Surface damage reduction effect of super long working face with large mining height in Shangwan Coal Mine

doi:10.21203/rs.3.rs-3871630/v1

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Surface damage reduction effect of super long working face with large mining height in Shangwan Coal Mine

https://doi.org/10.21203/rs.3.rs-3871630/v1

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Green mining is the basic background of high-quality mining, and source reduction is the most important part, among which the optimization of working face height and length is the most active. How to control the working face parameters, reasonably evaluate the surface subsidence characteristics of the super-long working face, and identify the limit working face length and the surface discontinuous deformation threshold is particularly important. In order to solve the problem of irreversible damage to the surface caused by the mining process of the working face, this paper discusses the whole process of surface movement in the 8.8 m super-large mining height working face of Shangwan Coal Mine through field investigation, in-situ monitoring and data analysis. Combined with the inflection point trajectory, advancing degree and subsidence coefficient, the 0.9 coefficient of the working face subsidence basin boundary and the loss reduction strategy far from the limit working face threshold are proposed. The results show that the subsidence basin of 12401 working face accounts for 34%, the continuous deformation area of surface accounts for 64%, and the influence area of discontinuous deformation area is within 10 times of mining height. With the help of borehole detection verification, the time effect of rock movement in different layers is further revealed, which provides a strong basis for the optimization design of working face and the timing of surface ecological management.

working surface

height mining

advancing degree

ground surface movement

As the main energy source of China 's strategic initiative and controllability, coal has effectively guaranteed China 's rapid economic and social development and energy security. In 2022, the national coal production will reach 4.56 billion tons, and the coal production of Shanxi, Shaanxi and Inner Mongolia will reach 3.355 billion tons, accounting for 74.6% of the country. The average single well (mine) production capacity of the three provinces (regions) is 2.51 million tons/year, which is much higher than the national average of 1 million tons/year. The Shanxi-Shaanxi-Inner Mongolia mining area has a high degree of intensification, high level of security and high resource recovery rate, which determines the status of coal resource center and production center in China 's energy security.

The quantitative relationship between overburden damage degree and mining parameters is established by comparing and analyzing the surface length and advancing speed effect of overburden damage degree and water-conducting fracture zone(He et al. 2021).The damage dissipation model of the relationship between damage coefficient and mining space in different zones of overburden ' three zones ' is established, and it is concluded that source reduction is the main way to control mining damage(Li et al. 2021 and 2022).When the crack width is greater than 20 cm, the effect is mainly manifested as vertical negative effect, and the increase of soil erodibility is mainly concentrated within the horizontal distance of 83cm (Wang et al 2021). The InSAR, GNSS and 3D laser scanning technology were used to monitor the surface subsidence in Shendong Shangwan Mine, and the distribution characteristics of surface subsidence were well analyzed (Zhang et al 2020)..

With the westward shift of China 's coal development focus strategy, the contradiction between high-intensity mining and fragile ecological environment protection in the Yellow River Basin is prominent. It shows that the surface ecological damage has a 'self-healing' trend under the action of natural forces (Zhang ea al 2013). However, the phenomenon of mining loss and surface ecological self-healing in Shendong mining area is obvious. It building China 's first 200 million tons of coal production base and accumulating 3.5 billion tons of coal mining, it has promoted the ecological management of 473 square kilometers of mining area. The vegetation coverage rate has increased from 3–11% at the beginning of development to 64%, and an oasis has been built in the transitional zone between the hilly and gully area of the Loess Plateau and the Mu Us Sandy Land. It has achieved a win-win situation between mining and ecology, turning deserts and gobi beaches into oasis, and rocky mountains into green water and green mountains, vividly demonstrating the role and potential of mining damage reduction and surface ecological self-remediation.

From the perspective of generalized resources, green mining can reduce the impact of coal mining on the environment from the source by controlling or utilizing the breaking movement of mining strata, and realize the co-mining or protection of coal seams and associated resources(Xu 2020 and Xu 2021). In order to quantify the mining loss reduction capacity, the mining activities and surface response of 200 million tons of mines in Shendong mining area were studied. In this paper, the most representative 12401 super-large mining height (8.8m) working face of Shangwan Coal Mine is selected as a sample. Based on the surface subsidence and morphological characteristics, the 'amount' (working face length) and green reduction (non-uniform settlement) are scientifically determined, which is of great significance to realize the source reduction and surface ecological improvement of high-intensity mining in the western mining area.

The excavation of underground engineering is bound to cause the redistribution of stress and displacement of overlying strata. The driving force of mining is not only destructive force, but also repair force. The most intuitive manifestation is the development and closure of surface subsidence and cracks. In order to study the surface damage reduction effect of super-long working face with large mining height in Shangwan Coal Mine, grasp the whole process change of mining surface movement, and characterize the dynamic response relationship of mining surface, the rock movement observation was carried out on the 12401 working face of Shangwan Coal Mine, and its cross line arrangement as shown in Fig. 1.

The working face of 12401 has a mining height of 8.6 m, advancing length is 5286 m, inclination length of the working face is 299.2 m, a dip angle of 1 ~ 5°, and a buried depth of 210 m. The surface adopts a cross-line layout, and its advancing direction there are 38 (K1 ~ K38)measuring points, the spacing of measuring points is 20m, and the length of working face inclination line is 760m. There are 43 ༈L1 ~ L43༉measuring points in the tendency of the working face, with a spacing of 20 m and a tendency line of 860 m. Among them, L22-K30 is the cross measuring point. The permanent measuring points (control points) A1, A2 and A3 are set along the K line, and the permanent measuring points B1, B2, C1 and C2 are set along the L line. The distance between the control points is 80 m, and the distance between the control points and the measuring points is 50 m.

The 12401 fully mechanized mining face began to be installed on March 2,2018 and began to be mined on March 20,2018. The surface movement began its first background observation on February 26,2018. By August 2019, a total of 24 surface movement process monitorings were carried out over 19 months

In order to directly reflect the response relationship between mining and surface subsidence, taking the advancing distance of working face as the time axis, the whole process change of 38 surface monitoring points from K1 to K38 is analyzed. The surface subsidence curve is shown in Fig. 2.

Figure 2 shows that the subsidence curve of the working face is L-shaped, and the surface subsidence change is mainly concentrated within the advancing distance of 400 m. After that, the surface subsidence no longer increases with the change of mining size. The subsidence values of the surface in different areas of the mining surface are different. The maximum subsidence is the K19 curve. The measuring point is located within the goaf, 80 m away from the open-off cut, the subsidence is 6.20 m, and the subsidence coefficient is 0.72. The K15 curve is located directly above the cutting eye, and its subsidence is 1.85 m, which is about 0.3 of the maximum subsidence value, less than half of the maximum subsidence value. The surface subsidence outside the open-off cut decreases with the increase of distance. The maximum subsidence of 80 m (K11) outside the open-off cut is 0.09 m, which means that the surface subsidence is less than 10 cm, indicating that the mining influence is not obvious. In order to vividly express the relationship between the progress degree of the advance, the ratio of the advance distance to the buried depth of 210 m and the face length of 299.2 m is used, that is, 400 m/210 m ≈ 1.9,400 m /299.2 m ≈ 1.34. It can be judged that the 12401 working face has reached the full mining state when the advance degree (ratio) is 1.34 ~ 1.9.

3.1 Mining subsidence and residual subsidence

The stability of surface subsidence requires a long process, especially the residual subsidence usually reaches the final stable state after N ( buried depth /100m ) years after mining. In order to visually display the ratio relationship between mining subsidence and residual subsidence, the subsidence at two time nodes of 300 m advance on May 30, 2018 (advance degree 1.4 ) and after stopping mining on August 12, 2019 (advance degree 25) was selected for comparison, and the ratio analysis of surface subsidence in the range of 200 m before and after cutting was carried out, as shown in Fig. 3.

When the advancing distance of 12401 working face is 300 m, the surface subsidence in the range of -80 m ~ 100 m on the surface nearby open-off cut occupies more than 95% of the final subsidence value. With the increase of the distance from the open-off cut, the proportion of mining subsidence decreases, but still occupies more than 80%. It can be seen that the surface of the open-off cut has basically reached the state of full mining when fully mined, and the proportion of mining subsidence accounts for 80% ~ 95%. The maximum subsidence threshold boundary can be set to 0.9, that is, the subsidence reaches 90% of the maximum subsidence value as full mining or full subsidence.

Generally speaking, the surface of mining boundary is characterized by S-shaped subsidence curve, which is mainly manifested as discontinuous deformation zone and uniform subsidence zone (Liu et al 2019). The inflection point controls the basic form of subsidence curve. The discontinuous deformation zone has the greatest influence on surface vegetation. In order to identify the boundary of discontinuous and continuous subsidence, the second derivative of S-shaped curve is equal to 0, that is, the inflection point position represents the boundary of discontinuous deformation of surface.

3.2 Trajectory analysis of advancing direction inflection point

The surface subsidence has reached the full mining state when the advance distance is 300 m. In order to fully express the change of the whole process, the curve shape dynamic change characteristics of the interval process from the working face open-off cut to the advance of 800 m (advance degree 3.8 ) are selected, and the inflection point trajectory is shown in Fig. 4.

In Fig. 4, B is the length of the working face, M is the mining height, H is the buried depth, and h is the thickness of the loose layer.

When the open-off cut (10m) is equivalent to the advance distance of 10m, the position of the inflection point is approximately equal to zero, indicating that the surface has almost no effect. With the increase of mining distance, the surface appearance began to appear, and the inflection point quickly shifted outside the mining boundary. When the advancing distance was 80 m, the position of the inflection point reached the peak, which was 60 m outside the open-off cut, indicating that the surface subsidence curve within the advancing distance of 0–80 m was basically a monotonically increasing state, and the shape did not change significantly. With the further increase of the advance distance, the inflection point begins to gradually shift from the outside of the cut to the inside of the cut, indicating that the discontinuous deformation of the surface has occurred after the advance distance of 80, and the shape of the subsidence curve has also changed. The inflection point position is zero when the advance distance is 100 m, and then finally stabilizes in the range of 25m ~ 28m in the goaf. The trajectory path of the inflection point can be simplified as 0m→-60m→0m→25m→28m.

Therefore, it can be judged that the main influence boundary of the 12401 working face to the surface is 60 m outside the open-off cut and 28 m inside the open-off cut.

4.1 Analysis of limit working face length

The threshold of 0.9 of the maximum sinking value above is used to identify the boundary of the fully sinking basin, and the data of 43 measuring points in the tendency of 12401 working face are analyzed, as shown in Fig. 5.

The maximum surface subsidence of the strike survey line is 5.04m, and the threshold boundary is 5.04×0.9 = 4.5 m. According to the data in the diagram, the limit length of the full settlement of the working face is 100m + 98m = 198m, that is, the limit length of the working face is 198m.Therefore, the farther the target working face length is from the threshold (198m), the better the effect of working face loss reduction. For example, the length of 12401 working face is 299.2m, and the boundary of fully sinking basin is 299.2m-100m-98m = 101.2m, that is, the length of basin accounts for 101.2 / 299.2 = 34% of the length of working face.

4.2 Trajectory analysis of the working face inclination inflection point

The dip angle of 12401 working face is 1 ~ 5 °, and the position of the inflection point of the two crossheadings is slightly different but not much different. It is 52 ~ 56m within the goaf. It can be seen that the discontinuous deformation range of the crossheading is 56m within the crossheading. The length of the continuous deformation area of the working face is 299.2m-56m-52m = 191.2m, and the continuous deformation area accounts for 191.2 / 299.2 = 64%.

It is known that the curve measuring line is 300 m away from the cutting hole. Similarly, in order to obtain the change trajectory of the inflection point of the tendency curve, the morphological characteristics of the sinking curve of the tendency measuring line with a propulsion distance of 100 m ~ 800 m are compared and analyzed, as shown in Fig. 6.

Comparing the trend trajectory, it can be found that the trajectory change of the tendency inflection point is also rapidly transferred from 0 to the outside of the mining, then quickly transferred to the side of the goaf, and finally stabilized at 58 m within the goaf. The inflection point path is 0m→-82 m→0m→75m→58 m. Therefore, 82 m outside the roadway boundary and 75 m inside the goaf belong to the mining influence boundary area. Compared with the strike influence boundary, the influence area of the roadway is greater than the open-off cut area, but the range is within 10M (mining height).

5.1 Borehole design

In order to understand the distribution characteristics of overlying strata movement in 12401 working face (Yang 2020,2021), three boreholes are constructed in the mining process. The boreholes are about 1850 m away from the cut hole, which are generally unevenly arranged in a " one-character " shape. The borehole SD1 is a pre-mining construction borehole, which is 175 m away from the return air trough and 125 m away from the main transport trough. The boreholes SD2 and SD3 are post-mining construction boreholes, which are located on both sides of the pre-mining hole. The specific location is shown in Fig. 7 and Table 1.

Table 1

List of basic situation of drilling holes
Number	Termination depth	Quality	logging depth	Beginning date	Completion date	loose layer	Bed rock	Fracture zone
SD1	187.43m	A grade	186.70m	2018.09.19	2018.09.24	69.6m	98.5m
SD2	177.25m	B grade	164.70m	2019.03.5	2019.03.17	44.4m	83.6m	46.8m
SD3	182.52m	B grade	174.80m	2019.03.19	2019.03.27	41.4m	74.3m	72.0m

5.2 Drilling data analysis

In order to accurately obtain the layered surface subsidence parameters of coal seam overburden before and after mining in 12401 working face, nine anchor claw displacement meters were installed in SD1 borehole (187m deep, 167m buried depth of coal seam ) to monitor the continuous surface subsidence of roof at different depths. The installation depth is 41 m, 57 m, 68 m, 79 m, 96 m, 115 m, 124 m, 133 m and 141 m respectively, and the height from the coal seam is 126 m, 110 m, 99 m, 88 m, 71 m, 52 m, 43 m, 34 m and 26 m respectively. The schematic diagram of installation location is shown in Fig. 8(a).

When the working face pushes through the borehole 6m, that is, the borehole enters the rear of the support top beam, the measuring points of the coal seam overburden begin to have displacement changes ; when the working face passes through the borehole for 10m, the displacement meter readings of No.5 ~ 9 measuring points (26 ~ 71m from the roof height ) change greatly at the same time, and the fracture or dislocation of the rock mass structure where the borehole is located is judged. In the range of 40 ~ 100m in the goaf, the displacement meters of No.3 ~ 9 measuring points settle rapidly, and the synchronization is good, and then gradually recovers to be stable, while the displacement meters of No.1 ~ 2 measuring points in the shallow part (110 ~ 126m from the roof height ) always have only a small surface subsidence. After the local surface observation hole enters the rear 180m of the goaf, the subsidence of the coal seam overburden gradually tends to be stable. As of April 30, 2019, the relative surface surface subsidence and total surface subsidence of each measuring point displacement meter are shown in Fig. 8(b).

By analyzing the relative change of displacement of each measuring point, it is found that the displacement of each measuring point is larger than that of the upper part, which is in line with the general law of stratum surface subsidence in goaf. The displacement of 9 measuring points shows the characteristics of rock strata. The overall displacement of No.1 and No.2 measuring points is the smallest, the overall displacement of No.3,4 and No.5 measuring points is medium, and the overall surface subsidence of No.6,7,8 and No.9 measuring points is larger. All the subsidence of 12401 overburden strata increased rapidly at 0–10 m after mining. Except for No.1 and No.2 measuring points, the subsidence rate of other points increased significantly at 33–69 m after mining, and the rate of No.9 and No.7 measuring points was the largest. The descending rate reflects that the overlying strata of the coal seam have experienced two rapid subsidence periods from subsidence to stability (0-300m after mining ). The analysis shows that the height of the coal seam overburden caving zone is 33.2-33.25m, and the height of the fracture zone is 118.08-132.83m.

The maximum subsidence of the 8.8 m ultra-large mining height working face is 6.2 m, and the subsidence coefficient is 0.72. The mining subsidence accounts for more than 80 ~ 95% of the total subsidence. The threshold value of 0.9 can be used as the identification boundary of the subsidence basin and the limit working face.

The surface of the mining boundary is characterized by an S-shaped subsidence curve. The inflection point trajectory with a secondary derivative of zero is used to characterize the shape of the surface subsidence curve. The inflection point path is first outside the mining area, and then quickly transferred to the side of the goaf. Finally, it is stabilized within 10M ( mining height ) of the goaf. The inflection point position can be used as the boundary point of the discontinuous deformation area.

The source damage reduction optimization space of the western high-strength working face is large, and the length optimization of the working face has a good damage reduction effect. The subsidence basin of the 12401 working face accounts for 34%, and the surface continuous deformation zone accounts for 64%.

Author Contribution

All authors reviewed the manuscript.

Acknowledgments

The research described in this paper was financially supported by National Energy Group (GJNY-21-44).

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

HE Xiang, ZHANG Cun, ZHAO Yixin, et al (2021). Parameters determination of high-intensity mining and reduction effect evaluation based on damage constitutive model of overburden rock [J]. Journal of Mining & Safety Engineering, 38 (03): 439-448.
LI Quansheng, GUO Junting, ZHANG Kai, et al (2021). Damage conduction mechanism and key technologies of damage reduction insources for intensive coal mining in Western China[J]. Journal of China Coal Society, 46(11): 3636-3644.
LI Quansheng, LI Xiaobin, XU Jialin,et al (2022). Research advances in mining fractures evolution law of rock strata and ecological treatment technology[J]. Coal Science and Technology, 50(1): 28-47.
Liu, XJ ,Cheng, ZB (2019) , Changes in subsidence-field surface movement in shallow-seam coal mining[J], Journal of the southern african institute of mining and metallurgy.119 (2): 201-209.
WANG Shuangming, DU Lin, SONG Shijie (2021). Influence of mining ground fissures on soil erodibility in Northern Shaanxi coal mining area of Yellow River Basin[J]. Journal of China Coal Society, 46(9): 3027-3038.
XU Jialin (2020). Research and progress of coal mine green mining in 20 years[J]. Coal Science and Technology, 48(9): 1-15.
XU Zhuhe (2021). Overburden Fracture Migration and Surface Damage in Shallow-buried Coal Seam with High-intensity Mining[D]. China university of mining and technology Beijing.
YANG Junzhe, LIU Qianjin, XU Gang, et al (2021). Strata behavior regularity and overlying strata broken structure of super large mining-height working face with 8.8 m support[J]. Journal of Mining & Safety Engineering, , 38 (04): 655-665.
YANG Junzhe, HU Bowen, WANG Zhenrong (2020). Study on distribution characteristics of collapse zone，fissure zone and curved subsidence zone and layered settlement of overburden on 8.8 m super－large mining height coal mining face[J]. Coal Science and Technology, 48( 6) : 42－48.
YANG Junzhe, LIU Qianjin (2020). Analysis and measured of strata behavior law and mechanism of 8.8 m ultra－high mining height working face[J]. Coal Science and Technology, 48 (1) : 69－74．
ZHANG Jian-min, LI Quan-sheng, HU Zhen-qi, et (2013). Study on Ecological Restoration Mode of Ultra Wide Fully-Mechanized Coal Mining in West China Aeolian Sand Area[J].Coal Science and Technology, 41(9):173-177.
ZHANG Kai, LI Quansheng, DAI Huayang, et al (2020). Research on integrated monitoring technology and practice of“space-sky-ground”on surface movement in mining area[J]. Coal Science and Technology, 48(2):207-213.

No competing interests reported.

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Surface damage reduction effect of super long working face with large mining height in Shangwan Coal Mine

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Abstract

Figures

1. Introduction

2. Mining surface monitoring of Shangwan 12401 working face

3 Analysis of advancing direction (strike) measuring points

3.1 Mining subsidence and residual subsidence

3.2 Trajectory analysis of advancing direction inflection point

4. Analysis of working face inclination measuring points

4.1 Analysis of limit working face length

4.2 Trajectory analysis of the working face inclination inflection point

5. Analysis of overburden rock characteristics

5.1 Borehole design

5.2 Drilling data analysis

6. Conclusions

Declarations

Author Contribution

References

Additional Declarations

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