
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13472-X
10.1016/j.heliyon.2024.e37441
e37441
Research Article
Effect of super-high water materials backfilling on stress decrease and energy release during strip coal pillar mining: A case study
Xing Keke TB20020032B4@cumt.edu.cn
ab
Cheng Jingyi 5759@cumt.edu.cn
a⁎
Zhen Zheng TB23020041A41LD@cumt.edu.cn
a⁎⁎
Wan Zhijun zhijwan@cumt.edu.cn
a
Han Zepeng hazp@cumt.edu.cn
a
Yan Wanzi wzyan@cumt.edu.cn
a
Lv Jiakun Jiakun_L11B4@cumt.edu.cn
a
Yang Yifei TS22020073A31LD@cumt.edu.cn
a
a School of Mines, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China
b Artificial Intelligence Research Institute, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China
⁎ Corresponding author. 5759@cumt.edu.cn
⁎⁎ Corresponding author. TB23020041A41LD@cumt.edu.cn
07 9 2024
30 9 2024
07 9 2024
10 18 e3744124 5 2024
3 8 2024
3 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Mining strip coal pillars left due to strip mining is important to resource-exhausted coal mines in eastern China. Backfilling mining is an effective means to mine strip coal pillars, which could decrease high stress and high energy release. However, materials with super-high water content were not widely applied in deeply isolated coal pillar mining due to lower strength characteristics and durability. In the paper, taking panel c8301 as engineering background, theoretical analysis, numerical simulation, and field monitoring were used to study the feasibility and application effect of the super-high-water backfilling technology in deep isolated coal pillar mining. The results showed that: (1) Panel c8301 is a strip-filling working face with insufficient mining on both sides, and the mining of the panel will trigger the rebalancing of the overlying rock structure on both sides, which increases the rock burst risk. (2) Simulation results indicate that the super-high-water filling method, in comparison to the traditional caving method, significantly reduces peak stress and elastic energy from 112.3 MPa to 4.6 × 106 J to 79.2 MPa and 2.23 × 106 J, respectively. This represents reductions of 29.6 % and 51.5 %, effectively mitigating the impact of mining activities on overburden movement. (3) On-site measurement data confirmed that the measured equivalent mining height was 1.25 m. The total number of microseisms and the amount of released energy decreased significantly, More specifically, three big energy release instances (>1.8 × 105 J) were recorded during the first roof weighting stage and panel in square meters. Super-high-water filling technology has achieved remarkable results in the mining of strip coal pillars and has significant application prospects.

Highlights

• After mining strip coal pillars, the key strata display a “T"-shaped distribution of hinge structure on both flanks.

• The peak stress and the elastic energy of the coal face can be reduced by 29.6 % and 51.5 % through super-high water filling.

• The field monitoring data prove that super-high water filling ensures the safe production of the working face.

• Filling body stress can be divided into four segments with time-space based on the on-site measured data.

Keywords

Backfilling mining
Strip coal pillar
Microseismic activity
Backfilling body stress
==== Body
pmc1 Introduction

With the diminishing coal resources in the eastern part of China, it is necessary to efficiently mine coal resources in the “three unders” and deep coal mine. The strip mining method was adopted for controlling the surface subsidence and overburden movement since the 1990s, resulting in a significant number of strip coal pillars being left leading to considerable coal resource wastage [1]. Mining strip coal pillars are critical to reducing resource waste and extending the span of resource-exhausted coal mines. Nevertheless, the mining of deep strip coal pillars presents various challenges, especially rock bursts, and overburden instability [[2], [3], [4]]. A strip coal pillar with insufficient mining on both sides and the mining of the panel will trigger the rebalancing of the overlying rock structure on both sides, significant rise of stress in the working face, which increases the rock burst risk [[5], [6], [7]].

Backfilling mining has emerged as a primary method for mining isolated coal pillars, effectively controlling and mitigating roof movement, and reducing rock burst incidents in deep coal mining. Traditional backfilling materials include dry backfilling with solid waste, paste backfilling, and high-water backfilling [8]. Dry backfilling with solid waste primarily employs solid wastes, such as gangue and fly ash, demonstrating notable efficacy in reducing ground sedimentation [9,10]. This technology requires the solid materials to be processed, and then vertically fed into the underground (or separated and filled in situ in the underground) through a large-aperture feeding system, finally transported to the space to be filled using conveyor belts and transloading devices [11]. However, this approach requires substantial investment in equipment and is constrained by the availability of gangue and fly ash, particularly in regions where these materials are in demand as raw materials for construction. Paste filling involves converting the material into a toothpaste-like slurry, which is then transported via pipelines to the depleting mining areas [12,13]. This method is effective in mitigating sedimentation but it entails significant investment due to the extensive crushing and conveying pipeline system required. Additionally, the reliability of this system is a concern, as it is prone to frequent pipe blockages. In contrast, super-high water material filling technology just requires super-high water material, which is transported via existing transportation systems to the headgate or tailgate near, was mixed and filled. Therefore, it offers a low initial investment, simple filling equipment, and rapid deployment, presenting substantial economic advantages for the mining of residual coal pillarsProfessor Feng Guangming researched and invented super-high water backfilling material in 2008, boasting advantages such as advantages of early strength and fast hardening, good flowability, adjustable initial setting time, flexible transportation, convenient mining and no environmental pollution [14,15]. Further, many scholars [[16], [17], [18], [19], [20]] have carried out a lot of research on the practical application of super-high water materials, mainly including in surface subsidence control, and gob-side entry retaining. The research result shows that super-high water materials provide better control of surface settlement, and meet the requirements for retaining roadway support along the goaf. However, due to the inferior strength characteristics and durability of super-high water content compared to solid or paste-filling materials, it is rarely employed in deep isolated coal pillar mining. Currently, research on super-high water material for filling under such mining conditions is scarce, with insufficient attention devoted to it. Therefore, it is imperative to investigate the effectiveness of applying this material in these mining conditions.

In this study, taking the super-high-water backfilling panel C8301 as the engineering background, the evolution of the overburden structure under isolated backfilling conditions was analyzed. Subsequently, a numerical model was established to analyze the stress and energy evolution in the panel. Finally, the control effect of stress accumulation in filling mining is assessed through on-site measurements. The study could provide guidance for the safe mining of deep strip coal pillars.

2 Engineering background

2.1 Geology of panel C8301

Panel C8301 was arranged to mine the 100m strip coal pillar in Jining, Shan-dong Province, China. This panel was 100 m wide and 424 m long with a 6 m wide barrier pillar between adjacent panels. The east of panel C8301 is the goaf of panel 8308 with a length of 130m, which was mined by the caving method. The west part is the goaf of panel 8307 with a length of 80 m, which was mined by the caving method, as illustrated in Fig. 1(a). The main coal seam was 800 m deep and 3 m thick and dipped 8° on average. The roof exhibits weak burst tendency and the panel is characterized by medium burst risk. According to the key strata theory [21], the coarse sand-stone located 1.5m from the coal seam is the No.1 inferior key strata (IKS), the medium sandstone located 19.3 m from the coal seam is the No.2 IKS and the fine sandstone located 119.7 m from the coal seam is the No.3 IKS, partial stratigraphic sequence is presented in Fig. 1(b).Fig. 1 Geological and mining conditions: (a) Plan layout of panel C8301, (b) Stratigraphic column.

Fig. 1

The panel C8301 was equipped with 66 ZC5500/20/40 backfilling shields. The headgate and tailgate were supported by ZQ4000/18/36 unit type shields with advanced support distances of 75m and 110m, respectively.

The super-high water bag filling mining technology with the DF-PACK super-high water filling material was employed. The DF-PACK super-high water filling material was composed of two kinds of materials, A and B. The A and B slurries were mixed near the working face to form supre-high water materials with an adjustable water content ranging from 91 % to 97 %. Mechanical parameters of super-high water consolidation bodies with different water volume ratios are shown in Table 3. The filling system mainly consists of ZC5500/20/40 filling powered supports, filling bags, bag spacer, mixing pipe, and branch filling pipe, as illustrated in Fig. 2. Beginning at the tailgate, five filling bags are arranged in sequence, with lengths approximately 14m, 22.5m, 22.5m, 22.5m and 28m, respectively, and a filling step distance of 2.4m.Fig. 2 The layout of the filling system in panel c8301.

Fig. 2

2.2 Overburden structure and theoretical analysis

Panel C8301 is situated between panel 8307 and panel 8308, both of which adopted the caving method. Surface subsidence observations indicate subsidence coefficients of 0.13 and 0.16, the mining of panels was defined as subcritical mining [22]. Panel C8301 can be characterized as a strip-filling working face with subcritical mining on both sides.

In the presence of subcritical mining on both sides, fracture movement in the overlying strata on either side of the mining area reaches a key stratum and halts. The overlying strata of the strip face display a “T"-shaped distribution of hinge structure on both flanks [23], as illustrated in Fig. 3. Strip face mining will initiate the rebalancing of the stable overburden structure on both sides, leading to the generation of additional complex abutment stresses, which in turn increases the burst risk during the mining process.Fig. 3 Structural characterization of inadequately mined overburden on both sides of C8301.

Fig. 3

The study reveals that the bed separation height of the subcritical mining panel is approximately half the width of the panel, with theoretical heights on the two sides measuring 65 m and 40 m. Based on the actual rock stratum, it is recommended that the two sides of panel C8301 be positioned at the bottom of the hard rock layer (56.8 m above the coal seam) and the bottom of IKS2 (19.3 m above the coal seam). This alignment closely aligns with similar simulations conducted by Chen [24] on panel C8301.In accordance with the theory of equivalent mining height for backfilling mining, as the equivalent mining height increases, the overburden rock of the C8301 working face undergoes stages of immediate roof breaking up, basic roof breaking up, instability in adjacent large caving goaf, and instability in adjacent small caving goaf. Dr. Chen Yang's research [7] indicates critical values of 0.8 m, 1.6 m, 2.2 m, and 2.8 m. Specifically, an equivalent mining height of 1.6m is crucial to avoid overall subsidence of the isolated panel and the adjacent mining goaf, thereby reducing the risk of destabilization of overlying structures on a large scale and the occurrence of rock bursts.

Super-high-water bag filling technology was adopted to fill the goaf of panel C8301. Factors that affect the equivalent mining height mainly include the roof-to-floor convergence before backfilling, lacking distance of roof contact and the compression rate of the filling body. The equivalent mining height can be calculated according to Eq. (1) [25],:(1) Mc=(hm+hq)(1−η)ηM

where hm is the roof-to-floor convergence before backfilling, m; hq lacking distance of roof-contact, m; M the mining height, m;η the compression rate of filling body. Relevant parameters, including hm = 0.2 m, hq = 0.3 m, η = 20 % [26], and M = 3 m, are selected. Then:(2) Mc=(0.2+0.3)×(1−0.2)+0.2×3=1m

Using the empirical calculation method, the preliminary calculation of equivalent mining height is 1 m, which is less than 1.6m and can basically meet the requirements. The actual equivalent height should be determined by on-site measuring roof-to-floor convergence (in section 5.2).

3 Numerical simulation study of backfilling mining in strip coal pillars

3.1 Establishment of the numerical model

Flac3D numerical software was used to study the stress evolution and energy release from backfilling mining in 100 m strip coal pillars. A total Flac3D simulation model was established, the length and width were the same as the actual onsite situation, as shown in Fig. 4. The model size was 520 m × 400 m × 120 m (length × width × height). Displacement constraints were used around the perimeter and bottom of the model. Vertical stress of 15.2 MPa was applied to the top of the model. The strain-softening model was used to simulate coal seam, and the Mohr-coulomb model was used for the rock strata and super-high water backfilling materials. Properties parameters of rock strata, coal seam, and super-high-water material were shown in Table 1, Table 2, Table 3. Numerical simulation was conducted using super-high-water materials with 95 % water content for filling.Fig. 4 Structural characterization of inadequately mined overburden on both sides of C8301.

Fig. 4

Table 1 Material properties parameter.

Table 1Lithology	Thickness (m)	Density (kg*m−3)	Bulk modulus (GPa)	Shear modulus (GPa)	Cohesion (MPa)	Friction angle (Deg)	
muddy sandstone	15	2380	1.28	1.12	1.52	32	
siltstone	20	2530	3.18	3.47	5.61	37	
Sandy mudstone	12	2380	1.28	1.12	1.52	32	
medium sandstone	25	2420	6.82	5.14	6.31	37	
Fine sandstone	2	2420	6.07	4.74	7.26	37	
muddy sandstone	6	2380	1.28	1.12	1.52	32	
Fine sandstone	2	2420	6.07	4.74	7.26	37	
coarse sandstone	8	2420	7.53	5.27	7.51	37	
siltstone	2	2530	3.18	3.47	5.61	37	
Coal	3	1470	0.93	0.36	1.44	32	
Fine sandstone	3	2420	6.07	4.74	7.26	37	
Sandy mudstone	10	2380	1.28	1.12	1.52	32	
Fine sandstone	12	2420	6.07	4.74	7.26	37	

Table 2 Physical and mechanical parameters of the coal pillar model.

Table 2Material	Softening rate(%)	Cohesion
Residual value (MPa)	Friction angle
Residual value (deg)	
Coal	0	1.44	32	
0.01	1	25	
0.05	0.43	22	
>0.05	0.43	22	

Table 3 Mechanical parameters of super-high water consolidation bodies with different water volume ratios.

Table 3water volume
ratios (%)	Density (kg*m−3)	UCS (MPa)	Young's modulus (MPa)	Poisson's ratio(%)	Cohesion (MPa)	Friction angle (Deg)	Tensile strength (MPa)	
0.93	1060	2.12	132	0.43	0.15	25	0.29	
0.95	1020	1.13	86	0.45	0.15	24	0.26	
0.96	950	0.99	63	0.45	0.11	22	0.21	

The specific simulation process is shown in Fig. 5. There are four steps: (1) establish a three-dimensional simulation model; (2) excavate the panel 8307 and 8308; (3) excavate the headgate and tailgate of C8301; (4) excavate the C8301 and fill the caved zone with a step length of 4 m.Fig. 5 Flowchart of numerical modeling.

Fig. 5

3.2 Analysis of stress evolution and energy reduction mechanism of backfilling mining

To investigate the stress evolution regulation and energy reduction mechanism of backfilling mining in 100m strip coal pillars, the differences including front abutment stress, side abutment stress and elastic energy between backfilling mining and caving method model were systematically analyzed in this section.

Fig. 6 denotes the front abutment stress distribution difference between the backfilling model and the caving model (the measuring curve, chosen for this comparison, is located 2 m from the floor and 6 m from the solid coal side in the headgate, aligned with the direction of advancement).Fig. 6 Front abutment stress distribution curve (The center of the working face is along the strike). (a) Backfilling model; (b) Caving model.

Fig. 6

As depicted in Fig. 6, with the excavation of C8301, the vertical stress of the filled area gradually increased due to the roof subsidence and filling body condition. In the backfilling model, the maximum front abutment stress is 79.2 MPa, whereas in the caving model, it peaks at 112.3 MPa, indicating a 29.6 % reduction in vertical stress with the filling method. In the single-square stage, the peak stress reduced from 104.3 MPa to 74.6 MPa when applying the filling method, illustrating a decrease of 29.7 MPa. In the first double-square stage (C8301 and 8307), the peak stress dropped from 111.2 MPa to 79.2 MPa with the filling method, signifying a reduction of 32 MPa. Similarly, in the second double-square stage (C8301 and 8308), the peak stress decreased from 112.3 MPa to 79.1 MPa, reflecting a reduction of 33.2 MPa. In the three-square stage, the peak stress diminished from 104.6 MPa to 72.5 MPa, showing a reduction of 32.1 MPa.

The significant reduction in stress peaks observed during these square stages can be attributed to the progressive strengthening of the filling body over time. This strengthening effectively restricts the movement of the roof plate within the goaf, as illustrated in Fig. 6(a), which shows the vertical stress distribution within the filling body in the goaf.

Fig. 7 denotes the side stress distribution difference between the backfilling and caving models. The peak stress for the filling method occurs approximately 6 m ahead of the face, while for the caving method, it is about 7 m ahead of the face. The vertical stress curves in the inclined direction are plotted by selecting the corresponding cross sections.Fig. 7 Side stress distribution curve(cross-section of 100m in advance of C8301). (a) Backfilling model; (b) Caving model.

Fig. 7

The side abutment pressure patterns in both caving and backfilling methods display similar characteristics. In the coal pillars, pressure initially increases and then decreases. In panel C8301, the pressure distribution on the solid coal side demonstrates a saddle-shaped curve. The peak stress on both sides of the coal pillar is relatively low, at just 10.62 MPa, signifying complete yield in the coal pillar region. Notably, stress in the larger goaf is significantly higher than that in the smaller goaf. This disparity is attributable to an increase in the width of the strip mining working face, which in turn leads to the proliferation of overburden fissures. Consequently, there is an augmented lateral stress exerted on the coal pillar of the strip. It's important to note that the average side front abutment stress decreases by 26 MPa, underscoring a remarkable filling effect.

In numerical modeling, elastic energy density W: can be calculated as follows [27]:(3) WE=δ12+δ22+δ32−2μ⋅δ1⋅δ2−2μ⋅δ2⋅δ3−2μ⋅δ1⋅δ32E

where μ and E are the Poisson ratio and elastic modulus respectively, and δ1, δ2, and δ3 are the maximum, intermediate, and minimum principal stresses of the element body, respectively.

Fig. 8 depicts the elastic energy distribution difference between the caving model and the backfilling model. The elastic energy peak of the caving model and backfilling model is 4.60 × 106 J/m³ and 2.23 × 106 J/m³, respectively, which also indicates that the elastic energy peak of the backfilling model is approximately 51.5 % lower than that of the caving model.Fig. 8 Elastic energy distribution contour. (a) Backfilling model; (b). Caving model.

Fig. 8

The elastic energy of the working face of the caving stope and backfilling stope showed similar distribution patterns. When the advancing distances were at 40 m, 100m, 180 m, 240 m, 320 m, and 360 m, the corresponding peak elastic energies at the center of the coal seam were 2.78 × 106 J/m³, 4.11 × 106 J/m³, 4.52 × 106 J/m³, 4.60 × 106 J/m³, 4.04 × 106 J/m³ and 3.59 × 106 J/m³, respectively, whereas in the backfilling model, peak elastic energies were 1.32 × 106 J/m³, 1.96 × 106 J/m³, 2.26 × 106 J/m³, 2.23 × 106 J/m³, 1.78 × 106 J/m³ and 1.30 × 106 J/m³. The overall destruction of the surrounding rock mass was mitigated, and the amount of energy released was significantly reduced with the filling method.

4 Field monitoring and analysis

4.1 Surface subsidence

Panel C8301 has a strike measuring line with a length of 685m, with 24 monitoring points(A01-A24), and an inclined measuring line with a length of 645m, with 18 monitoring points(C01-C18), as shown in Fig. 9. Surface subsidence was measured utilizing the Hi-Target H32 GNSS RTK system for acquiring the elevation coordinates of each surveyed point.Fig. 9 Layout of observation lines within panel C8301.

Fig. 9

There are 7 observation phases in the mining process of panel C8301, corresponding to distances from the set-up room 220 m, 246 m, 299 m, 343 m, 377 m, 398, and 424 m, respectively. In strike, taking A01 as the coordinate origin, from A01 to A24, the ground subsidence curve of panel C8301 is obtained, as shown in Fig. 10. After sinking, the maximum settlement value is located at point A45, which is 136 m in front of the open-off cut, and the cumulative subsidence is 334.2 mm. Taking the observation points with cumulative subsidence values greater than 10 mm as the boundary points of the subsidence basins, the mining influenced range is determined to be 314 m. Point A15, which is 106 m in front of the open-off cut, occurred 18 mm subsidence when the panel advanced 143.5 m, so the start distance of the C8301 working face is approximately 143.5 m, about 0.184H0.Fig. 10 Ground subsidence curves along the strike direction of panel C8301.

Fig. 10

In the inclined survey line, taking C01 as the coordinate origin, from C01 to C18, the ground subsidence curve of panel C8301 is obtained, as shown in Fig. 11. It can be found that the center of the subsidence basin is located at point C20 which is 23.7 m away from headgate, with a cumulative subsidence of 227.4 mm.Fig. 11 Ground subsidence curves along the inclined direction of panel C8301.

Fig. 11

The surface subsidence is minimal and exhibits no abrupt changes, indicating that the working face mining has not reached full subsidence, and the surface remains suitable for the normal use of cultivated land.

4.2 Front abutment pressure

Monitoring of the abutment pressure was conducted at the tailgate of the panel through observation unit pressure. Observations were made when the panel advanced 80 m, 160 m, and 320 m, as shown in Fig. 12. The pressure curve can be divided into three zones based on the distribution of pressure. In the significant impact zone, the peak abutment support strength was 0.85 MPa, occurring approximately 13 m from the longwall face, the stress concentration coefficient is 1.93, and the abutment pressure influence zone extends to 34 m. The general impact zone ranges from 34 m to 76 m, and the in-situ stress zone extends beyond 76 m. The intensity and range of the front abutment pressure increased significantly in square meters. The anti-burst measures such as large-diameter drilling holes to relieve pressure on the coal seam were implemented.Fig. 12 Front abutment pressure of C8301.

Fig. 12

4.3 MS events verification

MS events are caused by energy accumulation and the release of coal rock mass, and MS events tend to occur in areas with high stresses and large stress differences, which can respond to the macroscopic motion characteristics of the roof movement. KJ648 microseismic monitoring system was installed in panel C8301, and the count and energy of MS events were accumulated based on the microseismic data. Taking the setup entry as zero point and cumulative advance distance as the x-axis, the MS event cumulative energy, and cumulative count are presented in Fig. 13.Fig. 13 Statistics distribution of MS events energy and count: (a) Strike direction statistics of MS events energy, (b) Strike direction statistics of MS events count.

Fig. 13

The impact warning is activated when the dynamic ground pressure exceeds an energy level of 1.8 × 105 J. Only three instances of exceeding the warning threshold were recorded during the mining process, happening during the first roof weighting stage and panel in square meters. A noticeable increase in microseismic energy and frequency during the panel in square meters suggests that with the expansion of the goaf, both the intensity and frequency of roof crack development increase. Despite the substantial roof weighting in panel C8301, the occurrence of events and the release of energy are markedly lower in comparison to isolated working faces employing the caving method. Consequently, the utilization of super-high water materials effectively alleviates the intensity of roof strata movement, diminishes the accumulation and release of energy in the roof, and diminishes the risk of dynamic impacts in the panel.

5 Discussion

5.1 The law of abutment stress variation in isolated backfilling working face

The filling body is employed to fill the goaf and provide support to the roof, restricting the speed and amplitude of roof movement in the goaf, influencing the structural dynamics of overlying strata, and consequently affecting the stress distribution within the coal body and pillars in the working face [28]. The bagged filling method was used in the panel C8301. Before filling, a partial subsidence occurred in the lower roof strata. Following the filling process, as the roof strata continue to subside, it exerts pressure on and compacts the filling body. Ultimately, equilibrium is achieved under the conditions of roof loading and surrounding rock constraints.

Stress distribution of super-high water backfilling mining working face as depicted in Fig. 14. In the goaf, three zones are formed in the filling body sequentially from the setup room. Filing body stress stabilization zone (a), demonstrates stress levels are basically equal to the in-situ stress. The filling body, when amalgamated with the surrounding rock, forms an integrated load-bearing structure capable of consistently supporting the overburdened strata gravity. Filling body stress enhancement zone (b) undergoes a progressive increase in stress over time, concurrent with a gradual strengthening of the filling body. As the roof subsides, this zone acquires a specific load-bearing capacity. Filling body consolidation and roof-contact zone (c) manifests relatively lower stress levels. The filling body undergoes consolidation, and its strength gradually intensifies. As the roof subsides, the filling body makes contact with the roof. However, it is unable to effectively withstand the pressure exerted by the overlying strata, leading to the leakage of specific liquid backfill material.Fig. 14 Stress distribution of super-high water backfilling mining in panel C8301.

Fig. 14

Similarly to the panel in the caving method, four zones are formed at the front of the panel: the stress-increasing zone (d), coal body plastic zone (e), coal body elastic zone (f), and In-stu stress zone (g). In contrast to the caving method, the diminished movement of the overlying strata results in a reduction of the impact of front abutment stress on the working face, such as the decreased stress concentration factor and the contraction of the ranges of zones (e) and (f).

5.2 On-site analysis of backfilling body stress and roof subsidence displacement

Backfilling body stress and roof subsidence displacement were measured through stress and displacement sensors installed inside the backfilling body, which are No.20, No.35, No.50 stress station and No.34, No.36 displacement station. The station number corresponds to the number of the unit position in which the sensors were installed. Filling body stress sensor, model GPD300, employs a strain gauge to measure the stress, which consists of two sensors: one of high precision and low range (sensor 1) and another of low precision and high range (sensor 2), as shown in Fig. 15. The filling body displacement sensor, model GUD1500, utilizes a gear-rack structure to convert the linear displacement of the rod into angular displacement, as shown in Fig. 16. The stress and displacement sensors were fixed to the floor using bolts in the field.Fig. 15 GPD300 backfilling body stress sensor. (a) Sensor structure diagram; (b). Sensor Outline Diagram.

Fig. 15

Fig. 16 GUD1500 roof subsidence displacement sensor.

Fig. 16

Until the sensor signal was interrupted, the monitoring continued for 66 days, corresponding to a 76m advance in the panel, backfilling body stress and roof subsidence displacement curves were shown in Fig. 17.Fig. 17 Backfilling body stress and roof subsidence displacement.

Fig. 17

The pressure curve can be divided into four phases according to the characteristics of stress. In the first phase, termed the roof-contact segment (advancing distance/date: 0–7.8 m/4 d), the roof sank 180 mm, and the strength of the filling body was about 1.6 MPa. The second phase, identified as the filling body consolidation segment (advancing distance/date: 7.8–13.2 m/13 d), witnessed a roof subsidence of 410 mm. The filling body basically completes the solidification and strengthening, and it has a certain load-bearing capacity under the state of three-direction pressure. The third segment, designated as the filling body stress enhancement segment (advancing distance/date: 7.7–36.8 m/40 d), witnessed a gradual roof subsidence. The final phase, denoted as the filling body stress stabilization segment, marked the completion of adjusting the bearing structure of the roof and the filling body to establish a unified carrier. The final roof subsidence in goaf ranged from 850 mm to 1050 mm.

However, the displacement curve of roof subsidence, depicted in red, exhibits irregular fluctuations towards the end, This irregularity may be due to the following factors: After the roof breaks and sinks, the strength of the direct roof decreases significantly, leading to the development of cracks and irregular collapses, which can cause the sensor to become embedded in the roof. Furthermore, with a working face inclination angle of 8°, both the filling material and the collapsed roof move towards the transport roadway, causing the sensor to tilt or even topple, thereby altering its displacement readings. To enhance monitoring performance, it is recommended to install inertial guidance within the displacement sensor and a larger top linkage platform.

The stress distribution characteristics in different regions of the panel were analyzed. Stress stations No.20 and No.50 experience greater stress than Station No.35, similarly a saddle shape in the initial stage. After advancing distance/date: 36.8 m/40 d, the stress tendency shifts to be higher in the middle and lower on both sides. The reason is that the influence of stress superposition from the two sides of the goaf results in higher edge stress compared to center stress during the initial stage of filling body stress. The higher center stress indicates the roof in the goaf is basically stabilized.

Based on the on-site data, roof subsidence in goaf was about 1.05 m including lacking distance of roof contact (180 mm) and compression amount of filling body, the roof-to-floor convergence before backfilling was estimated to be 0.2 m, therefore, the measured equivalent mining height was 1.25 m.

6 Conclusions

This paper has studied the feasibility and practical application effects of super-high water materials backfilling in deep isolated coal pillar mining, taking panel C8301 as a case study. The specific conclusions are as follows.(1) It is determined that panel C8301 is a strip-filling working face with insufficient mining on both sides, and the mining of the panel will trigger the rebalancing of the overlying rock structure on both sides, which increases the rock burst risk. The key strata displays a “T"-shaped distribution of hinge structure on both flanks. The equivalent mining height should be less than 1.6 m for safe mining.

(2) Flac3D numerical model was established to compare and analyze the stress and energy evolution law of the filling and caving method. With the super-high-water filling method, peak stress and elastic energy were reduced from 112.3 MPa to 4.6 × 106 J to 79.2 MPa and 2.23 × 106 J, respectively, which reduced the accumulation of impact energy.

(3) The field monitoring results showed that the equivalent mining height was about 1.25 m, with the maximum surface subsidence reaching 355 mm. Throughout the mining process of the panel, there is a noticeable occurrence of roof weighting from the square meter, albeit with a relatively moderate intensity. This observation suggests that the super-high water bag filling technology effectively mitigates the mine pressure intensity in the island panel, ensuring the stability of the overburdened rock and promoting the safe mining of the panel.

(4) The stress characteristics of the isolated backfilling working face were analyzed, and four segments were formed including the roof-contact segment, filling body consolidation segment, filling body stress enhancement segment, and filling body stress stabilization segment with time-space based on the on-site measured data of backfilling body stress and roof subsidence of the goaf.

Data availability statement

Data included in article.

Funding

The work was supported by the 10.13039/501100012226 Fundamental Research Funds for the Central Universities (Grant Numbers 2018ONA24 ).

CRediT authorship contribution statement

Keke Xing: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. Jingyi Cheng: Writing – original draft, Resources, Funding acquisition, Conceptualization. Zheng Zhen: Writing – review & editing, Methodology. Zhijun Wan: Supervision, Resources. Zepeng Han: Visualization, Investigation. Wanzi Yan: Data curation. Jiakun Lv: Visualization, Supervision. Yifei Yang: Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the editors and the anonymous reviewers for their valuable comments and suggestions, which helped to improve the manuscript.
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