Overburden key strata (KS) have a significant influence on abutment pressure distribution. However, current calculation methods for working surface abutment pressure do not consider the influence of the overburden KS. This study uses KS theory to analyze the overburden load transferred to coalrock masses on both sides of the stope through fractured blocks in different layers of the KS in the fissure zone and KS in different layers of the curve subsidence zone. Using Winkler’s elastic foundation beam theory, we consider the fissure zone KS on the coal mass side and the curve subsidence zone KS as many elastic foundation beams interact with each other. A method to calculate the abutment pressure of the coal mass and the goaf was then established, considering the influence of the overburden KS. The abutment pressure distribution of working surface 207 after mining was then calculated using our method, based on mining conditions present in the Tingnan coal mine. The calculated results were verified using measurements from borehole stress meters and microseismic monitoring systems, as well as numerical simulations. In addition, the calculation results were used to determine a reasonable position for the stopping line and remaining width of the roadway’s protection coal pillar in working surface 207. The results of this study can be used to calculate the abutment pressure distribution of the working surface under a variety of overburden KS conditions. The results can also provide guidance for forecasting and preventing mine dynamic hazards, controlling the surrounding rock in mining roadways, determining reasonable widths for protection coal pillars, and designing the layout of mining roadways.
Primary rock stress is normally in equilibrium before coal seams are excavated. During the course of mining, the overlying strata are shifted and stress is redistributed to the surrounding rock, resulting in abutment pressure. Failure to account for abutment pressure distribution leads to coal mine hazards, such as rockbursts, coal and gas outbursts, and roadway deformation and instability [
Previous studies have found that abutment pressure distribution is closely related to the overburden KS (number, thickness, strength, and location of KS) [
This study uses KS theory [
Neglecting the influence of overburden KS on the distribution of abutment pressure often leads to catastrophic accidents, as described in the two following cases.
The overburden of the thick sandstone KS on abutment pressure distribution was not considered during the operation of the Tingnan coal mine, and as a result, the width of the protective coal pillars in the main roadway was insufficient. As a result, the main roadway 200 m from the stopping line underwent significant deformation. The Tingnan coal mine is located in the Changwu County, Xianyang, Shaanxi Province, and it has an average mining height of 7.0 m and a coal seam dip angle of 0° in the second panel. Figure
Layout sketch map of the working area in the second panel.
Location of the overburden KS in the Tingnan coal mine.
Photos of the main roadway damage.
The Haizi coal mine is located in Suixi County, Huaibei City, Anhui Province, China. The average coal seam thickness in the II102 mining area is 2.5 m, the average depth is 623 m, and a 140 m thick igneous KS is located approximately 170 m from the coal seam. After the extraction of working surfaces II1022 and II1024, a coal and gas outburst occurred 180 m from the goaf while a transport roadway was being excavated in working surface II1026, as shown in Figure
Layout sketch map of the working area in the Haizi coal mine.
The deformation and damage to the main roadway in the Tingnan coal mine and the coal and gas outburst in the Haizi coal mine [
In Chinese coal mines, the most commonly used mining method is longwall mining in which the stoping space created by the stoping of one or more working faces is rectangular in shape and the mining area contains two main sections [
Mechanical model for abutment pressure calculation.
In Figure
As the KS in Figure
Using KS_{m−1} in Figure
Stress analysis of the fissure zone KS on the coal mass side.
Using KS_{n−1} in Figure
Stress analysis of the curve subsidence zone KS: (a) side of the coal mass; (b) side of the goaf.
The relationship between the elastic foundation beam’s deflection, foundation pressure, and the load it bears is as follows [
According to the established mechanical model, the following groups of deflection curve differential equations can be formed:
Equation group (
As the expressions
The characteristic equation corresponding to equation (
The root of equation (
A special solution for equation (
Then, the general solution for equation (
As
As the expressions
Solving the equations in equation group (
As the expressions
The characteristic equation corresponding to equation (
The root of equation (
A special solution for equation (
Then, the general solution for equation (
Substituting equation (
In the paper,
There are a total of 4(
There are a total of 2
According to Section
When the coal lateral pressure in the proximity of the mining boundary is zero, i.e., when the area is under uniaxial compression, as the plastic failure [
The symbol
As (
If
According to Section
The distribution of the abutment pressure in the goaf can be calculated as follows:
Diagram of adjacent KS.
In Figure
In particular, the calculation of
Because the model assumes that the fractured rock in the caving zone and fissure zone satisfies Winkler’s elastic foundation hypothesis [
Our estimation method was applied to calculate the distribution of the abutment pressure of working surface 207 in the Tingnan coal mine. The location of the overburden KS and the height of the fissure zone in the Tingnan coal mine are shown in Figure
KS parameters.
KS number 







KS_{1} 





— 
KS_{2} 





— 
KS_{3} 




— 

KS_{4} 




—  — 
Other parameters (apart from KS) in the method used to calculate abutment pressure.








6.14  0.29  16  7  0.65  0.45  890 
Abutment pressure distribution of working surface 207.
It should be noted that it is nearly impossible to measure real in situ stress using stress sensors, and most stress monitoring systems thus record the vertical stress increment relative to the initial pressure of oil pumped into the sensors for coupling with the monitoring boreholes [
The initial pressure (
Variation curves of vertical stress increment.
Figure
A correlation is observed between the occurrence of microseismic events and the presence of abutment pressure, which has allowed for examination of the distribution of abutment pressure [
Locations of microseismic events relative to working face 207.
Figure
Based on geological mining conditions, a numerical model was established using UDEC, as shown in Figure
Model geometries.
Mechanical parameters of rock mass used in the numerical simulation.
Bulk modulus (GPa)  Shear modulus (GPa)  Cohesion (MPa)  Friction angle (°)  Tensile strength (MPa)  Normal stiffness of joint (GPa)  Shear stiffness of joint (GPa)  

Loess  3.3  1.5  0.11  10  0  2.7  2.5 
Fine sandstone  10.4  7.5  10.9  43.9  11.9  6.2  4.7 
Coarse sandstone  8.8  6.0  9.8  42  8.9  5.4  3.8 
Mudstone  6.0  3.6  5.1  38  4.7  3.2  2.9 
Coarse sandstone gravel  15  8.6  12.4  43.5  13.8  9.5  6.4 
Conglomerate  18  11.8  15.4  48  15.5  10.9  8 
Medium sandstone  9.2  6.5  10.2  42.5  9.2  5.6  4.3 
Sandy mudstone  7.4  4.5  6.2  39  6.6  4.5  3.2 
Siltstone  8.5  5.2  9.4  41  8.0  5.2  3.5 
Coal  3.5  2.1  3.6  37  1.8  2.5  1.6 
The results of the UDEC numerical model and the theoretical estimation of the abutment pressure for working face 207 are shown in Figure
Comparison of stress distributions predicted by UDEC and the proposed method.
Determining the stopping position and the width of the protection coal pillar must consider the characteristics of the overburden KS. On the basis of the integrated stratigraphic column in the Tingnan coal mine, the location of the overburden KS and the height of the fissure zone should be calculated based on previous researches [
The results of our calculations show that the abutment pressure width of working surface 207 after excavation is 283 m. If a protection coal pillar on the one side of the stopping line of working surface 207 was 200 m wide, according to the original design, the roadway would be significantly deformed and damaged in a manner similar to what happened at working surface 206. Consequently, working face 207 should stop mining outside of 283 m from the main roadway and leave no less than a 283 mwide protective coal pillar to ensure that the main roadway is outside the abutment pressure influence width of the working face, as shown in Figure
Setting the KS_{4} thickness
Abutment pressure distributions under different KS_{4} thicknesses.
Abutment pressure distribution is closely related to the overburden KS. Neglecting the influence of the overburden KS on the distribution of abutment pressure often leads to coal mine hazards, such as rockbursts, and so on. However, current calculation methods for working surface abutment pressure do not consider the influence of the overburden KS. Using KS theory and Winkler’s elastic foundation beam theory, we consider the fissure zone KS on the coal mass side and the curve subsidence zone KS as many elastic foundation beams interact with each other. A method to calculate the abutment pressure of the coal mass and the goaf was then established, considering the influence of the overburden KS. The abutment pressure distribution of working surface 207 in the Tingnan coal mine was then calculated using our method and was verified using measurements from borehole stress meters and microseismic monitoring systems, as well as numerical simulations.
Results of using the proposed calculation method show that as the thickness of the KS_{4} in the Tingnan coal mine increased, the abutment pressure width significantly increased. The working faces 205 and 206 neglected the impact of 222.14 m thick, hard sandstone KS_{4} in the overburden on the distribution of abutment pressure, and only a 200 mwide protective coal pillar was left, causing serious deformation and damage to the roadway. Based on the results that show the abutment pressure width to be 283 m for working surface 207, the working face must stop 283 m from the main roadway and leave a, no less than 283 m wide, protection coal pillar.
The abutment pressure calculation method established in this paper can calculate the abutment pressure distribution of a working face under different overburden KS conditions, providing guidance for forecasting and preventing mine dynamic hazards, controlling the surrounding rock in mining roadways, and designing the layout of mining roadways. However, when the overburden strata have a thick alluvium layer, the thick alluvium layer will also have a strong impact on the abutment pressure. It is necessary to further study the calculation method of abutment pressure under the condition that both KS and thick alluvium layers exist in the overburden.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors gratefully acknowledge the financial support from the State Key Laboratory of Groundwater Protection and Utilization in Coal Mining. This research was funded by the Open Fund of State Key Laboratory of Groundwater Protection and Utilization in Coal Mining (Grant number SHJT1630.11).