Study on the evolution and transport pattern of overburden fracture induced by mining in cave mines

Abstract

Coal mines located in southwestern China are generally faced with the complex situation of repeated mining of coal seams in areas with dense karst caves. In view of the problems brought about by overburden fracture transport-induced karst cave instability during repeated mining of karst mines, this paper takes the repeated mining under the karst mines of Qinglong Coal Mine in Guizhou, China, as a background for the study.Using a combination of theoretical analyses, physical experiments and numerical simulations, the influence of karst caves on the transport of the overlying rock layers and the height of the development of water-conducting fissure zones was analysed from the conditions of repeated mining in karst mines.The results show that the existence of repeated mining karst caves accelerates the overburden fracture fissure penetration into the upper coal seam air-mining zone, and at the same time, the analysis of the stress distribution of the caves obtains the judgement basis for the instability of the caves.The DIC digital scatter monitoring system found that the displacement of the overlying rock layer of the cave model at the monitoring point was significantly larger than the displacement of the overlying rock layer of the no-cave model as the working face progressed.The displacement rate of the cavern model is obviously larger under repeated mining, but the cavern plays a key role in controlling the fracture of the overburden rock when the working face advances to the bottom of the cavern, and the cavern is re-stabilised after instability, and it still has a certain supporting effect on the overburden rock above.In order to verify the above analysis using 3DEC numerical simulation to verify it again, it was found that the results are still shifted more at the karst cave, and showed asymmetric subsidence after the cave was perturbed to return to the steady state, which is highly similar to the results of the physical experiments.Therefore, before the coal seam is mined to the vicinity of the karst hole, the roof of the working face can be grouted and reinforced in advance, and the working face can be pushed through the vicinity of the karst hole quickly, and the results of the above research provide certain reference for the safe mining in karst mines.

Introduction

The hosting conditions of coal seams are very complex, especially in Southwest China, where large areas of karst mines exist^1–4.Mining under karst mines will face two problems: first, karst water poses a threat to mine safety; second, karst areas have fragile ecological environments, and underground mining of coal seams causes rock movement, inducing the development of mining fissures and karst fissures, which has a greater impact on the ecological balance of the dissolved rock areas⁵. At the same time, repeated mining of multiple coal seams is likely to cause overburden force instability, exacerbating overburden deformation, breakage, and transport^6–10. The overburden rock fissure evolution law and development height caused by mining has many influencing factors, such as the thickness of the coal seam mining, the span of the working face, the structure of the overburden rock layer, etc. Many scholars have used the methods of theoretical analysis, numerical simulation, physical modelling, and the DIC monitoring system to investigate the overburden rock movement law and the fissure evolution law^11–22. In particular, repeated mining under karst mines will make the change of overburden more complicated, so it is necessary to conduct research on the study of overburden transport and fissure evolution law for repeated mining of coal seams containing hidden cavities.

Currently, many scholars have studied the transport law and fracture development of overburden rock under mining as well as repetitive mining in karst mines, for example, Zhang, J²³. et al. constructed a mechanical model using thin plate theory and cloud key layer, analysed the initial and periodic roof breakage of conventional working face, and studied the movement and instability characteristics of the roof layer under the conditions of variable amplitude mining of shallow buried thin coal seams. Li, X. P²⁴ et al. used theoretical analysis, numerical simulation and engineering verification in order to investigate the cracking characteristics and pressure law of coal seams under close coal seams. Zhao, Q²⁵. et al. studied the coal mining activities under karst mine, revealed the relationship between controlling the karst fissure development and surface slope deformation, and used UDEC to simulate the overall deformation response of mining under the rock-bearing karst caves, and the results showed that the bottom of the karst fissure is the first to be easily cracked during the mining of coal seams due to the concentration of stresses, and the serious areas of slope deformation overlap with the mining pressurised areas. Wang, Y. L²⁶et al. designed a similar model for mining under karst aquifers for coal seam mining under karst aquifers, and elucidated the fracture evolution law and fracture seepage dynamic evolution law for mining under karst aquifers. Zhao, B²⁷. et al. proposed an elastic wave exploration method based on key stratigraphic theory to predict the height of hydraulic fracture zones in order to understand the development of hydraulic fracture in the overlying strata during coal seam mining. Liu, Y²⁸., Ju, J. F²⁹., Mokhov, A. v³⁰. and others have studied the development of overburden fissures in the course of mining activities to varying degrees by different research methods. Yang, Z. P³¹. et al. study the spatial and temporal evolution of mining-induced destabilisation of overlying karst mountains, and analyse the changing law of overburden pressure and displacement during the mining process.The above scholars have studied the development of fissures under karst mines and the overburden rock transport law and fissure development during repeated mining. However, few scholars have studied the transport and fissure evolution law of overlying rock during repeated mining of karst cavities under karst mines. Therefore, this paper takes Qinglong Coal Mine in Guizhou, China, as the background, and uses similar simulation and numerical simulation to study the transport and fissure evolution law of the overlying rock during repeated mining of coal seams containing hidden cavities, so as to provide certain reference for the safe mining of karst mines.

Project overview

The coal seams studied in this paper are mainly 16 and 18 coals of Qinglong Coal Mine in Guizhou, China. 16 coal has an average thickness of 2.4 m, a dip angle of about 9°, a direct top of siltstone and a basic top of fine sandstone. The average thickness of 18 coal seam is about 3.1 m, the dip angle is about 5°, the direct top is siltstone, and the basic top is carbonaceous siltstone. The average spacing between the two coal seams is 15.1 m, and the cavern is located about 40 m above the 16 coal seam, and the comprehensive histogram of the coal seam production is shown in Fig. 1. In this paper, 16 and 18 coals are taken as the research objects to investigate the influence of karst holes on the transport and fissure evolution of the overlying rock layer in the repeated mining of coal mines. Some karst caves are shown in Fig. 2.

Fig. 1.

Histogram of coal seams.

Fig. 2.

Some karst caves.

Analysis of fissure development pattern and destabilisation damage of the cavern

Analysis of fissure development pattern

With the advancement of the working face, the stress balance of the quarry is destroyed, and the overlying rock layer in the mining area is also destroyed. According to the degree of destruction of the overlying rock movement, from top to bottom can be divided into bending zone (or the overall movement of the zone), crack zone and collapse zone ‘three zones’³², as shown in Fig. 3

Fig. 3.

Schematic diagram of the three zones of the overburden.

The development pattern of cleavage and the general development to within the cleavage zone, according to the empirical formula of the National Coal Industry Bureau, the height of cleavage development is calculated as follows (Eq. 1).

1

The test formula is obtained based on the analysis of a large number of observation data in the field, which does not take into account the factors affecting the movement of the strata, such as the penetration of the upper coal seam and the lower coal seam of the air-mining zone in the repetitive mining of the coal seam in close proximity in this study, which will make the change in the development of the fissure increase. Therefore, this study uses the correction formula for calculation³³. Where M is the total thickness of the two coal seams. Bringing in the values gives H_i = 59.35 m.

2

Destabilisation damage analysis of the cavern

The morphology of the overlying caverns in karst mining area is strange and irregular, and this thesis simplifies them into circular caverns to analyse their force instability. For the calculated height of the water-conducting fissure zone, it is found that when the coal seam is mined, the overlying rock caverns are located in the water-conducting fissure zone, and the caverns will be penetrated with the development of the fissure and destabilisation occurs, and there are two kinds of destabilisation damage of caverns, one is that there exists a weak structural surface around the caverns, and another is that there does not exist a weak structural surface³⁴, which is analysed as follows:Idealising the cavern as a homogeneous elastomer, the force applied to the cavern can be divided into vertical stress and horizontal stress according to the elasticity theory, and its stress analysis is shown in Fig. 4. The horizontal stress P₂ and vertical stress P₁ suffered by the cavern are regarded as an infinite stress concentration problem, and according to the Ziersey solution for solving the plane problem in the theory of elasticity, its solution formula can be obtained as:

3

where R is the radius of the cavern, m; L is the distance from a monitoring point to the centre of the cavern, m; σr, σ_t,—are the radial, tangential and shear stresses, respectively; P₁ is the vertical stress; P₂ is the horizontal stress; and θ is the angle of the horizontal direction.

Fig. 4.

Force analysis of the cavern.

(1) When there is a weak structural surface around the cavern, the integrity of the rock body will be damaged and the strength of the rock body will be reduced, therefore, the weak structural surface plays a decisive role in the stability of the cavern, and its stress is analysed as follows:

According to the geometrical relations in Fig. 5, it is obtained that the distances L_A and L_B from the centre of the cavern at both ends of the structural surface AB are:

4

Fig. 5.

Structural surface analysis model for karst presence.

After simplification its expression is as follows:

5

The analysis of the single structural surface theory proposed by Jaeger gives the normal stress σ_n and the shear stress τ_n, respectively:

6

According to the stress analysis of the circular cavern, it can be seen that the stress conditions around the cavern are:σ₃ = σ_r, σ₁ = σ_t, Substituting into Eq. (6) simplifies to give:

7

The cave instability criterion takes the average value of the stresses applied at both ends of the structural face as follows:

8

Style: σ_A, σ_B—are the normal stresses at the ends of the structural plane AB, respectively;

τ_A, τ_B—are the shear stresses at the ends of the structural plane AB, respectively;

The structural surface stability can be obtained from the Moore Cullen criterion with the following criterion:

9

Style: c—Structural surface cohesion;

φ—Angle of internal friction on structural surfaces.

In Eq. (9), when K < 1, the structural surface is damaged and the distance of the damage range of the cavern is AB; when K > 1, tNo damage to structural surfaces.

(2) When there is no weak structural surface around the cavern, the rock structure is not damaged and the integrity is good, then there exists L = R around the cavern, and its stress distribution is obtained as follows:

10

Then analysed according to the Griffith’s strength criterion, it is obtained that the, Maximum principal stress σ₁ and Minimum principal stress σ₃ relate to σ₁ + 3σ₃ < 0, Its cave instability criterion is:

11

Fail σ1+3σ3>0, Its cave instability criterion is:

12

R_L is the uniaxial mineral compressive strength of the rock at the layer where the karst cave is located, which is 45 MPa.In the periphery of the karst cave without weak structural surface, there are σ_r = σ₁ and σ_t = σ₃ = 0, which can be brought into Eqs. (11) and (12) to make a judgement on the destabilisation damage of the cave:

Experimental design and physical similarity simulation modelling

Physical modelling

The physical similarity model is constructed with the stratigraphic data of Qinglong coal mine in Guizhou, China as the reference background, and the column diagram of its coal seams is shown in Fig. 1, and this simulation mainly mines the 16th and 18th coal seams, in which the location of the cavern is located in the position of 40 m apart vertically above the 16th coal seam, and 100 m away from the boundary of the model horizontally, and the shape of the cavern is simplified as a sphere with the diameter of 10 m, and the model is shown in the schematic diagram of Fig. 6.

Fig. 6.

Schematic diagram of the experimental model.

Physical experiments were used to analyse the overlying rock transport and fissure evolution law of repeated mining of coal seams 16 and 18 under the influence of karst. Similar simulation experiments were designed to summarise the study of the overlying rock transport and fissure evolution law of coal seam mining with hidden cavities by studying the damage law of the overlying rock layer under the influence of repeated mining.

Analogue tests with similar materials

Physical simulation test is based on scientific rationality and similarity theory, the geological conditions of the quarry of Guizhou Qinglong Coal Mine as the engineering background for the experiment, in order to make the experiment operable and not affect the reliability of the test results, a certain degree of simplification of the model, according to the size of the laboratory similarity simulation test bench model to choose a reasonable similarity constants, the design of the similar material for sand, gypsum, lime, mica, the specific distribution is shown in Table 1. The geometric similarity ratio of the model is: C_L = C_l1/C_l2 = 1:100(C_l1 is the experimental model size, C_l2 is the experimental prototype size), the similarity ratio of capacity to weight is: C_γ = C_γ1/C_γ2 = 1:1.62(C_γ1 is the experimental model capacity, C_γ2 is the experimental prototype bulk weight), the stress similarity ratio is:C_σ = C_L*C_γ = 1:1.162.

Table 1.

Similar material ratios for each rock formation.

Serial number	Lithology	Thicknesses/cm	Ration number	Sand/kg	Lime/kg	Plaster/kg	Water/kg
1	Limestone	77.2	537	467.50	168.30	392.70	56.10
3	Shale	3.1	737	18.29	0.78	1.83	2.09
5	2 Coal	1.4	873	13.69	1.20	0.51	1.54
6	Siltstone	6.2	673	43.37	506.00	216.86	5.06
8	Silty sandstone	18.2	637	171.60	8.58	20.02	20.02
9	Limestone	4.6	537	42.17	15.18	35.42	5.06
10	Siltstone	10.8	746	101.83	6.79	10.18	11.88
11	Siltstone	6.6	673	46.20	5.39	2.31	5.39
13	16 Coal	2.4	873	23.47	2.05	0.88	2.64
14	Siltstone	5.5	673	51.86	605.00	259.29	6.05
15	Charcoal siltstone	7.2	773	69.30	6.93	2.97	7.92
16	Siltstone	2.4	673	22.63	264.00	113.14	2.64
17	18 Coal	3.1	873	30.31	2.65	1.14	3.41
18	Shale	7.2	737	28.88	1.24	2.89	3.30

Laying and excavation of the model

According to the ratio in Table 1, after calculating the materials needed for each layer of the corresponding thickness, use electronic scales to weigh and record, add water to similar materials for mixing, then load the materials into the model in layers and compact them sufficiently until they are laid into the cavern-containing rock layer, and load the similar materials containing the cavern-containing rock layer into the platform so that they are uniformly distributed, and then bury the ice cubes that have been prepared beforehand into the materials, and then the materials will be buried in the ice, the model was then compacted, and after compaction, the model continued to be laid upwards, and after the model was laid, the ice was left to melt to form a spherical cavern. After the model was laid and waited for air-drying, a layer of lime was applied to the surface, and scattering was applied to the lime, and then excavation was carried out, and the DIC numerical scattering instrument monitoring system was used to record and analyse the excavation process. The simulated protective coal pillar of 30 cm was left on both sides of the seam to eliminate the boundary effect of the model. The length of the simulated actual excavation working face is 110 m, and the cutting eye is made 30 cm away from the left boundary, and then each time it is advanced by 10 cm. 16 coal seam is excavated firstly, and after 16 coal seam is excavated, it is left for one day, and then the 18 coal seam is excavated. Figure 7 shows the numerical scatter plot after model laying, and Fig. 8 shows the DIC numerical scatter gauge monitoring system.

Fig. 7.

Similar model laying process.

Fig. 8.

DIC digital scattering spot monitoring system.

Comparison of fracture and fissure evolution patterns in overlying rocks under repetitive disturbances

The research object of this paper is the overburden rock fracture evolution law and the height of fissure development under repeated mining, so the similar simulation is the mining study of the 18 coal seam after the 16 coal seam is fully mined. Under one disturbance, in the cave model, when the mining disturbance has no effect on the stability of the cave, its overburden rock fissure development law and mineral pressure appear similar to the no-cave model, due to the continuous mining of the working face, the top plate of the mining area occurs overhanging broken, overburden rock bending and sinking, overburden fracture fissures are gradually developed upwards, and its initial pressure step is 27 m, 29 m, and the cycle pressure is about 12 m. After mining disturbance affects the stability of the cavern, the overburden rock fracture development law changes, the overburden rock sinking rate accelerates, the overburden rock fissure rapid upward development, and the two sides of the mining area and the cavern is located in the location of the vertical fissure, the formation of the overburden in the vertical direction of the water conduction channel.

Evolutionary pattern of overburden fracture under mining in karst mining area

According to Fig. 9(a), after the 16 coal seam was mined, excavation of the 18 coal seam was started, and after the cutting eye was opened, the roof of the mining area was in good condition and stable; according to Fig. (b), when the working face was advanced to 20 m, the rock strata of the roof of the coal seam was fractured, and the initial collapse occurred; According to figure (c), when the working face advances to 35 m, the transverse fissure of the coal seam roof plate develops upward rapidly, and the overlying rock layer produces obvious off-layer phenomenon, and in the process of advancing the roof plate, the second fracture occurs, but at this time, the fissure is not connected with the mining fissure of the upper coal seam; According to Fig. (d), when the working face advances to 45 m, as the overhanging distance of the roof plate increases, the amount of roof sinking also increases, and the off-layer fissures between the rock layers of the roof plate develop and expand again, and vertical fissures begin to be produced on the two sides of the mining hollow area; According to Fig. (e), when the working face advances to 55 m, the vertical fissures on both sides of the mining airspace penetrate through the rock layer of the top plate to fracture, the overlying rock layer continues to sink, and the rock layer of the off-story fissures continue to develop upwards, but the development rate is relatively slow, and not connected with the 16 coal seam mining after the mining of the airspace, which is due to the lithology of the rock layer between the 18th and 16th seams is mainly siltstone, which is relatively hard; According to figure (f), the working face has advanced to 65 m, at this time from the left side of the cavern is only about 5 m, the lower coal seam overlying rock rapid fracture, fissure rapid development and the upper coal seam overlying rock fissure through, the upper coal seam has been broken rock layer in the second mining disturbances in the influence of the fracture, the overlying rock fissure with the fracture of the rock layer of the secondary fracture, subsidence and once again development, expansion, the upper left side of the cavern fissure is also relatively developed, there is a more obvious subsidence, indicating that at this time the cave due to the influence of the secondary disturbance once again destabilisation damage occurs, the upper part of the left side of the cave is also relatively developed, and there is more obvious subsidence, which indicates that the cave is destabilised and damaged once again due to the influence of the second disturbance; According to Fig. (g), the working face advances to 75 m, although the direct top of the coal seam breaks again, but the collapsed rock block is compacted in the hollow area, and the overlying rock layer at the front end of its working face presents a cantilever beam structure, no fracture occurs, and it is basically in a stable state, and the fissure of the overlying rock layer does not show obvious development and expansion phenomenon, either, it shows that as the working face gradually moves away from the location of the cave, the influence of its mining disturbance on the stability of the cave gradually decreases, and the cave gradually returns to a stable state after another instability and collapse, and its influence on the development of the fissure gradually decreases; According to Fig. (h), at this time, the working face advanced to 15 m to the right of the cavern, and the direct roof of the extraction zone collapsed again, but the fissure of the overlying rock layer did not appear to develop significantly.

Fig. 9.

Simulation of overlying rock fracture evolution pattern under secondary disturbance.

Fracture and fissure evolution of overburden rock under repeated extraction

According to Fig. 10(a), after the 16 coal seam was mined in the similar model without cavern, the 18 coal seam started to be mined, and the roof condition of the mining area was stable after the opening of the cutting eye was completed; According to Fig. b, the working face advances to 30 m, at this time the working face roof still did not appear fracture phenomenon, this is due to the 16th coal and 18th coal between the rock stratum lithology is mainly for siltstone and charcoal siltstone, the rock layer is more hard, and the thickness is larger, so its tensile and shear strength is relatively large, so at this time the roof did not reflect the first time to come to the pressure; According to Fig. c, the working face advances to 50 m, at this time, the coal seam roof plate has a large area of fracture, and collapsed rock broken, this is due to the roof plate overhanging a long distance, the overlying rock layer load and its own load is larger, making the roof plate at the same time produce tension bending damage and shear damage, with the roof plate fracture, the overlying rock layer also occurs bending and sinking, resulting in a fissure, but at this time, the fissure is not with the once disturbed through the fissure; According to Fig. d, the working face advances to 65 m, at this time the top plate is fractured again, and the overlying rock layer is suspended, presenting a cantilever beam structure, but because the top plate of the overlying rock layer is also relatively hard, the overlying rock fissure is still slowly developed, and it is not penetrated with the primary disturbing fissure, whereas comparing with the scenario 1, at this time, the overlying rock fissure has been penetrated with the primary disturbing fissure due to the unstable collapse of the cavern; According to Fig. e, when the working face advances to 75 m, the top plate of the coal seam breaks again, and the overlying rock layer is unable to bear the collapsed rock block of the first coal seam and its own load, and also breaks, and the overlying rock fissure and the primary disturbed fissure pass through, and a water-conducting channel is created between the two coal seams; According to Fig. e, the working face advances about 85 m, at this time the coal seam roof plate produces fracture, the overlying rock layer at the front end of the working face presents a cantilever beam structure and does not fracture, and the overlying rock fissure returns to slow development.

Fig. 10.

Simulation of fracture and fissure evolution of overburden rock under repeated perturbation of cave-free model.

Combined with the above, combined with Fig. 11 can be seen that the cavern model working face advances to 65 m when the 18 coal seam fissure development and 16 coal seam mining after mining the airspace through, and no cavern model in the working face advances to 75 m when the 18 coal seam fissure and 16 coal mining airspace fissure through, the difference between the two is 10 m, indicating that the existence of the karst cavern makes the mining process of the roof plate fissure development rate increases.

Fig. 11.

Comparison of advancing distance between 18 coal seam roof fissure and upper coal seam fissure penetration in hollow area.

Characterisation of overburden displacement evolution based on DIC monitoring

Characteristics of displacement evolution of repetitively disturbed overburden rocks under karst mine area

According to the digital scattering system calculation, the overburden rock displacement evolution law under the secondary disturbance of the cavern-containing model was obtained as shown in Fig. 12. According to Fig. a, after the 16 coal seam was mined, 18 coal seam started to be mined, the karst cave is located at the position of 100 m from the left boundary and 40 m from the lower boundary, after the opening of the cutting eye was completed, it can be seen from the figure that the maximum displacement of the overlying rock is located in the right side below the karst cave and below the karst cave, and the displacement of which is 21.646 mm; As shown from b to d, with the continuous advance of the working face, when advancing to 45 m, although the displacement of the overlying rock layer in the second coal seam mining area is increasing, the maximum displacement of the overlying rock is still located in the lower part of the cavern, and the displacement is 22.205 mm, which indicates that the displacement of the overlying rock layer is only affected by the disturbance of the second mining in the mining process, and secondary subsidence occurs, and the cavern is in a stable state; According to Fig. e, when the working face advances to 65 m, the displacement of overburden rock in the hollow area suddenly increases from 22.205 mm to 23.309 mm, and its displacement is not much different from the right part below the cavern, which indicates that at this time, the left side of the cavern is affected by the secondary disturbance, and it starts to produce destructive failure once again, which increases the amount of overburden rock sinking; According to Fig. f, the workface continued to advance, the cavern continued to collapse under the secondary disturbance, and eventually the maximum displacement of the overburden rock shifted to the left part below the cavern, and the maximum displacement at the cavern at this time was 24.040 mm; According to Fig. g, h shows, the working face advances to 75 m, 85 m its displacement change is relatively small, respectively, 24.567 mm and 24.750 mm, indicating that at this time, the secondary disturbance does not continue to have an impact on the cave, the cave once again back to a stable state.

Fig. 12.

Displacement evolution of rocks underlying karst mines.

Characteristics of the displacement evolution of overburden rocks from repeated mining of karst-free caves

According to the digital scattering system calculations to obtain the overburden rock displacement evolution law under the secondary disturbance of the model without cavities is shown in Fig. 13. According to the Fig. a and b shows, the second coal seam began to mine, study the displacement change of overburden rock at the same position, that is, the same position as the cavern, and found that the working face opened the cutting eye until the mining to 35 m only 0.08 m displacement, overburden rock displacement concentration area is almost no change; According to Fig. c to Fig. f, it can be seen that under the second disturbance, when the working face advances to 50 m, 65 m, 75 m and 85 m, the displacement of the overlying rock layer is 14.396 mm, 14.463 mm, 14.519 mm and 14.722 mm, respectively, and the amount of overlying rock displacement increases again, and the area of expansion of the displacement begins to develop to the left, but there is only one area of concentration of displacement, and this area does not produce obvious changes, indicating that with the continuous advancement of the working face, although the subsidence of the rock layer above the open area occurs again, its subsidence amount is not large. and the area did not produce obvious changes, indicating that as the working face continues to advance, although the overburden rock layer of the mining airspace under the secondary disturbance once again occurred subsidence, but its subsidence is not large, and according to the figure can be seen, the displacement expansion area is gradually to the left side of the development, indicating that the overburden rock did not suddenly fracture and subsidence at a certain moment in this process.

Fig. 13.

Displacement evolution of overburden for the model without cavities.

In summary, the monitoring point is located at the modelled cave, as shown in Fig. 14. The monitoring point is set around the model cave, and the no-cave model is set at the same position, Fig. 15 and Fig. 16 show the displacement of the monitoring point and the amount of change of displacement, the displacement of the cave model is higher than that of the no-cave model at the monitoring point. The maximum displacement at the monitoring point of the cavern model is 24.750 mm, while the maximum displacement of the model without cavern is 14.722 mm.The displacement change of the roof plate during the secondary mining process of the cavern model is larger than that of the model without cavern under the same advancing distance.

Fig. 14.

Layout of monitoring sites.

Fig. 15.

Comparison of displacements at monitoring points under repetitive extraction.

Fig. 16.

Comparison of displacement change of monitoring points under repeated quarrying.

Numerical simulation programme design

Numerical simulation modelling

Based on the coal mine construction borehole data and coal seam geological data of Qinglong coal mine area in Guizhou, this study constructs a 3DEC numerical model to explore the water-conducting fissure development law of coal seams containing karst cave openings under repeated mining disturbances. In view of the irregularity of the cavern hollow zone, in order to facilitate the study, the irregular shape is simplified to a spherical regular shape during the model construction process in this paper. However, due to the special three-dimensional void zone characteristics of this numerical model, the use of traditional 3DEC command flow modelling is not conducive to the construction of this research model.Therefore, in this study, we first constructed the required 3D geological model of the spherical hollow zone with cavities in Rhino software, exported the file with the suffix of 0.3dgrid after the mesh was divided, and imported it into 3DEC7.0 software, so as to complete the 3D stratigraphic model of the spherical hollow zone with cavities. The process of constructing the 3D geological model of the hollow area with spherical cavities is shown in Fig. 17

Fig. 17.

The process of constructing the 3D model of the empty area containing spherical cavities.

According to the research background geological information, combined with the comprehensive column map of the coal seam, the dimension of this numerical simulation model length (X direction) × width (Y direction) × height (Z direction) is: 250 m × 40 m × 156 m, and the position of the centre point of the spherical cavern empty zone is (100, 20, 60), and its diameter is 10 m. Numerical simulation of the physical and mechanical parameters and jointing parameters of each coal formation are shown in Tables 2 and 3.

Table 2.

Physical and mechanical parameters of each coal seam.

Stratum name	Capacity Kg/m3	Tensile strength/MPa	Compressie strength/MPa	Angle of internal friction/°	Bulk modulus/GPa
Limestone	2600	3.24	8.87	30	4.57
Shale	2350	2.26	3.68	32	1.65
Limestone	2500	3.19	5.42	31	4.28
Coal	1250	0.8	1.23	28	1.32
Siltstone	2550	7.62	8.56	34	8.24
Silty sandstone	2500	3.46	6.79	31	3.94
Siltstone	2300	3.74	7.42	35	5.42
Charcoal siltstone	2450	6.84	8.16	30	7.21

Table 3.

Physical and mechanical parameters of each joint between coal seams.

Name of coal seam	normal stiffness /MPa	Tangential stiffness /MPa	angle of internal friction/°	compressive strength /MPa	tensile strength /MPa
Limestone	3580	2250	17	0.84	0.32
Shale	3510	2380	14	0.94	0.34
Limestone	3500	2230	16	0.79	0.33
Coal	2300	2320	15	0.42	0.31
Siltstone	5550	5980	21	2.14	1.47
Silty sandstone	3560	2610	16	1.04	0.38
Siltstone	3620	4400	17	1.24	0.80
Charcoal siltstone	5500	5850	19	0.95	0.41

Numerical simulation programme

In this simulation, 30 m coal pillars are left on each side of No.16 and No.18 coal seams, and the downward mining sequence is adopted to mine the coal seams, so as to study the change characteristics of the karst void zone of overburden rock and the development law of water-conducting fissures under repetitive mining. In the simulation process, a step-by-step excavation programme was adopted for the coal seam, and the length of each step was 10 m. After each step, the coal seam was excavated by the model. After each step of excavation, the model calculates to a stable state before the next step of excavation, so as to achieve the simulation of the real underground mining coal cutting process.

Analysis of numerical simulation results

Fracture and fracture evolution and transportation patterns of the mining overburden in the no. 16 seam

As shown in Fig. 18, after opening the cutting eye of No. 16 coal seam, its overlying rock strata fractured and showed a slight subsidence phenomenon, while the bottom plate also showed a slight bottom bulge, but the overlying rock did not produce obvious fissures. When the No.16 coal seam advances 30 m, the overlying rock further fracture sinking range gradually increases, the amount of roof sinking and floor bottom drum also grows, but at this time the vertical displacement of the overlying rock in the hollow area of the cavern affected by mining is almost zero. As the No. 16 coal seam advanced to 30 m, the sinking range of the overlying rock strata gradually expanded, and the displacement between the top and bottom slabs increased accordingly. However, at this stage, the vertical displacement of the overlying rock layer in the hollow zone of the cave affected by mining was almost zero. When the seam advances to 50 m, the direct roof comes under pressure for the first time and forms an articulated structure with the coal wall side at the eye side of the open cut. At this time, the basic top still has the bearing capacity, and the first off-seam fissure with the upper layer occurs. The maximum subsidence of the roof is located in the middle of the voided area and amounts to 2.3 m. As the seam continued to advance up to 70 m, the voided area was located just below the contained karst cave void. At this point, the basic roof subsided and formed an articulated structure with the direct roof and the overlying rocks on both sides. The off-seam fissures between the basic top and the upper level were further developed and widened, while more off-seam fissures were generated. At this time, the water-conducting fissures in the overburden reached a height of 40 m. At the same time, in the position of overburden rock 70 m away from No.16 coal seam and close to the cavern empty area, slight off-stratum fissures have also been developed. It can be seen that the existence of the cavern hollow area will not only increase the development height of the overburden water-conducting fissures, but also make the development location mainly concentrated in the vicinity of the cavern.As No.16 coal seam advances to 110 m, the change of overburden rock in the mining area gradually appears. Compared with the state at 70 m advancement, the overburden rock near the eye end is gradually compacted, and the off-seam fissures are gradually closed. However, the overburden rock exhibits asymmetric subsidence in the vicinity of the cavern due to the presence of the cavern void zone. Specifically, the overburden rock loses its bearing capacity near the cavern, leading to the expansion of subsidence to the cavern side. In addition, the cavern void also makes it more difficult to compact the surrounding overburden, and the development of water-conducting fissures is gradually concentrated near the cavern. In this case, the maximum vertical displacement of the asymmetrically subducted overburden reaches 2.4 m, the minimum vertical displacement is 1.6 m, and the height of the water-conducting fissure development is about 50 m in comparison with the extraction zone. When the coal seam advances to the position of 150 m, the development direction of water-conducting fissures turns to horizontal direction. Due to the asymmetric subsidence of the overburden rock, horizontal off-seam fissures appeared in the vertical direction at a position higher than 70 m above the mining area, and the scope of the asymmetric subsidence was further expanded in the horizontal and vertical directions. As the seam continued to advance, the water-conducting fissures near the cavern were gradually compacted under mine pressure and continued to develop horizontally to the side of the coal wall, compared with the situation when it advanced to 110 m. Although the maximum displacement of the subsidence of the asymmetric overburden remains at 2.4 m, its range continues to expand and is on a par with the height of the cavern. Meanwhile, the minimum displacement of asymmetric overburden subsidence increased to 2.1 m. The development height of the water-conducting fissure was stable at 70 m after the mining of No. 16 coal seam was completed. In addition, its expansion was mainly concentrated on the coal wall side after mining was completed, while the water-conducting fissures on the side of the cutting eye had been compacted under long-term mining pressure, and at the same time, the upper and lower edge arcs of the cavern hollow zone were compacted to a flat state under the influence of the mining action. In this process, the highest point of overburden rock subsidence is exactly located directly above the cavern. Accordingly, it can be inferred that the existence of the cavern empty zone has significantly affected the morphology of the overlying rock subsidence in the mining zone, forming an asymmetric structure, and the highest point of this asymmetric structure is located directly above the cavern. In addition, during the mining process of the No. 16 coal seam, the water-conducting fissures underwent a process of first development, then expansion, and finally began to close, an observation that is highly consistent with the fissure evolution pattern derived from similar simulation experiments.

Fig. 18.

Vertical displacement characteristic cloud map of No.16 coal seam.

Evolution and transport patterns of fracture rifts in the mining overburden of No. 18 coal seam.

As shown in Fig. 19, after the mining of No. 16 coal seam is finished, the top plate of No. 18 coal seam still has a certain bearing capacity after analysed, so in the process of opening the cutting eye of No. 18 coal seam, it did not have obvious influence on the evolution of overburden fissure, and the overburden subsidence displacement did not see any significant change. Therefore, the cloud map of displacement characteristics in vertical direction after opening the eye of No.18 coal seam is not analysed in depth here, and the relevant cloud map is not inserted for illustration. When No.18 coal seam advances to the position of 40 m, the first incoming pressure phenomenon occurs in the direct roof. At this time, the direct roof fractured, and the maximum subsidence displacement of the direct roof of No.18 coal seam reached 3 m, which was similar to the thickness of the coal seam. The maximum subsidence displacement occurred on the top plate of No.16 coal seam corresponding to the mining hollow area, and its maximum displacement value was 5.072 m. At this stage, the water-conducting fissure was re-developed by the influence of roof subsidence, and the development height reached up to 70 m, and the subsidence displacement near the cavern was 1.5 m. As the No.18 seam continued to advance up to 70 m, the water-conducting fissure was further developed horizontally, and gradually formed the off-stratum fissure. At this time, the height of overlying rock subsidence caused by No.18 coal seam mining reaches 50 m, and the subsidence range is still in a symmetrical structure. In this symmetrical range, the maximum vertical displacement is 5 m, and the minimum vertical displacement is 3.5 m. And in the process, the top of the spherical cavern first showed a slight collapse phenomenon. When the coal seam advances to 110 m, due to the mining of No.18 coal seam, the range of overlying rock subsidence changes from the previous symmetrical subsidence to asymmetrical subsidence. Compared with the previous mining conditions, the distance of coal seam advancement has exceeded the lower part of the cave. At this time, the water-conducting fissure tends to be closed in the side of the cutting eye, and the water-conducting fissure at the position of 70 m is also compacted, while the development of the water-conducting fissure close to the side of the coal wall is more concentrated, and the scope of development is mainly concentrated in the position of the overlying rock around the cavern. At this time, the development height of water-conducting fissure is 50 m. When the No.18 coal seam advanced to the position of 150 m, two water-conducting fissures were formed below the cavern. Similar to the previous stage, the development of water-conducting fissures was mainly concentrated on the side close to the coal wall, and the scope of asymmetric subsidence of overburden rock was further expanded. Within this range of overburden subsidence, the maximum subsidence of 5.5 m occurred at the 16th coal seam at the mining zone, while the minimum subsidence was located at the top of the model at 3.75 m. When the advancement of the seam was completed, the water-conducting fissures of the previous stage began to close gradually, while the water-conducting fissures of this stage developed horizontally along the previous stage and were concentrated on the side of the coal wall. The highest point of asymmetric subsidence of the overlying rocks due to the mining of the No. 18 coal seam is located directly above the left side of the cavern. The top of the spherical cavern collapsed further after the coal seam was fully mined, but the overall structure remained relatively intact. The maximum vertical displacement of the overburden rock occurred in the mining area of No.16 coal seam, reaching 5.5 m, while the maximum displacement of the top plate of No.18 coal seam was 3 m.

Fig. 19.

Vertical displacement characteristic cloud map of No. 18 coal seam.

Conclusions and recommendations

• 1. The study of overlying rock damage and roof breakage found that the overlying rock layer is detached from the layer or bending and sinking to form a water-conducting fissure zone, and the height of which was calculated by empirical formulae and improved empirical formulae, and the height of the water-conducting fissure zone developed under repeated mining is 59.35 m. The cave located in the water-conducting fissure zone will be destabilised due to the development of the fissure. Combined with the elasticity theory Zierse solution, we can get the horizontal and vertical stress distribution of the cave and the basis for the judgement of the destabilisation.

• 2. The influence of mining disturbance on the stability of the cave is changed in the cave model under repeated mining. When the working face is mined to 65 m, the cave destabilises and destroys, the overburden rock breaks, and the secondary disturbed fissure and primary disturbed fissure pass through; in scheme 2, the secondary disturbed fissure and primary disturbed fissure pass through when the working face is pushed forward to 75 m, which is 10 m different from that of scheme 1, indicating that the destabilising damage of the cavern speeds up the development of overburden rock fissure and improves the time for the water-conducting channel to pass through.

• 3. Using the DIC numerical scattering monitoring system, it is found that the maximum displacement of the overlying rock layer after repeated mining is 24.750 mm, which is larger than that of the no-cavern model (16.722 mm), and the rate of displacement is significantly larger, and the cavern is destabilised and re-stabilised after the working face advances up to 65 m, which has a certain supporting effect on the overlying rock above it, and the cavern tends to be stabilised in the end. Numerical simulation shows that the overburden rock sinks asymmetrically, and the highest point of sinking is on the left side of the karst hole, the maximum vertical displacement of the overburden rock is 5.5 m in No.16 coal seam, and the maximum displacement of the roof plate of No.18 coal seam is 3 m. The water-conducting fissure develops and expands first, and finally closes, which is in line with the results of the physical experiments.

• 4. At the same time, most of the caves are located in hard rock layers, such as greywacke, which can be used as a key layer to control the collapse of overburden rock in mining activities, and at the same time largely affect the development height of the water-conducting fissure zone, so it is suggested that research can be strengthened in this direction in the future.

Author contributions

Yuan Zhang: Conceptualization, Methodology, Writing – review & editing; Mengtang Xu: Data curation, Writing- Original draft; Dezhong Kong: Software, Validation; Yujun Zuo: Validation Guiyi Wu: Project administration,Supervision; Fei Chen：Software,Visualization; Yang Zhou: Visualization.

Funding

National Natural Science Foundation of China,No.52564015,No.52104123,No. 52164002,No. 52164005,Guizhou Science and Technology Support Programme,Qiankehe Support [2021] General 351,Guizhou Provincial Basic Research Program(Natural Science),Qianke He Foundation-ZK[2024] Key 022

Data availability

Data Availability Statement All data, models, and code generated or used during the study appear in the published article.

Declarations

Competing interests

The authors declare no competing interests.