# A Study on the Instability Mechanisms of Coal Pillars in Shallow Coal Seams Group Mining

Authors: Tielin Shang; Jindong Wang; Yuwei Liu; Yonglu Suo; Zhiyi Wang; Sixin Fu; Fei Zhang
Journal: Advances in Civil Engineering
 (2023)

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## Abstract

To explore the instability mechanisms of coal pillars in the upper coal during coal seam group mining in the Yulin area and hence to achieve safe and green mining of the lower coal seams, the engineering geological condition for no. 3−1, no. 4−2, and no. 5−2 coal seams in the north-second panel area of Hongliulin Coal Mine was investigated in this article. Using the combination of physical simulation, FLAC3D numerical calculation, and theoretical analysis, the instability mechanisms, the characteristics of the fracture structure, and fracture evolution between the coal pillars and the interval rocks were all studied. The results showed that a layout position existed that induced instability and subsidence of the coal pillars of the upper coal seam. The instability mechanism was such that the concentrated stress of the upper and lower coal pillars caused shear plastic damage in the interval rock along the direction of stress-transfer influence angle. The phenomenon of “inclined step beam” fracture structure, falling fracture zone, and severe mine pressure happened during seam group mining. Furthermore, the minimum center offset formula was put forward to study the instability of the upper coal pillars. This study provides a theoretical basis for a reasonable layout on how to position coal pillars for shallow coal seams group mining.


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## Body

## 1. Introduction

In the process of coal seam group mining, the concentrated stress formed by the residual section of a coal pillar in the floor of an upper coal seam has a great influence on the safe mining of the lower coal seam [1]. The presence of goaf in the upper segment makes the stress distribution of the surrounding rock more complicated than would otherwise have been the case [2]. A coal pillar may enter a residual state and the solid coal side of the next panel can become the main bearing body under the abutment load [3]. According to the position of the roof relative to the goaf, the area above the goaf is divided into three regions: a curving zone, a water-conducting fracture zone, and a falling zone [4]. The effect of lower coal seam pillar stress concentration and surface subsidence gradient decreased as a function of increasing the section coal pillar stagger distance of the upper and lower coal seams, and that a reasonable section coal pillar stagger distance should be more than 40 m [5]. In previous studies, the caving roof structure of the upper coal seam was activated after mining of the lower coal seam, and this lower seam was combined with a three-hinge arch structure to form an “asymmetric three-hinged arch with arch shell load structure” [6]. By reasonably arranging the coal pillars, the stress concentration caused by the coal pillar can be decreased, and the uneven surface subsidence and cracks can be reduced [7]. The ratio, G, of the thickness of the interval strata to mining height for the lower coal seam was found to be the key index influencing the characteristics of the roof pressure. A calculation method for a coal pillar staggered distance was also proposed [8–12]. At Bulianta Coal Mine, the stress at the bottom of the upper pillar and the deformation of the surrounding rock in the lower roadways were analyzed by theoretical analysis, field observations, and numerical simulation. The size of the coal pillar and the staggering distance of the extraction roadway were studied in the process of downward combined mining in shallow coal seams [13]. The proposed methods were found feasible to study the reasonable position of the roadway for ultraclose lower seams [14]. The stress distribution in the floor around the pillars was significantly nonuniform, and the variation rate-coefficient in the stress fields was defined to evaluate this nonuniformity [15]. Taking Chenghe Bailiang Xusheng Coal Mine as an engineering practice background, FLAC3D was used to analyze the plastic failure, vertical stress, and the deformation characteristics of the roadways at inward, overlap, and outward in the lower coal seam [16]. To determine a reasonable section coal pillar size in Nanliang Coal Mine, the planar and spatial models in different section coal pillar sizes were designed, and the stability of the section coal pillar was studied based on the simulation experiments [17]. In the previous study, the disturbance mechanism of the stresses on coal pillars on the floor area before and after the overlying coal seam was mined was studied using the propagation law of disturbance of the coal pillars in the remaining section, and the disturbance width of the remnant coal pillar area during underlying coal seam mining was calculated and analyzed [18]. The influencing factors of the coal pillar in the goaf on the mine pressure of the mining face of the lower coal seam were analyzed, and it was concluded that vertical stress is the most important element, followed by horizontal stress [19]. For close-distance coal seam mining, the lower coal seam is impacted by the mining stress and tectonic stress of the coal seam. It bears the impact of the residual coal pillar stress transfer in the higher coal seam, resulting in a more severe occurrence of intense rock pressure in the coal seam’s working face [20]. In multiple seam mining, severe deformation of roadways and coal bursts caused by overlying coal pillars can occur in the lower seams. The abutment stress in the lower seam is affected by overlying coal pillars as suggested by the bubble and the cantilever beam models. The high-energy seismic events caused by large overburden movement under the overlying pillars were the sources of the dynamic loads [21]. The disturbance caused by mining of the underlying working face may cause instability of the high-level fractured roof [22]. Gangue backfilling mining can effectively alleviate the overburden aquifer destruction [23]. When fault fracture zones are encountered in the mining tunnel excavation, the water-conducting pathway is easily formed by the granular structure of the broken surrounding rock [24]. These previous studies mainly addressed engineering practice problems such as the concentration law of the stress-section coal pillars in the bottom plate, section coal pillars in the lower coal seam, a reasonable layout position of the mining roadway, and surface subsidence control. Furthermore, these studies focused, in general, only on the two layers of coal. Studies on the instability of a coal pillar left in the goaf of an upper coal seam are few. The instability movements of the coal pillars in the left section of the upper coal seam are the sources of induced roof weighing, deformation, failure of mining the roadway, and spalling of the lower coal seam working face. In the mining process of a multilayered shallow coal seam group, the instability mechanism, structural characteristics of fractures of the interval strata, and how it moves in the residual section of the coal pillars in the upper coal seam with different layout positions of the lower coal pillars need to be further studied.In this article, based on the engineering geological conditions of the coal seam in the north-second panel of Hongliulin Coal Mine, similar material simulation, numerical simulations, and theoretical analysis, as in the previous study, were used to understand the instability mechanisms and rock-breaking structure characteristics and motion the coal pillars in the lower coal seam section and the upper coal seam section during the mining process of the coal seam group (three-layer coal). The coal pillar offset-calculation method that ensures the stability of the coal pillar and the roadway in the lower coal seam section is proposed to avoid roof accidents and rock burst phenomena, which, in turn, provides a sound, scientific theoretical basis for the safe and efficient green mining of the shallow coal seam group in Yulin area.At present, the related concepts such as the layout position and the stress transfer influence angle of coal seam group mining need to be further clarified and unified. The concept for a reasonable layout location on mining roadways of the lower coal seam in coal seams group mining, which mostly refer to  the thick coal seam mining [25, 26]. It was proposed that the layout of the coal pillars (roadways) in the lower coal seam could be classed into three kinds: internal fault, external fault, and overlapping layout. It was suggested that the alignment arrangement form of the upper and lower section coal pillars be labeled “overlapping layout.” No matter whether the lower section coal pillar was on the left or the right side of the left section coal pillar in the upper coal seam, the unified name was “offset layout” in this article. The unified name of the distance of the section coal pillar is “center offset,” as shown in Figure 1. The residual section of the coal pillar of the upper coal seam forms stress concentration on its floor. To determine the range of stress influence, some studies in the past have used the pressure transfer influence angle, some studies have used the internal friction angle and the stress influence angle of the foundation as used in soil mechanics, and some other studies have used the included angle of the decompression zone and the concentrated stress transfer angle. The scholars believe that the lower roadway could be arranged so as to avoid an increase in the stress area and also that the stress-transfer influence angle of the coal pillar was outside the line. It was suggested that the stress-transfer influence angle (φ) should be standardized in this article, as shown in Figure 1.Figure 1
Center offset of section coal pillar and stress transfer influence angle.


## 2. Physically Similar Material Simulation of Sectional Coal Pillar Instability

### 2.1. Engineering Background

The main coal-bearing section of the Jurassic coalfield in northern Shaanxi is Yan’an Formation (J2y), containing 10–15 coal layers, but mainly 5–7 layers can be mined, and the distance between coal seams is 20–40 m. The coal layers have the characteristics that the distance are close each other. Many large coal mines have completed the first layer of coal mining, and attention gradually turns to the lower coal seam mining [27–30]. Most coal mining faces in the coalfield adopted the double lane layout and section coal pillar roadway protection. The first coal seam in the upper part was fully mined and left many-section coal pillars. During the mining process of the lower coal seam, the instability and subsidence of the left section coal pillars on the upper coal seam will inevitably  occur the development of surface cracks, which may affect a fragile ecological environment around the mining area.The physically similar material simulation experiment is based on the actual engineering geological conditions of no. 3−1, no. 4−2, and no. 5−2 coal seams in the north-second panel area of the Hongliulin Coal Mine. No. 3−1 coal seam is located at the top of the third section of the Yan’an Formation, no. 4−2 coal seam is located at the top of the second section of Yan’an Formation, and the average distance of no. 4−2 from no. 3−1 coal seam is 43.51 m; no. 5−2 coal seam is located at the top of the first section of Yan’an Formation, and the average distance from no. 4−2 coal seam is 61.54 m. The coal mining method adopted fully mechanized longwall mining and downward mining sequence [31, 32]. The width of the section coal pillar is 20 m.


### 2.2. Similar Simulation Experiment Model Design

The model experiment frame-height of each rock layer was referred to the data of no. 6-HB6 drilling histogram in the middle of the northern second panel in Hongliulin Coal Mine. The physical and mechanical parameters of the coal and rock mass according to the geological exploration report are shown in Table1.Table 1
Mechanical parameters of rock and coal.
LithologyBulk weight (g/cm3)Elastic modulus (GPa)Poisson ratioTensile strength (MPa)Cohesive force (MPa)Internal friction angle (°)Loess1.861.80.150.090.9229.4Red clay2.0220.180.221.2827.4Silty fine sand2.442.10.191.936.1836.2Medium-grained sandstone2.282.30.182.186.0737.1Coal1.340.620.261.244.7235.2Fine sandstone2.292.320.22.295.637.3Mud-sandstone2.31.080.240.291.8528.6Similar materials used aeolian sand in the Yulin area, such as red clay, loess and gypsum, calcium carbonate, coal ash, and water, and used mica powder as layered materials. According to the physical model, mechanical parameters, engineering geological condition, and similarity ratio, the matching number of similar materials was determined. Mix ratios of similar simulation material were weight ratio of sand : gypsum : calcium carbonate. For example, the mix ratio 837 represent 8 : 0.3 : 0.7. The parameters of the physical model are shown in Table2. The density of rock and coal seam, respectively, is 1600 kg/m3 and 1300 kg/m3.Table 2
Similar simulation material parameters of the physical model.
LithologyStrata thickness (m)Model thickness (cm)Mix ratioMaterial consumption (kg/cm): each layerSandGypsumCalcium carbonateCoal ashAeolian sand189Aeolian sand9.6Loess21Sand : loess3.26.4Red clay6432Sand : red clay3.26.4Fine sandstone1688378.530.320.747Medium-grained sandstone638378.530.320.747Medium-grained sandstone848288.530.2130.853Coal no. 3−131.520 : 1 : 5 : 203.390.170.173.39Silty fine sand429378.640.2880.672Silty fine sand52.59378.640.2880.672Fine sandstone1058378.530.320.747Silty fine sand73.59378.640.2880.672Mud-sandstone1059288.640.1920.768Silty fine sand429378.640.2880.672Coal no. 4−24220 : 1 : 5 : 203.390.170.173.39Mud-sandstone52.59288.640.1920.768Silty fine sand849378.640.2880.672Mud-sandstone52.59288.640.1920.768Silty fine sand1269378.640.2880.672Fine sandstone638378.530.320.747Medium-grained sandstone1688378.530.320.747Silty fine sand849378.640.2880.672Medium-grained sandstone31.58378.530.320.747Coal no. 5−26320 : 1:5 : 203.390.170.173.39Fine sandstone1898378.530.320.747Silty fine sand1269378.640.2880.672Medium-grained sandstone94.58378.530.320.747The plane model size was 300 × 20 × 134.5 cm, and the geometric similarity ratio was 1 : 200. These ratio numbers were used to determine the weight of each layer, each layer were paved with mica powder and artificial cutting joints. The physical integral model of the simulation is shown in Figure2.Figure 2
Integral model of physical simulation.


### 2.3. Test Procedure

The excavation scheme was designed after natural air-drying of the model, and the instability process, fracture structure, fracture development characteristics, movement law of the section coal pillar, and the interval rock were monitored during the mining process for its photographic description. The experimental prototype was 600 m in length and 269 m in height. To eliminate the influence of the boundary effect, 60 m protective coal pillars were left on the left and right boundaries of each coal seam excavation. The coal seams were excavated 1 m each time using a steel saw blade from one side to section coal pillar in the middle of the model, which gradually changed the length of the working face.The specific mining scheme was as follows: (1) According to the actual production of the coal mine, the transport return airway on both sides of the no. 3−1 section coal seam was excavated, and the 20 m section coal pillar was retained in the middle of the model. The right side retained the boundary coal pillar, and the right working face coal seam was fully excavated. Similarly, the left working face coal seam was also excavated. (2) The boundary coal pillar of the no. 4−2 coal seam was retained on the left, and the length of the working face on the left was gradually increased. After this, the following scheme was adopted: the central offset of the section coal pillar between the no. 4−2 coal seam and no. 3−1 coal seam on the left was reduced, vertical layout was done, and the length of no. 4−2 coal working face on the left was increased, post which the right central offset of the section coal pillar began to occur between no. 4−2 coal seam and no. 3−1 coal. After the section coal pillar and interval strata were in a stable state, a 20 m section coal pillar of no. 4−2 coal seam was set, and finally, the right side of the no. 4−2 coal seam was excavated. (3) A boundary protection coal pillar of the no. 5−2 coal seam was set on the right side, following which the length of the right working face of no. 5−2 coal seam was gradually increased. Then, center offset of the section coal pillar between the no. 4−2 coal seam and no. 5−2 coal seam was reduced on the right, vertical layout was done, the length of the right working face of no. 5−2 coal seam was made to increase, and the center offset of section coal pillar between no. 4−2 coal seam and no. 5−2 coal seam was increased on the left. After the interval strata and the intrusive rock entered the stable stage, a 20 m coal pillar was set, and the coal seam of the working face of no. 5−2 coal seam was mined on the left.


## 2.1. Engineering Background

The main coal-bearing section of the Jurassic coalfield in northern Shaanxi is Yan’an Formation (J2y), containing 10–15 coal layers, but mainly 5–7 layers can be mined, and the distance between coal seams is 20–40 m. The coal layers have the characteristics that the distance are close each other. Many large coal mines have completed the first layer of coal mining, and attention gradually turns to the lower coal seam mining [27–30]. Most coal mining faces in the coalfield adopted the double lane layout and section coal pillar roadway protection. The first coal seam in the upper part was fully mined and left many-section coal pillars. During the mining process of the lower coal seam, the instability and subsidence of the left section coal pillars on the upper coal seam will inevitably  occur the development of surface cracks, which may affect a fragile ecological environment around the mining area.The physically similar material simulation experiment is based on the actual engineering geological conditions of no. 3−1, no. 4−2, and no. 5−2 coal seams in the north-second panel area of the Hongliulin Coal Mine. No. 3−1 coal seam is located at the top of the third section of the Yan’an Formation, no. 4−2 coal seam is located at the top of the second section of Yan’an Formation, and the average distance of no. 4−2 from no. 3−1 coal seam is 43.51 m; no. 5−2 coal seam is located at the top of the first section of Yan’an Formation, and the average distance from no. 4−2 coal seam is 61.54 m. The coal mining method adopted fully mechanized longwall mining and downward mining sequence [31, 32]. The width of the section coal pillar is 20 m.


## 2.2. Similar Simulation Experiment Model Design

The model experiment frame-height of each rock layer was referred to the data of no. 6-HB6 drilling histogram in the middle of the northern second panel in Hongliulin Coal Mine. The physical and mechanical parameters of the coal and rock mass according to the geological exploration report are shown in Table1.Table 1
Mechanical parameters of rock and coal.
LithologyBulk weight (g/cm3)Elastic modulus (GPa)Poisson ratioTensile strength (MPa)Cohesive force (MPa)Internal friction angle (°)Loess1.861.80.150.090.9229.4Red clay2.0220.180.221.2827.4Silty fine sand2.442.10.191.936.1836.2Medium-grained sandstone2.282.30.182.186.0737.1Coal1.340.620.261.244.7235.2Fine sandstone2.292.320.22.295.637.3Mud-sandstone2.31.080.240.291.8528.6Similar materials used aeolian sand in the Yulin area, such as red clay, loess and gypsum, calcium carbonate, coal ash, and water, and used mica powder as layered materials. According to the physical model, mechanical parameters, engineering geological condition, and similarity ratio, the matching number of similar materials was determined. Mix ratios of similar simulation material were weight ratio of sand : gypsum : calcium carbonate. For example, the mix ratio 837 represent 8 : 0.3 : 0.7. The parameters of the physical model are shown in Table2. The density of rock and coal seam, respectively, is 1600 kg/m3 and 1300 kg/m3.Table 2
Similar simulation material parameters of the physical model.
LithologyStrata thickness (m)Model thickness (cm)Mix ratioMaterial consumption (kg/cm): each layerSandGypsumCalcium carbonateCoal ashAeolian sand189Aeolian sand9.6Loess21Sand : loess3.26.4Red clay6432Sand : red clay3.26.4Fine sandstone1688378.530.320.747Medium-grained sandstone638378.530.320.747Medium-grained sandstone848288.530.2130.853Coal no. 3−131.520 : 1 : 5 : 203.390.170.173.39Silty fine sand429378.640.2880.672Silty fine sand52.59378.640.2880.672Fine sandstone1058378.530.320.747Silty fine sand73.59378.640.2880.672Mud-sandstone1059288.640.1920.768Silty fine sand429378.640.2880.672Coal no. 4−24220 : 1 : 5 : 203.390.170.173.39Mud-sandstone52.59288.640.1920.768Silty fine sand849378.640.2880.672Mud-sandstone52.59288.640.1920.768Silty fine sand1269378.640.2880.672Fine sandstone638378.530.320.747Medium-grained sandstone1688378.530.320.747Silty fine sand849378.640.2880.672Medium-grained sandstone31.58378.530.320.747Coal no. 5−26320 : 1:5 : 203.390.170.173.39Fine sandstone1898378.530.320.747Silty fine sand1269378.640.2880.672Medium-grained sandstone94.58378.530.320.747The plane model size was 300 × 20 × 134.5 cm, and the geometric similarity ratio was 1 : 200. These ratio numbers were used to determine the weight of each layer, each layer were paved with mica powder and artificial cutting joints. The physical integral model of the simulation is shown in Figure2.Figure 2
Integral model of physical simulation.


## 2.3. Test Procedure

The excavation scheme was designed after natural air-drying of the model, and the instability process, fracture structure, fracture development characteristics, movement law of the section coal pillar, and the interval rock were monitored during the mining process for its photographic description. The experimental prototype was 600 m in length and 269 m in height. To eliminate the influence of the boundary effect, 60 m protective coal pillars were left on the left and right boundaries of each coal seam excavation. The coal seams were excavated 1 m each time using a steel saw blade from one side to section coal pillar in the middle of the model, which gradually changed the length of the working face.The specific mining scheme was as follows: (1) According to the actual production of the coal mine, the transport return airway on both sides of the no. 3−1 section coal seam was excavated, and the 20 m section coal pillar was retained in the middle of the model. The right side retained the boundary coal pillar, and the right working face coal seam was fully excavated. Similarly, the left working face coal seam was also excavated. (2) The boundary coal pillar of the no. 4−2 coal seam was retained on the left, and the length of the working face on the left was gradually increased. After this, the following scheme was adopted: the central offset of the section coal pillar between the no. 4−2 coal seam and no. 3−1 coal seam on the left was reduced, vertical layout was done, and the length of no. 4−2 coal working face on the left was increased, post which the right central offset of the section coal pillar began to occur between no. 4−2 coal seam and no. 3−1 coal. After the section coal pillar and interval strata were in a stable state, a 20 m section coal pillar of no. 4−2 coal seam was set, and finally, the right side of the no. 4−2 coal seam was excavated. (3) A boundary protection coal pillar of the no. 5−2 coal seam was set on the right side, following which the length of the right working face of no. 5−2 coal seam was gradually increased. Then, center offset of the section coal pillar between the no. 4−2 coal seam and no. 5−2 coal seam was reduced on the right, vertical layout was done, the length of the right working face of no. 5−2 coal seam was made to increase, and the center offset of section coal pillar between no. 4−2 coal seam and no. 5−2 coal seam was increased on the left. After the interval strata and the intrusive rock entered the stable stage, a 20 m coal pillar was set, and the coal seam of the working face of no. 5−2 coal seam was mined on the left.


## 3. Physical Model Test Results

The experiment mainly studied the mechanism, processes, and the fracture characteristics of the instability phenomenon of the coal pillars in the remaining section, and the plane model was used as the direction of coal seam strike observation.


### 3.1. Movement Law of Overlying Strata after 3−1 Coal Mining

After mining of no. 3−1 coal seam, the overlying strata naturally collapsed, and the roof caving angle on both sides of the section coal pillar was about 60°. An “inverted trapezoid” structure had formed above the left section coal pillar of no. 3−1 coal, which caused the transfer of concentration of stress in the floor. The whole model formed a caving state section of “goaf-left section coal pillar-goaf,” as shown in Figure 3.Figure 3
Fracture structure characteristics after no. 3−1 coal.The roadway of no. 3−1 coal left section coal pillar on both sides did not cause damage and deformation and was in a stable state which indicated that the 20 m section coal pillar did not undergo instability failure, which ensured the safety of the first coal seam mining. Due to the small thickness, the thick red soil layer was the key layer with strong integrity, which formed an internal separation structure. The strata on the goaf did not fully collapse, and the development height of the collapse line on both sides was short.


### 3.2. Left Section Coal Pillar of No. 3−1 Instability Fracture Characteristics Coal during 4−2 Coal Mining

A 60 m boundary coal pillar was set on the left side of model no. 4−2 coal seam, and the length of the working face was gradually increased. With this increase, the center offset between the section coal pillar of the no. 4−2 coal seam and the left section coal pillar of the no. 3−1 coal seam was gradually reduced, and the separation layer-height of the interval strata was gradually developed upward; due to all these, upward expanding longitudinal cracks were formed. When the center offset between the section coal pillar of no. 4−2 coal seam and the section coal pillar of no. 3−1 coal was 46 m on the left, the caving roof strata of the goaf of no. 3−1 coal seam showed an activation trend. When the center offset was 32 m (the length of the left working face of the no. 4−2 coal seam was 198 m), the intervening strata between the no. 4−2 coal seam and no. 3−1 coal seam broke and got connected with the caving goaf roof strata of no. 3−1 coal, which resulted in its rapid subsidence under rock pressure. The longitudinal fissures of the overlying strata of no. 3−1 coal seam developed upward rapidly, and the strata including the key layer of the red clay continued to develop upward separately. The roof fully collapsed and developed to the ground surface. The subsidence of the ground surface had appeared, and also, the erosion rock movement was relatively strong. Continued excavation increased the length of the left working face, and the overlying strata and the interval strata broke and collapsed simultaneously. The overall movement of the overlying strata was gentle. The breaking structure of the overlying strata of no. 4−2 coal is shown in Figure 4. When the layout of the overlapping strata was carried out, the caving angle of the interval strata was about 62°. Vertical and longitudinal cracks ran in the surface of the aeolian sand.Figure 4
Fracture structure characteristics for overlapping layout of no. 3−1 and no. 4−2 coal.From the overlapping position, the central offset on the right side was formed between the section coal pillar of coal seam no. 4−2 and the left section coal pillar of coal seam no. 3−1. When the central offset reached 28 m, cracks started to develop gradually along the floor of no. 3−1 coal to the right side. When the right center offset reached 32 m (the length of the left working face is 262 m), the left section coal pillar of no. 3−1 coal suddenly lost its stability, and “inclined step beam” fracture structure characteristics occurred in the interval rock that formed a caving line along the right side of the left section coal pillar of no. 3−1 coal seam which was inclined to the right lower side. At the same time, a caving line formed from the left side of the section coal pillar of no. 4−2 coal seam that was inclined to the left upper side, which was also roughly parallel to the caving line, and due to this, the whole model collapsed violently. The left section coal pillar of no. 3−1 coal seam continued to sink sharply and formed a broken-structure belt of short masonry beam inclined to the lower right between the two collapse lines, where the collapse angle was about 71°, as shown in Figure 5. This structural instability of mine pressure caused deformation and failure of roadway in the no. 4−2 coal seam, which had a great influence on its safety and stability. Deformation and failure of the coal pillar itself did not occur in no. 4−2 coal.Figure 5
Fracture structure characteristics for remaining coal pillar of no. 3−1 coal.During the continued increase in the center offset to the right, the overlying strata did not collapse violently until it reached the right center offset of 60 m (the length of the left working face was 290 m). A 60 m boundary coal pillar was set on the right side of the model, and mining of the right working face was performed in no. 4−2 coal seam. In the process of mining, the separation structure of the interval strata developed gradually upward, and there were caving cracks also developed upward. When the mining length of the right working face was 130 m, the caving roof of the overlying goaf of no. 3−1 coal seam was activated, and because the interval strata were connected, these strata caused a whole downward-facing violent phenomenon. The overlying red soil layer developed gradually upward, and the grazing rock collapsed through the surface. The length of the right working face continued to increase, and the roof caving was relatively flat. Finally, the 20 m section coal pillar was set in the no. 4−2 coal seam, and the center offset between it and the right center of the section coal pillar left by the no. 3−1 coal seam was 60 m (the length of the right working face was 170 m).


### 3.3. Left Section Coal Pillar of 4−2 Instability Fracture Characteristics Coal during 5−2 Coal Mining

A 60 m boundary coal pillar was set on the right side of the no. 5−2 coal seam, and the length of working face increased from the right side to the left side. During the gradual decrease of the center offset between the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal seam on the right side, the caving structure and the movement law of the weathered rock were similar to those of no. 4−2 coal mining. The roof of interval strata was separated and developed gradually upward to form caving cracks. When the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal seam overlapped (the length of working face on the right side is 170 m), the separation structure of the interval strata developed to the floor of no. 4−2 coal seam which resulted in gradual activation of the caving rock in the goaf of no. 4−2 coal floor. The structural collapse and the instability of the interval strata and all the upper caving roofs suddenly occurred, and the strata pressure was very intense. At this time, the self-weight stress of the overlying strata on the no. 3−1 coal seam, no. 4−2 coal seam, and no. 5−2 coal seam were released, and the roof overburden fully collapsed. The caving angle of the interval strata was about 60°. The fracture structure characteristics of overburden rock are shown in Figure 6.Figure 6
Fracture structure characteristics for overlapping layout of no. 4−2 and no. 5−2 coal.As the length of the right working face continued to increase, the central offset began to form between the coal pillar of no. 5−2 and the coal pillar of no. 4−2 on the left. When the central offset on the left side was about 48 m, upward developed cracks appear on the right side of the section coal pillar for no. 5−2 coal. The left section coal pillar of no. 4−2 coal seam and the interval strata showed an overall trend of subsidence and collapse. When the central offset reached 52 m (the length of the right working face was 222 m), the interval strata suddenly showed kind of a “inclined step beam” structural instability, and the left section coal pillar of no. 4−2 coal and the lower interval strata showed a sharp synchronous collapse phenomenon. The characteristics of the rock fracture structure are shown in Figure 7. It was observed that the caving interval strata were similar to the fracture structure that was formed during the mining process of no. 4−2 coal seam. The caving line developed along the left side of the coal pillar left in the residual section of no. 4−2 coal to the lower left side and was roughly parallel to the upper right side of the coal pillar left in the residual section of no. 5−2 coal seam. The two caving lines were the short masonry beam fracture structure zone tilted to the lower left side, and the caving angle was about 70°.Figure 7
Fracture structure characteristics for remaining coal pillar of no. 4−2 coal.The central offset between the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal on the left side continued to increase, and the separation layer and fracture structures developed slowly; it was also discovered that the behavior of the strata was not severe. When the central offset was 60 m between no. 5−2 coal seam and no. 4−2 coal seam, the section coal pillar on the left overlapped between section coal pillar of no. 5−2 coal seam and section coal pillar of no. 3−1 coal seam, and also, the rock movement was gentle. The left central offset of no. 5−2 coal seam and no. 4−2 coal section coal pillar was 92 m (the left central offset of no. 3-1 coal section coal pillar was 32 m). At this time, the right working face length of no. 5−2 coal was 262 m. No. 5−2 coal seam and no. 4−2 coal interval strata appeared as inclined cracks along no. 5−2 coal section coal pillar from bottom to top and which developed to no. 4−2 coal floor. The coal pillar of no. 3−1 coal seam had subsidence trend, but there was no strong mine pressure phenomenon. It showed that the instability phenomenon of the section coal pillars occurred only between the adjacent coal seams during coal seam mining under the shallow seam group. The remaining section of the coal pillar of no. 3−1 coal seam had little influence on the safety and stability of no. 5−2 coal face and the section coal pillar. When the central offset between no. 5−2 coal and no. 4−2 section coal pillars was 112 m (right working face length of 282 m), no. 5−2 section coal pillar roadway was in a stable state. A 20 m of section coal pillar was left, and the left working face of no. 5−2 coal seam could be excavated. When the length of the left working face of no. 5−2 coal was 178 m, the interval rock layer of no. 5−2 coal roof was integrated with the caving roof of the upper no. 3−1 coal and the coal goaf of no. 4−2, which formed an overall synchronous caving phenomenon, and which further resulted in strong mine pressure. Finally, a 20 m section coal pillar was set up, and the collapse structure characteristics are shown in Figure 8. By observing the whole excavation model, it can be seen that an inverted ladder-shaped stress structure was formed above the coal pillars of no. 5−2, no. 4−2, and no. 3−1 coal sections, and the surface of the aeolian sand showed vertical fracture cracks.Figure 8
Fracture structure characteristics after coal seam mining.


## 3.1. Movement Law of Overlying Strata after 3−1 Coal Mining

After mining of no. 3−1 coal seam, the overlying strata naturally collapsed, and the roof caving angle on both sides of the section coal pillar was about 60°. An “inverted trapezoid” structure had formed above the left section coal pillar of no. 3−1 coal, which caused the transfer of concentration of stress in the floor. The whole model formed a caving state section of “goaf-left section coal pillar-goaf,” as shown in Figure 3.Figure 3
Fracture structure characteristics after no. 3−1 coal.The roadway of no. 3−1 coal left section coal pillar on both sides did not cause damage and deformation and was in a stable state which indicated that the 20 m section coal pillar did not undergo instability failure, which ensured the safety of the first coal seam mining. Due to the small thickness, the thick red soil layer was the key layer with strong integrity, which formed an internal separation structure. The strata on the goaf did not fully collapse, and the development height of the collapse line on both sides was short.


## 3.2. Left Section Coal Pillar of No. 3−1 Instability Fracture Characteristics Coal during 4−2 Coal Mining

A 60 m boundary coal pillar was set on the left side of model no. 4−2 coal seam, and the length of the working face was gradually increased. With this increase, the center offset between the section coal pillar of the no. 4−2 coal seam and the left section coal pillar of the no. 3−1 coal seam was gradually reduced, and the separation layer-height of the interval strata was gradually developed upward; due to all these, upward expanding longitudinal cracks were formed. When the center offset between the section coal pillar of no. 4−2 coal seam and the section coal pillar of no. 3−1 coal was 46 m on the left, the caving roof strata of the goaf of no. 3−1 coal seam showed an activation trend. When the center offset was 32 m (the length of the left working face of the no. 4−2 coal seam was 198 m), the intervening strata between the no. 4−2 coal seam and no. 3−1 coal seam broke and got connected with the caving goaf roof strata of no. 3−1 coal, which resulted in its rapid subsidence under rock pressure. The longitudinal fissures of the overlying strata of no. 3−1 coal seam developed upward rapidly, and the strata including the key layer of the red clay continued to develop upward separately. The roof fully collapsed and developed to the ground surface. The subsidence of the ground surface had appeared, and also, the erosion rock movement was relatively strong. Continued excavation increased the length of the left working face, and the overlying strata and the interval strata broke and collapsed simultaneously. The overall movement of the overlying strata was gentle. The breaking structure of the overlying strata of no. 4−2 coal is shown in Figure 4. When the layout of the overlapping strata was carried out, the caving angle of the interval strata was about 62°. Vertical and longitudinal cracks ran in the surface of the aeolian sand.Figure 4
Fracture structure characteristics for overlapping layout of no. 3−1 and no. 4−2 coal.From the overlapping position, the central offset on the right side was formed between the section coal pillar of coal seam no. 4−2 and the left section coal pillar of coal seam no. 3−1. When the central offset reached 28 m, cracks started to develop gradually along the floor of no. 3−1 coal to the right side. When the right center offset reached 32 m (the length of the left working face is 262 m), the left section coal pillar of no. 3−1 coal suddenly lost its stability, and “inclined step beam” fracture structure characteristics occurred in the interval rock that formed a caving line along the right side of the left section coal pillar of no. 3−1 coal seam which was inclined to the right lower side. At the same time, a caving line formed from the left side of the section coal pillar of no. 4−2 coal seam that was inclined to the left upper side, which was also roughly parallel to the caving line, and due to this, the whole model collapsed violently. The left section coal pillar of no. 3−1 coal seam continued to sink sharply and formed a broken-structure belt of short masonry beam inclined to the lower right between the two collapse lines, where the collapse angle was about 71°, as shown in Figure 5. This structural instability of mine pressure caused deformation and failure of roadway in the no. 4−2 coal seam, which had a great influence on its safety and stability. Deformation and failure of the coal pillar itself did not occur in no. 4−2 coal.Figure 5
Fracture structure characteristics for remaining coal pillar of no. 3−1 coal.During the continued increase in the center offset to the right, the overlying strata did not collapse violently until it reached the right center offset of 60 m (the length of the left working face was 290 m). A 60 m boundary coal pillar was set on the right side of the model, and mining of the right working face was performed in no. 4−2 coal seam. In the process of mining, the separation structure of the interval strata developed gradually upward, and there were caving cracks also developed upward. When the mining length of the right working face was 130 m, the caving roof of the overlying goaf of no. 3−1 coal seam was activated, and because the interval strata were connected, these strata caused a whole downward-facing violent phenomenon. The overlying red soil layer developed gradually upward, and the grazing rock collapsed through the surface. The length of the right working face continued to increase, and the roof caving was relatively flat. Finally, the 20 m section coal pillar was set in the no. 4−2 coal seam, and the center offset between it and the right center of the section coal pillar left by the no. 3−1 coal seam was 60 m (the length of the right working face was 170 m).


## 3.3. Left Section Coal Pillar of 4−2 Instability Fracture Characteristics Coal during 5−2 Coal Mining

A 60 m boundary coal pillar was set on the right side of the no. 5−2 coal seam, and the length of working face increased from the right side to the left side. During the gradual decrease of the center offset between the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal seam on the right side, the caving structure and the movement law of the weathered rock were similar to those of no. 4−2 coal mining. The roof of interval strata was separated and developed gradually upward to form caving cracks. When the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal seam overlapped (the length of working face on the right side is 170 m), the separation structure of the interval strata developed to the floor of no. 4−2 coal seam which resulted in gradual activation of the caving rock in the goaf of no. 4−2 coal floor. The structural collapse and the instability of the interval strata and all the upper caving roofs suddenly occurred, and the strata pressure was very intense. At this time, the self-weight stress of the overlying strata on the no. 3−1 coal seam, no. 4−2 coal seam, and no. 5−2 coal seam were released, and the roof overburden fully collapsed. The caving angle of the interval strata was about 60°. The fracture structure characteristics of overburden rock are shown in Figure 6.Figure 6
Fracture structure characteristics for overlapping layout of no. 4−2 and no. 5−2 coal.As the length of the right working face continued to increase, the central offset began to form between the coal pillar of no. 5−2 and the coal pillar of no. 4−2 on the left. When the central offset on the left side was about 48 m, upward developed cracks appear on the right side of the section coal pillar for no. 5−2 coal. The left section coal pillar of no. 4−2 coal seam and the interval strata showed an overall trend of subsidence and collapse. When the central offset reached 52 m (the length of the right working face was 222 m), the interval strata suddenly showed kind of a “inclined step beam” structural instability, and the left section coal pillar of no. 4−2 coal and the lower interval strata showed a sharp synchronous collapse phenomenon. The characteristics of the rock fracture structure are shown in Figure 7. It was observed that the caving interval strata were similar to the fracture structure that was formed during the mining process of no. 4−2 coal seam. The caving line developed along the left side of the coal pillar left in the residual section of no. 4−2 coal to the lower left side and was roughly parallel to the upper right side of the coal pillar left in the residual section of no. 5−2 coal seam. The two caving lines were the short masonry beam fracture structure zone tilted to the lower left side, and the caving angle was about 70°.Figure 7
Fracture structure characteristics for remaining coal pillar of no. 4−2 coal.The central offset between the section coal pillar of no. 5−2 coal seam and the left section coal pillar of no. 4−2 coal on the left side continued to increase, and the separation layer and fracture structures developed slowly; it was also discovered that the behavior of the strata was not severe. When the central offset was 60 m between no. 5−2 coal seam and no. 4−2 coal seam, the section coal pillar on the left overlapped between section coal pillar of no. 5−2 coal seam and section coal pillar of no. 3−1 coal seam, and also, the rock movement was gentle. The left central offset of no. 5−2 coal seam and no. 4−2 coal section coal pillar was 92 m (the left central offset of no. 3-1 coal section coal pillar was 32 m). At this time, the right working face length of no. 5−2 coal was 262 m. No. 5−2 coal seam and no. 4−2 coal interval strata appeared as inclined cracks along no. 5−2 coal section coal pillar from bottom to top and which developed to no. 4−2 coal floor. The coal pillar of no. 3−1 coal seam had subsidence trend, but there was no strong mine pressure phenomenon. It showed that the instability phenomenon of the section coal pillars occurred only between the adjacent coal seams during coal seam mining under the shallow seam group. The remaining section of the coal pillar of no. 3−1 coal seam had little influence on the safety and stability of no. 5−2 coal face and the section coal pillar. When the central offset between no. 5−2 coal and no. 4−2 section coal pillars was 112 m (right working face length of 282 m), no. 5−2 section coal pillar roadway was in a stable state. A 20 m of section coal pillar was left, and the left working face of no. 5−2 coal seam could be excavated. When the length of the left working face of no. 5−2 coal was 178 m, the interval rock layer of no. 5−2 coal roof was integrated with the caving roof of the upper no. 3−1 coal and the coal goaf of no. 4−2, which formed an overall synchronous caving phenomenon, and which further resulted in strong mine pressure. Finally, a 20 m section coal pillar was set up, and the collapse structure characteristics are shown in Figure 8. By observing the whole excavation model, it can be seen that an inverted ladder-shaped stress structure was formed above the coal pillars of no. 5−2, no. 4−2, and no. 3−1 coal sections, and the surface of the aeolian sand showed vertical fracture cracks.Figure 8
Fracture structure characteristics after coal seam mining.


## 4. Numerical Modeling Test

In order to further explore the internal mechanical mechanism of the external macroscopic failure phenomenon of the residual section coal pillar at the specific center offset distance position in the process of similar material simulation experiment in the laboratory, FLAC3D numerical simulation software was used to construct a three-dimensional model of 600 × 8 × 269 m, according to the same conditions in similar material simulation experiment. The mechanical parameters of rock and coal refer to Table 1. The model flank is a ball support, which limits its lateral and horizontal movement, and the bottom of the model is fixed to limit its horizontal and vertical displacement. Mohr–Coulomb failure criterion used in FLAC3D numerical simulation. The gravity of in-situ stresses is −9.81. The plastic failure characteristics of the interval strata are shown in Figure 9 when the interval between the coal pillar of no. 4−2 coal seam and the coal pillar of no. 3−1 coal seam was 32 m in the center right. The plastic failure characteristics of the interval strata are shown in Figure 10, when the central offset of the left side of the coal pillar between no. 5−2 coal section and no. 4−2 coal left section was 52 m. The plastic failure state includes shear failure and tensile failure in FLAC3D numerical simulation.Figure 9
Plastic failure characteristics of interval strata for no. 3−1 coal instability position.Figure 10
Plastic failure characteristics of interval strata for no. 4−2 coal instability position.Through Figures9 and 10, it may be seen that both shear failure and tensile failure occurred during coal seam excavating. Shear failure mainly occurred in the interval strata between section coal pillars, and tensile failure mainly occurred in the roof and floor of the coal seam. The plastic failure mode is consistent with similar material simulation experiment. The reason for the instability of the coal pillar in the upper residual section is that the interval rock occurred as an inclined connected structure, and the plastic failure is mainly manifested as shear failure. Therefore, the mechanical mechanism of the instability for the remaining coal pillar is that the interval rock reaches the limit shear span in a specific center offset position. Under the action of the concentrated stress of the upper and lower coal pillars, the interlayer rock moved up and down along the inclined direction, and the internal shear failure occurred, which formed a “inclined step beam” fracture structure zone, resulting in severe mine pressure. These phenomena are macroscopic manifestations of beam mechanical fracture.


## 5. Rationale Central Offset of Section Coal Pillar

To reduce damage to the surface ecology, the shallow-buried coal seam group mine in Yulin should try to make the surface subsidence uniform and prevent large cracks on the surface. During mining of the lower coal seam, all the pressed coal under the residual section coal pillar of the upper coal seam must be mined, so as to make the residual section of the coal pillar of the upper coal seam stable. The residual coal pillar instability of no. 3−1 coal seam occurs at the right central offset of 32 m. If the excavation design of the similar material simulation model was to be increased continuously along the length of the right working face of no. 4−2 coal seam, the residual coal pillar instability should occur symmetrically at the left central offset of 32 m. Similarly, for no. 5−2 coal, if the length of working face was to be increased from the left side of the model, the instability of the coal pillar in the left section of no. 4−2 coal will occur near the central offset of 52 m on the right side. In order to ensure the stability of the section coal pillar and the roadway during mining of the lower coal seam, the section coal pillar of the lower coal seam should not be arranged in an area where the compressive stress on the floor of the residual section coal pillar in the upper coal seam increases. A reasonable coal pillar central offset must ensure that the roadway in the lower coal seam was outside the stress concentration area of the residual coal pillar in the upper coal seam (the pressure transfer influence angle). According to the physically similar material simulation experimental and numerical simulation analysis, a reasonable coal pillar should offset the lower coal seam roadway outside the caving line, as shown in Figure 11. The minimum central offset must be greater than L, which was calculated according to the following equation :(1)L>a2+b+c+d+a2,where a is the section coal pillar width (upper and lower coal pillar take the same value); b is the influence distance between the coal pillar concentrated stress and the upper coal seam section; c is the horizontal distance of the broken-structure zone; and d is the width of the coal seam roadway under breaking .Figure 11
Calculation drawing of rational center offset for section coal pillar.The influence distance,b, of the coal pillar concentrated stress in the upper coal seam section can be calculated by using the following equation:(2)b=h·tanφ,where h is the thickness of the interval rock, and φ is stress-transfer influence angle of the interval rock.The width of the section coal pillar in Hongliulin Coal Mine is 20 m, and the distance between no. 3−1 coal seam and no. 4−2 coal seam is 40 m, and the stress-transfer influence angle is 19° in the experiment. The horizontal distance of the fracture structure belt is 10.8 m. The width of no. 4−2 coal transportation roadway is 5.6 m. The central offset of the section coal pillar must be greater than 40.2 m, which was calculated by using equation (1). The spacing between no. 4−2 coal seam and no. 5−2 coal seam is 63 m, and the stress-transfer influence angle is 20° in the experiment. The horizontal distance of the fracture structure belt is 8.48 m, and the width of the transportation roadway of no. 5−2 coal is 6.1 m. According to equation (1), the central offset of the section coal pillar must be greater than 57.5 m.


## 6. Conclusions

(1)
The residual section coal pillars of shallow coal seam group mining all formed an “inverted trapezoidal” roof support structure with the roof with collapse lines on both sides as the boundaries above them. The stress concentration occurred in the floor of the coal pillar floor and transferred downward in the interval strata, which resulted in an increase of the stress area within the influence angle of pressure transfer. The width of the coal pillar in the upper residual section was large, and there was an “elastic core” in the middle part, which did not cause instability and failure.(2)
When the working face of the current part of the coal seam achieve a certain length, the interval rock strata will break and lose the stability due to the limit imposed by caving, which induces activation of the roof of the goaf in the upper coal seam. The two form a whole, and the mine pressure phenomenon of synchronous collapse occurs, which together form mine pressure on the mining face.(3)
During mining the shallow coal seam group, the instability mechanism of the coal pillar in the upper coal seam is the structural instability of the interval strata. At present, the coal pillar in the coal seam section is in the central dislocation position of ultimate instability. Shear plastic failure would occur in the interlayer rock, and an “inclined step beam” instability structure would also occur. The short masonry beam fracture structure belt will form, and severe collapse pressure will also appear, which will affect the safety and the stability of the coal pillar and the roadway in the lower coal seam.(4)
To avoid structural instability and rock pressure in the upper residual section coal pillar in the inclined direction of the coal mining face, a calculation method for a reasonable layout of the section coal pillar in the lower coal seam in the shallow coal seam group mining was proposed in Yulin. This method also takes into account the safety and stability of the section coal pillar and the roadway in the lower coal seam. Combined with the specific engineering geological conditions and the conclusions derived from experiment of Hongliulin Coal Mine, it is suggested that the central offset of the coal pillar in the lower coal seam section under the subsequent coal seam group mining should be greater than the value of the calculation results.



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*Source: 4646450-2023-03-21.xml*