The upper layer of the 101 mining face was seriously damaged because of the small-scale old coal mine, and the excavation efficiency of the roadway was low. The system for the prevention and control of old goaf water under conditions of coal mine re-mining was adopted, and the efficient formation and safe mining of the mining face were realized.
5.1 Engineering for the Prevention and Control of Old Goaf Water in the Process of Roadway Excavation
Ground Comprehensive Geophysical Engineering
Before mining took place, three-dimensional seismic and TEM exploration were carried out on the ground, and a large amount of water-rich anomalous area in the mining face was identified, with an estimated water accumulation of 13.8×104 m3 and a water accumulation area of 17.64×104 m2, which accounted for 68.36% of the mining face area.
Engineering for Diversified Comprehensive Prevention and Control of Old Goaf Water in Underground Mine
1) Engineering for the Prevention and Control of Old Goaf Water in the Auxiliary Haulage Roadway
The length of the auxiliary haulage roadway was 1200 m, the excavation time was 1.8 years, and the average excavation efficiency was 1.8 m per day. Multiple geophysical exploration were combined with conventional boreholes and advanced short-distance exploration technology to ensure the real-time safety of the roadway, with a total of 30 iterations of exploration and drainage, the longest lasting 47 days in a single exploration and drainage cycle. The maximum number of boreholes in a single exploration and drainage cycle was 35, and the shortest excavation distance of a single excavation was 5 m (the longest was 70 m). As can be seen in Fig. 2, the distribution of the old goaf in the auxiliary haulage roadway was relatively less, which was consistent with the actual exposure. In the process of excavation, nine lower layered old roadways were exposed, multiple boreholes encountered old goaf and became stuck, the outflow boreholes were few, and the roadway had been safely formed.
2) Engineering for the Prevention and Control of Old Goaf Water in the Haulage Roadway
The length of the haulage roadway of the 101 mining face was 1200 m. It can be seen in Fig. 2 that the water accumulation in the haulage roadway was severe, the excavation time was 2.7 years, and the excavation efficiency was low. The exploration and drainage process for water was conducted 33 times, the longest duration for a single run of exploration and drainage was 7 months, the maximum number of boreholes in a single exploration and drainage cycle was 45, the shortest excavation distance for a single excavation was 5 m (the longest was 70 m), and the maximum rate of water outflow for the conventional boreholes was 108 m3/h. The haulage roadway encountered old goaf dynamically recharged water at 277 m, and multiple geophysical exploration techniques combined with conventional boreholes were adopted until 440 m. The time spent from 277 m to 440 m was 1.5 years, and the average excavation efficiency was 0.3 m/d. It was clear that the old goaf water severely restricted the excavation efficient of the roadway. The water level and water volume at 440 m of the haulage roadway cannot drop for a long time, therefore, multiple geophysical exploration combined with conventional boreholes and directional boreholes were adopted to prevent and control the old goaf water at 440 m and 658 m, which greatly improved the excavation efficiency.
a. Engineering for the Prevention and Control of Old Goaf Water at 440 m in the Haulage Roadway
The TEM and DC exploration at 440 m of the haulage roadway showed that there was water accumulation in front of the head on the left. Therefore, 21 conventional boreholes were constructed for water exploration and drainage, numbered 1–16 and B1–B5, among which the B5 borehole had the largest amount of water outflow (16.7 m3/h). Further, there was only a small amount of water seepage in a few boreholes, so it can be inferred that the distribution of the old goaf was scattered and isolated, and their connections were unknown. Within two months, the excavation and drainage of the haulage roadway were stopped, and the total water outflow from the borehole had been stable for a long time (at 36 m3/h), the water pressure was 0.3 MPa, and the drainage effect was poor. Two months later, TEM and DC were carried out again to compare the drainage effect. The exploration results showed that the strong water-rich area was particularly significant at the front-left, and there were four boreholes in this area, including the B5 borehole. Therefore, it can be determined that the front-left was water-rich; therefore, five conventional boreholes were constructed east of the auxiliary roadway to intercept the old goaf water (B17–B21), and the interception effect was poor. The exploration results at 440 m in the haulage roadway are shown in Fig. 12.
After 6 months of drainage, the no. 1 directional borehole was arranged at a position where the elevation was 34 m lower than the level of the old goaf water. The no. B5 borehole was taken as the target spot, and the strong water-rich anomalous area was taken as the target area. In accordance with the principle of avoiding drilling into coal seams and lower stratified old roadways, the orifice inclination angle was designed to be negative, and the borehole structure was designed as a six-section arc structure. This allowed stable drilling along the sand–mudstone strata below the coal seam floor, which ran obliquely through the old goaf at a large angle from its bottom. This can not only drain the old goaf water in the lower layered old roadway, but also improve the formation rate of the borehole. The no. 1 directional borehole was finally drilled into the old goaf at a depth of 526.4 m, and the initial rate of water outflow was 134 m3/h, which was much larger than that of the no. B5 borehole. The stable rate of water outflow was 89 m3/h, and the total old goaf water was 3.6 × 104 m3. The drainage effect was obviously better than that of conventional boreholes, and a proper thru-borehole can ensure the long-term stable drainage. After 17 days of drainage, the conventional boreholes no longer experienced water outflow and can be excavated safely again, indicating that there was an accumulation area of interconnected old goaf water on the north side of the haulage roadway. Subsequently, it took 6 months to adopt multiple geophysical exploration combined with conventional boreholes to safely excavate to 658 m, with an average excavation efficiency of 1.2 m/d. This efficiency exhibited an increase of 4 times, which showed that the directional borehole was successfully drilled to the bottom of the old goaf and effectively intercepted the old goaf water. The directional boreholes information is shown in Table 1.
b. Engineering for the Prevention and Control of Old Goaf Water in Ground Geophysical Anomalous Areas
According to the results of three-dimensional seismic and TEM exploration, in combination with experience regarding the control of old goaf water in haulage roadways, it was considered that dynamic recharge of the old goaf water took place in the northeast of the mining face. At low positions, the no. 2 directional borehole was arranged along the sand–mudstone strata below the coal seam floor to drill into the geophysical anomalous area, and the final borehole depth was 585 m (Table 1). The initial rate of water outflow was 65 m³/h, and the total old goaf water was 3.93×104 m³ (Fig. 2). No. 4 and no. 4-1 Directional boreholes were also designed to explore geophysical anomalies in advance at low levels, and all of them were drilled into the old goaf, but there was no stagnant water in the goaf (Fig. 2, Table 1).
c. Engineering for the Prevention and Control of Old Goaf Water at 658 m in the Haulage Roadway
The underground TEM exploration of the haulage roadway at 658 m showed that there was a large areas of old goaf water in front of the head on the left and right sides of the roadway, within the anomalous area of geophysical exploration, there were 28 boreholes for conventional exploration and drainage. There were nine boreholes with water, among which the maximum rate of water outflow in the no. 8 borehole was 108 m3/h, and after 15 days of drainage, the rate of water outflow attenuated to 50 m3/h. Seventeen conventional boreholes were constructed on the left side. There were two boreholes with water, among which the maximum rate of water outflow in the no. 4 borehole was 68.85 m³/h, and the rate of water outflow in the no. 8 borehole was suddenly reduced to 0.6 m³/h, showing that the no. 4 borehole can effectively intercept the dynamic recharge water for the no. 8 borehole. The initial rate of water outflow for the no. 17 borehole (on the right) was 78 m³/h, which continued to supplement the other boreholes and speed up the process of water drainage, after 8 days of drainage, the rate of water outflow reduced to 0.2 m³/h; the analysis showed that there was no connection between the water accumulation area of the old goaf in front of the right side and that on the left side, which was in an isolated island state. Using conventional borehole verification, it was found that only a small amount of water in the old goaf remained in the low-lying position, in an unpressurized and self-flowing state, 80 m away from the roadway side. It had no effect on the excavation. The exploration results at 658 m in the haulage roadway are shown in Fig. 13.
After 20 days of drainage, the rate of water outflow from the no. 4 conventional borehole did not decrease, so the no. 4 conventional borehole located in the geophysical water-rich anomalous area was taken as the target spot, and the no. 1 geophysical water-rich anomalous area was taken as the target area. Particularly at lower elevations, the no. 3 directional borehole was designed to steadily drill along the sand–mudstone strata below the coal seam floor to the low-lying area of the old goaf to intercept the old goaf water, with a borehole depth of 699 m and water outflow of 67 m3/h (Table 1). At this time, the rate of water outflow from no. 4 conventional borehole was suddenly reduced to 0.1 m³/h. After 5 days of drainage, TEM and DC exploration were carried out again, and it was found that the area of the no. 1 geophysical anomalous area was significantly reduced (by 63%), indicating that the directional borehole had achieved very good results in intercepting old goaf water at low positions. At the same time, the no. 3 geophysical anomalous area had disappeared, and it was verified once again that the stagnant water area was of the isolated island type and was easy to release. However, the no. 2 geophysical water-rich anomalous area was detected in front of the roadway, and then verified by a number of conventional boreholes, all of which did not come out of the water. To speed up the drainage progress of the no. 1 water-rich abnormal area, branch 3-1 and 3-2 boreholes were constructed in the no. 3 directional borehole, all of which were drilled into the old goaf, with borehole depths of 690 m and 741 m, respectively, and a water outflow of 63 m³/h and 73 m³/h, respectively (Table 1). The analysis showed that the space at the front-left of the old goaf was narrow and continuous, and after 15 days of continuous drainage, all conventional boreholes had no water outflow and could be excavated safely again. A total of 4.53×104 m³of the old goaf water was drained from the no. 3 directional borehole, and it took 44 days to explore and drain the water there. After using the no. 3 directional borehole to prevent and control the old goaf water, it took 5.6 months to complete the haulage roadway by using multiple geophysical exploration combined with conventional boreholes. The average excavation efficiency was 2.6 m/d, which represented an increase of 9 times.
3) Engineering for the Prevention and Control of Old Goaf Water in the Initial Cut off the Set-up Entry
The length of the initial cut off the set-up entry was 215 m. Fig. 2 shows that the water disaster in the north-eastern portion of the initial cut off the set-up entry was severe. During the process of excavation, multiple geophysical exploration combined with conventional boreholes were adopted, with a total of nine iterations of water exploration and drainage. The average distance for a single exploration was 21 m, and the total excavation time was nine months. The average excavation efficiency was 0.8 m/d, the excavation speed was very slow, the longest time for a single exploration and drainage was 34 days, and the maximum number of boreholes for a single exploration and drainage cycle was 58. The shortest excavation distance was 6 m and the longest was 50 m for a single exploration and drainage, and the maximum rate of water outflow from a single borehole was 20 m3/h. In the process of excavating the initial cut off the set-up entry, it was found that there was dynamic recharge of old goaf in the north-eastern direction, and the no. 5 directional borehole was specially designed for long distances and large drops to intercept the old goaf water (Table 1). However, the borehole was drilled into the coal seam at a location of 600 m, resulting in a sticking accident, resulting in a serious efficiency drop for excavation.
The TEM and DC exploration at the intersection of the haulage roadway and the initial cut off the set-up entry showed that there was an anomalous area in the north-eastern direction. Therefore, 17 conventional boreholes were arranged in this direction, and two boreholes run water, of which the no. +8 borehole had a flow rate of 71.3 m3/h and the no. +4 borehole had a flow rate of 53 m3/h (Fig. 2). The water accumulation area in the old goaf was about 20 m from the initial cut off the set-up entry, and the water volume was not attenuated after one month of drainage. Therefore, the no. +8 borehole – with the largest water outflow capacity – was taken as the target spot, the geophysical water-rich anomalous area was taken as the target area, and the long-distance directional borehole at a lower elevation was used to intercept the old goaf water. The no. 6 directional borehole was specially designed to steadily drill along the sand–mudstone strata below the coal seam floor to the low-lying area of the old goaf to intercept the old goaf water (Table 1). Finally, the borehole depth was 670 m and the rate of water outflow was 40 m3/h. At this time, the rate of water outflow for the no. +8 borehole was suddenly reduced to 13 m³/h, and the rate of water outflow from the no. +4 borehole was suddenly reduced to 0. Because the no. 6 directional borehole could drain the old goaf water about 39.25×104 m³, the safe excavation of the roadway was ensured. The plane distance between the no. 5 and no. 6 directional boreholes was only 50 m, but the no. 5 borehole entered the coal seam and the no. 6 borehole entered the old goaf, which showed that connections within the old goaf were complex and the distribution was disorderly.
5.2 Engineering for the Prevention and Control of Old Goaf Water in the Mining Face
According to the data available for the small-sacle old coal mine and underground exploration, it was considered that the height of the old roadway was 2.2 m, and there was generally the recharge of sandstone fissure water in the roof and the residual production water in the goaf. Therefore, there may be ‘isolated island’ static old goaf water in the mining face. To ensure safe mining, it was necessary to fully cover the old goaf water inside the mining face.
Audio-frequency electric perspective and radio-wave perspective detection of the roof and floor were carried out in the haulage roadway and auxiliary haulage roadway. The detection length was 1200 m, and the results shown that there were two strong water-rich anomalous areas in the upper layer, along with one fault and three old goaf areas in the mining face.
According to the detection results for the water-rich anomalous area, 37 conventional boreholes were designed and constructed in the haulage roadway, including nine boreholes in the lower-layer old goaf, 23 boreholes in the upper-layer old goaf, and five boreholes in the roof sandstone, further, 18 conventional boreholes were designed and constructed in the auxiliary haulage roadway.
In view of the geological anomalous areas, 43 groups of geological exploration boreholes were designed and constructed in the haulage roadway – each group was 20 m apart. Considering the exploration results, there were 3–4 boreholes in each group, and the maximum length of the designed borehole plane was 200 m. Fifty-one groups of boreholes were designed and constructed in the auxiliary haulage roadway, with one borehole in each group. The plane length of the designed boreholes was generally 30 m. Two groups of boreholes were designed and constructed in the initial cut off the set-up entry, with four boreholes in each group, and the maximum length of the designed borehole plane was 100 m. These geological boreholes fully covered the exploration area of the mining face and provided a geological guarantee for safe mining through the old goaf. The boreholes trajectory of the 101 mining face are shown in Fig. 14.
5.3 Engineering for the Prevention and Control of Goaf Water in the Process of Mining
During the process of mining, the advanced short-distance exploration was carried out stringently, and the mining reached lengths of 61 m and 347 m. Advanced short-distance exploration found that the old goaf stagnant water was left in low-lying areas in front of the coal wall, and advanced short-distance exploration boreholes were added to ensure safe mining. Owing to the mining process, on the rock strata of the roof and floor, the destruction phenomenon of ‘three zones above coal seams’ appears. To ensure that the no. +8 borehole – at the intersection of the haulage roadway and the initial cut off the set-up entry – had long-term stable drainage of dynamic recharge water, the seamless steel pipe was buried here and extended to the outside of the roadway. After 13 months of mining, the rate of water outflow from the seamless steel pipe was still about 30 m³/h. At present, the mining face has been safely mined to 850 m and 1.37 Mt of coal resources have been liberated.