Relationship between advancing abutment pressure and deformation of surrounding rock in a roadway: a case study in Helin coal mine in China

The determination of propulsive abutment pressure and deformation characteristics of roadway surrounding rock mass in top-coal caving mining is of great significance for further research on mining stress, roadway support and equipment selection. This paper adopts the method of combining theoretical analysis and field test, taking Helin coal mine j7401 working face as the research object. The relationship between abutment support pressure and roadway surrounding rock mass deformation is revealed by monitoring the stress of transportation and ventilation roadway and observing the deformation of mining roadway. The main conclusions are as follows: when the distance between top-coal caving mining and borehole is about 65 m, the borehole stress begins to exceed the original rock stress, and the peak of the abutment pressure is 15–22 m in front of the working face, ranging from 33 to 39 MPa, which is 3.2–3.8 times the original rock stress. The intersection of propulsive abutment pressure curve and original rock stress curve is located at 8.1 m in front of the working face, the boundary of roadway rapid deformation stage and deceleration deformation stage is 8.9 m ahead of the working face, the distance between the two stages is 0.8 m, and the peak position of propulsive abutment pressure is 19.8 m. The boundary of roadway decelerative deformation stage and stationary small deformation stage is 20.1 m, and the distance between the two stages is 0.3 m. The boundary between the fracture zone and the limit equilibrium zone is the intersection of the propulsive abutment pressure curve and the original rock stress curve. The plastic zone is constituted by the damage zone and the limit equilibrium zone, and the boundary between the plastic zone and the elastic zone is located at the peak of the propulsive abutment pressure. The above study results provide a basis for roadway support and efficient coal mining.


Introduction
As a result of the continuous improvement of coal mining technology and mining equipment in China, large-scale mechanized caving mining has been used widely (Akulshin et al. 2020;Fazelabdolabadi and Golestan 2020;Wang and Xiong 2020). The determination of the advancing abutment pressure of fully mechanized caving mining and the characteristics of the deformation of the surrounding rock in the roadway is of great significance for in-depth studies of mining pressure, roadway support, and equipment selection. Studies of the advancing pressure and the deformation of the surrounding rock in the roadway following the methods of Qian and Shi (2003) and Hou (2012) have resulted in constructive achievements. Xu and Ju (2011) and Xu et al. (2012) investigated the influence of the key layer on 1 3 763 Page 2 of 10 the characteristics of the mining pressure in a fully mechanized working face. Wang et al. (2012) and Wang and Zhang (2015) obtained good results in a study of the influence of the caving ratio on the characteristics of the mining pressure in a fully mechanized working face. Zhang et al. (2015), Zou et al. (2015) and Liu et al. (2013) determined key rules regarding the characteristics of the mining pressure in different areas and different mines in a fully mechanized working face. Ma et al. (2015a,b) conducted in-depth theoretical and experimental research on the stress and failure characteristics of roadways during mining. Meng et al. (2017) and Kang et al. (2012) conducted in-depth research on roadway deformation damage and control technology and the methods are widely recognized in the industry.  and  investigated the mechanical processes of roadway destruction.
To sum up, many scholars have achieved numerous achievements on roadway advancing abutment pressure, deformation characteristics of surrounding rock mass and roadway support. However, there are few researches on the relationship between stress distribution and surrounding rock deformation. Therefore, the paper adopts the method of combining theoretical analysis and field test, taking Helin coal mine j7401 working face as the research object. Through the stress monitoring of the transportation roadway and the ventilation roadway and the deformation observation of the mining roadway that reveals the relationship between abutment support pressure and the roadway surrounding rock mass deformation, the study results provide a basis for roadway support and efficient coal mining (Fig. 1).

Overview of the study background
The 7(4) coal seam in the j7401 working face of the Helin coal mine has a surface elevation of 255-310 m, a level elevation of − 150 to − 270 m, a length of the working face of 240 m, and an advancing direction length of 1520 m; the inclination range of the working face is larger and the average angle of the working face is about 6°. The range of the coal seam thickness of the working face is 7.8-13.6 m and the average thickness of the coal seam is about 11.4 m. Because the hardness of the coal seam is relatively low, the stability is low, a 3.4-m mining height and an 8-m top-coal caving height are used in the fully mechanized top-caving coal mining. The overburden structure of the working surface is relatively simple. The top and bottom plate consist of mudstone and sandstone and the bottom is carbonaceous mudstone, which is black-gray with a thin, dense, and hard layer and an average thickness of 6.70 m. The old bottom is also black and brown-gray and consists of silty sandstone that is dense and hard with an average thickness of 8.65 m.

Theoretical analysis
Due to the existence of a beam structure, a zone of increasing stress is formed in front of the working face as mining progresses, as shown in Fig. 2. Prior to the structural failure of the beams, the advancing abutment pressure increases continuously, the surrounding rock of the roadway moves closer, and the moving speed increases (Qian and Shi 2003;Hou 2012;Zou et al. 2021;Yang and Wang 2010). During this process, the working face continuously progresses. Under the influence of the advancing abutment pressure, two areas are formed in front of the working face, i.e., the plastic zone (zone A) and the elastic zone (zone B).
The equation of the hydraulic equilibrium in the horizontal direction and vertical direction at any location in the plastic zone is: where m is the thickness of the working face, f is the friction coefficient, x is the horizontal stress of the unit, and y is the vertical stress of the unit.
Based on the Mohr-Coulomb criterion, Eq. (1) is obtained: where C is the cohesion of the coal, and is the internal friction angle of the coal.
We define R = 1+sin 1−sin , R C = 2C √ 1+sin 1−sin . The vertical stress expression in the plastic zone can be reduced to: where N 0 is the supporting capacity of the coal wall, and k is the advancing stress concentration factor.
When y = k H , the advancing abutment pressure reaches its peak and the distance between the peak position and the coal wall, x 0 , is represented as: where is the support resistance to the flank.
The peak of the advancing abutment pressure in front of the working face occurs at x 0 m and represents the boundary between the elastic deformation area (area B), which is x 0 m in front of the working face and the plastic deformation zone, where o to x 0 m in front of the working face. The plastic deformation zone is divided into the failure zone (zone C) and limits equilibrium zone (zone D) by the intersection of the advancing abutment pressure curve and the original rock stress curve.
In zone C, the coal strata that lost its integrity and has poor carrying capacity causes large and sharp deformation of the surrounding rock in the roadway; this is referred to as the rapid deformation stage of the surrounding rock of the roadway. In zone D, although the coal bed has higher integrity and bearing capacity and experiences plastic deformation, it is still in a discontinuous state. There is less roadway deformation in this area and the rate of deformation is slower; this is referred to as the deceleration deformation stage of the surrounding rock of the roadway. The failure zone and the limit equilibrium zone are collectively called the plastic zone with large deformation and is called the large deformation stage.
The elastic deformation zone, where the coal rock formation is in good condition and has good bearing capacity, is referred to as the stable small deformation stage. Although the roadway deformation is small in this area, it is larger than the deformation of the roadway under the original rock stress.

Observation station of the advancing abutment pressure
Four groups of observation stations were set up 100 m ahead of the j7401 fully mechanized working face; each observation covered two boreholes located in the transportation roadway and the ventilation roadway, respectively. The depths of the two drilling holes were 3 m in the first group, 6 m in the second group, 12 m in the third group, and 15 m in the fourth group as shown in Fig. 3. One borehole stress meter was placed in each borehole and was located 1.5 m from the floor of the mining roadway. The layout The distance between the observation point and the working face lager than 50 m and the observation data were recorded every 2 days during the morning shift. The distance between the observation point and the working face was 30-50 m and the observation data were recorded daily during the morning shift. The distance between the observation point and the working face smaller than 30 m and the observation data were recorded daily during the morning shift and midday shift.

Observation station of the surrounding rock deformation
In the fully mechanized mining face, the deformation of the back mining roadway was observed to determine the deformation characteristics of the surrounding rock of the transport roadway and the ventilation roadway and analyze the influence of the mining on the surrounding rock deformation. Observation lines with three stations located 30 m apart were laid out in a cross pattern to observe the surface displacement of the mining roadway. The layout parameters of the tunnel test station are shown in Fig. 4.

Results and discussions
Variations in the advancing abutment pressure As the fully mechanized caving face is pushed forward, the coal body is gradually deformed and broken and the borehole wall in the mining roadway is filled between the borehole and the borehole stress gauge. As a result, the borehole stress gauge in the roadway is in full contact with the coal body in the fully mechanized working face. The measurements of the borehole stress meter from eight boreholes at the four test stations are shown in Tables 1, 2, 3 and 4. The borehole stress measurements of the advancing working face at different drilling depths are shown in Fig. 5.
The red curve in Fig. 5 represents the original rock stress curve, the green curve represents the transportation roadway pressure, and the blue curve represents the ventilation roadway pressure. The following are observed:  (i) When the distance between the fully mechanized caving and drilling is about 65 m, the borehole stress begins to exceed the rock stress, indicating that the advancing abutment pressure range of the j7401 fully mechanized working face is about 65 m. (ii) The peak of the advancing pressure is between 15 and 22 m in front of the working face, the peak range of the advancing pressure is 33-39 MPa, and the peak of the advancing abutment pressure is 3.2-3.8 times that of the original rock stress.

Variations in the surrounding rock deformation
The observation data of the surrounding rock deformation are shown in Tables 5 and 6. The data were used to determine the influence range of the mining roadway on the fully mechanized mining face. We used the data in Tables 5 and 6 to plot the velocity changes in the surrounding rock deformation of the transportation roadway and ventilation roadway (Fig. 6).
The following results were observed: (i) In front of the fully mechanized working face, the roof activity is intense at 0-20 m. The maximum daily subsidence rate of the roof is 18 mm in the range of 5-11 m in the front of the working face. The daily convergence of the roof subsidence at 20 m in front of the working face is less than 5 mm for the ventilation roadway and less than 3 mm for the trans-    These results indicate that the deformation of the surrounding rock is very large in the range of 0-20 m in the front of the working face, is weak in the range of 20-60 m, and the surrounding rock is basically unaffected by mining beyond 60 m.

Relationship between advancing abutment pressure and roadway deformation
The method of least square is used to process the above four sets of data, which obtain the change of advancing abutment pressure in different positions ahead of the top-coal caving mining face. The 96 data points in the four tables (x i , y i ) , (i = 1, 2, … , 96) , were used to find the approximation function y = S(x) . The principle of this method is to minimize the sum of squares between the fitted value and the actual value to determine the approximation function of f (x) in Φ = span{ 0 (x), 1 (x), … , n (x)} (where i (x) is the i vector function of x) to find a k (k = 0, 1, … , n ). Let S(x) = ∑ n k=0 a k k in Φ space be the best approximation function for f (x) Zou et al. 2021).
We used the distance between the measuring point and the working face (Tables 1, 2, 3, 4) as the independent variable and the advancing abutment pressure as the dependent variable; the advancing abutment pressure change at different distances from the working face is shown in Fig. 7.
The following are observed in Fig. 7:  The data in Tables 5 and 6 are also analyzed using the least squares fitting method. We made no distinction between the convergences of the two sides and the roof subsidence and considered only the daily roadway deformation. The measuring point and working face distance (Tables 5, 6) were used as independent variables and the daily deformation of the roadway in front of the working face as the dependent variable, as shown in Fig. 8.
The following are observed in Fig. 8: (i) The maximum daily deformation occurs at 8.9 m in front of the working face and the daily deformation is larger than 12 mm in the range of 0-8.9 m, which is the rapid deformation stage.
(ii) In the range of 8.9-20.1 m the daily deformation of the roadway decreases from 14.5 mm to about 3 mm; this is the deceleration deformation stage. (iii) The daily deformation of the roadway beyond 20.1 m in front of the working face is very small. The deformation is zero at 65 m. In the range of 20.1-65 m, the daily deformation of the roadway decreases slowly from 3 to 0 mm; this is the stable small deformation stage.
A comparison of the results in Figs. 6 and 7 indicates that the intersection of the advancing abutment pressure curve and the original rock stress curve is located at 8.1 m ahead of the working face. The boundary between the rapid deformation stage and the deceleration deformation stage of the roadway is 8.9 m in front of the working face and the distance between the stages is 0.8 m. The peak position of the advancing abutment pressure is at 19.8 m. The boundary between the deceleration deformation stage and the stable small deformation stage of the roadway is at 20.1 m and the distance between the stages is 0.3 m.
Based on the results of the theoretical analysis and the field measurement, we obtained the relationship between the advancing abutment pressure and the surrounding rock deformation stage of the roadway (Fig. 9).
The results show that the boundary between the failure zone (rapid deformation stage) and the limit equilibrium zone (deceleration deformation stage) is the intersection of the advancing abutment pressure curve and the original rock stress curve. The damaged area (rapid deformation phase) and the limit equilibrium area (slow deformation stage) constitute the plastic zone (large deformation stage). The boundary between the plastic zone (large deformation stage) and elastic zone (stable small deformation stage) is located at the peak value of the advancing abutment pressure.

Conclusions
A theoretical analysis and field measurements are used to analyze the relationship between the advancing abutment pressure and the surrounding rock deformation characteristics of the roadway of the j7401 fully mechanized working face. The following conclusions are drawn: 1. When the distance between top-coal caving mining face and borehole is about 65 m, the borehole stress begins to exceed the original rock stress, which indicating that the range of advancing abutment pressure of J7401 topcoal caving mining face is about 65 m. The peak point of advancing abutment pressure is 15-22 m ahead of the working face and the range is 33-39 MPa, which is 3.2-3.8 times of the original rock stress. 2. Ahead of the working face, the deformation of surrounding rock mass in the range of 0-20 m is relatively large, and the deformation of surrounding rock mass in the range of 20-60 m is weak, and the surrounding rock mass above 60 m is basically unaffected by mining. The maximum daily subsidence of the roof, within the range of 5-11 m ahead of the working face, is 18 mm and the maximum daily convergence displacement of the two sides is 19.4 mm. However, the daily convergence of the roof and the two sides, within the range of 20-60 m ahead of the working face, is less than 3 mm. 3. The intersection of propulsive abutment pressure curve and original rock stress curve is located at 8.1 m in front of the working face, the boundary of roadway rapid deformation stage and deceleration deformation stage is 8.9 m ahead of the working face, the distance between the two stages is 0.8 m, and the peak position of propulsive abutment pressure is 19.8 m. The boundary of roadway decelerative deformation stage and stationary small deformation stage is 20.1 m, and the distance between the two stages is 0.3 m. 4. The boundary between the fracture zone (rapid deformation stage) and the limit equilibrium zone (slow deformation stage) is the intersection of the propulsive abutment pressure curve and the original rock stress curve. The plastic zone (large deformation stage) is constituted by the damage zone (rapid deformation stage) and the limit equilibrium zone (slow deformation stage), and the boundary between the plastic zone (large deformation stage) and the elastic zone (stable minute distortion) is located at the peak of the propulsive abutment pressure.
In this paper, the evolution law of propulsive abutment pressure at different distances ahead of working face is defined and the deformation characteristics of roadway surrounding rock mass are clarified. Based on the above characteristics, the relationship between propulsive abutment pressure and roadway surrounding rock mass deformation is further elucidated. In the meanwhile, the above research results provide a basis for roadway support and efficient coal mining.