Study on Overlying Rock Movement and Mine Pressure Behavior in Shallow-buried Close Coal Multi-seam Mining

： In order to reveal the failure morphology and mine pressure behavior of each overburden strata during shallow buried close multi-coal seam mining, this research takes the Shenmu Zhangjiamao mining area as the engineering background, the migration characteristics of shallow buried close-range overburden structure and the distribution law of mine pressure were studied. The results show that: with the gradual mining of 2 -2 coal seam, the front-rear supporting pressure of the unmined coal seam increases first and then decreases, and the compaction strength of the collapsed overburden in the middle of the mined-out area increases gradually. With the mining of 2 -3 coal seams, the displacement of the direct roof of the lower mined-out area and the rock layer of the upper compaction area increases gradually. The peak stress of the concentrated stress area on both sides of the 2 -3 coal seam gradually decreases with the increase of layer spacing. The compaction stress of overburden in the upper goaf decreases gradually, and the peak value of compaction stress in the middle of the lower goaf increases gradually. Through the measured results of the borehole stress sensor , the obvious influence range of the advance support pressure is about 40 m.


Introduction
Shallow-buried close distance coal seams group exist in Shenmu Coalfield, and the coal seam interlayer spacing is generally about 40 m (Fan et al., 2017).Repeated mining of close coal seam will produce secondary disturbance to overlying rock, aggravate the failure of rock stratum, and cause the problems such as ground falling, and cracking, lead to the destruction of surface vegetation and buildings, and also aggravate the difficulty of rock layer control (Sui et al., 2019;Xie et al., 2019;Huang et al., 2020;Zhang et al., 2019;Li et al., 2020).Therefore, it is of great theoretical significance to study the changes of strata migration caused by multi-seam mining in Zhangjiamao Coal Mine, and to reveal the failure morphology and mine pressure behavior, so as to prevent the catastrophe of overburden instability and formulate effective control strategies.Qian et al. (1998) and Huang et al. (2023) studied the transmission law and strata control of dynamic load of rock strata.Yang et al. (2022) studied the evolution law of overlying strata structure by similar experimental method.The fracture development law of overlying strata in Gaojialiang Coal Mine was studied by Yang et al. (2019) through theoretical analysis and experimental methods.Li et al. (2020) studied the movement and fracture law of overlying strata under the condition of multi-coal seam mining.Guo et al. (2020) studied the failure characteristics of overlying rock caused by longwall mining by discrete element method.Yan et al. (2022) studied the law of surface movement caused by multi-coal seam mining based on UDEC software.Yu et al. (2016) studied the rules for roadway deformation during ultra-close coal seam mining.
Aiming at the problem of repeated disturbance of roof and floor strata caused by coal seam mining in Zhangjiamao Coal Mine and the formation of a large area of plastic zone, the failure characteristics of overburden rock and the distribution characteristics of mine pressure in shallow-buried close coal multi-seam mining are studied using field experiment, numerical calculation, and theoretical analysis.The research results provide scientific guidance for formulating effective control strategies.

The general situation of coal mine
Zhangjiamao Coal Mine is located in Shenmu County, Shaanxi Province.The earth's surface is mainly andy beach landform.There are small valleys in some areas, and the terrain is relatively flat.
The coal seam is shallowly buried with a gentle dip angle, mainly in medium-thick-thick coal seams.
The 2 -2 and 2 -3 coal seams are mainly mined.The comprehensive histogram is shown in Table 1.

Stress characterization analysis of overburden structure
Fig. 2 shows the vertical stress distribution of overlying strata when 2 -2 coal seam is mined.In Fig.      E is elastic modulus and  is Poisson's ratio.
To obtain the solution in Eq. 1, the Dirac delta function is introduced as the boundary condition.
Eq.1 is expressed as Eq.2: ( ) Where the Dirac delta function is expressed as: When studying the problem of rock subsidence caused by multi-seam mining, the superposition principle is used to calculate the subsidence of rock strata.Taking the mining of 2 layers of coal as an example (in Fig. 12), the mining depth of the first coal seam is H1 and the thickness is d1.The mining depth of the second coal seam is H2 and the thickness is d2.The rock subsidence caused by only mining the upper coal seam is expressed in Eq. 4, and the rock subsidence caused by double-coal seam mining is shown in Eq. 5.
where ( ) Combined with the geological conditions of Zhangjiayuan coal mine, L=100m, mining thicknessd1=7.7m,d2=2.3m,mining depth H1=71m, H2=104m, 22 .0 =  , substituting the above parameters into Eq.6, the rock subsidence curve is obtained.To verify the feasibility of the theoretical formula, the dynamic displacement meter is installed on the roof of the mined-out area to test the roof subsidence value.The results are compared with the numerical simulation results(Fig.13).The amount of roof subsidence increases with the advancement of the working face, and the numerical values are approximately the same, which verifies the reliability of the theoretical model.

Test of mine pressure behavior
To obtain the mine pressure behavior of the working face in 2 -2 coal seam mining, a GPD60 pressure sensor is used to monitor the working resistance of hydraulic support.Three detection areas are arranged, and three hydraulic supports are set as monitoring targets in each detection area ( the layout scheme is shown in Fig. 14).The pressure sensor is installed on the left and right columns of each hydraulic support.The data acquisition is carried out once every 10 minutes, and the data acquisition range is from 50m to 150m.The working resistance of 12 # and 56 # hydraulic supports exceeds the rated value more times than other test supports, indicating that there is a certain safety hazard due to the influence of roof collapse and suspension.
To analyze the basic roof pressure law, the average working resistance and its covariance are selected as pressurization judgment basis (Li, 2021).The pressurization of the hydraulic support in each measurement area is shown in Tab. 3 by sorting out the collected data.
Through analysis, the dynamic load coefficients of each test area during the initial weighting period are not much different, and the maximum value is 1.18.The dynamic load coefficient of each measuring area is large during the periodic pressure period, and the maximum value is 1.32.The periodic weighting step of the second survey area is larger than that of the other two survey areas, showing the characteristics that the middle is slightly larger than the two sides.The experimental results of the first weighting step and the periodic weighting step are similar to the results obtained by numerical simulation, which verifies the feasibility of the proposed model.To obtain the influence law of the vertical stress field on the adjacent coal in the process of 2 -2 coal seam mining.The borehole stress sensor is used for observation.The measuring points are set at the return air chute and the transport roadway at 100m away from the cut-hole.Three measuring points are set on each side, and one measuring point is set every 3m.The drilling depth is 5m, 7m, 9m, and the borehole diameter is 45mm.The drilling height is 1.2 m.Fig. 16 is the layout of the station.(2) The peak stress of the concentrated stress area on both sides of the 2 -3 coal seam gradually decreases with the increase of layer spacing.The compaction stress of overburden in the upper goaf decreases gradually, and the peak value of compaction stress in the middle of the lower mined-out area increases gradually.The subsidence range of the upper collapsed overburden is gradually reduced, and the maximum subsidence of the overburden is gradually decreasing.
(3) The bending deflection function of overburden structures in multi-coal seam mining is constructed.By comparing with numerical simulation results, it is concluded that the results are close to each other, which verifies the reliability of the theoretical model.
(4) Through the measured results of the borehole stress sensor, the obvious influence range of the advance support pressure is about 40 m.

3
Numerical simulation analysis 3.1 Rock Strata Distribution Model UDEC simulation software is used to study the movement characteristics of overburden structure and mine pressure behavior when mining 2 -2 ,2 -3 coal seam.Based on the data obtained from field measurement, the mechanical model is constructed.Model size: 200m long×100m wide×122m high, and it is divided into 688,000 cells.Considering the small dip angle of rock formation, the simulation model is established as a horizontal distribution, as shown in Fig.1.Displacement constraints are imposed on the surrounding and bottom layers of the model.The rock mechanics parameters are shown in Tab. 2. The numerical calculation criterion adopts the Mohr-Coulomb criterion.Using step-by-step excavation to simulate mining.

2
(a), the stress concentration phenomenon appears on both sides of the coal wall.The maximum vertical stress is 26.7 MPa.In Fig.2(b), the initial collapse of the immediate roof leads to the decompression of the overlying strata in the mined-out area, and there is only local stress concentration at the back-end coal seam.The maximum vertical stress is 30.5 MPa.In Fig.2(c), the immediate roof has completely collapsed, the main roof begins to bend and sink, resulting in the initial collapse, and the overlying strata produce large cracks.In Fig.2(d), the first periodic weighting of the main roof occurs, and the weighting step is 18 m.The collapsed rock formations are gradually compacted, and the compaction stress exceeds the stress of the roof-floor strata(Fig.2(e)), and the vertical stress in the front and rear of the mining coal seam shows a decreasing trend, which is due to the formation of a small triangular rock layer distribution area here, which plays a supporting role.In Fig.2(f), the stress of the overburden rock in the compacted area gradually increases, forming a distribution law of small stress on both sides and large stress in the middle, and the maximum stress value reaches 34.2 MPa.

Fig. 4 Fig. 5
Fig. 4 Overburden Stress Distribution Diagram During 2 -2 Coal Seam Mining (a)Mining 15m (b)Mining 30m (c) Mining 38m (d)Mining 56m (e)Mining 70m (f)Mining 126m When the 2 -3 coal seam is mined for 25 m(Fig.5(a)),the roof overlying strata does not collapse and sink, and only the middle part of the immediate roof produces bending deformation.When the 2 -3 coal seam is mined for 40 m(Fig.5(b)), the middle part of the immediate roof begins to fall, and it is suspended on both sides of the coal seam.The upper collapsed rock formation also sinks, and the maximum subsidence value reaches 7.79 m.In Fig.5(c), the collapse range of the direct top is expanded, and the subsidence range of the upper collapsed rock formation also expands.The maximum displacement is 9.68 m.In Fig.5(d), the subsidence range of the upper collapsed rock is further expanded.The displacement of the direc top increases, and the maximum displacement reaches 9.85 m.

Fig. 6
Fig. 6 Overburden Stress Contour of Rock Formation, When the Interlayer Spacing Is 22 m

Fig. 9
Fig. 9 Displacement Contour of Rock Formation, When the Interlayer Spacing Is 22 m

，
Based on the boundary conditions, the solution of Eq.1 is obtained by inverse Fourier transform(as shown in Eq. 3):

Fig
Fig. 12 Two-layer Coal Mining

Fig. 13
Fig. 13 Comparison Diagram of Roof Subsidence Value

Fig
Fig. 14 Layout of Survey Area

Fig. 17
Fig.17shows that when each measuring point is about 80 m, the advance support pressure begins to increase gradually, but the increase is not large.When the measuring point is about 40 m, the vertical

Fig
Fig. 16 Measurement Position Layout Diagram

Table 3
Statistics of Working Face Pressure