Influence of the extra-thick coal seam exploitation on the deformation characteristics of the overlying rock mass in an open-pit mine slope

Abstract

Coal seam mining causes movement and deformation of the overlying rock mass and ground surface, directly resulting in serious damage to the surface construction facilities. To investigate the deformation mechanism and characteristics of the overlying rock mass, three-dimensional numerical simulation and model test are carried out on an open-pit mine slope. The results show that the sliding instability characteristics of the slope can be identified based on the numerical analysis. Coal mining affects on the deformation characteristics of the overlying rock mass near the inner dump. The closer to the slope boundary, the greater the subsidence of overlying rock mass. The maximum displacement occurs near the inner dump. The slope mining has great influence on the deformation characteristics of inner dump. Moreover, the damage process of overlying rock mass can be further studied via the model test. After coal seams mining out, the overlying rock mass occurs overall settlement failure. There is a large difference between the lower end side of working face and the upper end side at the caving angle of roof. This work can provide a basis for the safe operation of coal mine disaster prevention and mitigation.

Keywords:

• Overlying rock mass
• ground surface
• deformation characteristics
• extra-thick coal seam mining
• numerical simulation
• model test

1. Introduction

In recent years, the problem of coal seam compressed at end slope in open-pit mines has become increasingly prominent, such as Baiyinhua Coalfield, Shengli Coalfield, Fushun Coalfield, and Jungar Coalfield in China (Carlà et al. 2017; Zhou et al. 2019; Lu et al. 2022; Yuan et al. 2022). The open-pit mining end-wall coal accounts for about 20% of the resources (Tarolli and Sofia 2016; Ma et al. 2018; Li et al. 2021). At present, there is no mature theoretical and technical system for efficient mining of end-slope coal. The more effective method is the combination of open-pit mining and underground mining to form a compound mining mode (Xu et al. 2015; Liu et al. 2020; Cao et al. 2021). The mining effect of the two mining methods influences each other, and finally forms a dynamic complex system with diversified spatial forms. Due to the complexity of mining, open-pit side slope mining has become a new issue to be solved urgently (Ordin and Vasil’ev 2014; Ulusay et al. 2014; Whittle et al. 2018; Du et al. 2020). Moreover, to further improve the recovery rate of high-quality resources and economic benefits, it is necessary to mine the coal seam at the first mining area. Due to the damage to the integrity of the strata caused by the open-pit mining in the initial mining area of the coal mine, the overburden composition and the mechanical properties of the roof change substantially. Therefore, special attention paid to the deformation characteristics of overburden and ground surface under the action of coal seam mining (Yanli et al. 2011; Li et al. 2019).

Surface subsidence caused by underground mining is the most common disaster in mining area, and overburden deformation directly results in lots of environmental geological problems (Marschalko et al. 2015; Zhu et al. 2019; Song et al. 2020a, 2020b). According to the thickness of the coal seam, it can be divided into thin coal seam (<1.3 m), medium thick coal seam (1.3–3.5 m), thick coal seam (3.5–8 m), and extremely thick coal seam (>8 m) (Ghose 1984; Qiao and Teng 2018). The surface subsidence occurred in many provinces of China, such as in Inner Mongolia, Heilongjiang, Shanxi, etc (He et al. 2017). Overlying strata deformation has a significant effect on the stability of slopes in the process of underground mining (Li et al. 2016; Li et al. 2017; Li et al. 2020; Zhang et al. 2020; Wang et al. 2022). Many scholars have investigated the deformation of overlying strata and surface under the action of coal seam mining (Dou et al. 2014; Zhu et al. 2014; Wang et al. 2017) investigated the movement characteristics of overlying rock mass of mining-induced fractures from the coal seam to ground surface. Yang et al. (2019) observed the movement of overlying rock in the mining process with the three-dimensional optical displacement monitoring system, and analyzed the deformation process of the slope via the numerical simulation method. Wang et al. (2019) systematically studied the fracture process and structural evolution characteristics of overburden by combining physical simulation and numerical simulation. Deng et al. (2018) adopted field investigation and monitoring methods to comprehensively evaluate the caving characteristics of overlying strata. At present, many research achievements have been made on the influence of coal mining on overlying rock mass. However, due to the complex geological structure of open-pit mine, the influence of different coal mining technology on the deformation characteristics of overlying rock mass is different (Du and Song 2022). Hence, the deformation characteristics of the overlying rock mass need to be further studied.

In this work, taking an open-pit mine as the main engineering background, according to the numerical method and scaled model test, the influence of extra-thick coal seam mining on the deformation characteristics of overlying rock mass and ground surface is investigated in depth. The high-precision three-dimensional numerical slope model containing complex geological structure was established, and the deformation characteristics of the overlying rock mass and ground surface are analyzed. The numerical results are verified by the scaled model test, and the deformation characteristics in coal mining are further studied. This work can provide reference for the prevention and control of the disasters in open pit mines.

2. Study area

The open-pit mine is located in Ordos City, Inner Mongolia Autonomous Region, China (Figure 1). The open-pit mine includes three mining areas. After the completion of the first mining area, an unfilled 175 × 478 m pit is left. After the working side turns to the second mining area, the southwest side of the initial mining area leaves an unmined area, and the overall shape is elongated. Underground mining is adopted in this area. Several faults distribute in this area (Figure 2). The roof of composite coal seam is mainly composed of coarse sandstone, medium sandstone and fine sandstone. The direct roof is dominated by mudstone, carbonaceous mudstone, high ash coal and clay rock. The lithology of the direct roof is weak, which is very beneficial to caving coal mining and reducing the compressive strength of the basic roof. Above the direct roof is a thick layer of yellow coarse sandstone, with a total thickness of 50–60 m and a coal seam in the middle. The yellow coarse sandstone is argillaceous and poorly consolidated, with strong water-disintegrated property, small unidirectional compressive strength and large difference in strength. The thickness of the single layer above the yellow coarse sandstone is not large, generally less than 2 m, which mainly consists of mudstone, fine sandstone and siltstone; and then the topsoil layer, mainly yellow clay. The direct floor is mainly mudstone and claystone, followed by sandstone. 3DMine software was used to visualize the excavations. The lithologic distribution is shown in Figure 3. The rock mechanics parameters of the open-pit coal mine are listed in Table 1, according to a series of rock tests in laboratory and field tests (Figure 4).

Figure 1. Location of the open-pit mine in the Inner Mongolia Autononmous Region, China.

Figure 2. Geological structure distribution map of the open-pit mining area.

Figure 3. Distribution map of soil discharge thickness and bedrock of the coal seam area.

Figure 4. Laboratory test for physical and mechanical parameters of rock mass: (a) Partial rock samples; (b) Uniaxial compressive test of coal sample; (c) Splitting tensile test; (d) Variable angle shear test of coal sample; (e) Poisson’s ratio test of rock sample.

Table 1. Rock mechanics parameters of the open-pit coal mine.

Stratigraphic name	Density (kg/m³)	Modulus of elasticity (MPa)	Poisson’s ratio	Cohesion (MPa)	Friction angle (°)	Tensile strength (MPa)
Quaternary surface soil	2000	800	0.30	0.12	14.4	0.05
Yellow sandstone	2700	1750	0.20	1.60	28.8	0.55
Coal rock	1400	1400	0.30	1.20	34	0.83
West excavation layer	1400	1400	0.30	1.20	34	0.75
South excavation layer	1400	1400	0.30	1.20	34	0.75
Bottom	2550	3500	0.13	6.27	40	3.25
Waste dump	2400	1600	0.26	1.20	25	0.80

3. Analysis of deformation characteristics of the overlying rock mass using numerical method

3.1. Numerical modeling

Due to the complex geological structure and geomorphology of open-pit mines, a high-precision three- dimensional (3 D) numerical model should be constructed first during numerical calculation. The construction process of the 3 D numerical model is shown in Figure 5. The detailed modeling steps are as follows. The twenty horizontal and vertical exploration line profiles of the open-pit mines were rotated in three-dimensional space to form important modeling data sources and shape constraints for the solid model. The exploration line spacing was 50 m (Figure 6a). Layered modeling is based on the sampling points of different geological levels. After necessary interpolation and fitting, Digital Elevation Model (DEM) of each layer is established respectively, and then all interfaces are superimposed together. Firstly, DEM of each formation is generated by borehole data, 3 D exploration line profile line and interpolation points (Figure 6b,c). Then, a side triangular network between the roof and floor of the coal seam is constructed to suture the top and bottom of the coal seam to form a spatial envelope (Figure 6d,e). The 3 D geological surface model of the first mining area of open pit mainly includes yellow sandstone, bedrock and roof and floor surface model of coal seam. Its construction steps mainly include delineate modeling boundary line, extraction of borehole data points, spatial interpolation. TIN model is shown in Figure 6f. The solid model of the coal seam, quaternary system, yellow sandstone, above yellow sandstone and floor is shown in Figure 6g.

Figure 5. Flow chart of construction of high-precision three-dimensional numerical model.

Figure 6. Construction of the high-precision numerical model: (a) Spatial sequence section of the open-pit mine; (b) Multi-layer DEM construction; (c) Multi-layer DEM rendering; (d) Side triangular mesh; (e) Coal seam envelope model; (f) Certain a coal level model; (g) Geological model; (h) Internal dump model; (i) Reserved pit area in the model; (j) Transformation diagram of ANSYS and FLAC3D partial body units; (k) FLAC3D numerical model.

In addition, the working face of the open-pit mine has been shifted to the second mining area. Most of the first mining area has been backfilled. At the southwest end of the first mining area, adjacent to the working side of the boundary, a rectangular pit surrounded by inner drainage materials is formed. According to the requirements of soil discharge construction technology, the surface model of the internal dump is stretched to form a solid model (Figure 6h). The west and south sides of the pit area are solid slope at the boundary of the initial open-pit mining area, and the west and north sides are the internal waste dump slope. The mechanical characteristics of surrounding rocks obviously do not conform to the basic assumption of two-dimensional plane calculation, hence, the two-dimensional calculation method cannot be used. FLAC3D (Fast Lagrangian Analysis of Continuum) numerical simulation was adopted for calculation and analysis. A rectangular calculation area was determined with the pit area as the center, and the southern and western sides were determined taking into account the influence range of surface deformation and the boundary effect of the model (Figure 6i). Moreover, in the modeling process, based on the previously established 3 D geological model, the 3 D surface of the geological model is successfully converted to ANSYS finite element platform through the program interface. The 3 D geologic body model of each stratum is generated by sealing the curved surfaces. Then, the unit conversion interface program between ANSYS and FLAC3D is written to make the volume unit type in ANSYS be converted to the unit type in FLAC3D (Figure 6j). Finally, the FLAC3D numerical model is generated (Figure 6k). The colors of different groups represent different strata, and there are 8 groups in total. The number of grid nodes and cells is 376319 and 2159815, respectively.

According to field sampling and rock mechanics experiment results, the rock mass studied is mainly sandstone, and the rock has obvious elastic-plastic deformation characteristics under different confining pressures. The Mohr-Coulomb constitutive model is used to simulate the mechanical properties of rock and soil near the surface and working face. The boundary stress and the constraint conditions of the calculation model are important contents of the calculation model, which directly affect the reliability and accuracy of the calculation results. Therefore, appropriate boundary constraints must be adopted for the calculation model. The boundary conditions of the model are determined as follows: (a) Horizontal constraints are imposed on the left and right, front and rear boundaries of the model, and the initial horizontal displacement of the boundary is zero; (b) The initial horizontal and vertical displacements of the bottom boundary of the model are zero; (c) The top of the model is the free boundary. The physical and mechanical parameters of the model materials are shown in Table 1.

3.2. Numerical simulation procedure of excavation

The ground surface movement and deformation characteristics at different locations after the excavation of the lower coal seams at west side are shown in Figure 7. Before coal seam excavation, the model is in the initial equilibrium state without plastic failure. Under the action of self-weight stress, the vertical displacement of the overlying strata on the four boundary angles and the working surface of the model is relatively large. After coal seam excavation, the original stress field equilibrium state in rock mass will be destroyed, and the system will gradually form a new equilibrium state by means of rock mass deformation and failure, stress transfer, etc. With the gradual progress of coal mining, the failure scope of overlying strata is gradually expanded, and the stress is released constantly, deformation, movement, straddle and fracture of strata will occur. Figure 7a,c shows that after coal mining excavation, plastic failure appeared on the platform area of the slope surface. The main failure mode is tensile failure, and the stress concentration area is mainly in the overburden above the working face and the surface. Figure 7b,d shows that the largest subsidence area of the west side is mainly located in the open stepped position dominated by yellow sandstone. This phenomenon suggests that the upper roof strata of the coal seam affected by mining disturbance, and its stability becomes weak. When the coal seam is exploited, the thin bedrock fault zone is cut off, and the open-pit sandstone steps above sink in a large area, resulting in the failure of pull shear, and the settlement movement scope gradually expands. The subsidence is the largest in the middle of the step and gradually weakens to both sides, and the subsidence in severe areas can be 10–16 m. Part of the dump area in contact with the west end will also sink, with the settlement reaching 6–10 m. Because a 20 m long and narrow shaped coal pillar is reserved at the main well mouth, the amount of subsidence at the step position near the main wellhead is small, approximately 1–2 m. After mining in the south coal pressure zone, the area with the greatest degree of subsidence is also concentrated in the open-air step position dominated by yellow sandstone. The subsidence trend is the largest in the middle part of the step, which gradually decreases to both sides and shows a long and narrow ellipse along the working face strike. There is a trapezoidal coal pillar with a width of about 170 m at the working surface clamping place on both of west and south sides, and the serious area near the trapezoidal coal pillar can subsidence of about 9–14 m.

Figure 7. The plastic contours and the displacement contours after excavation in western and southern slope.

In addition, to further investigate the influence of coal mining on the deformation of the overlying strata and ground surface, the deformation characteristics of typical sections of the west and south working faces were analyzed (Figure 8). Taking typical sections as an example, Figure 9a,b shows the vertical displacement cloud maps of the sections of the west and south slope, respectively. Figure 9c,d shows that after coal seam mining, the goaf is directly subjected to the gravity action of the roof strata and the overlying strata, resulting in their downward movement and bending. In the meanwhile, Figure 9 shows that the closer to the boundary of the model, the greater the subsidence value of the overlying strata and the ground surface, that is, coal mining has a great influence on the deformation of overlying strata and ground surface near the dump. With all coal seams being mined out, the stress of rock around the tunnel shifts and transmits, and the movement direction is mainly toward goaf. The overlying strata release energy through movement. After coal mining, the settlement curves of measurement points in each profile are shown in Figure 10a–d. The maximum displacement occurs on the side close to the inner dump. This is because the overlying strata are disturbed and damaged by open-pit mining. The upper strata are stripped into step-like slopes and replaced by loose materials, resulting in weaker stability of the strata. The exploitation of coal seam causes the surface and yellow sandstone steps to suffer tensile failure, produces stress concentration area and plastic failure around them, and moves towards the center line of stopping.

Figure 8. Profile of the open-pit location in the numerical model.

Figure 9. Vertical displacement contours 2 D map of the section at the west and south retaining pit.

Figure 10. The settlement of the monitoring point along the west and south working face.

3.3. Analysis on the influence of coal mining extraction on the side slopes

To clarify the influence of coal mining on the overall deformation characteristics of the slopes, the plastic yield and horizontal displacement nephogram of the open-pit mine after slope mining were shown in Figure 11a–d. Figure 11a,b shows that with the west slope mining, the overlying rock mass of the slope produces the movement deformation, and gradually transfers to the ground surface. Coal mining has little influence on the deformation of the step of the dump near the pit, and the plastic zone of the overlying rock formed by mining disturbance is very small. However, along the working face of the west slope, the plastic zone gradually increases, and the maximum plastic yield occurs at the boundary of west slope, resulting in a horizontal displacement with a maximum displacement of 3.4 m. It can be seen that this area is a potentially dangerous slide body. Figure 11c,d shows that with the south slope mining, large plastic yield and horizontal displacement can be found in the overburden above the working face of the south slope. The maximum displacement reaches 8.6 m, and the upper part of the plastic zone is the potential sliding body.

Figure 11. Deformation characteristics of the open-pit mine after western and southern slope mining.

4. Scaled model test

4.1. Experimental method and equipment

The overlying rock mass in the west and south side slopes of the open-pit mine is affected by excavation. Some strata have been stripped and backfilled with the abandoned substances from the open-pit mine, which changes the state of the original stratum. The mechanical properties of some strata media have changed, and the overburden activity and roof pressure development law of the working faces are different from the previous ones. Hence, similar scaled model experiments are carried out to further reveal the law of roof strata movement, deformation failure and instability collapse. Taking certain a working face of the side slope as an example, and the model was constructed according to the formation structure of the west side slope. The topsoil and backfill in the model are homogeneous materials, and the other strata are divided into several layers according to the strength of different strata to be stacked layer by layer. The main coal seam is a composite coal seam, with an average thickness of 22 m and a dip angle of 0–29°. The actual working face of the simulated profile is 180 m long, cutting 4 t of coal per day, with a cut-off depth of 0.85 m and a daily advancement of 3.6 m, as shown in Figure 12a. Coal mining method is one time full height mining technology, and roof management method is all caving method. The original length of the open-pit working face is 555 m and the maximum buried depth is 300 m. In this test, the length L was taken as the controlling parameter, and the size similarity ratio between the prototype and the model was 200:1. The length of the model test bench was 3 m. In the test, the materials and test parameters of the model were obtained based on the Buckingham π theorem of similarity, which is shown in Table 2. The experimental equipment mainly includes the plane strain model test bench, eight vibrating string pressure boxes, intelligent digital static resistance strain gauge, and desktop computers. The scaled model and the arrangement scheme of eight pressure sensors in the working face bottom plate are shown in Figure 12b. There was not any intermediate layer constituting a slip between the layers.

Figure 12. Scaled model and arrangement scheme of pressure sensors in the working face bottom plate: (a) Model section; (b) Layout of 8 pressure sensors.

Table 2. The similarity ratios of the model.

No.	Parameters	Similarity coefficient	Similarity ration
1	Physical dimension (L)	Controlling parameter (aL)	0.005
2	Time (t)	at=(aL)^−0.5	0.071
3	Volumetric weight (r)	ar=rm/rp	1.5
4	Stress (σ)	aσ=ar×aL	0.0075
5	Cohesion (c)	—	1
6	Internal fraction angle (φ)	—	1
7	Displacement (s)	as=aL	0.005
8	Elastic modulus (E)	aE=aL	0.005

4.2. Production and mining of scaled model

According to the actual geological data of coal strata in the open-pit mine, the mechanical parameters of each coal strata in the model are determined by laboratory tests. The mechanical parameters of the model material are shown in Table 3. The similar materials can be divided into aggregate and cementing materials. The mixture ratio of aggregate and cementing material is selected. Fine sand is used for the aggregate. Lime and gypsum are mainly used for the cementing material. The scaled model is divided into 33 layers, and the model is stacked layer by layer according to the matching number. After the model is made, the measuring points are arranged. The model adopt all straddle method management roof, and one full height mining technology. To better observe the characteristics of strata movement, observation points are set before model mining. There are 14 measuring points in the model, and 60 observation points are set in each layer, with each point spaced about 5 cm apart. The measuring material is matches with different colors of pigments. When rock strata move and deform, marks are left on the glass plates to observe their deformation characteristics.

Table 3. The experimental mechanical parameters of the coal and rock.

Stratum and lithology	Moisture content (%)	Compressive strength (MPa)	Tensile strength (MPa)	Internal fraction angle (°)	Cohesion (MPa)	Elastic modulus (MPa)	Poisson’s ratio
5 coal roof yellow sandstone	0.6	31.92	2.01			3030.5
5 coal roof yellow sandstone (water absorption softening)	6.35	18.71				2838
6 coal roof yellow sandstone	1.03	25.41	1.09	34.6	2.69	2523.7	0.35
5 coal roof yellow sandstone (water absorption softening)	5.38	11.85				2493.8
6 coal dirt band	0.6	23.24	1.64			3744.3
6 coal	2.14	12.28	0.83	32.3	2.15	1544.8	0.23
6 coal floor mudstone	1	29.87	1.73			3034.5

In this test, the raw materials that make up the similar material can be divided into two categories: aggregates and cementing materials. Aggregates make up a larger proportion of the material and are the object of cementing, and their physical and mechanical properties have a significant influence on the properties of the similar material. Secondly, the aggregate and cementing materials were selected for proportioning. In this experiment, fine sand was used for the aggregate and lime and gypsum were mainly used for the cementing materials. The experiment is divided into 33 layers and the model is stacked layer by layer according to the proportioning number. The specific mixture ration of aggregate and cementing material are listed in Table 4.

Table 4. Specific mixture ration of aggregate and cementing material proportioning numbers and stratification design calculation table.

Model layer sequence	Lithology	Thickness (m)	Model thickness (cm)	Number of layers	Thickness of layers	Prototype strength	Model strength	No. Ratio
33	Discharge of soil			Unstratified		5.00	0.016	673
32	Topsoil and Loess	22.00	11.0	3	3.7	5.00	0.016	673
30 ∼ 31	Fractionated fine sandstone, mudstone	18.00	9.0	3	3.0	15.00	0.039	673
28 ∼ 29	Caystone	15.00	7.5	3	2.5	15.00	0.039	673
27	Mudstone	9.22	4.6	2	2.3	40.00	0.096	655
26	Medium sandstone and sandy mudstone	5.63	2.8	1	2.8	40.00	0.096	655
25	Fine sandstone	2.60	2.0	1	2.0	50.00	0.115	555
24	Sandy mudstone, mudstone, and siltstone	3.20	2.0	1	2.0	40.00	0.096	655
22 ∼ 23	Mudstone, fine sandstone, coarse sandstone, mudstone	21.03	10.5	3	2.1	50.00	0.115	555
21	Coarse sandstone and mudstone	12.37	6.2	3	2.1	30.00	0.078	655
20	Carbonaceous mudstone	1.82	2.0	1	2.0	50.00	0.115	555
19	Mudstone and fine sandstone	10.04	5.0	2	2.5	50.00	0.115	555
18	Siltstone and mudstone	4.45	2.2	1	2.2	15.00	0.039	637
17	Fine sandstone	2.35	2.0	1	2.0	50.00	0.115	637
16	Coarse sandstone	9.04	4.5	2	2.3	30.00	0.078	655
15	Fine sandstone	4.22	2.1	1	2.1	50.00	0.115	555
14	Carbonaceous mudstone	3.51	2.0	1	2.0	15.00	0.039	637
13	Claystone	3.15	2.0	1	2.0	15.00	0.039	637
12	Fine sandstone	2.50	2.0	1	2.0	50.00	0.115	555
11	Medium sandstone, coarse sandstone and fine sandstone	3.85	2.0	1	2.0	30.00	0.078	573
9 ∼ 10	No. 6 composite coal seam	34.48	6.9	3	2.3	20.00	0.052	673
7 ∼ 8	Coal seam floor 7 ∼ 8		4.0	2	2.0	60.00	0.138	355
1 ∼ 6	Coal seam floor 1 ∼ 6		10.0	3	3.3	60.00	0.138	355

4.3. Influence of coal mining on the deformation of overlying rock mass

Mining subsidence is an extremely complex rock mass mechanics problem. The overburden and surface deformation of mining area is a gradual process that the strata near the goaf transfer upward gradually. To investigate the overburden and ground surface deformation, it is necessary to make dynamic analysis of the strata’s moving deformation and fracture process first. In the process of underground mining, the test model of roof collapse and rock strata movement of the coal seam is shown in Figure 13. In the scaled model test, the movement, deformation, failure and pressure of the overlying rock mass in different stages of coal mining are observed and recorded. During the mining test process, the deformation and failure processes of the models at different stages are shown in Figures 13 and 14. Figure 13a shows that the overlying rock mass and ground surface deformation are very small at the initial stage of coal mining. Figure 13b shows that the overlying rock mass and surface deformation are intensified, and cracks appear in the overlying rock, in particular, in the part near the drainage soil. Then, with the development of mining, fractures further expand, and the overburden is further subsidence and deformation. Finally, when the top of the coal seam is completely excavated, the overburden and the surface show large-scale deformation and failure.

Figure 13. Model failure phenomena in the process of underground mining: (a) initial excavation stage; (b) overburden breaks and sinks with top coal caving.

Figure 14. Local failure characteristics of the model after the model failure: (a) roof caving angle; (b) movement track and displacement of overburden; (c) ground surface fractures caused by overburden movement.

To further observe the deformation characteristics of the model, the local deformation after the model failure is shown in Figure 14. Coal mining causes deformation and movement of surrounding rock mass. When the movement and deformation of the overlying rock mass exceed its limit deformation, the deformation and failure occur layer by layer from the direct roof of the coal seam. The overlying strata can be divided into three zones according to the degree of damage, that is, the caving zone (straddle zone), fissure zone and curved subsidence zone (Figure 14). Figure 14a,b shows that, in the test, three zones formed after the deformation and failure of the overlying rock mass are different in the shallow part of the mine boundary and the deep part of the mine boundary. In the deep part, the caving zone and fracture zone of the model are formed, which are mainly manifested as penetrating fractures in the shallow and dislocation subsidence of the whole stratum.

In the shallow part, a thin curved subsidence zone is formed, which only appears in the shallow bedrock and topsoil. Figure 14c shows that the ground surface deformation of the dump and topsoil is characterized by terrace settlement and tensile fracture. The maximum surface crack width of the topsoil reaches 5 cm, and that of the dump is 2.5 cm. In addition, there is a great difference of roof caving angle between the lower side of the working face (buried deep) and the upper side (buried shallow). The deep caving angle is much larger than the shallow caving angle, which are approximate 86° and 49°, respectively (Figure 14a). In the process of coal mining, when 80% of the coal seam is mined, the remaining 20% of the coal pillar will collapse as a whole, and the overburden will fail as a whole. This indicates that after partial original strata were excavated on one side, when underground longwall mining was carried out under overlying strata of backfill loose media of the open-pit mine slope, the key strata in roof strata could still form a protective structure and play a protective role on working face.

In addition, during the mining process, the pressure changes of the eight sensors are shown in Table 5. The vertical pressure in the coal face and the nearby floor has changed obviously. The pressure of pressure sensors No. 1 and 8 increases, which conforms to the distribution law of abatement pressure transfer. Sensors No. 2, 3, 4 and 7 below the goaf show a decrease in pressure, while the No. 5 and 6 sensors below the goaf show an increase in pressure. Based on the stress distribution characteristics around the coal face, it can be seen that the lower and middle pressure of the working face decreases, while the middle and upper part of the working face increases. The lower average pressure in goaf decreases, which also indicates that the key layer structure in the complex rock still plays a protective role.

Table 5. Stress sensor measurement results of the model test.

Sensor number No.	Elastic modulus of sensor (kPa)	Strain change	Model stress variation (kPa)	Prototype stress variation (kPa)
1	151.8	2	0.3	101.1
2	515.7	−39	−20.11	−6697.4
3	130.6	−22	−2.87	−956.78
4	492.9	−19	−9.37	−3118.58
5	254.6	30	7.64	2543.45
6	721.2	14	10.1	3362.23
7	765.7	−4	−3.06	−1019.91
8	365.8	9	3.29	1096.3

5. Discussion

In this work, numerical simulation and model test are used to study the deformation of the overlying rock mass and ground surface under the action of extra-thick coal seam mining. A high-precision 3 D numerical model was constructed and FLAC3D was used for the simulation analysis of coal mining, The influence on the deformation characteristics of the side slope, overburden and surface was studied by using FLAC3D. However, the model test was difficult to fully describe the above deformation characteristics. In addition, the scaled model tests are to scale the main geometric and physical mechanical parameters of the prototype slope based on strict similarity criteria, which can better reflect the deformation characteristics of the original slope subject to mining (Zhang et al. 2019; Tao et al. 2020; Xue et al. 2020). According to the scaled model test, the mining pressure development law of the overburden roof is studied to obtain the characteristics of the roof breaking, collapse and movement of the working face. It is revealed that the overburden movement, roof structure characteristics and instability law of the coal seam subject to mining after the backfill in the end wall of the open-pit mine. The mechanical structure that protects the working face still exists in the overburden of the coal seam. The joint analysis of 3 D high-precision numerical simulation and scaled model test can better clarify the deformation characteristics of the overlying rock mass.

6. Conclusions

Based on the numerical simulation and scaled model test, the failure and surface movement deformation law of the overlying rock mass caused by extra-thick coal seam mining are investigated. Some conclusions can be drawn as follows.

1. In view of the complex geological structure of multiple coal seams in open-pit mine, a numerical modeling method of complex slopes is presented. By constructing the surface element combination of top and bottom plate of coal seam, infinite approximation is made to the surface of top and bottom plate of coal seam, and then the curved surface of ore body is generated to enclose the ore body and further solidify to form solid model. FLAC3D numerical simulation test is used to study the deformation law of the ground surface and overburden under coal mining in the west and south side slopes. The influence of coal mining on the stability of slopes is analyzed.

2. Based on the numerical simulation and model test, by analyzing the subsidence and plastic zone distribution characteristics of the overlying rock mass and ground surface, after the mining of the coal seam in the west side slope, the shear failure of the overlying thin bedrock caused a large area of subsidence above the yellow sandstone platform and working face. The area with the largest degree of subsidence is mainly located in the platform position dominated by yellow sandstone. The subsidence tendency is the largest amount of subsidence in the middle of the platform, which gradually decreases to the both sides. The subsidence tendency is characterized as the U type. The mining of the west side slope has great influence on the deformation of the dump, while the mining of the south side slope has little influence on the deformation of the dump.

3. According to the model test, after all coal seams are mined out, the overlying strata shows the subsidence deformation as a whole. The caving angle of roof has a big difference between the lower end side of working face (buried deep) and the upper end side (buried shallow). The deep caving angle of the overlying rock mass is larger, which is approximately 86°. The shallow caving angle of the overlying rock mass is smaller, which is approximately 49°. The deformation development trend of the overlying rock mass is upward gradually. The maximum crack width of the surface reaches about 5 cm, and the maximum crack width of the dump reaches 2.5 cm.

However, for the future operation of the open-pit mine, the following points should be noted. Mining and geological conditions make the deformation characteristics of the open-pit slope very complex. For the mining of coal seams in the slopes, attention should be paid to the influence on the deformation of the overlying rock mass and the ground surface, and their deformation should be controlled within a certain range, so as to avoid large deformation and instability of the slope. Moreover, when mining the slope of complex open-pit mine, the slope stability of important areas should be analyzed carefully by numerical simulation and model experiment, so as to ensure the safety and normal operation of the open-pit mine.

Data availability statement

Some or all data, models, or code generated or used during the study are available from the corresponding author by request.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is financially supported by the National Natural Science Foundation of China (52109125, 52090081), the China Postdoctoral Science Foundation (2020M680583), the National Postdoctoral Program for Innovative Talent of China (BX20200191), and the Excellent Sino-foreign Youth Exchange Program of China Association for Science and Technology in 2020 (No. 58).