa and the vertical stress is 62.77 MPa. For the backfill part in line-25, the vertical stress is 64.52 MPa, and the horizontal stress data is missing due to instrument damage.
For the internal part of the backfill, the horizontal stresses in line-8, line-10, line-14, and line-16 are 6.95 MPa, 6.74 MPa, 29.79 MPa, and 74.36 MPa, respectively. The vertical stresses in line-8, line-10, line-12, line-14, line-16, and line-21 are 25.21 MPa, 11.89 MPa, 25.61 MPa, 1.16 MPa, 13.99 MPa, and 33.47 MPa, respectively. Both horizontal and vertical stresses of the internal part of the backfill are much smaller than in the boundary part.
The backfill is a passive support measurement. The exterior load acted on the surrounding rock and then transferred to backfill through the rock material.
The vertical stress of the internal backfill is much less than that of the boundary part of the backfill. The point with the minimum value of stress is located on exploratory line-14. This part of the backfill is the center part of backfill along its strike.
The stress of the internal parts of the backfill was less than that in the boundary parts of the backfill. The backfill located on exploratory line 14 is the internal part, where the stress was only 1.16 MPa. This part of the backfill is the center of the backfill along its strike. Corresponding to the ground movement, the settlement center is also located in exploratory line-14, and several main fissures were concentrated near the settle center in exploratory line-14. It should be noted that the bed separation displacement of the backfill in line-14 increased during the monitoring duration. Combined with the corresponding ground subsidence, the internal part of the backfill located on exploratory line-14 released its accumulated stress. This situation led to the stress of backfill in exploratory line-14 is much lower than other parts of the backfill.