Exploitation of shallow thick coal seams that are overlain by phreatic aquifers may cause loss of the water resource and destruction of the surface ecological environment. In order to explain the phenomenon that the actual leakage of phreatic water is greater than the predicted value, field investigation and analogue simulation were carried out, and the nonpenetrative fractured zone (NFZ) was proposed based on the original three zone theory. Further, a "vertical four-zone model" was established and the overlying strata was divided into a caved zone (CZ), through-going fractured zone (TFZ), NFZ, and continuous zone (COZ) from the bottom to the top. The characteristics of fractured rock within NFZ and the determination method of its height were studied. The results showed that the height of NFZ ranged from 11.55 to 21.20 m, which was approximately 0.17 times the combined height of the TFZ and the CZ. To reveal the mechanism of phreatic water leakage, the permeability of rock within NFZ was studied for their premining and postmining using an in situ water injection test and laboratory test. The results showed that the permeability of the rock in NFZ was increased by 7.52 to 48.37 times due to mining, and the magnitude of the increase was nonlinear from top to bottom. The increase of permeability of tested specimens was also related to the lithology. The results of the study are helpful to the prediction of the potential loss of phreatic water and the determination of the mining thickness.
Study on mining induced fractures in overlying strata, and their influence on rock mass permeability is very important for underground engineering such as coal, oil, and metal mining and tunnel excavation [[
Many theories including "pressure arch," "cantilever beam," "articulated rock mass," "preformed fracture," "voussoir beam," "transferring rock beam," and "key stratum" have been put forward to explain the mining pressure and movement of overlying strata [[
The height of fractured zone in overlying strata is mainly influenced by rock mechanics parameters, lithology combination, stress, mining space size, and geological structure [[
The study on permeability variation of fractured rock during coal mining was mainly based on numerical simulations and field tests [[
The loss of sand layer water has a serious impact on coal mine production and longevity and the ecological environment in the arid and semiarid areas in NW China. The traditional three zone model takes the strata above the water conducting fractured zone as a whole, without considering its permeability change. However, the reduction of the water-resisting property of the rock in nonpenetrative fractured zone (NFZ) is the main reason for the loss of water resources. Using Stope #101 of Jinjitan Coal Mine as the case study, we designed and drilled nine boreholes from the ground surface down to the underground workings and established a "vertical four-zone model" with nonpenetrative fracture zone (NFZ) being proposed. The height of each zone was determined using hydrogeological borehole investigations and analogue simulation. Three types of rock, argillaceous sandstone, fine-grained sandstone, and siltstone within the NFZ, were studied for their premining and postmining water-resistant capacities using in situ water injection tests and laboratory permeability test. The results are of significance for designing the mining thickness of coal seams and determining the risk to loss of phreatic water resources, which are important theoretical bases for green mining.
Jinjitan Coal Mine is located in the northern part of Shaanxi province and to the south of the Mu Us desert at longitudes 109
Graph: Figure 1 Location of Stope #101 and Jinjitan Coal Mine.
Graph: Figure 2 Typical geological log profile of No. 101 panel overlying strata.
The fully mechanized Stope #101 has elevations between +991.7 and +1010.4 m, whereas the corresponding surface elevations are between +1229.8 and +1263.7 m, respectively. The panel has a designed advance length of 4,548 m and a designed net face length of 300 m (Figure 1). Although the coalbed average thickness is 9.4 m, however, the mining thickness is only 5.5 m in order to avoid a large volume of phreatic water leakage and an ecological deterioration. The direct roof of coal seam 2-2 consists of dark gray argillaceous sandstone with a thickness of 3.03 m. The integrity of overlying strata is good with an average thickness of approximately 10 m. The roof is mainly composed of mudstone, argillaceous sandstone, siltstone, fine-grained sandstone, and medium-grained sandstone. The argillaceous sandstone, siltstone, and fine-grained sandstone are the main strata, accounting for 27.91%, 31.81%, and 29.52% of the total bedrock thickness, respectively.
Hydrogeological borehole investigation is one of the most commonly used methods to study the development characteristics of fractures in overlying strata. It can also be used to evaluate the rock permeability with simple hydrogeological observation. In this study, nine boreholes were drilled at 2 months after mining when the gob had been stabilized. Figure 3 shows locations of the boreholes. Borehole JE5 was drilled in the nonmining area 150 m south of #2 return airway tunnel on Stope #101 to obtain the background data as a reference for other drilling data. Two profiles, A-A
- (i) Rock quality designation (RQD) can reflect the degree of rock integrity with higher values indicating better integrity. The appearance and density of fresh vertical or oblique fractures of the extracted cores were also visually observed. The RQD values of the normal rocks in the Jurassic Yan'an and Zhiluo formations of study area are generally greater than 60%. There are obvious differences of the rock permeability at different positions in overlying strata because of the differences in fracture shape, density, and width. The DCA method is particularly applicable to determining the height of TFZ and CZ but not to determining the height of NFZ
- (ii) The DFLM was conducted according to Standard MT/T865-2000. Drilling fluid consumption per unit time and the fluid depth were monitored dynamically. Where drilling fluid consumption started fluctuating and slowly increased with drilling depth and the fluid depth started to slightly decline or fluctuate, the position is regarded as the top interface of the NFZ. Where the fluid depth began to decline rapidly and the fluid loss rate began to increase significantly with depth, the position is regarded as the top interface
- (iii) WDY-HW-60 video camera was used to visualize fractures in the borehole walls. It must be noted that the observations are possible only when the water in the borehole is clear and still. As a result, the image of the CZ cannot be obtained. Based on the images, the development characteristics of fractures in the inner walls of the borehole were analyzed, and the top boundary of TFZ and NFZ could be confirmed
- (iv) The FPT was conducted by injecting water into the boreholes at 5 m intervals. The injection sequence was from bottom to top. Five pressures (0.3 MPa~0.6 MPa~1.0 MPa~0.6 MPa~0.3 MPa) were applied to each interval. Based on the amount of water injected, the rock permeability could be calculated. The position where the permeability increased by at least one order of magnitude and was above the TFZ was considered the top boundary of NFZ
Graph: Figure 3 Boreholes layout at No. 101 working face.
Similar material physical models have been used to study fracture development of overburden [[
Table 1 Similarity condition of similar material simulation model to actual mine system.
Model size (mm3) Geometric Time Bulk density Poisson ratio Stress Strength 1 : 200 1 : 14.14 1 : 1.5 1 : 1 1 : 300 1 : 300
Permeability tests of the rock with different lithology were designed and carried out to investigate the permeability change in the NFZ in response to longwall mining. Nine core samples were taken from the NFZ in the boreholes and were cut into cylinders with 50 mm in diameter and 100 mm in length according to the suggestion by International Society for Rock Mechanics. The average dry densities of the fine-grained sandstones, siltstone, and argillaceous sandstone specimens are 2,395.21, 2,406.30, and 2,385.63 kg/m
Table 2 Tested sandstone specimens in this research.
Specimen Length Diameter Mass Dry density Clay content Position of the NFZ F1 99.26 49.57 458.88 2395.51 28.3 Upper F2 100.05 49.57 462.88 2397.31 31.2 Middle F3 99.63 49.33 455.63 2392.82 29.6 Lower S1 97.92 49.48 453.04 2406.11 32.5 Upper S2 99.96 49.58 464.61 2407.46 34.9 Middle S3 98.73 49.59 458.67 2405.32 32.7 Lower A2 100.82 49.58 464.77 2387.75 44.9 Lower A4 99.89 49.4 456.66 2385.21 40.1 Middle A5 100.79 49.58 463.89 2383.94 39.0 Upper
The experiments for the sandstone specimens were carried out on a France-made TAW-1000 rock servo-controlled rock mechanics experimental system. Distilled water was chosen as the pore fluid. The test ambient temperature was kept at around 25 degrees to eliminate the effect of temperature on the experimental results.
With the advance of panel, the overlying strata undergo a series of mechanical processes, such as movement, deformation, and fracture. According to the deformation and fracture characteristics of the overlying strata and its hydraulic conductivity, the overlying strata could be divided into several regions [[
The CZ results from caving of the immediate roof strata into the mined-out area following coal extraction. The strata in the CZ lose not only the continuity of the formation but also the stratified bedding. The rock blocks in this zone are of different sizes and cluttered. The voids and thus connectivity are typically greater at the lower section of the zone. Because of the presence of collapsed rocks, one or more of the followings may occur when drilling into the CZ:
- (i) There are frequently dropped drills
- (ii) The drilling speed is unstable, and there are phenomena of being jammed and aggravated vibration of the drilling tools
- (iii) The drilling process has obvious air suction phenomenon with audible whirring sound
The above indicators help determine the position of the top boundary of the CZ. The elevation difference between the top boundary and the coal seam floor is the height of the CZ. The observed phenomenon may not be the same for all boreholes. For example, in borehole JE2, the drill tool was stuck at a depth of 240.10 m with strong vibration, and at 241.22 m, there was a suction phenomenon, whereas in borehole JE9, a suction phenomenon was observed at 238.00 m and the drill dropped with 45 cm distance at a depth of 247.72 m. Therefore, the depths of 240.10 m and 238.00 m were considered the top boundaries of the CZ in boreholes JE2 and JE9, respectively. Table 3 presents the top boundary of the CZ at each borehole and the pertinent lines of evidence.
Table 3 Top boundary depths and heights of the CZ in boreholes.
Holes Special phenomena in the drilling process DCA Results (m) Basis for determination Top boundary depths the CZ (m) Heights of the CZ (m) Basis for determination Top boundary depths the CZ (m) Heights of the CZ (m) JE1 At the depth of 240.00 m, drill dropped with 25 cm distance; at 241.40 m, suction phenomenon 240.00 22.70 Below 238.00 m, extremely broken core, 238.00 24.70 24.70 JE2 Drill tool was stuck at a depth 240.10 m with strong vibration; at 241.40 m, suction phenomenon 240.10 22.37 Below 239.00 m, broken core, 239.00 23.47 23.47 JE3 During 237.00~238.00 m, drill dropped with 60 cm distance 237.00 23.55 Below 238.28 m, fragmental core, 238.28 22.27 23.55 JE4 At 238.12 m, drill dropped with 30 cm distance; at 238.56 m, suction phenomenon 238.12 23.16 Below 237.20 m, fragmental core, 237.20 23.08 23.08 JE5 None — 0 Integrity core, — 0 0 JE6 None — 0 Integrity core, — 0 0 JE7 At 240.80 m, drill dropped with 15 cm distance; at 243.15 m, suction phenomenon 240.80 21.30 Below 239.15 m, broken core, 239.15 22.95 22.95 JE8 At 247.02 m, drill dropped with 35 cm distance; at 239.00 m, suction phenomenon 247.02 19.18 Below 241.70 m, broken core, 241.70 24.50 24.50 JE9 At 238.00 m, suction phenomenon; at 247.72 m, drill dropped with 15 cm distance, and drill tool was stuck 238.00 20.58 During 234.80~238.00 m, disorder bedding, 234.80 23.78 23.78
Figure 4 shows the drill-core photographs of borehole JE1. Figure 4(a) shows that the value of RQD is zero below 238.00 m. In comparison, the RQD value at the same position in the background borehole JE5 is approximately 80%. As a result, the depth of 238.00 m was considered the top boundary of the CZ in borehole JE1. The heights of the CZ in boreholes JE1, JE2, JE3, JE4, JE7, JE8, and JE9 were identified as 24.70, 23.47, 23.27, 23.08, 22.95, 24.50, and 23.78 m, respectively. In addition, the CZ was not observed in the JE6, which is 15 m away from the mining area.
Graph: Figure 4 Drill-core photographs of JE1 borehole: (a) RQD is zero of CZ; (b) first fresh vertical fracture and smaller RQD of TFZ; (c) initial high angle fracture and larger RQD of NFZ.
The height of the CZ in Stope #101, as determined by the anomalies in the drilling process, ranges from 19.18 to 23.16 m with an average of 21.83 m. However, the height determined by RQD ranges from 22.27 to 24.70 m with an average of 23.54 m. As shown in Table 3, except for borehole JE3, the results determined by RQD are slightly larger than those determined from the drilling observations. In most cases, the RQD method has a higher resolution in determining the top boundary of the CZ. The larger value of the two methods was selected.
Therefore, determination of the height of the CZ is based on multiple lines of evidence. The final height of the CZ ranges from 22.95 to 24.70 m with an average of 23.72 m.
Figure 5 shows the magnitude and spatial distribution of the overburden failure zones in the analog model. The size of the grid is
Graph: Figure 5 The characteristics of overburden failure on the model.
According to the above results and analyses, the height of the CZ is relatively stable at around 24 m, whether it is along the advancing or sloping direction. This average height is approximately 4.36 times of the mining thickness. In addition, the fact that no CZ was observed in borehole JE6 proves that the CZ develops only in the area above the working face.
The TFZ is located above the CZ, where the rock retains the stratified bedding. The mining-induced fractures are well interconnected and provide the pathways for groundwater entering the gob. In many cases, the strata breakage gradually reduces upwards, resulting in the decrease of fracture network development thus the permeability in the upper section of the TFZ.
The presence of fresh vertical fractures in the rock strata and significantly lower RQD values than the background one at borehole JE5 are two indicators in determining the position of the top boundary of the TFZ. Figure 4(b) shows that the first fresh vertical fracture was identified at the depth of 162.15 m in borehole JE1 and the RQD value decreased significantly at the same location. More fractures were found with the increase of depth.
The method of determining the position of the TFZ based on the characteristics of fluid loss rate and fluid surface depth during drilling is used and considered to be accurate [[
Graph: Figure 6 The change of drilling fluid loss rate and fluid surface depth in JE8 borehole.
Graph: Figure 7 Video camera images of JE3 borehole: (a) no fracture; (b) initial high angle no-though fracture; (c) first fresh penetrative vertical fracture; and (d) TFZ.
Although the degree of fracture development of the overlying strata can be observed intuitively and the relative quantitative evaluation can be carried out from core logs, however, the following two defects should be taken into account:
- (i) Due to the impact of the drill pipe, the rocks tend to be broken in the process of the core being raised, leading to an overestimate of the height
- (ii) Due to the small range of drilling, there might be no obvious longitudinal fractures in the core even if the TFZ is encountered, resulting in an underestimate of the height
With the video camera images, the fractures in the hole-walls can be visualized. The overestimate error could be reduced to a great extent by comprehensive analysis of DCA and BVCO results. DFLM reflects the development of both macrofractures and microfractures in the borehole. The underestimate error could be reduced by comprehensive analysis of DCA and DFLM results. Such analyses lead to the following procedures in determining the top boundary of the TFZ:
Firstly, the results of the DCA were compared with the results of the BVCO, and the larger value (
Secondly, the results of DCA and DFLM were compared to choose a smaller value (
Finally, the arithmetic mean (
The estimated positions of the top boundary and the height of the TFZ are presented in Table 4.
Table 4 Top boundary depths and heights of the TFZ in boreholes.
Boreholes Top boundary depths of the TFZ (m) Heights of the TFZ (m) DCA DFLM VCO Final results DCA DFLM VCO Final results JE1 162.15 161.7 162.15 161.93 75.85 76.3 75.85 76.07 JE2 199.43 199.47 199.55 199.51 39.57 39.53 39.45 39.49 JE3 155.98 155.55 156.9 156.23 81.02 81.45 80.1 80.77 JE4 191.47 195.28 196.78 194.13 46.73 42.92 41.42 44.07 JE5 0 0 0 0 0 0 0 0 JE6 0 0 0 0 0 0 0 0 JE7 172.2 172.1 172.2 172.15 46.95 47.05 46.95 47 JE8 155 155.2 155.3 155.15 86.7 86.5 86.4 86.55 JE9 145.83 145.58 145.63 141.71 88.97 89.22 89.17 93.09
The results of the physical simulation showed that the height of the TFZ reached its maximum value of 85.23 m when the mining advanced 300 m. Unlike the CZ, the height of the TFZ varies with the spatial position. Figure 8 shows cross-sectional views of the induced height of the TFZ above Stope #101. The TFZ is arch-shaped along the sloping direction of the panel (Figure 8(a)), whereas the height gradually increases in the advance direction (Figure 8(b)). The height at the end of the mining panel is 1.22 times of the height at the beginning of the mining panel.
Graph: Figure 8 Profiles of top boundaries of CZ, TFZ, and NFZ.
Graph: (b)
The NFZ is located above the TFZ and deflects upward without apparent open fractures. Mining-induced fractures are present but small and isolated. Rock permeability tends to get smaller from bottom to top of the NFZ. The changes of rock permeability of TFZ are of great significance to design the mining height and to prevent leakage of overlying phreatic sand aquifer. If the sand phreatic water overlie directly the NFZ or the NFZ is missing, the leakage can be significant.
Because the fractures are not fully penetrated and their lengths are small, there are no significant changes in the RQD values. For the same reasons, the video camera images failed to identify the position of the top boundary of the NFZ in the boreholes. The techniques used to determine the top boundary of the NFZ included DCA, VCO, DFLM, and packer testing.
Based on the DCA and VCO, in particular, where high angle fractures start to emerge (Figures 4(c) and 7(b)), the top boundaries of the NFZ were estimated in boreholes JE1, JE3, JE4, JE7, and JE9. Figure 6 shows the fluid loss rate and fluid surface depth in boreholes JE5 and JE8. The fluid loss rate fluctuated in the B-C segment between 135.15 m and 155.20 m. However, there is no dramatic increase, and the fluid surface depth slowly fell. Packer testing was conducted in all boreholes with the exception of boreholes JE5 and JE6 where no NFZ was developed.
The results of the top boundary and height of the NFZ are presented in Table 5. The results of DCA are similar to those of VCO, and the results of DFLM are similar to those from FPT. It appears that the estimated heights from DFLM and FPT are smaller than those from DCA and VCO. The reason is the results of DCA and VCO were based on field observations, while the results of DFLM and FPT were based on the hydrogeological properties of the rock. It is more accurate to determine the top boundary of the NFZ from the changes of hydrogeological properties. The minimum value of the results from the four methods was used as the conservative estimate of the top boundary of NFZ. The results of analogue simulation show that there are independent small fractures in the range of 19.02 m above TFZ.
Table 5 Top boundary depths and heights of the NFZ in boreholes.
Boreholes Top boundary depths of the NFZ (m) Heights of the NFZ (m) DCA DFLM VCO FPT Results DCA DFLM VCO FPT Results JE1 143.93 143.73 143.79 143.7 143.7 18.00 18.20 18.14 18.23 18.23 JE2 — 187.99 — 187.96 187.96 — 11.52 — 11.55 11.55 JE3 137.43 137.41 137.45 137.42 137.41 18.80 18.82 18.78 18.81 18.82 JE4 182.04 182.1 182.04 182.31 182.04 12.09 12.03 12.09 11.82 12.09 JE5 — — — — — 0 0 0 0 0 JE6 — — — — — 0 0 0 0 0 JE7 167.8 167.73 167.75 167.73 167.73 14.35 14.42 14.40 14.42 14.42 JE8 — 135.15 — 135.11 135.11 — 20.00 — 20.04 20.04 JE9 — 120.51 120.68 120.61 120.51 — 21.20 21.03 21.10 21.20
The estimated height of the NFZ ranges from 11.55 to 21.20 m (Table 5). The height of the NFZ is approximately 0.17 times the combined height of the TFZ and the CZ, and this ratio is also confirmed by the analogue simulation results.
The results of FPT in the background borehole JE5 show that the estimated permeability values of the argillaceous sandstone, siltstone, and fine-grained sandstone are
The transient-pulse permeability tests were used determine the permeability of postmining rock core samples collected in the boreholes. It took approximately one hour to reach a balance of the upstream pressure and downstream pressure, but after 0.1 h permeating, an obvious change trend occurred. If a transient pulse test lasts too long, the creep deformation of specimen under high compression stress occurs, which could cause a larger error of transient pulse tests. As a result, a proper test time ranging from 10 to 20 min was selected to short the total test time of a compression test.
Figure 9 shows the permeability test results. In this figure, P0 represents the position not affected by mining; P1, P2, and P3 represent the upper, middle, and lower sections of NFZ, respectively. Figure 9(a) shows that the permeability of the argillaceous sandstone was significantly increased after mining. The degree of change is closely correlated with the locations of the argillaceous sandstone. The permeability coefficients of the upper, middle, and lower sections of the argillaceous sandstone of NFZ increased from
Graph: Figure 9 Permeability increase curves of rock with nonpenetrative fracture affected by coal mining: (a) argillaceous sandstone; (b) siltstone; (c) fine-grained sandstone.
Graph: (b)
Graph: (c)
The increasing extent of rock permeability is related not only to the lithology of the rock but also to the location in the NFZ (the distance from the coal seam). The estimated rock permeability coefficient of the lower part of the NFZ is an order of magnitude larger than that of the upper part. The permeability coefficients of rock in the upper, middle, and lower parts of the NFZ are approximately 10 times, 20 times, and 40 times, respectively, higher than those estimated for their premining rock samples.
According to the characteristics of fracture distribution and its potential effect on the leakage of sand phreatic water, the overlying strata was divided into four zones: CZ, TFZ, NFZ, and COZ, from the bottom to the top. The developmental height of CZ is relatively stable, which is roughly 4.36 times of the thickness of the mining coal seam. Along the advancing direction of the working face, the height of TFZ gradually increases to the maximum value of 93.09 m, approximately 16.93 times of the mining thickness. In the advancing direction, the vertical profile of TFZ was arch-shaped. The height of the NFZ ranges from 11.55 to 21.20 m, which is approximately 0.17 times of the combined height of CZ and TFZ. Affected by coal mining, the permeability of the rock in NFZ was increased by 7.52 to 48.37 times, and the magnitude of the increase is nonlinear from top to bottom. The results of the study are helpful to predict the potential loss of phreatic water and determine the mining thickness. It can provide the basis for water preserved mining in the arid and semiarid mining areas in western China.
The data used to support the findings of this study are included within the article.
The authors declare no conflict of interest.
Yu Liu, Qimeng Liu, and Wenping Li designed and wrote the paper, Yu Liu and Qimeng Liu performed the experiments, and Youbiao Hu supervised the paper writing.
This work was supported by the Natural Science Foundation of Anhui Province (2008085QD191 and 1908085ME144), the National Key Research and Development Program of China (2017YFC0804101), and the Independent Research fund of the State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines (Anhui University of Science and Technology) (No. SKLMRDPC19ZZ06). The authors are grateful to Prof. Wei and Dr. Xu for their assistance.
By Yu Liu; Qimeng Liu; Wenping Li and Youbiao Hu
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