Study on the Distribution Trend of Rockburst and Ground Stress in the Hegang Mining Area

Abstract

As the depth of coal mining increases, rockbursts become more severe, and multiple rockburst accidents have previously occurred, seriously affecting coal mine production safety. With the Hegang mining area as the research object and using field measurement and numerical simulation as the research methods, the geological and tectonic characteristics of each impact mine in the Hegang mining area are analyzed to obtain the tectonic stress field distribution characteristics of each mine and the tectonic stress field distribution trend in the Hegang mining area. The maximum horizontal principal stress and the minimum horizontal principal stress on the energy field in the overrun area of the retrieval working face is analyzed via numerical simulation, and the influence trend of the ground stress on the impact ground pressure is finally obtained. Results show that using the typical working face where rockburst occurs as the geological proto-type, the influence of the direction of the ground stress field, the maximum horizontal principal stress and the minimum horizontal principal stress on the energy field in the overrun area of the retrieval working face is analyzed via numerical simulation, and the influence trend of the ground stress on the impact ground pressure is finally obtained. When the angle is 70°~90°, an energy peak in the overrun area of the working face reaches its maximum. The ratio of the minimum horizontal stress to the vertical stress is positively correlated with the energy concentration in the overrun area of the working face, but its change has a minimal effect on the energy distribution in the overrun area of the working face. When this ratio is increased from 0.6 to 1.4, the peak energy of the simulated working face only increases by 8.22%, and the energy concentration area remains basically unchanged.

1. Introduction

With the increase in mining depth, the impact pressure situation in the Hegang mining area is becoming severe, with many rockburst accidents occurring that seriously affect the safe production of coal mines. The high ground stresses at deep mining levels and the high stress fields created by the superimposed stresses of mining are two of the root causes of rockburst hazards [1,2]. Rockburst is a typical dynamic disaster in coal mining [3]. Usually, when the mechanical system of coal and rock reaches its ultimate strength, it releases its elastic properties in a sudden, rapid and violent manner, causing instantaneous damage to the coal and rock layers; this is accompanied by the impact of coal powder and rock, resulting in damage to the shaft and personal injury accidents [4,5,6]. When rockburst occurs, it often causes great damage to the support of the roadway, leading to large-scale roof fall, wall spalling and a floor heave in the roadway, causing great damage to personnel and equipment and seriously affecting coal mine production safety. Moreover, the occurrence of rockburst is accompanied by the ejection of coal and rock, causing disasters such as coal and gas outbursts, coal and gas dust explosions, fires, floods and even ground vibration, which can cause the collapse of ground buildings and pose great harm [7,8]. Stress is the most fundamental cause of rockburst, and the stress of a coal–rock mass includes crustal stress and disturbance stress; thus, monitoring and controlling the stress in a coal–rock mass is very important for the research and prevention of rockburst.

The safe, efficient and green mining of deep coal seams are some of the major concerns with ensuring the healthy development of a country’s modern energy economy and an important guarantee of environmental sustainability. Deep coal seams not only have the essential property of high static load characteristics, but also have the additional property of strong disturbances resulting from large-scale mining activities, leading to the frequent occurrence of dynamic hazards, such as impact ground pressure, which can easily cause significant casualties and economic losses. Impact pressure control can overburden deformation, reduce surface subsidence and prevent impact pressure disasters, and is an effective way to achieve safe, efficient, sustainable and green mining in deep coal seams. Therefore, this study has far-reaching significance for the prevention and control of impact pressure and sustainable development.

Much research has been carried out by domestic and international scholars and field engineers on the relationship between ground stress and rockburst. Lianpeng Dai [9] focused on the geological and mining technical factors that influence the occurrence of impact ground pressure in a roadway. The impact ground pressure area of the roadway was quantitatively divided into several different risk level areas, and an impact ground pressure roadway zoning and grading management system was proposed. Masoud Ghorbani [10] investigated the behaviour of rockburst in high stress rock masses, described different types of energy absorbing bolts and other support components, and determined that a suitable support system was the main condition for preventing rockburst. Jian Zhou [11] used the stochastic gradient boosting (SGB) method to study impact ground pressure data, analyzed the effect of different indicators, such as the ground stress condition factor on the impact ground pressure and predicted the impact ground pressure damage dataset with accurate and reliable results. Charlie C. Li [12] analyzed a combination of seismic monitoring and numerical modeling as a promising approach to forecasting. Using existing knowledge and technology to predict impact ground pressure and large seismic events was extremely challenging, but prediction was possible. Jian Zhou [13] used a hybrid technique combining an artificial neural network (ANN) and an artificial bee colony (ABC) to establish a complex relationship between susceptibility to ground pressure and its influencing factors, which could be used as an effective tool for predicting susceptibility to ground pressure. Yigai Xiao [14] proposed a metal mine pressure monitoring and warning method based on deep learning data analysis. The theoretical basis of rockburst was elaborated, the inducing factors of deep well rockburst were analyzed and the classification of rockburst was examined. Hongpu Kang [15] conducted an on-site experiment involving large-scale hydraulic fracturing to weaken the hard and refractory rock layer above the long wall panels of underground coal mines. The strength of the long wall working face was significantly reduced, and the strong dynamic load pressure generated by the large-scale fracture of the roof was basically eliminated. Shengquan He [16] used the simultaneous compressional transform (SST) method and kinetic modeling to analyze the mechanism of remotely triggered rockbursts around coal mine roadways. Based on seismic monitoring data, the seismic waveform at the source was estimated using the empirical scaling method and calibrated in the model.

At present, with the further increase of mining depth and the further deterioration of the geological environment, the original research results are not suitable for deep underground engineering. The numerical simulation analysis methods of a geostress field are mainly applied in hydraulic and hydropower engineering, and the study area is small and the geological structure is simple. Reasonable boundary conditions can be obtained by using a linear regression analysis method, a nonlinear mathematical method, an optimization function and other methods to carry out numerical simulation of a geostress field in the study area. However, for the inversion of a geostress field in complex geological conditions and at large range, the existing numerical simulation analysis method is not suitable. With the advancement of technology, there have been great advances in the development of ground stress measurement methods and inversion analysis methods for ground stress fields [17]. Moreover, with the development of [18], the engineering problems encountered have become increasingly complex and serious, causing many issues for construction safety. With the increase in mining intensity and depth, rockburst is now a frequent occurrence in mining activities and a serious impediment to the safe development of coal mine production. As a fundamental source of force for the occurrence of impact ground pressure disasters in mines, ground stress is highly valued by scholars at home and abroad and has been thoroughly studied with effective results. However, with the further increase in mining depth, the geological environment further deteriorates, and the original research results are not applicable to deep underground studies.

The Hegang mine is located in the city of Hegang in the northeastern part of Heilongjiang Province at the southern foot of the Xiaoxinganling Mountains. The coal strata are 800 to 1200 m thick and contain 36 coal seams, of which thicker seams greater than 3.5 m account for 75.5%. The mines in this region are mostly exploited by vertical-slope mixed multilevel mining, with coal hardness coefficients ranging from 1.4 to 3.0 in mostly high gas mines; several mines in the Hegang mine area have already experienced impact ground pressure. At present, the mining depth of the mine has extended to 500~1100 m, and the situation of impact ground pressure is more severe. The high ground stress at depth and the high stress field formed by the superimposed stress of mining are the root causes of the impact ground pressure disasters and the large deformation damage of the roadway. The main objectives of our study are to test the ground stress in typical impact mines in the Hegang mining area to understand the distribution pattern of the deep ground stress field in the mines, to analyze the influence of ground stress on impact ground pressure and to evaluate the regional risk of impact ground pressure hazards in the Hegang mining area.

2. Geological Overview

2.1. Geological Formations

The mine is located in Hegang City, Heilongjiang Province. It is 100 km long from north to south and 28 km wide from east to west, with a total area of approximately 2800 km². The average surface elevation of the mine area is 290 m, with elevations ranging from 250 to 340 m. The whole area has a topographic pattern of high elevation in the southwest and a low elevation in the northeast. A geological structure map of the Hegang mining area is shown in Figure 1.

2.2. Ground Stress Conditions

In situ crustal stress measurements are the most effective and direct means to obtain the state of crustal stress in a region. There are many measurement methods. The most widely used measurement methods are hydraulic fracturing [19,20] and stress relief [21,22]. Small-aperture hydrofracture measurement techniques and hollow envelope stress relief methods have been more widely used in ground stress testing at coal mine sites. The 56 mm diameter of the hydraulic fracture test borehole greatly reduces the need for deformation in the borehole, reduces the size of the test rig and increases the speed of testing. The measurement of ground stress is a precise measurement task, and factors such as geological formations, tunnel distribution characteristics, mining activities, construction space and the rock deposit characteristics of the test location can all have an impact on the measurement of ground stress. In addition, coal mine production is a complex production and operation system, and the operation of any link affects the operation of the entire production system.

The geostress measurements were mainly carried out for seven impact mines in the Hegang mining area. According to the principles of determining the ground stress measurement points, the mine area was arranged through the analysis of the geological conditions on site and combined with the actual production situation on site. The locations of the measurement points are shown in Figure 2 and

Table 1. Technical characteristics of the in situ measurement points in the Hegang mining area.

Measuring Points		Burial (m)	Location	Borehole
				Depth (m)	Azimuth (°)	Dip Angle (°)
Junde Coal Mine	1#	940	Belt stone gate	7.3	181	3
	2#	470	Return air stone gate	7.4	144	5
	3#	627	Track stone gate	7.1	26	4
Xing ’an coal mine	1#	730	Switchboard road	7.2	129	3
	2#	754	2 #Switchboard Road	6.9	350	4
	3#	563.5	Boundary stone gate	7.4	188	3
Fuli coal mine	1#	720	−450 roadway	7.5	265	4
	2#	800	−530 roadway	7.2	170	3
	3#	880	−610 roadway	7.3	188	3
Xinlu coal mine	1#	940	−650 Stone gates	7.2	173	3
	2#	990	−700 water silos	7.5	143	5
	3#	830	−540 Return air lane	7.1	82	4
Nanshan coal mine	1#	539	−120 return roadway	7.310	30	4
	2#	539	−120 return roadway	6.9	30	5
	3#	508	−180 roadway	6.8	136	5
Yixin coal mine	1#	487	Floor runway	7.2	121	3
	2#	635	Oblique well	7.3	94	4
	3#	561	Air inlet lane	7.1	79	4
Xingshan coal mine	1#	459	Belt lane	7	225	4
	2#	706	−340 bottom bend	7	175	4
	3#	458	−90 rock lane	7	5	4

.

The arrangement of the ground stress measurement points was determined in each typical impact mine in the Hegang mining area. The feasibility of the construction of the ground stress measurement points was examined by testing the plastic zone and the structure of the surrounding rock, and the depth of the ground stress test borehole was also determined. The strain data for each measurement point were then obtained according to the ground stress test procedure, and the optimal position for the ground stress measurement was obtained by screening the raw data and results. The results of the ground stress at each measurement point in the Hegang mine are shown in

Table 2. Results of the in situ stress measurement in the Hegang mining area.

Measuring Points		Location	Burial (m)	Principal Stress				Vertical Stress
				Principal Stress	(MPa)	Azimuth (°)	Dip Angle (°)
Junde Coal Mine	1#	Belt stone gate	720	σ1	33.42	87	−8.2	21.16
				σ2	18.73	177	−23
				σ3	10.81	267	−52
	2#	Return air stone gate	470	σ1	22.87	79	8.7	13.2
				σ2	10.87	237	−13
				σ3	8.29	152	−62
	3#	Track stone gate	627	σ1	32.5	111.5	−2.5	17.49
				σ2	16.42	−62	−14.4
				σ3	14.41	205	−56.4
Xing ‘an coal mine	1#	Switchboard road	730	σ1	30.10	113.07	12.12	23.61
				σ2	16.89	20.93	9.88
				σ3	14.97	252.75	74.26
	2#	2 #Switchboard Road	754	σ1	32.72	69.16	4.16	24.03
				σ2	16.85	167.23	62.63
				σ3	14.75	259.35	26.99
	3#	Boundary stone gate	563.5	σ1	30.48	86.6	1.72	13.95
				σ2	14.47	193.6	84.14
				σ3	14.09	103.7	−5.60
Fuli coal mine	1#	−450 roadway	720	σ1	35.9	90.9	1.6	21.67
				σ2	22.3	181	19.8
				σ3	10.6	−4.3	73.1
	2#	−530 roadway	800	σ1	39.2	76	2.59	18.69
				σ2	21.2	261	32.6
				σ3	10.1	170	57.2
	3#	−610 roadway	880	σ1	41.2	95.4	4.3	22.37
				σ2	25.7	184.3	−5.9
				σ3	18.9	78.8	64.2
Xinlu coal mine	1#	−650 Stone gates	940	σ1	39.64	264.89	10.24	26.14
				σ2	21.27	−32.27	78.43
				σ3	18.81	175.86	−5.34
	2#	−700 water silos	990	σ1	39.79	116.66	7.57	27.93
				σ2	24.09	25.68	7.35
				σ3	19.95	252.00	79.42
	3#	−700 water silos	830	σ1	35.49	90.44	4.61	20.98
				σ2	17.57	−62.50	84.83.
				σ3	15.40	180.62	2.34
Nanshan coal mine	1#	−120 return roadway	539	σ1	27.878	132.817	−8.138	14.223
				σ2	23.383	−80.934	−78.472
				σ3	19.972	228.426	7.949
	2#	−120 return roadway	539	σ1	25.729	133.117	−8.638	14.438
				σ2	23.267	−80.934	78.072
				σ3	20.027	228.436	7.949
	3#	−180 roadway	508	σ1	23.892	124.116	18.002	12.946
				σ2	20.672	226.342	40.549
				σ3	18.837	177.917	20.877
Yixin coal mine	1#	Floor runway	487	σ1	21.7	107.3	7.74	13.6
				σ2	9.8	25.5	13.3
				σ3	8.7	236.7	74.5
	2#	Oblique well	635	σ1	19.0	89.3	0.23	13.4
				σ2	10.5	179.4	24.6
				σ3	9.9	269.9	65.3
	3#	Floor runway	561	σ1	20.3	70.7	−0.22	12.0
				σ2	11.5	157.7	85.7
				σ3	8.9	247.7	4.22
Xingshan coal mine	1#	Belt lane	459	σ1	17.9	116.5	3.01	11.7
				σ2	9.4	−33.2	79.8
				σ3	9.3	219.3	9.65
	2#	−340 bottom bend	708	σ1	21.6	107.2	11.01	15.76
				σ2	11.3	−39.3	17.8
				σ3	8.6	162.8	72.0
	3#	−90 rock lane	458	σ1	18.6	−84.3	−14	11.10
				σ2	16.5	184	−6.6
				σ3	13.2	249.7	74

and summarized in Figure 3.

The maximum horizontal principal stress at each measurement point in the Hegang mining area has a clear tendency to increase with depth, with the maximum horizontal principal stress ranging from 17.9 to 22.8 MPa at burial depths of 400 m to 500 m. The maximum horizontal principal stress ranges from 19 to 32.5 MPa at depths of 700 m to 800 m. The maximum horizontal principal stress ranges from 35.5 to 41.2 MPa at depths of 800 m to 900 m. The maximum horizontal principal stress ranges from 39.6 to 39.8 MPa at depths of 900 m to 1000 m. Although the maximum horizontal principal stress increases as the depth of burial increases, the difference between the maximum and minimum values of the maximum horizontal principal stress at each burial level is 10–20 MPa. There are evident regional characteristics that are influenced by the regional structure as shown in Figure 4.

3. Characteristics of Rockburst in the Hegang Mining Area

3.1. Characteristics of Rockburst during Driving

(1) Range of rockburst pressure: In the Hegang mining area, most rockburst occurs within the range of 15 m~110 m from the heading face, while most rockburst occurs within 50 m from the heading face, accounting for 86.7% of the total [23,24].

(2) Release energy: Use microseismic monitoring measurements of the energy released by rockbursts during roadway excavation in the Hegang mining area is calculated on the Richter scale, ranging from 1.25 to 2.86, with an average of 1.86. The energy ranges from 2.0 × 10⁵ to 1.07 × 10⁸ J, and the average energy is 2.15 × 10⁶ J. By analyzing the impact energy and the impact energy rate during the mining process, solid coal mining and goaf mining, the impact energy curve as shown in Figure 5 is obtained.

(3) Failure characteristics: The failure characteristics of rockburst in the Hegang mining area are mainly manifested in four aspects: roadway deformation, supporting body failure, equipment damage and casualties. When it is subjected to artificial mining and other geological activities, it has a severe impact and destruction phenomenon, which can easily cause a coal mine rockburst accident, resulting in fully mechanized coal mine underground working face or roadway, resulting in sheet wall, floor bulge, roof collapse, support failure, roadway blockage, equipment damage, casualties and other destructive consequences.

Roadway deformation: The floor bulging ranges from 0.3 to 1.8 m, and the two sides move closer to the range from 0.1 to 1.7 m. The minimum section of the roadway after deformation is 1.3 m². Roadway deformation is the most serious in the range from 15 m to 50 m outwards from the heading head. There is a patch phenomenon in the head, and the depth of the patch is 0.5 m.

Support body damage: The rigging equipment and anchor cable rope can be damaged, and the U-shaped steel can become deformed.

Equipment damage: The transport machine and boring machine can be sprung up or tilted; the rig can be overturned, plus many others.

Casualties: Partial rockburst can cause casualties.

3.2. Characteristics of Rockburst of the Working Face

(1) Impact range: The impact ground pressure in the Hegang mine area is most serious in the Junde and Nanshan coal mines. The location where the impact ground pressure causes serious damage occurs initially in the return air duct, followed by the working face, with relatively little impact damage in the machine duct. The impact damage in the return duct is generally distributed within 110 m from the exit position on the working face to the overhead working face, and the serious damage is generally within 20 m to 60 m of the overhead working face. A few shocks occur in the range of 100 m to 255 m of the overburden. The number of impacts on the working face is low, and they occur in the range of 20 to 100 groups of hydraulic supports, and the severe sections are mostly distributed in the range of 50 to 90 groups of hydraulic supports. The impact pressure in the machine path occurs less frequently, and the impact range is from the lower exit position of the working face to 110 m of the overrun working face.

(2) Release energy: The energy released from the impact ground pressure occurring during the workings retrieval process in the Hegang mining area ranges from 0.81 to 4 on the Richter scale, with an average of 2.23. The energy ranges from 3.57 × 10⁴ J to 9.12 × 10⁹ J, and the average energy is 9.12 × 10⁶ J.

(3) Damage characteristics: The damage caused by the impact ground pressure that occurs during the workings retrieval process in the Hegang mining area can be studied in terms of both roadway damage and workings damage.

Roadway damage: Roadway damage is most serious in the return airway, the bottom drum range is 0.5~1.3 m, the top sink is 0.5~1.0 m and the two helpers shrink 0.6~2.0 m. Roadway deformation is serious, the section sharply shrinks, a part of the impact ground pressure causes the return airway to severely bubble and the number of impact ground pressure that causes bubbles accounts for 18.5% of the total statistics; when impact ground pressure occurs, it is accompanied by an upper helper, a lower helper and a broken bottom plate. The rocks are thrown out, equipment and pipelines near the upper and lower helpers are also ejected and overturned; the overhead support body is severely damaged, the bottom and top beams are bent and cracked, the single hydraulic pillar fails and tilts, the U-beam is bent and deformed, and the anti-impact bracket is damaged. Moreover, a part of the impact ground pressure leads to casualties.

Damage to the working face: the coal mining machine tilts towards the coal gang, the working face shelf shed tilts towards the soft gang, and some of the hydraulic brackets are damaged, accompanied by bottom drum, flaky gang and other phenomena.

4. Influence of Ground Stress on Rockburst

4.1. Model Establishment

The working section in the third level north of the 17 layers of the third four sections in the Junde coal mine is used as a simulation prototype; this area has a strike length of 920 m, an inclination length of 168 m, a mining height of 3.5 m and a mining depth of 451–551 m. The coal seam is 9.63 to 15.92 m thick and consists of bright and dark coals with an inclination of 25° to 35°, with an average of 30°. The direct top is fine sandstone with a thickness of 4–7 m, the old top is medium and fine sandstone with a thickness of 30–40 m, and the direct base is tuffaceous siltstone with a thickness of 4.5–5.5 m. The old bottom is coarse sandstone with a thickness of 48–50 m. The western windway side of the working face is the upper section of the mining void, the coal column of the section is 4–47 m wide and the eastern machine side is solid coal; in this section, the back of the northern working face is a stratified mining void, and the southern stop line is near the L1 and F7 large faults. The coal seams adjacent to this face are #3, #9, #11, #21 and #17, which are not fully mined, leaving a large number of irregular coal pillars. Due to the influence of various factors, such as ground stress, coal rock seam impact tendency, stage coal pillars, interval coal pillars left in the upper section, adjacent mining area and high intensity production, this working face has experienced several impacts of ground pressure.

The size of the model is 500 m × 300 m × 500 m, the width of the working face is 150 m, the dip angle of the rock seam is 30 degrees, the thickness of the coal seam is 4 m, the thickness of the direct top is 6 m, the thickness of the old top is 35 m and the thickness of the direct bottom is 5 m. This simulation adopts long-wall mining and roof caving management. According to the initial state close to the stratum, the initial stress caused by the dead weight of the overlying rock is simulated. The full thickness method was used for a simulation analysis, and the stress balance was calculated at each advance step before the next advance. The model is divided into 81,252 units and 93,340 nodes. According to the measured stress value at measurement point #3 of the Junde coal mine, the load value of the upper boundary was determined, and the horizontal load of the model was also determined according to the angle between the back mining face and the direction of ground stress. The bottom of the model is surface constrained. The model uses the Moore–Coulomb criterion (Figure 6), and the physical and mechanical parameters of each rock formation are shown in

Table 3. Physical and mechanical parameters of the rock masses for the numerical model.

Rock Mass	Density/kg·m−3	Bulk Modulus/GPa	Shear Modulus/GPa	Cohesion/MPa	Friction Angle/(°)	Tensile Strength/MPa
Overlying strata	2560	4.2	2.9	5.0	34	1.5
Medium-fine sandstone	2721	3.47	2.08	5.2	37.6	2.81
fine sandstone	2558	2.01	1.45	2.4	41.98	2.16
Coal	1420	0.46	0.19	0.8	20	0.01
siltstone	2630	5.0	3.8	6.0	35	2.5
Coarse sandstone	2560	4.2	2.9	5.0	34	1.5
goaf	2010	0.46	0.19	0.8	20	0.01

.

The superposition of high ground stresses and mining stresses is the root cause of many impact ground pressures, and the effect of the direction of ground stress on impact ground pressures during workface recovery is investigated. The influence of the angle between the direction of the maximum horizontal principal stress and the direction of the advance of the working face on impact ground pressure is analyzed, and the variation in energy in the area of the overrun working face is studied when the angles are 0, 20, 45, 70 and 90 degrees.

4.2. Results Analysis

According to the calculation results of each boundary load at different pinch angles, the load is applied to the model and calculated; the fish program is written to extract the energy value of the overrun area of the working face, and the energy value of the overrun area of the working face during the retrieval process is plotted as a graph. The calculated results and graphs are shown in Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11.

As shown in Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11, the area of energy concentration in the working face is biased towards the lower chute (machine path) due to the influence of the inclined rock formation. When the initial pressure is applied, the energy in the leading area of the working face is the highest. During the mining process, the energy of the solid coal seam on the lower side of the goaf along the dip direction is greater than that of the solid coal seam on the upper side. As the face advances, the energy in the solid coal seam below the mining area and in the area ahead of the face gradually increases and shifts away from the face, while the energy in the solid coal seam below the mining area is gradually greater than the energy in the area ahead of the face. This feature explains the concentration of severe impact pressure sections in the windrow (i.e., the upper chute) during face retrieval. Therefore, after the upper section is mined, the coal pillar is reserved and the mining of the next section begins; specifically, the mining of the next section is influenced by the empty area and the coal pillar of the upper section, and the mining activities of the working face and the return windway (i.e., the upper chute) are also carried out in the area of its influence and can definitely become an important cause of the occurrence of impact ground pressure.

As shown in

Table 4. Energy distribution situation of the area in front of the work face at different angles.

Angle	Peak Energy/J	Distance from Working Face/m	Energy Concentration Areas/m
0	2.7 × 105	10	10~50
20	3.3 × 105	10	10~55
45	5.9 × 105	15	10~80
70	9.5 × 105	25	15~95
90	9.3 × 105	20	15~70

, energy values extracted from the overrun area of the working face are as follows: (1) The peak energy of the coal seam at the initial incoming pressure is 2.7 × 10⁵ J when the maximum principal stress direction is at an angle of 0 degrees to the working face advance direction. At an angle of 20 degrees, the peak initial incoming pressure energy is 3.3 × 10⁵ J. At an angle of 45 degrees, the peak initial incoming pressure energy is 5.9 × 10⁵ J. At an angle of 20 degrees, the peak initial incoming pressure energy is 3.3 × 10⁵ J. At an angle of 45 degrees, the peak initial incoming pressure energy is 5.9 × 10⁵ J. At an angle of 70 degrees, the peak initial incoming pressure energy is 9.5 × 10⁵ J. At an angle of 90 degrees, the peak initial incoming pressure energy is 9.3 × 10⁵ J. The angle between the direction of the maximum horizontal principal stress and the direction of the workface advance during the recovery process has a greater influence on the energy distribution in the overfront area of the workface; a larger angle correlates to more energy gathered in the overfront area of the workface. When the angle is greater than 70 degrees, the peak energy in the overfront area of the workface reaches its maximum. The gathered energy reaches the conditions for the occurrence of impact ground pressure, which also releases the most energy and causes the most damage. (2) When the angle between the direction of the maximum principal stress and the working face advance direction is 0 degrees and 20 degrees, the peak energy gathering position at the initial incoming pressure is 10 m from the working face. At 45 degrees, the peak energy gathering position is 15 m from the working face; at 70 degrees, the peak energy gathering position is 25 m from the working face; at 90 degrees, the peak energy gathering position is 20 m from the working face. As the angle between the direction of maximum horizontal principal stress and the working face advance direction increases, the energy in the overrun area of the working face is gradually transferred to the interior.

4.3. Horizontal Principal Stress and Weight Stress

The ratio of the maximum horizontal principal stress to the self-weight stress is K₁, and the ratio of the minimum horizontal principal stress to the self-weight stress is K₂. At K₁ = 1.8, K₂ has very little effect on the energy distribution in the overrun area of the working face, its peak energy changes minimally as K₂ increases and the energy concentration area is relatively constant. As K₂ increases, the peak energy in the overrun area of the working face increases, but to a lesser extent. Compared to the peak energy at K₂ = 0.6, the peak energy increases by 1.59% at K₂ = 0.8, 4.26% at K₂ = 1.0, 6.29% at K₂ = 1.2 and 8.22% at K₂ = 1.4. The energy value of the coal seam in the overrun area of the working face is U₁, the energy value of the coal seam in the stable area of the working face is U₂, and the ratio of U₁ to U₂ is f. An area with f > 1.2 is considered the energy concentration area, and the distribution range of the energy concentration area under different K₂ conditions is calculated as shown in

Table 5. Energy distribution situation of the area in front of the work face in different K₂ conditions when K₁ is 1.8.

K2	Peak Energy/J	Increase Rate	Distance from the Working Face/m	Energy Concentration Areas/m
0.6	324,810	0	10	10~40
0.8	329,960	1.59%	10	10~40
1.0	338,660	4.26%	10	10~40
1.2	345,250	6.29%	10	10~40
1.4	351,500	8.22%	10	10~40

. From the table, K₂ has a minimal influence on the distribution range of the energy concentration area when K₁ = 1.8 (see Figure 12).

As the value of K₁ increases, the energy in the overrun area of the face and in the coal seam area below the mining area gradually increases, and the energy concentration area also increases. Figure 13 shows the graph of the energy distribution in the overrun area of the working face for different K₁ conditions at K₂ = 0.8. From the graph, the effect of the change in K₁ value on the energy distribution in the overrun area of the working face has the following characteristics: (1) The peak energy is affected in the overrun area of the working face; with the increase in K₁, the peak energy of the overrun area of the working face gradually increases. When K₁ changes from 1.8 to 2.0, the greatest change in peak energy in the overrun area of the working face is observed. When K₁ = 2.2, the peak energy in the overrun area of its working face doubles relative to the peak energy at K₁ = 1.4; moreover, the change in K₁ has a certain influence on the location where the peak energy gathers, and as K₁ increases, the peak energy tends to shift towards the interior of the coal seam. When K₁ = 2.2, the location where the peak energy appears in the overburden area of the working face shifts from 10 m to 15 m. (2) The energy concentration zone is also affected; the change in the K₁ value has a greater impact on the distribution of the energy concentration zone of the coal seam. As shown in

Table 6. Energy distribution situation of the area in front of the work face in different K₁ conditions when K₂ is 0.8.

K1	Peak Energy/J	Increase Rate	Distance from the Working Face/m	Energy Concentration Areas/m
1.4	239,950	0	10	10~40
1.6	270,170	12.59%	10	10~45
1.8	329,960	37.51%	10	10~45
2.0	442,580	84.45%	10	10~50
2.2	480,020	100.05%	15	10~55

, with an increasing K₁ value, the energy concentration area of the coal seam in front of the working face gradually increases.

4.4. Stress Control Technology

Controlling the occurrence of impact ground pressure hazards essentially avoids high stress concentrations due to the superposition of the two by controlling ground stress or mining stress to ensure that sudden impact destabilization damage does not occur in the coal–rock body.

(1) Mine ground stress measurements: Using a technically mature and highly accurate ground stress test method, the magnitude and direction of the ground stress are measured at three or more different levels in the mine by selecting measurement points.

(2) Three-dimensional geomechanical modeling: Combined with the engineering geological data of the mine, a three-dimensional geomechanical model including geological anomalies, such as faults and folds and different rock formations in the study area, is established, and the influence of these geological anomalies is fully considered when analyzing the ground stress field.

(3) Regional prevention of impact pressure: From the start of coal mining design, the requirements of ground stress are used, reasonable coal mining methods and techniques are selected, the mining sequence of coal seams is optimized and the coal seam is selected that is most conducive to impact pressure prevention and control as the first to be mined as a protective layer. After determining the order of coal seam mining, the orientation of the roadway layout for the identified coal seams is optimized such that the direction of the roadway axis is predominantly consistent with the direction of the maximum horizontal principal stress and the level of ground stress around the roadway is minimized, thereby decreasing the degree of stress concentration that can induce impact ground pressure and reducing the impact risk at the working face. For the identified working face, numerical simulation methods are combined to simulate the dynamic mining process in the coal mine, analyze the stress distribution state, evaluate the impact hazard at different locations of the working face and carry out impact ground pressure monitoring and prevention for areas with different risk levels.

5. Conclusions

(1) A numerical model was established using a typical impact ground pressure working face in the Hegang mining area as the geological proto-type; the influence of the direction of the ground stress field, the maximum horizontal principal stress and the minimum horizontal principal stress on the impact ground pressure was analyzed. As the angle between the maximum horizontal principal stress direction and the working face advance direction increases, the energy peak in the overtopping area of the working face shifts to the interior of the coal seam, the distribution range of the energy concentration area gradually increases, and the energy peak in the overtopping area of the working face reaches the maximum when the angle is 70°~90°.

(2) The ratio of the maximum horizontal principal stress to the vertical stress, K1, has a greater influence on the distribution of energy in the coal seam in the overrun area of the working face. As K1 increases, the peak energy in the overrun area of the working face increases, and the energy concentration area also increases. The ratio of minimum horizontal stress to vertical stress, K2, is positively correlated with the energy concentration in the overburden area of the working face; however, its change has less influence on the energy distribution in the overburden area of the working face. When K2 increases from 0.6 to 1.4, the peak energy of the simulated working face only increases by 8.22%, and the energy concentration area remains basically unchanged.

(3) The impact pressure area stress control technology includes four steps: ground stress measurement, 3D geomechanical modeling, 3D ground stress field inversion and impact pressure area prevention. The concept of the impact pressure area stress control technology is to cause the design and layout of the mining face to conform to the requirements of the ground stress to reduce the base stress level of the working face and to avoid the impact pressure occurrence caused by the superposition of the ground stress and the mining stress to form a high stress concentration.