Roof weighting and support of a largely mined shallow coal seam

To study roof weighting and the support of a largely mined shallow coal seam, the hydraulic support resistances in both small and large periodic weightings were derived by different theoretical formulae. Support working resistances of the 12,401 mining face in small and large periodic weighting were 10,442 and 17,064 kN, which represented a loading-increase coefficient of up to 1.64. Mining cracks were formed up to land surface and were visible to the naked eye, and numerical simulation of roof stratum structure by 3DEC has been proven to be realistic. A 1.2-MPa supporting intensity represented a critical threshold for effectively reducing horizontal displacement and inhibiting rib spalling using FLAC3D. Hydraulic fracturing and micro-seismic monitoring were used to ensure the safety of the 12,401 mining face. These findings can explain the mechanism of mining a shallow coal seam for effective prevention and controlled design. The results show high accuracy and are consistent with reality.


Introduction
It is well known that largely mining shallow coal seams are being widely used in China's main northwest coal-production area [1]. It has many advantages such as productivity, lower amounts of discharged gangue, and less coal dust: problems include severe rib spalling, frequent support crushing, and roof water disasters [2]. More research has been developed in recent years: Yang [3] revealed the specific mechanism of main roof weighting with a large mining height, Huang et al. [4] investigated the cantilever fracture pattern of the equivalent immediate roof. He et al. [5] conducted physical simulation experiments of roof breaking angles from 56.2 to 69.3°, Huang et al. [6] proposed a method of determining the reasonable support load and rated working resistance. He et al. [5] established mechanical models for key strata and equivalent immediate roof of large-mining-height working faces in a shallow coal seam. Yin [7] established a cutting block model for hydraulic supports. Wen et al. [8] proposed a method for determining support working loads under given deformation and limited deformation. Liu et al. [9] estimated the effects of mining face length and the underground pressure distribution when mining over a large height. Wang and Pang [10] discussed the mining or caving method for use in a thick coal seam, Wang et al. [11] evaluated coal rib stability effects over large mining heights, Xu et al. [12] explored the supporting stress fields with large mining height using an elastic beam model. Fan et al. [13] established a mining intensity evaluation model for coal seams with a large mining height. Wang et al. [14] investigated the relationship of hydraulic supports to the coal wall in coal seams with a large mining height, Zhao et al. [15] simulated the undermined ground pressure distribution on geographic information system, Kong et al. [16] determined the specific support capacity needed for the stability control of a coal face, Huang and Tang [17] assessed the mining roof structural changes in a shallow coal seam, Zhang et al. [18] studied the roof leakage mechanism of fully mechanized faces with a large mining height. Huang and Zhou [19] explored the roof weighting behavior and patterns of largely mined-out shallow coal seams, Wang et al. [20] discussed the effects of broken key strata. Yang et al. [21] investigated the strata failure process and the required support resistance, Guo et al. [22] discussed the given mechanism of overburden strata deformation with a large mining height around geological structures. Wang et al. [23] elucidated mechanisms of rib spalling through different types of faults, Xie et al. [24] debated different characteristics of strata movement and support systems. Yang and Kong [25] studied the reinforcement mechanism of rib spalling, Chang et al. [26] discussed the different rib spalling mechanisms of fully mechanized top caving seams with and large mining height, Yin et al. [27] studied the different criteria and safety evaluation of rib spalling in shallow coal seams. RayChowdhury et al. [28] got visible light communication and long rang wireless technology for transmitting underground information to the aboveground control room.
Roof stratum control in shallow coal seams has been effective, but the relationship between roof weighting and support strength remains to be found in specific geological conditions. In the present work, hydraulic support working resistances in small and large periodic weightings were determined. Numerical simulations in 3DEC and FLAC 3D were used to reveal specific mechanism of roof weighting and support intensity in mining shallow coal seams. These findings explain the mechanisms prevailing when mining shallow coal seams for effective accident prevention and controlled design.
2 Roof weighting and support resistance in small or large periodic weightings

Roof weighting and support resistance under small periodic weighting
The immediate roof stratum of a shallow coal seam is regarded as the likely rigid body or the given load stratum over the hydraulic support, and the main roof stratum is considered to be the given deformation stratum above the immediate roof and hydraulic support [29]. The morphological structures of a composite cantilever beam and a hinged rock beam in small periodic weightings are shown in Fig. 1. In small periodic weightings, hydraulic supports are never up to the maximum pressure, so the safety valve is less likely to open and roof subsidence remains under control [30]. The maximum support pressure is bearing the whole load of immediate roof stratum and the part-load of the main roof stratum. In Fig. 1, the immediate roof stratum is seen as the composite cantilever beam including blocks A 1 , B 1 , A 2 , B 2 , A 3 , and B 3 . Blocks A 1 , A 2 , and A 3 are firstly formed above the coal seam. Blocks B 1 , B 2 , and B 3 are then formed above the hydraulic support. The main roof stratum is usually regarded as a hinged rock beam formed from blocks A 4 , B 4 , and C 4 , which are above A 3 , B 3 and mined gob. Block A 4 is first formed above A 3 , block B 4 is then formed above B 3 , and block C 4 is finally formed above the mined gob. Mechanical analysis of composite cantilever beams in small periodic weighting is illustrated in Fig. 2. Points O 1 , O 2 , and O 3 are the fracture locations of A 1 and B 1 , A 2 and B 2 , A 3 and B 3 . x 1 O 1 y 1 , x 2 O 2 y 2 , and x 3 O 3 y 3 are the rectangular coordinate systems of B 1 , B 2 , and B 3 . Q z is the hydraulic support pressure, c denotes the distance between a hydraulic support and fracture location O 1 , α is the roof fracture angle, l 1 is the length of B 1 , h 1 represents the depth of B 1 , p 1 is the weight of B 1 , R 1 and R ' 1 denote the added loads from B 2 , l 2 is the length of B 2 , h 2 refers to the depth of B 2 , p 2 is the weight of B 2 , R 2 and R ' 2 are the added The immediate roof stratum reaches a mechanical equilibrium as given by formulae (1)(2)(3)(4)(5)(6).
Q z1 is the hydraulic support pressure, P j is the weight of the immediate roof stratum, h j represents the thickness of the immediate roof stratum, l j denotes the immediate roof stratum length, is the roof stratum fracture angle, C represents the distance between the support force and coal wall, and R is the added load from the main roof stratum. The roof stratum interactive force can be omitted, so the immediate roof and main roof can be generalized as an integrated whole; formula (6) is simplified to the form given in (7). Mechanical analysis of a hinged rock beam under small periodic weighting is shown in Fig. 3, and it can be regarded as a hinged structure.
The relationship between blocks A and B can be generalized by formulae (8)(9)(10).
Key blocks A and B are the same length and depth in small and large periodic weightings, so P A = P B = P . Taking formulae (9) into (10), the hydraulic support working resistance under small periodic weighting over a large mining height is given by formula (11).
where L is the periodic weighting interval, H is the key block depth, P represents the key block weight, p denotes the upper uniform load, f is the block friction coefficient, K represents the gob gangue stiffness, s is the gob gangue h j , is the coal recovery ratio, and m is the large mining height.

Roof weighting and support resistance under large periodic weighting
Chinese mining of shallow coal seams usually advances by 15 to 20 m a day, so the collapsed roof stratum cannot fully backfill gob which is then poorly compacted after being periodically broken. In big periodic weightings, hydraulic supports are usually up to the maximum pressure, so the safety valve is often likely to open and roof subsidence is out of control [30]. Key stratum caving spans are much longer than periodic weighting intervals, so the broken roof stratum conforms to an elastic long beam model ( [31] in Fig. 4 (a) showing the mechanical analysis, and (b) the compressed analysis). S is the compression degree of the elastic long beam and breaking roof stratum, which can be described by parabolic functions from O to L. K j is compression stiffness of key block A, P (x) is the force acting between the beam and surrounding rock stratum (formula 12): Research Article SN Applied Sciences (2022) 4:119 | https://doi.org/10.1007/s42452-022-05013-z L 0 is the span of the beam, x is the distance from the coordinate origin to the point of contact between the long beam and caving gangue, K j refers to the bracing stiffness of the roof, support and floor. The main roof stratum is synchronously broken by the elastic long beam, whose lower strata are considered as a composite cantilever beam. The upper strata of the long beam are considered as an articulated set of rock blocks, so a long beam model can be considered as a cantilever beam-articulated block system (formula 13).
R longbeam is the internal force between the long beam and the lower block (formula 14), x longbeam denotes the virtual distance from the point of contact to that where the force acts between the long beam and the rock stratum.
Taking formula (14) into (13), the hydraulic support working resistance under large periodic weighting when mining a shallow coal seam is given by formula (15). P j is the immediate roof block weight, h j is the immediate roof block depth, l j is the immediate roof block length, is the roof rock fracture angle,K z represents the compression stiffness, h i is the roof stratum depth below the key strata, E i refers to the roof stratum elastic modulus below the key strata, h f is the immediate floor depth, E f is the immediate floor elastic modulus, S is the caving rock compression caused by the long beam, L 0 is the long beam caving length, and C denotes the distance between the line of action of the resultant force and the coal wall. Because hydraulic support working resistances under large periodic weighting over a large mining height are given by formula (16).

Numerical simulation of roof stratum structure by 3DEC
3DEC software is used to simulate large differential displacements, which can allow multiple contact modes, providing multiple material models [32]. Numerical simulation of this roof stratum structure by 3DEC covered a 300-m long, 1-m wide, 237-m high model. The mining height of the 12,401 seam is 8.8 m, the roof stratum is 215 m deep, and the Mohr-Coulomb yield criterion is used. The related rock mass physico-mechanical parameters are listed in Table 1, and rock mass joint fissure parameters are displayed in Table 2.
When the stimulation reached 50,000 feet (15.24 km), the mining roof stratum can be broken and collapsed by longwall caving method. Considering the boundary effect, coal mining begins from 50 m to the left until 50 m to the right, giving an excavation length of 200 m. The roof stratum structure simulation is illustrated in Fig. 5 showing 113,637 steps: (a) is the initial model diagram; (b) is the roof failure distribution, and (c) is the roof displacement distribution. Discontinuous surface cracking of the 12,401 ground subsidence basin is shown in Fig. 6. The height of the water-induced fracture zone exceeds 200 m, so mining cracks reach the land surface, and are visible to the naked eye. This arises because the arc-shaped separation space is not even, one big periodic weighting of the large mining height usually appears after two or three small periodic weightings: the main and inferior key strata are broken and collapsed by the longwall caving method.  So numerical simulation of this roof stratum structure by 3DEC is shown to conform to the reality in this mine.

Numerical simulation of graded support load by FLAC 3D
FLAC 3D is an explicit finite-difference formulation used to imitate complex behaviors, but some problems include several stages, large displacements, and nonlinear material behaviors [33]. According to the 12,401 seam mining information and exploration borehole histogram, support pressures upon coal failure, as modeled by FLAC 3D reached 0.5 MPa under similar operational conditions, when using similar working procedures. This operation was covered a 200-m long, 1-m wide 237-m high model (Tables 1 and  2), whose upper strata were 1-m long, 1-m wide in Figs. 7 and 8. Figure 7a shows the initial stress equilibrium, When P was 0.8 MPa, the immediate roof stratum was mainly broken in the front of the hydraulic support, and partial rib spalling was less likely to happen. The shear failure range of the coal seam was decreased, tensile failure occurred within 1 m in front of the coal wall, but the worst extent of failure remained in the central coal wall.
When P was 1 MPa, the shear failure range and roof subsidence velocity in the immediate roof stratum decreased. Tensile failure mainly occurred within 1 m in front of the coal wall, but the greatest extent of failure remained in the middle of the coal seam. Figure 10 shows contours of szz at 30 m when P is 1.2 MPa; (a) shows contours of szz at 30 m and (b) shows contours of szz at 130 m. The shear failure range and roof subsidence velocity in the immediate roof stratum had been effectively controlled, but the immediate roof stratum was mainly broken next to the hydraulic support. The worst failure still occurred in the middle of the coal wall, penetrating to a depth of about 1.0 m.
When P was 1.4 MPa, the non-shear failure range of the immediate roof stratum was increasing, and the immediate roof stratum was mainly broken above the rear of the hydraulic powered support. The worst extent of failure remained in middle of the coal wall, penetrating to a depth of about 0.5 m. Provided the supporting intensity is less than 1.2 MPa (Table 3), horizontal displacement in coal wall gradually decreased with increasing support pressure. When P was greater than 1.2 MPa, the horizontal displacement of the coal wall would be stable with increasing support pressure, so 1.2 MPa could be seen as the critical threshold at which engineers could effectively reduce horizontal displacement and inhibit rib spalling. Numerical simulation of graded support load by FLAC 3D is well able to identify the optimum support pressures upon coal failure.

Engineering calculation
Shangwan Colliery is located in Ordos Ejinholo, Inner Mongolia; the overlying stratum of the 12,401 mining face has a thickness ranging from 124 to 244 m, the thickness of its loose layer ranges from 0 to 27 m k, and the dip angle ranges from 1 to 5°. The designed mining height of the 12,401 face is 8.8 m, its coal mining length is 299.2 m, the advanced mining length reaches 5254.8 m, and the predicted mining production is up to the historical record of 17.58 Mt.
According to site investigation, f is 0.5, ∑ P j is 2000 kN, ∑ l j is 6 m, ∑ h j is 3 m, is 60°, p is 20 kN/m 3 , L is 10 m, H is 5 m, P is 200 kN, K is 40 kN/m 3 , s is 0.3 m, Δ is 8 m, and c is 1 m. The 12,401 hydraulic support working resistances under small periodic weightings over a large mining height are given by formula (11).
According to site investigation, c is 1 m, f is 0.5, ∑ P j is 2000 kN, ∑ l j is 6 m, ∑ h j is 3 m, is 60°, S is 3 m, L 0 is 10 m,

Engineering verification
According to Shangwan Colliery field observations, the first weighting of 12,401 seam occurred after 40 m, its maximum weighting strength reached 507 bar (50.7 MPa).
Broken key strata and discontinuous surface cracks were formed after 85 m; the first periodic weighting and partial roof fall occurred after advancing about 100 m. Hydraulic fracturing technology (Fig. 11) and a micro-seismic monitoring system (Fig. 12) were used in the 12,401 mining face. Hydraulic fracturing is of paramount importance to enhance fracturing effects in colliery hard roof control; it is a widespread process that involves blasting water and chemicals underground at high pressure to shatter the shale rock by inclined drilling [34,35]. Micro-seismic monitoring systems have developed to a high degree in recent years as a new reference to guide safe mining: precise location of micro-seismic incidents and their intensity is realized by acoustic event monitoring, seismology, and computational geophysics [36,37]. Micro-seismic monitoring system could be used to record micro-seismic events in both time and frequency domains to predict roof   After implementing these two measures, the 12,401 had been quickly advanced by some 4000 m and safely mined to produce 13 Mt of coal (Fig. 13), (a) shows the situation before taking measures, (b) the situation thereafter.

Conclusion
1. Support working resistances of the 12,401 mining face in small and large periodic weightings were 10,442 and 17,064 kN, which represented a loading-increase coefficient of up to 1.64. Mining cracks reached the land surface and were visible to the naked eye. The arc-shaped separation space was uneven, so the main and inferior key strata were broken and collapsed by the longwall caving method. 2. A 1.2-MPa support pressure represented the critical threshold at which engineers can effectively reduce horizontal displacement and inhibit rib spalling. Hydraulic fracturing technology and a micro-seismic monitoring system were used in the 12,401 mining face, which had been quickly advanced by 4000 m and safely mined of 13 Mt of coal. 3. The dynamic evolution of roof weighting and support, the disaster mechanism of geological structure  will be studied by separated layer water and tectonic stress for future research direction, which will provide new support for early warning of roof water disasters when mining coal at complex conditions. 2019MS05053).
Author contributions All the authors contributed extensively to the work. MS derived the formulas and wrote the paper. ZY and XZ undertook the numerical simulation. CG and XZ conducted the field work. LQ drew the related figures and tables.

Data availability
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