Study on development height of water flowing fractured zone in shallow thick coal seam mining of Daheng Coal Mine

doi:10.21203/rs.3.rs-1737104/v1

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Study on development height of water flowing fractured zone in shallow thick coal seam mining of Daheng Coal Mine

https://doi.org/10.21203/rs.3.rs-1737104/v1

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The height of water flowing fractured zone is a key parameter to implement water-retaining mining technology. In order to study the development height of water flowing fractured zone in shallow thick coal seam mining in ecologically fragile areas of western China, taking 91105 working face of Daheng Coal Mine as the research object, the height of water flowing fractured zone is comprehensively determined by empirical formula, rock tensile rate formula, field test and numerical simulation. The results show that the formula calculation and numerical simulation both are close to the measured results, the upward slant hole observing method is reliable for the height observation of water flowing fractured zone. The maximum height of water flowing fractured zone is 53.62 m, the fracture production ratio is 12.32, and the development form is ' saddle '. Compared with the empirical formula, the calculation results of rock tensile rate formula are closer to the measured results, which improves the prediction accuracy of water flowing fractured zone height. It shows the feasibility of water flowing fractured zone height calculation based on rock tensile rate, and provides a new idea for water flowing fractured zone height prediction. Based on the geological conditions of 91105 working face, the numerical simulation analysis shows that the development of water flowing fractured zone undergoes three stages of slow increase, sudden increase and stability. The research results can provide guidance for the implementation of water-preserved coal mining and ecological environment protection in thick coal seams in ecologically fragile areas in the west.

With the strategic Westward shift of china's coal industry, western coal production increased year by year. However, there is a lack of groundwater resources and fragile ecological environment in the western region, most areas belong to ecological fragile areas, the negative impact of coal mining on groundwater resources and ecological environment under the condition of high intensity mining is far greater than that of eastern mining area. Maintaining a coordinated development relationship between safe and efficient coal mining and groundwater resources protection has become a key problem for the realization of green coal mining in the ecologically fragile areas of western China^1–3. The height of water flowing fractured zone is a prerequisite for the safety analysis of water preserving mining^4–6. Therefore, it is of great significance to study the development height of water flowing fractured zone to scientifically guide water-retaining mining and ecological environment protection in ecologically fragile areas of western China.

Many scholars have done a lot of research on the height of water flowing fractured zone in coal mining in western ecologically fragile areas. Fan et al.⁷ summarized the research results of water preserving mining in western mining area in the past 30 years, and gave the prediction model of the development height of water flowing fractured zone in Yushenfu mining area under certain mining height. Xu et al.⁸ put forward the method of predicting the height of water flowing fractured zone based on the position of key strata and applied it in Shendong mining area, guiding the prevention and control practice of roof water inrush disaster in Bulianta Coal Mine and Qidong Coal Mine. By using the method of similar material simulation, numerical simulation and field test, Hou et al.⁹ analyzed the overburden's destructive rules and the development characteristics of water flowing fractured zone in shallow seam mining of Ningtiaota Coal Mine in Yushen Mining Area, which provided theoretical guidance for the groundwater resources protection in shallow seam mining in western mining area. By means of similar material simulation and numerical simulation, Xu et al.¹⁰ studied the development and evolution law of water flowing fractured zone of overlying strata in coal mining of Dananhu mining area, and analyzed the feasibility of water retaining coal mining in this area. Taking Shendong mining area as the research object, Ma et al.¹¹ comprehensively analyzed the degree of overburden distruction under mining conditions by using physical simulation, numerical simulation and field test methods, and proposed a formula for calculating the height of water flowing fractured zone which is widely used in Shennan mining area.

The above research results enrich the theoretical research on the height of water flowing fractured zone, but the research on the height of water flowing fractured zone in shallow-buried thick coal seam mining in western ecological fragile area is not systematic and comprehensive. 91105 working face of Daheng Coal Mine is a typical shallow-buried thick coal seam mining in the western ecological fragile area. There are multi-layer aquifers in the roof, which seriously threatens the safety production of the working face. Therefore, it is necessary to accurately determine the development height of the water flowing fractured zone, so as to provide accurate theoretical parameters for the implementation of water-preserved mining technology.

The author studied the development height of water flowing fractured zone in 91105 working face by means of empirical formula calculation, rock tensile rate formula calculation and field measurement. The numerical simulation is used to verify it, and the dynamic development process of water flowing fractured zone in the roof of the working face is further studied, which provides a reference for the application of water-retaining coal mining technology under similar mining geological conditions in the western ecological fragile area.

Daheng coal mine is located in northwest Shanxi loess plateau area, ravines vertical and horizontal distribution, semi-arid climate, windy sand, rare surface vegetation, belongs to the typical western ecological fragile area. The 91105 working face of Daheng Coal Mine in the study area has a strike length of 876.3 m and a tendency length of 145.5 m. The strike long wall layout is adopted, and comprehensive mechanized mining 9-1 coal seam. The roof is managed by fully caving method, the 4,8 coal seams on the working face are not mined. The thickness of 9-1 coal seam is 4.1~5.1 m, the average thickness is 4.35 m, the dip angle is 0°~6°, the average dip angle is 3°, and the buried depth of coal seam is about 185 m, most of the surface is covered by loess coating, which belongs to shallow buried and near horizontal thick coal seam. The XK9 borehole histogram of the working face is shown in Figure 1.

The roof strata are mainly composed of siltstone, sandy mudstone and sandstone, showing the distribution of soft and hard phases, and the overall water insulation performance is good. The main aquifers in the upper part of working face include: (1) Sandstone fissure aquifers of shanxi formation and taiyuan formation. The local fractures are relatively developed and have certain water-bearing conditions. The permeability coefficient is 0.037m/d, the unit water inflow is 0.017 L/s·m, and the water abundance is weak. (2) Sandstone fissure aquifers of upper and lower stone box formation. The local fractures are more developed, and a small amount of bedrock outcrops in the valley. The atmospheric precipitation infiltration recharge is extremely limited, and the water abundance is weak. (3) Tertiary and quaternary pore aquifer. The aquifer is widely distributed in the mine field, with poor recharge conditions, poor continuity and weak water-richness.

Empirical formula

The roof of 91105 working face belongs to medium hard rock stratum. According to ' Specification for Coal Pillar Retention and Coal Mining in Buildings, Water Bodies, Railways and Main Roadways '¹² (hereinafter referred to as ' Specification '), the calculation formula of height of water flowing fractured zone under the condition of medium hard rock is

$${H}_{li}=\frac{100M}{1.6M+3.6}\pm 5.6 \left(1\right)$$

In the formula, ${H}_{li}$ is the height of water flowing fractured zone, m. $M$ is the cumulative mining thickness, taking 4.35 m.

Through formula (1) calculation, the predicted height of water flowing fractured zone in 91105 working face is 35.59 ~ 46.79 m.

The height calculation of water flowing fractured zone based on tensile rate of rock stratum

According to the research^13,14, under the influence of mining, the tensile deformation of roof strata determines whether the fractures produced by overlying rock breaking penetrates and conducts water. The tensile deformation degree of rock formation can be expressed by the tensile rate of intermediate layer of rock stratum. Therefore, this paper takes tensile rate of intermediate layer of rock stratum as the criterion, the development degree and water conductivity of rock fracture are analyzed. Taking the nth layer above the direct roof as the analysis rock formation, the tensile rate of intermediate layer of rock stratum¹⁵ is

$$\epsilon =\left({l}_{1}-{l}_{0}\right){l}_{0} \left(2\right)$$

Where

$${l}_{0}=h\left({cot}\delta +{cot}\psi \right) \left(3\right)$$

$${l}_{1}=\left({w}_{i}^{2}+{l}_{0}^{2}\right){arcsin}[2{w}_{i}{l}_{0}/\left({w}_{i}^{2}+{l}_{0}^{2}\right)]\pi /(180\cdot 2{w}_{i}\left) \right(4)$$

In the formula, ${l}_{0}$、${l}_{1}$ are the length before and after tensile deformation of intermediate layer of rock stratum, m. $h$ is the horizon height of intermediate layer of rock stratum of coal seam roof, m. $\delta$ is the coal seam mining boundary angle. $\psi$ is the angle of full subsidence. ${w}_{i}$ is the maximum subsidence of intermediate layer of rock stratum, m.

After coal seam mining, it provides free movement space for roof strata bending subsidence, and the goaf will be filled by the broken rock formation. Considering the broken expansion coefficient of each rock formation, the maximum subsidence¹⁶ of intermediate layer of rock stratum of the $i$th layer above the coal seam is

$${w}_{i}=M-{\sum }_{j=1}^{n}{h}_{j}\left({k}_{j}-1\right)-{\sum }_{l=1}^{n}{h}_{l}\left({k}_{l}-1\right) \left(5\right)$$

In the formula, M is the mining thickness, m. ${h}_{j}$ is the thickness of the $j$th lower layer direct roof strata, m. ${k}_{j}$ is the expansion coefficient of the immediate roof strata of the $j$th lower layer. ${h}_{l}$ is the thickness of the lower $l$th layer basic roof strata, m. ${k}_{l}$ is the expansion coefficient of the basic roof strata of the $l$th lower layer.

By the above formula (2)~(5) calculation, the tensile rate of intermediate layer of rock stratum in the roof of the coal seam can be obtained. According to the critical tensile rate of the upper limit strata of the water flowing fractured zone given in the literature¹⁷, that is, the hard rock layer is less than 0.04%, the medium hard rock layer is 0.1%~0.24%, and the soft rock layer is more than 0.4%, the development height of the water flowing fractured zone is finally determined.

Based on the above analysis, the 91105 working face in this paper is taken as the research object for calculation. The study¹⁸ shows that in the vertical direction, the relationship between the average expansion coefficient of overlying strata in goaf and the distance from the roof of the coal seam is

$$k={k}_{z}-0.017{ln}{h}_{m},\left({h}_{m}<100\right) \left(6\right)$$

In the formula, ${k}_{z}$ is the residual expansion coefficient of the immediate roof strata of the lower layer.

According to the literature¹⁹ and combined with the site, 91105 working face ${k}_{z}$ takes 1.08, ${cot}\delta$ and ${cot}\psi$ take 0.577. Combined with the XK9 borehole histogram of the working face, the calculation parameters of the tensile rate of intermediate layer of rock stratum at different horizon heights are shown in Table 1.

Table 1

calculation parameters of the horizontal tensile rate
Serial number	Rock formation	Expansion coefficient	The horizon height of intermediate layer (m)	The subsidence of intermediate layer (m)	Tensile rate (%)
14	Mudstone	1.01	54.37	2.76	0.09
13	Medium sandstone	1.01	48.13	2.89	0.13
12	Sandy mudstone	1.01	43.92	2.94	0.17
11	4 − 1 coal	1.02	37.34	3.10	0.29
10	Mudstone	1.02	32.49	3.13	0.41
9	Fine sandstone	1.02	28.51	3.26	0.60
8	Sandy mudstone	1.03	24.40	3.32	0.87
7	4 − 2 coal	1.03	20.89	3.47	1.32
6	Fine sandstone	1.03	15.87	3.62	2.53
5	Sandy mudstone	1.04	12.44	3.69	4.31
4	Siltstone	1.04	8.64	3.92	9.96
3	8 coal	1.05	5.19	3.98	27.21
2	Siltstone	1.08	2.30	4.35	23.06

It can be seen from Table 1 that the tensile rate of rock formation gradually decreases with the increase of the horizon height of intermediate layer. The roof strata of 91105 working face belongs to medium hard rock formation, and the critical tensile rate of the top boundary strata in the water flowing fractured zone is 0.1%~0.24%, while the tensile rate between No.13 middle sandstone and No.14 mudstone decreases from 0.13–0.09%. Therefore, No.13 middle sandstone belongs to the water flowing fractured zone, and No.14 mudstone belongs to the bending subsidence zone. At this time, the development height of water flowing fractured zone is 49.79 m.

Observation method

There are many field test methods to determine the height of water flowing fractured zone^20–23, among which the borehole method is the method with high observation accuracy at present. Therefore, this field test adopts the upward slant hole observing method, the borehole field is excavated in the section roadway of the adjacent working face of the underground coal mining face or the stop line of the measured working face or the roadway outside the open-off cut. According to the observation scheme, the oblique hole is constructed to the roof above the goaf. The borehole construction length should exceed a certain distance above the predicted top boundary height of the fracture zone, and ensure that the borehole does not pass through the caving zone. The double end water shutoff device is used to block water injection step by step, and the leakage of each section after pressurized water injection is observed, so as to understand the fracture and loosening of the overlying strata and determine the upper boundary height of the fracture zone. The observation principle is shown in Fig. 2.

Observation program design

The maximum height of the water flowing fractured zone calculated by the above prediction formula is 49.79 m. In order to ensure that the observation boundary exceeds the height of the actual failure zone, the vertical depth of the borehole is extended by 11 ~ 15 m. The dip angle of 91105 working face is 3° on average, belonging to the near-horizontal coal seam. It is expected that its stable form is that the starting point is within the coal body, and the boundary gradually shifts to the coal body, and the highest point is in the saddle shape of the inner side of the goaf of the mining boundary²⁴.

Considering the stability of surrounding rock, convenience of construction and limitation of construction conditions at the observation position (the inclination angle of borehole is between 45° and 55°), the borehole field is arranged in the transport contact roadway about 41 m away from the final mining line of the working face, and a pre-mining borehole (CQ1) and two post-mining boreholes (CH1 and CH2) are designed and constructed. of which the CH1 borehole is used to observe the middle position of the saddle top, and the CH2 borehole is used to observe the highest point of the saddle top. The layout of the observation borehole is shown in Fig. 3.

During the observation, a contrast borehole is arranged before the mining of the working face to observe the development of primary fractures in cover rock, two post-mining boreholes were arranged after the mining of the working face to observe the development of fractures in cover rock after mining. Due to the influence of mining, the overlying strata movement and deformation, borehole is difficult to form, considering the influence of lithology of overlying rock and mining thickness, the reasonable observation time of water flowing fractured zone height should be one month after the end of working face mining.

3.3 Analysis of observation results

By sorting out the observation data of each borehole, the distribution of water injection leakage in borehole is shown in Fig. 4. Comparing the change of water injection leakage in each hole section, the height of water flowing fractured zone is determined.

(1) CQ1 pre-mining borehole

From the Fig. 4, it can be seen that the average water injection leakage of the Pre-mining contrast borehole CQ1 is about 2.5 L/min under the condition that the overlying strata on the working face is not affected by mining, indicating that the rock mass structure of the roof is complete before mining the working face. The local leakage changes in the range of 10.2 ~ 12.3 L/min, indicating that there are relatively developed primary microcracks or caves in local rock strata.

(2) CH1 post-mining borehole

Compared with water injection leakage of pre-mining borehole, the leakage of water injection in the post-mining borehole of CH1 is 19.8 ~ 32.5 L/min at the borehole depth of 61 ~ 73 m (vertical height 43.13 ~ 51.62 m), and the leakage is significantly increased, indicating that the rock in this section is seriously damaged. The borehole has penetrated the fracture zone, and a large number of new fractures were produced in the rock affected by mining. In the range of borehole depth 74.5 ~ 89 m (vertical height 52.68 ~ 62.93 m), there are two abnormal boreholes, namely borehole depth 82 m (vertical height 57.98 m) and borehole depth 86.5 m (vertical height 61.16 m), and the leakage is 11.8 ~ 13.4 L/min. Compared with the pre-mining borehole analysis, it is considered that the hole section is the original micro-fracture area and the non-mining fracture area. The leakage of other hole sections is 2.1 ~ 3.5 L/min, indicating that the height of fracture zone is not developed here. Therefore, the maximum development height of the water flowing fractured zone in the working face determined by the CH1 borehole is at the borehole depth of 74.5 m, developing to mudstone 52.68 m away from the roof of working face.

(3) CH2 post-mining borehole

Compared with water injection leakage of pre-mining borehole, the leakage of water injection in the post-mining borehole of CH2 is 15.8 ~ 28.3 L/min at the borehole depth of 61 ~ 68.5 m (vertical height 46.73 ~ 52.47 m), and the leakage is significantly increased, indicating that this section is the top of the water flowing fractured zone. In the area of 70 ~ 82 m in borehole depth (53.62 ~ 62.82 m in vertical height), the leakage of most hole sections is about 2.7 L/min, which is not significantly different from that of the corresponding section of the pre-mining borehole, indicating that the rock stratum of this hole section is relatively complete. Other local leakage is 9.1 ~ 11.0 L/min, which is obviously higher than that of the same hole section of the pre-mining borehole, indicating that there are small primary fractures in this section. Therefore, the maximum development height of the water flowing fractured zone in the working face determined by the CH2 borehole is at the depth of 70 m, developing to mudstone 53.62 m away from the roof of working face.

In summary, the height of water flowing fractured zone measured by CH1 borehole is 52.68 m, and that measured by CH2 borehole is 53.62 m. Taking the maximum value of the two, the height of water flowing fractured zone in 91105 working face is 53.62 m, and the fracture mining ratio is 12.32. The top boundary of water flowing fractured zone is located in the mudstone 53.62 m away from the roof of working face. According to the observation results and the borehole layout position, it is determined that the water flowing fractured zone of 91105 working face presents the characteristics of saddle shape, as shown in Fig. 5.

Model building

In order to verify the reliability of the measured results and further study the dynamic development process of the roof water flowing fractured zone under different advancing distances, based on the geological conditions of 91105 working face, the FLAC^3D numerical calculation model is established to analyze the development height of the water flowing fractured zone and the overburden rock failed state in 91105 working face. The strike length of the model is 300 m, and the dip length is 145 m. The front, rear, left and right of the model are set with 50 m boundary coal pillars, and the design size is long × wide × high = 400 m × 245 m × 215 m. The front, rear, left, right and bottom of the model are fixed boundary, and the top is free boundary, and the Mohr-Coulomb yield criterion is used as rock failure criterion. The excavation step is 25 m, and the total excavation is 300 m. The numerical calculation model is shown in Fig. 6, and the physical and mechanical parameters of coal strata are shown in Table 2.

Table 2

Rock mechanics parameters of working face
Lithology	Density (kg/m³)	Cohesion (MPa)	Tension (MPa)	Friction (°)	Bulk (GPa)	Shear (GPa)
Loess formation	1970	0.96	0.80	20	3.73	1.56
Gritstone	2750	3.10	1.90	35	7.92	5.42
Medium sandstone	2700	2.43	1.52	34	8.38	5.75
Fine sandstone	2830	4.30	1.40	36	8.54	5.82
Mudstone	2476	2.10	0.89	29	5.20	3.43
Sandy mudstone	2418	2.52	0.95	32	6.37	3.94
Siltstone	2380	3.00	1.50	33	7.33	4.60
Coal seam	1400	2.20	0.50	31	3.89	2.96

Simulation result analysis

After coal seam mining, the stress of surrounding rock of working face is redistributed. The overlying strata can be divided into tensile failure zone, shear failure zone and elastic zone from bottom to top²⁵. The failure modes of water flowing fractured zone are mainly shear failure and tensile failure. Therefore, the plastic failure height of roof strata can be regarded as the height of water flowing fractured zone. The variation curves of the maximum plastic failure height of the roof under different excavation steps are shown in Fig. 7. The plastic zone of the roof failure when the working face advances 250 m is shown in Fig. 8.

It can be seen from Fig. 7 that the plastic failure height of roof strata shows an overall increasing trend with the advancement of working face, and its variation curve can be divided into the following three stages:

(1) Slow increase stage: after the working face advancing from the open cut, the direct roof breaks down to the goaf after reaching the limit span, the plastic failure of the roof begins to transfer upward, and the failure height increases slowly.

(2) Sudden increase stage: with the increase of the advancing distance, the plastic failure of the roof develops to the overlying hard strata, and the rock stratum fracture loses the bearing capacity, and the failure height increases rapidly.

(3) Stabilization stage: when the working face advances to 250 m, the full extraction of overburden destruction is achieved, and the roof plastic failure height develops to the maximum. Since then, as the working face continues to advance, the overburden failure height will no longer increase.

It can be seen from Fig. 8 that when the working face advances to 250 m, the plastic zone of the roof at the position of the opening and cutting hole of the working face and the position ahead of the coal wall is mainly shear failure, while the plastic zone of the middle roof above the goaf is mainly tensile failure, and the shape of the plastic zone of roof is saddle-shaped with low in the middle and high on both sides. At this time, the height of the water flowing fractured zone reaches the maximum of about 58 m, which is basically consistent with the height of the water flowing fractured zone determined by the upward slant hole observing method.

The development height results of water flowing fractured zone obtained by traditional empirical formula, rock tensile rate formula, field test and numerical simulation are comprehensively compared. The results are shown in Table 3. It can be seen from Table 3 that the calculation results of the predicted formula and the numerical simulation analysis results are all close to the measured results, indicating the accuracy and reliability of the measured results. Therefore, this paper takes the measured value as the height of water flowing fractured zone in 91105 working face of Daheng Coal Mine, which provides reference value for the detection of the height of the water flowing fractured zone in the mining area. In the prediction formula, the difference between the calculation results of the traditional empirical formula and the measured results is large, while the difference between the calculation results of rock tensile rate formula and the measured results is small, which further illustrates the feasibility of the calculation of the height of the water flowing fractured zone based on the rock tensile rate, and provides a new idea for the prediction of the height of the water flowing fractured zone in the mining area.

Table 3

Comprehensive comparison table of water flowing fractured zone height
Prediction formulas		Field test	Numerical simulation
Empirical formula	Rock tensile rate formula	Field test	Numerical simulation
35.59 ~ 46.79m	49.79m	53.62m	58m

(1) The comparative analysis of borehole leakage between pre-mining borehole and post-mining borehole shows that the height of water flowing fractured zone is 53.62 m, the fracture mining ratio is 12.32, and its development morphology is saddle-shaped. At the same time, the numerical simulation is used to verify, the numerical simulation results are basically consistent with the measured results, and the reliability of field test were determined.

(2) Based on the geological conditions of Daheng Coal Mine, the height of water flowing fractured zone calculated by empirical formula and rock tensile rate formula is compared with the measured results. The calculation results of rock tensile rate formula are closer to the measured results, indicating the feasibility of the calculation of the height of water flowing fractured zone based on the rock tensile rate, which provides a new idea for the prediction of the height of water flowing fractured zone in shallow buried thick coal seam mining in the ecologically fragile area of the west.

(3) The numerical simulation results further show that the maximum plastic failure height of roof strata presents an overall increasing trend with the advancement of the working face, and its change can be divided into three stages: slow increase, sudden increase and stability.

Data availability

Source data are provided with this paper. Other data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

X.C.: Conceptualization; Methodology; Analysis; writing—original. M.W.: Field testing; Lab testing; Analysis; Writing; Review & Editing. W.Z.: investigation; data collection. J.M.: Field testing; Review & Editing.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to M.W.

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No competing interests reported.

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Study on development height of water flowing fractured zone in shallow thick coal seam mining of Daheng Coal Mine

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Abstract

Figures

Introduction

Project profile

Prediction Of Development Height Of Water Flowing Fractured Zone

Empirical formula

The height calculation of water flowing fractured zone based on tensile rate of rock stratum

Field Test Of Water Flowing Fractured Zone Height

Observation method

Observation program design

3.3 Analysis of observation results

Numerical Simulation Analysis Of Height Of Water Flowing Fractured Zone

Model building

Simulation result analysis

Comprehensive Analysis Of Water Flowing Fractured Zone Height

Conclusion

Declarations

References

Additional Declarations

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