Coupling control on pillar stress concentration and surface cracks in shallow multi-seam mining

Qingxiang Huang,Junwu Du,*,Jie Chen,Yanpeng He

a College of Energy Engineering,Xi’an University of Science and Technology,Xi’an 710054,China

b Key Laboratory of Western Mine Exploitation and Hazard Prevention,Ministry of Education,Xi’an 710054,China

c College of Materials Science and Engineering,Xi’an University of Science and Technology,Xi’an 710054,China

Keywords:Shallow multi-seam Coal pillar offset Stress concentration Surface cracks Strata control

ABSTRACT In order to ensure safe mining and reduce surface damage in shallow multi-seam mining,the failure characteristics of interburden strata with different coal pillars offset distances between pillars in the upper and lower seams,the distribution characteristics of stress concentration in coal pillars,and the development characteristics of stratum cracks and subsidence were investigated by physical and UDEC2D simulation.Meanwhile,the effect of different coal pillar offset distances on stress concentration of coal pillar and development of stratum cracks were studied.Based on those results,a formula for safe mining and reducing surface damage was established,which provided a theoretical basis for safe and environmentally friendly mining in shallow multi-seam.According to the results,the optimal coal pillar offset distance (the side to side horizontal distance of the upper and lower coal pillars) between the upper and lower coal seams was developed to reduce the stress concentration of coal pillars and surface damage.The results of this study have been applied in Ningtiaota coal mine and have achieved good results in safe and environmentally friendly mining.

1.Introduction

The Jurassic Coalfield in northern Shaanxi is one of the seven largest coalfields in the world,of which 60%of the reserves belong to the shallow multi-seam mining.There are two main problems in shallow multi-seams mining.One is the stress concentration of coal pillars in the upper coal seam that causes the strong weighting and affects the lower coal face safe mining (Fig.1),and another is that the surface cracks formed by shallow multi-seam mining seriously damage the surface environment (Fig.2).Therefore,achieving a safe,environmentally friendly and sustainable mining of coal is an important subject in future mining [1-4].

For a long time,Chinese scholars have made great achievements in the single shallow seam mining.However,research on the shallow multiple-seam mining is in its infancy in China.Zhang et al.studied the abutment pressure of coal pillars on the floor,and proposed a method to determine the optimal location of entry by using the rate of stress change [5].Zhang et al.studied the depth of floor failure after mining extremely close upper coal seam,and proposed an optimum pillar offset distance 7.5 m between upper and lower seam when the interburden thickness was 9 m[6].Zhou used the Lame-Maxwell Equation to derive analytic solutions for the plane wedge and semi-plane subjected to stress concentration[7].Huang et al.simulated the crack height of multi-seam mining in Wanli Coal Mine by UDEC2D[8].Yao et al.simulated the development of overburden cracks under repeated mining [9].Li et al.studied the overburden movement and cracks development in multi-seam pressure relief mining [10].Yu studied the roof structure of multi-seam and its influence on the lower coal seam mining[11].Huang et al.established two models of coal pillar offset distance determination,one to avoid the stress concentration of lower seam pillar,and the other to decrease the uneven subsidence[12,13].Through physical simulation,Cheng et al.studied the stress-fracture of surrounding rock in closely-spaced multi-seam mining [14].Gao et al.established the floor stress calculation model to determine the damage zone of floor [15].Xu et al.suggested that the reasonable coal pillar width of high-gas coal seams mining should be optimized by considering the coal pillar stability,the pressure relief effect and the coal recovery rate [16].Hu et al discussed the surface movement and subsidence with different thicknesses of soil layer in high-intensive coal mining[17].Huang et al.studied the evolution characteristics of overburden and surface cracks,and found that the surface cracks alone the coal pillar boundary are the main cracks need to be controlled in shallow multi-seam mining [18-20].However,there are few studies that have looked at both the impact to mine safety and surface protection in shallow multi-seam mining.

Fig.1.Strong roof weighting under coal pillar.

Fig.2.Surface cracks.

Based on the mining condition of shallow multi-seam in Ningtiaota Coal Mine,through physical and numerical simulations,as well as measured surface cracks data,this paper studied the stress concentration of coal pillars with different coal pillar offset distances and the development characteristics of stratum cracks.The research results can provide scientific basis for environmentally friendly mining in shallow multi-seam.

2.Physical simulation

2.1.Design of physical model

According to the drilling core data of Ningtiaota Coal Mine,the physical and mechanical properties and similarity ratio of coal and strata are shown in Tables 1 and 2.

The experimental prototype is the east panel of Ningtiaota Coal Mine,which mainly mines 1-2 seam (upper seam) and 2-2 seam(lower seam),both of which are flat seams.The average thickness of 1-2 coal seam is 1.84 m,2-2 coal seam is 5 m,and the average thickness of interburden strata between the two seams is 33 m.The depth of 1-2 coal seam is about 110 m.The thickness of bedrock is 70 m,and the thickness of soil layer is 40 m.The panel width in the 1-2 coal seam is 245 m,while that in the 2-2 coal seam is 295 m,and the width of coal pillars are 20 m.

The physical model is a 3 m plane model with geometric similarity ratio of 1:200.According to the similarity theory,the similarity ratio for time is 0.071,for the stress is 0.0033,and for the bulk density,Poisson’s ratio and internal friction angle is 1.0.Eighty weighing sensors were laid at the bottom of the model,and displacement measuring points were set up on the strata and surface.During mining,the experimental data of overburden displacement,coal pillar stress and overburden cracks were recorded.

2.2.Characteristics of overburden caving and pillar stress distribution

Due to the high cost and long period of physical simulation experiment,the characteristics of overburden caving and pillar stress distribution are carried out on the same model.The experimental process is,firstly,that the left working face of the upper coal seam is mined,with a width of 245 m.After the coal pillar with a width of 20 m is reserved,then the right working face of the upper coal seam is mined,with a width of 245 m.After the overburden caving is stable,the lower coal seam is mined.The working face of the lower coal seam is excavated to the right(lower pillar is a moving face to simulate the different side-toside distances with upper pillar of 1-2 coal seam),and the overburden caving characteristics and pillar stress distribution with different offset distances between the left coal pillar of lower seam and the right boundary of the coal pillar in upper coal seam is mainly observed.

According to the physical simulation,when coal pillars overlapping layout between the 2-2 and the 1-2 coal seams (Fig.3a) is used,pillar stress in the 2-2 coal seam was the largest.When coal pillar offset distance (the side to side horizontal distance of the upper and lower coal pillars)was 0 m(Fig.3b),between the upper and lower seams,obvious cracks appeared in the interburden strata.With the increase of offset distance between coal pillars,the stress concentration reduced.When coal pillars offset distance was 40 m (1.3 times the interburden thickness) (Fig.3c),the coal pillar of the upper coal seam sank with the roof caving of the lower coal seam,and the stress in the upper and lower coal pillars reduced.

As the offset distance increases,when the pillar in the 2-2 seam is located in the compacted zone of the 1-2 seam goaf,pillar stress begins to increase.The vertical stress of pillar in the 2-2 coal seam decreases first and then increases (Fig.4).

2.3.Development characteristics of overburden cracks with different coal pillar offset distance

2.3.1.Overburden cracks

After mining,the overburden cracks include bed separations and upward cracks.The separation cracks caused by the nonuniform overburden bending were gradually closed in the goaf far from the working face.The upward cracks develop upward along the boundary around the working face at 50°.

2.3.2.Surface cracks

The surface cracks include the permanent boundary cracks at the setup entry,along the pillars between the faces,and the parallel cracks that appear periodically in the coal face (Fig.5).Among them,the boundary cracks along the pillars between the working faces are the main object of surface cracks control.

When the pillars overlapped layout between 2-2 and 1-2 coal seams are used,the roof of 2-2 coal seam interburden strata rotates and fails to the left goaf,resulting in the boundary cracks of overlapping goaf on the left side of 1-2 coal seam pillar being obviously larger than those of single coal seam mining on the right side of the pillar (Fig.3a).The left boundary cracks of upper coal seam increases from 0.5 mm wide after mining of the lower coal seam,to 1.5 mm.

When coal pillars offset distance was 15 m(side-to-side layout)between the upper and lower seams,there were obvious strataseparations and upward cracks in the interburden strata.The interburden strata rotate,the pillar of 1-2 coal seam sank,and the width of boundary cracks reduced.When coal pillars offset distance was 40 m,the interburden strata caved completely at a 50° caving angle.At this time,the pillar in the 1-2 coal seam and floor strata subside completely.In particular,the abutment pressure influence zone,shaped like an inverted trapezoid,sank totally.Meanwhile,the upward cracks and surface cracks on the boundary of coal pillar tended to close up (Fig.6).

Table 1.hysical and mechanical properties coal and strata.

Table 2.atio of component materials in simulated materials.

Fig.3.Overburden caving formed in different offset distance of pillars.

As shown in Fig.7,the panels of 1-2 coal seam was located at 60-305 and 325-570 m with pillar 20 m.There are two panels in 1-2 coal seam,and the width of two panels is 245 m.The coal pillars of 2-2 coal seam and 1-2 coal seam panels were offset at various distances from 0 to 50 m.When the pillars of 2-2 and 1-2 coal seams were overlapped,the differential subsidence was the largest within the pillar influence zone(when the face was at about 300 m location in Fig.7)with the maximum being 3.8 m.The differential subsidence was 2.6 m when the offset distance was 30 m;when the offset distance was 40 m,the differential subsidence decreases rapidly to a minimum of 1.6 m.The simulation results indicated that surface subsidence reduced by 58% with optimal offset distance of coal pillars,and greatly improving the uniformity of surface subsidence (Fig.7).

Fig.4.Maximum vertical stress of 2-2 coal pillar in different offset distance.

Fig.5.Distribution of surface cracks of working face.

3.UDEC2D numerical calculation

Based on UDEC2D modeling results,the distribution characteristics of vertical stress of coal pillar with different offset distance was obtained (Fig.8).

When the coal pillars were overlapped or the offset distance of pillars was less than 0 m (the offset distance of pillar center line between the upper and lower seam was 0-20 m) (Fig.8a and b).The outby area of the panel in the lower coal seam was completely in the superimposed stress zone of the upper and lower seam coal pillars.The maximum stress was at the center of the coal pillar.The abutment pressure reached 8 m wide in the horizontal direction and the stress was maintained above 20 MPa.As a result,it is difficult to control the surrounding rock of the roadway in the lower coal seam panel and rib spalling is easy to happen.

With the increase in offset distance,the vertical stress of pillar in the lower coal seam decreases gradually.When the offset distance of pillars was 20 m,the stress concentration zones of coal pillar were hardly superimposed (Fig.8c).When the offset distance of pillars was 40 m,the stress of 2-2 coal seam pillar decreases obviously,and the caving strata on the left side of 1-2 coal seam pillar were compacted(Fig.8d).Meanwhile,the width of pillar stress greater than 20 MPa was about 3 m,and the stress distribution on the coal pillar was basically uniform.When the offset distance reached 50 m,the lower coal pillar was in the compacted area of the upper coal seam mining,and the vertical stress of the lower coal pillar increased slightly,but the increase was not significant.

Fig.6.Development of surface cracks.

Fig.7.Surface subsidence curve with different offset distance (2-2 seam right offset).

Fig.8.Vertical stress of different offset distance.

With the increase of coal pillar offset distance,the peak vertical stress of coal pillar in the lower coal seam reduced first and then increased,and there existed an optimum interval.When the pillars were in the overlapped layout,the peak stress of the lower coal seam is the largest.With the increase of pillar offset distance,the peak stress of the lower coal seam pillar reduced continuously.When the pillar offset distance between the center of pillars was 40 m,the peak stress of the lower coal seam pillar was the smallest.When the offset distance between pillars was more than 50 m,the lower pillars were in the compacted area of the goaf of the upper coal seam,and the peak stress began to rise again.

The vertical stress distribution of coal pillars with different offset distances in 2-2 coal seam is shown in Fig.9,and the maximum stress variation characteristics is consistent with the physical simulation (Fig.4).

In conclusion,according to the mining conditions of Ningtiaota Coal Mine,the optimal offset distance in the layout of upper and lower coal pillars is about 40 m.

4.Coal pillar offset distance to ensure safe mining

According to physical simulation and UDEC2D numerical analysis,the model for determining the optimal coal pillar offset distance to ensure safe mining (Fig.10) was developed as shown in Eq.(1).

where φ1and φ2are stress transfer angles,°;h the distance of interburden strata,m;a the pillar width of lower seam,m;b the roadway width,m;and Lσthe coal pillar offset distance for avoiding stress concentration,m,L1≤Lσ≤L2.

5.Coal pillar offset distance for reducing surface damage

Physical simulation showed that different coal pillar offset distances had a significant effect on the uniformity of surface subsidence.The surface subsidence curves for the overlapped layout and 40 m offset distance (side-to-side layout) of pillars in the 1-2 and 2-2 coal seams are shown in Fig.11.It can be seen that in the surface subsidence basin (the middle part of the curve),subsidence above the coal pillar was the smallest when the coal pillar was overlapped,forming a W-shaped surface subsidence curve.With the increase of coal pillar offset distance,surface subsidence in the middle of the basin tended to be flat.When the offset distance of coal pillar was 40 m,differential surface subsidence decreases in the central area of the basin by about 45%.Pillar of 1-2 coal seam is located at 305-325 m,and pillar of 2-2 coal seam is offset toward right.

Fig.9.Vertical stress distribution of 2-2 coal seam pillar with different offset distances.

Fig.10.Offset distance model to ensure safe mining.

Fig.11.Surface subsidence curves of different coal pillar offset distance.

According to physical simulation,when the coal pillar of upper coal seam is located in the fully caving and compacted zone of the goaf of the lower seam,the coal pillar of lower coal seam subsides fully,eliminating the non-uniform subsidence of the coal pillar affected zone,and significantly reducing or closing up the coal pillar boundary cracks(Fig.12).Thus,the equation of the optimal coal pillar offset distance for reducing surface damage is

where l1is the distance between the coal pillar of upper coal seam and roof compacted zone,m;and l2the distance between the coal pillar of lower coal seam and roof compacted zone,m.

According to the physical simulation,the roof compacted zone is caused by the rotation movement of the roof structure from the coal pillar boundary to the goaf (Fig.13).The equations for l1and l2are

where Lεis the coal pillar offset distance for reducing surface damage,m;M1the mining height of upper coal seam,m;M2the mining height of lower coal seam,m;α1the average rotation angle of upper coal seam roof,°;and α2the average rotation angle of lower coal seam roof,°.

6.Optimal coal pillar offset distance for safe and environmentally friendly mining

6.1.Optimal coal pillar offset distance between upper and lower coal seams

According to the physical and numerical simulations,an environmentally friendly and safe mining can be realized by adopting an optimal coal pillar offset distance,by making the coal pillar of lower coal seam located in the stress-relaxation zone Eq.(1) of the upper seam mining,and at the same time,taking into account the uniform surface subsidence Eq.(2).So,the equation for coal pillar offset distance for safe and environmentally friendly mining is

where L is the optimal coal pillar offset distance between the upper and lower coal seams,m.

6.2.Case analysis

According to the geological conditions of Ningtiaota Coal Mine,coal seam inclination is about 1°.At present,the coal mine mainly mines the 1-2 seam(upper seam)and 2-2 seam(lower seam).The average thickness of 1-2 coal seam is 2.0 m,2-2 coal seam is 5.0 m,and the average thickness of interburden strata between the two seams is 33 m.The pillar width of lower seam is 20 m.The roadway width is 5 m.According to the physical simulation results,the stress transfer angles φ1is 40° and φ2is 70°.The average rotation angle of the upper and lower coal seam roof is 10°.Substituting those parameters into Eqs.(1),(2) and (4),the calculation results are 33 m≤Lσ ≤66 m,Lε≥11.3+28.4≈40 m,L=40-66 m.

In conclusion,the optimal coal pillar offset distance for safe mining and reducing surface damage is 40-66 m,which is consistent with the physical simulation results.The results of this study were tested in Ningtiaota Coal Mine,which has obtained good effect in environmentally friendly mining.

Fig.12.Mechanism for uniform surface subsidence in multiple-seam mining.

Fig.13.Pillar offset distance calculation model to reduce surface damage.

7.Conclusions

(1) During the mining process of shallow multiple-seam,with the increase of the coal pillar offset distance in the upper and lower coal seams,the vertical stress of the coal pillar in the lower coal seam section is firstly decrease and then increase,and there is a stress-relaxation zone.The surface boundary cracks caused by the section coal pillars are the main cracks on the surface,and the surface cracks can be reduced or closed by an optimal coal pillar offset distance.

(2) The stress concentration of coal pillars and surface cracks are related to non-uniform subsidence.By selecting an optimal offset distance of the upper and lower coal pillars,the nonuniform subsidence of the coal pillar support zone can be decreased to reduce pillar stress concentration and surface damage.There is a coupling control effect for safe and environmentally friendly mining.

(3) The equation calculation formula for safe mining and reducing surface damage was established,which provided a theoretical basis for safe and environmentally friendly mining in shallow multi-seams.Combined with the panel geological conditions in Ningtiaota Coal Mine,it is concluded that the optimal coal pillar offset distance of 1-2 and 2-2 coal seams is 40-66 m.Furthermore,the application in Ningtiaota Coal Mine has achieved a good effect and realized safe and environmentally friendly mining.

Acknowledgements

The article was funded by the National Natural Science Foundation of China (Nos.51674190 and 52074211) and the Natural Science Basic Research Program of Shaanxi (Nos.2019JQ-798 and 2019JLP-08).The authors also thank the reviewers for their patient work.