A Case Study on Strong Strata Behaviors Mechanism of Mining Reserved Roadway and Its Prevention Techniques

Abstract

The underground roadway of the Buertai Coal Mine adopts the double-roadway layout. Double-roadway layout mode has a roadway that is affected by repeated mining, called reserved roadways. The reserved roadway is strongly affected by mining, and the strata behaviors appear violently. This paper studies the strata behaviors that occur in auxiliary haulage roadway (AHR) during the mining of panel 42106. By analyzing geological conditions, mining influencing factors, and roadway layout, the mechanism of strong rock behavior has been clarified. Then, based on the theoretical analysis, we put forward the treatment method for the manifestation of strong strata behaviors by using hydraulic fracturing technology to break the key stratum. In this way, the high stress of the surrounding rock can be reduced by forcing the hard roof to be broken. After the application of hydraulic fracturing technology, we monitored the deformation of roadways and the periodic weighting law of the working face. The strength of strata behaviors has significantly weakened, and the application of this technology ensures the safe production of coal mines.

1. Introduction

The degree of roof collapse after coal mining is related to the strength and structure of the overlying rock [1,2,3]. Overlying rocks with stronger rock strength and more complete structure are not easy to collapse. Therefore, the cantilever beam structure is formed at the edge of the goaf, leading to the phenomenon of strong strata behaviors in the roadway and working face [4,5]. The key stratum breakage block size and form determine the drastic degree of manifestation of strata behaviors [6,7]. Strong strata behaviors’ manifestation is often accompanied by severe deformation of the roadway [8,9], wall caving [10], rock bursts [11], and other disasters [12,13].

In the traditional practice of prevention and control of the manifestation of strong strata behaviors, disasters such as roof collapse and coal wall caving are often prevented passively by optimizing support parameters [14,15]. However, passive support cannot effectively prevent hazards, such as large deformation of the roadway and comprehensive cutting of the roof of the mining goaf [16]. These strong strata behaviors’ manifestation is caused by the rapid release of enormous potential energy stored in the rock layers during the crushing of high-strength and large overhanging overlying rocks [17,18]. Therefore, it is necessary to adopt the method of “active elimination” to reduce the potential energy stored in rock strata [19]. Hydraulic fracturing has been promoted for its effectiveness in dealing with various dynamic hazards [20,21]. Nguyen et al. [22] provide methods for hydraulic fracturing modeling, such as grid generation, execution time, convergence, and numerical integration problems, and indicate that Newton–Cotes orthogonality must be used for cohesive interface elements of secondary flow, at least for the proposed problems. Wu et al. [23] investigated the effect law of primary laminar orientation and horizontal stress difference magnitude on hydraulic fracture initiation pressure, extension pattern, hydraulic pressure–time curve, and fracture volume by large true triaxial directional hydraulic fracture test with a high-energy industrial CT scan. Cheng et al. [24] studied the propagation process of multi-well fractures through physical model experiments, simulated the dynamic fracture initiation and propagation process of cross-hole fractures during directional hydraulic fracturing, and analyzed the evolution law of pore elastic stress and pore pressure between multiple wells.

At present, the technology to solve the strata behaviors after coal mining in the Shendong Mining Area is more inclined to carry out strong support in the working face and roadway, and there is no precedent to use hydraulic fracturing technology to weaken the key layer of overlying rock in strong mining face [25,26]. Therefore, it is crucial to study the prevention and control principles of hydraulic fracture based on the analysis of the mechanism and dominance of strong strata behaviors’ manifestation. In this paper, we take panel 42107 of the Buertai Coal Mine as the engineering background. Based on the theoretical analysis, the prevention and control principles and technical solutions of hydraulic fracturing technology were studied. The practical application is remarkable.

2. Project Profile

2.1. Panel Profile

Buertai Coal Mine is the largest underground mining coal mine in China with an annual output of 20 Mt. The mine is located in the central part of Dongsheng Coalfield in Ordos City, southern Inner Mongolia Autonomous Region. The mining roadway in Buertai Coal Mine adopts the double-roadway layout mode, i.e., two mining roadways are excavated at the same time, one of which is affected by repeated mining and needs to serve two working faces (called reserved roadways). The geographic location, related photos, and mining layout of the Buertai Coal Mine are shown in Figure 1.

Comprehensive mechanized top-coal caving is adopted in panel 42107 of coal seam 4-2 in Buertai Coal Mine. The length of the working face is 300 m, the average depth of the coal seam is 457 m, the thickness of the coal seam is 7.6 m, the dip angle of the coal seam is 1~3°, and the length of the panel is 4808 m. The average distance between coal seam 4-2 and overlying coal seam 2-2 is 75 m. The upper part of the coal seam 4-2 roadway is coal seam 2-2 goaf. There are two key strata in the overlying strata between coal seam 4-2 and coal seam 2-2, which are fine sandstone subcritical stratum 1 with a thickness of 8.6 m and siltstone subcritical stratum 2 with a thickness of 22.6 m. Rock parameters of the roof and floor of 4-2 coal seam are given in

Table 1. Geological parameters of coal and rock.

Lithology	Burial Depth/m	Thickness/m	Cohesion/MPa	Compressive Strength/MPa	Notes
Sandy sandstone	390.9	19.2	6.6	59
Clip coal	391.6	0.7	1.9	12
Sandy sandstone	406.8	15.2	5.4	43
Siltstone	428.4	22.6	6.6	54	Subcritical stratum 2
Sandy sandstone	436.1	5.7	4.2	39
Fine Sandstone	444.7	8.6	6.2	52	Subcritical stratum 1
Sandy sandstone	451.0	6.4	3.6	34
4-2 Coal seam	457.4	7.6	2.5	17
Sandy sandstone	465.0	6.1	3.5	33

.

2.2. Strong Strata Behaviors Survey

Panel 42106 of coal seam 4-2 is adjacent to panel 42107, as shown in Figure 1. During the mining of panel 42106, the leading area of 42106AHR (reserved roadway of panel 42105) appeared to manifest strong strata behaviors, and the influence range could reach 100 m ahead. Strong strata behaviors’ manifestation seriously affected the normal mining of the working face, as shown in Figure 2.

The working resistance of the hydraulic bracket was greater than 23,000 kN, and the coal wall flakes were serious during periodic weighting of the working face. The floor vibrated and caused the coal cutter to pop up and led to the overhead hydraulic bracket being buried, as shown in Figure 2a. The roadway in front of the working face shows the heaving floor and hydraulic bracket tilt, as shown in Figure 2b. The roadway adjacent to the goaf (coal pillar 25 m) was severely deformed within 40 m in front of the working face, with ribs bulging greater than 2 m and the heaving floor greater than 1.5 m, as shown in Figure 2c. The anchor net broke, resulting in the nets bag and leakage of gangue, as shown in Figure 2d.

3. Mechanism and Treatment Principle of Strong Strata Behaviors

3.1. Main Controlling Factors of Strong Strata Behaviors

(1) Geological conditions

The roof of coal seams 4-2 in Burtai Coal Mine is thicker and stronger, with strong self-sustaining ability. The upper part of the goaf is prone to forming a cantilever roof, which increases the fracturing step of the basic roof. This leads to an increase in the elastic energy accumulated before the roof breaks. In the process of a hard roof breaking or sliding, the elastic energy stored in the roof is prone to suddenly release, inducing strong strata behaviors disaster [27]. When the buried depth of the coal seam is greater than 350 m, the frequency and intensity of strong strata behaviors will gradually increase. The buried depth of coal seam 4-2 is 457 m, which is the internal factor of strong strata behaviors’ manifestation.

(2) Influence of mining stress

Under the influence of repeated mining, the reserved roadway is located in an extremely complex superimposed stress field composed of original rock stress, abutment pressure, mining dynamic load, and expansion pressure in the plastic zone [28]. When the roof periodically intensifies, the position of the fracture line will affect the manifestation of strata behaviors. If the fracture line is located in the goaf, a cantilever beam is formed at the edge of the pillar. The pressure from the insufficient span of the overlying strata in the goaf is transmitted to the coal pillar through a cantilever beam. If the fracture line is located inside the coal pillar, the rotational deformation of the basic roof on the coal pillar will transmit several times the initial stress to the coal pillar. All these are not conducive to the stability and maintenance of the roadway.

When 42106AHR (reserved roadway) is subjected to secondary mining, it is 3~5 times of original rock stress under the influence of advanced abutment pressure, abutment pressure of side mined-out area, and concentrated stress of residual coal pillars in coal seam 2-2. Under the disturbance of multiple stress superposition, it will cause rapid deformation and failure of the mining roadway, which can easily lead to the occurrence of strong strata behaviors’ manifestation [29,30].

(3) The impact of mining spatial relationship (coal pillar in overlying goaf)

The goaf of coal seam 2-2 is located 75~80 m above panel 42106 of coal seam 4-2. The siltstone between the two coal seams is thick and hard. The horizontal projection distance between 42106AHR and the overlying coal pillar (the coal pillar between panel 22104 and panel 22105) is 43.5 m. The fault protection pillar of coal seam 2-2 also has an important influence on the stress of the lower strata [5]. Stress concentration occurs in a certain range below the coal pillar, and the difference within stress distribution is greater as it gets closer to the edge of the pillar. The spatial location relationship of mining is shown in Figure 3.

3.2. Technical Principle of Disaster Prevention and Control of Strong Strata Behaviors

The integrated hydraulic fracturing of the coal seam roof is similar to the artificial creation of a “relief layer” in advance. After fracturing above the working face, the overlying strata produce fractures within the fracturing range. These fractures destroy the previously intact overburden structure within the fracture range. When the overburden load acts on the fractured area, the fractured rock mass is compressed to fill the fracture, which reduces the elastic deformation of the fractured rock mass. Therefore, after fracturing, the support of the upper roof is reduced due to the deformation of rock mass within the fracturing range, as shown in Figure 4. Pan et al. have carried out a detailed theoretical derivation according to the ground prefracturing coal seam roof, and the research results can be a certain reference value for the paper [31].

In order to obtain the stress control effect of hydraulic fracturing, a numerical mining model (FLAC 3D) was established, and the calculation results are shown in Figure 5. When hydraulic fracturing technology is adopted, the goaf fully collapses, and the caving line overlaps with the pressure relief line, which is distributed along the hydraulic fracturing hole. The maximum vertical stress in the area affected by mining is 56.15 MPa. When hydraulic fracturing is not implemented, we set a 20 m overburden overhanging area at the goaf boundary. The maximum vertical stress in the area affected by mining is 64.09 MPa. After hydraulic fracturing, the peak vertical stress was reduced by 7.94 Pa. The vertical stress in the coal pillar decreases obviously.

Compared with before fracturing, artificial cracking caused by a thick hard roof on the coal seam of the working face roof can make long beams become short beams and large pieces become smaller. Thus, the hard roof no longer has the cantilever function. The impact risk of the dynamic load caused by roof fracture with an increased overhanging area is reduced. At the same time, the hard roof can collapse and fill the goaf in time after fracturing, which reduces the high stress concentration in the coal pillar and also reduces the concentrated dynamic load caused by the roof pressure. It can be seen that hydraulic fracturing can significantly weaken the risk of a dynamic disaster of strong strata behaviors.

3.3. Technical Basis for Controlling Strong Strata Behaviors

To avoid the occurrence of strong strata behaviors’ manifestation in 42107AHR and 42107 working face during the mining of panel 42107, we decided to adopt hydraulic fracturing technology to relieve concentrated stress according to the above analysis. According to the relevant literature and our rich engineering practice experience, hydraulic fracturing roof strata to reduce the strong mining pressure in stope should be based on the key stratum theory of rock strata control. The working condition of the project site is very complicated (the overlying coal seam 2-2 is mined out, but there is 300~360 m fault protection coal pillar locally, the coal pillar is 20 m in the section of the coal working face of 2-2 coal, and the overlying strata are also affected by the fault, all of which lead to a complex overlying rock stress environment. In addition, it is difficult to establish a realistic theoretical model due to the large number, thickness variation, and strength difference of rock strata), as shown in Figure 3. Therefore, we only establish a simplified theoretical model under ideal conditions for schematic analysis. There are some differences between the calculation results of this model and the actual working conditions, but it can be used for reference.

The key strata control the deformation movement of local and whole strata, and the mechanical properties of the key stratum are important factors affecting the periodic breaking step. The stress state of the key stratum before breaking and turning was simplified into a pure bending fracture model of the beam structure, as shown in Figure 6. According to Figure 4, the relationship between the tensile strength of the key stratum and its periodic breaking step can be qualitatively analyzed.

According to the mechanics of materials, the maximum normal stress of a pure bending beam occurs at the furthest distance from the neutral axis. The relationship between the maximum tensile stress and the maximum bending moment can be obtained as shown in Equation (1).

$${\sigma }_{\max }=M_{\max }h/\left(2I_{\mathrm{z}}\right)$$

(1)

where σ_max is the maximum tensile stress (MPa), M_max is the bending moment (kN·m), Iz is the moment of inertia for the Z-axis in the rectangular section (m4), and h is the thickness of the key stratum (m).

$$M_{\max }=-q{y}_{\max }^2/2$$

(2)

where q is the interface compressive stress on the key stratum (MPa), and y_max is the maximum roof spacing (m).

$$I_{\mathrm{z}}=b{h}^3/12$$

(3)

where b is the width of the rectangle interface (m). Take b = 1 m, substitute Equations (2) and (3) into Equation (1), and the relationship between the periodic breaking step distance of the key stratum and its tensile strength can be obtained. If σ_max = RT, then:

$$y_{\max }=h\sqrt{\frac{RT}{3q}}$$

(4)

where RT is the tensile strength of the key stratum (MPa).

According to Equation (4), the larger the tensile strength of the key stratum is, the larger the maximum suspended roof distance is. In the process of panel propulsion, too large overhead distance will make the hydraulic bracket and coal body bear more load. This will lead to working face pressure frame, wall caving, and roadway deformation. If the strength of the key stratum can be weakened, the strong strata behaviors can be controlled to a certain extent. Therefore, the selection of a hydraulic fracturing horizon is the key to controlling strong strata behaviors.

In the mining process of panel 42107, subcritical stratum 1 and 2 will form the fracture structure of “cantilever beam + masonry beam”, which will affect the stope strata behaviors. According to the geological parameters of the overlying strata of coal seam 4-2 combined with Equation (4), it can be calculated that the periodic weighting distance of subcritical stratum 1 is 36.75 m, and that of subcritical stratum 2 is 53.25 m. Subcritical stratum 2 is the key to influencing the stope strata behaviors.

4. Hydraulic Fracturing Technology and Scheme

4.1. Design Basis of Hydraulic Fracturing Program

Many scholars have studied the law of strata behaviors in the process of advancing the working face [32,33,34,35]. They obtained the “square position” characteristic of the working face [36]. That is, when the advancing length of the working face is an integer multiple of the width of the working face, the strata behaviors will be more severe [37,38].

From the perspective of “square position”, we should design a long fracturing borehole when the panel advances to integral multiples of the width of the working face. The design can promote the full collapse of the hard roof and weaken the “square position”. The vertical height of the long fracturing borehole should be close to the upper boundary of the siltstone (subcritical stratum 2) to ensure its collapse. The horizontal width of the long fracturing borehole should ensure that the fracture range covers the width of the working face. The short fracturing borehole is designed to prevent cantilever beams from forming in the hard rock above the lateral coal pillar. The short fracturing borehole shall be at least 10 m within the siltstone bed. The horizontal length of the borehole is determined according to the depth of the borehole and the construction angle of the drilling equipment.

The spacing of the fracturing borehole is set according to the effective fracturing radius of the borehole [39,40]. The fracturing radius of the borehole is 3.2~4.3 M when the water injection pressure is 12~16 MPa under the condition of crustal stress and surrounding rock strength. Therefore, 1/3 (7 m) of the average periodic weighting was selected as the fracturing borehole spacing. In this way, sufficient fracturing can ensure that the edge of the goaf caved in time after mining, so as to reduce the stress concentration.

4.2. Parameter Design of Implementation Scheme

Based on the above analysis, hydraulic fracturing drilling holes were arranged to the roof through 42107AHR, 42107BTR, and 42108 AHR. The vertical depths of the long and short holes are 38 m and 30 m, respectively. The drilling hole opening is about 2.2 m away from the floor.

Single-hole multiple fracturing technology is adopted, and the part of drilling in the coal seam is not fractured. The fracturing boreholes are arranged before mining, and the fracturing operation is carried out ahead of the working face in the process of mining. The sealing pressure of hydraulic fracturing is 12~16 MPa. The hydraulic fracturing roof fracturing process is divided into three steps, that is, drilling, sealing, and fracturing, respectively.

The hydraulic fracture drill holes were drilled by ZDY1200S fully hydraulic drilling rig along the right rib of 42107AHR (Fracturing drilling A), the left rib of 42107BTR (Fracturing drilling C), and 42108AHR (Fracturing drilling B) to the roof of panel 42107, respectively, with a diameter of 56 mm, as shown in Figure 7.

In 42107AHR, a total of 18 hydraulic fracturing drill holes with a length of 150 m are arranged. The six drill holes in the first group are arranged within the range of 80~140 m from the open-off cut of panel 42107, as shown in Figure 6a. In 42108AHR, a total of 15 hydraulic fracturing drill holes with a length of 150 m are arranged. The three drill holes in the first group are arranged within the range of 120~140 m from the open-off cut of 42107, as shown in Figure 6b.

The second group of three drill holes is 300 m away from the open-off cut, and then each group is 300 m away from the other. The spacing of the drill holes in the group is 7 m, the drilling angle is 17°, and the vertical height of the drill holes is 38 m. In 42107BTR, a total of 65 hydraulic fracturing drill holes with a length of 43 m are arranged in the range of 560~1080 m away from the open-off cut, the drilling angle is 51°, and the vertical height of the drill holes is 30 m, as shown in Figure 8c. The area where the borehole is located in the coal seam will not be fractured (red section). To prevent the loose gangue from leaking in front of the hydraulic bracket, the yellow section of the borehole will not be fractured. The green part is fractured every 3 m, and the fracturing duration is 10–15 min.

5. On-Site Industrial Test Monitoring

5.1. Monitoring of Roadway Deformation

After hydraulic fracturing, the integrity of 42107AHR at the front of the working face has been significantly improved compared to 42106AHR, as shown in Figure 9. To monitor the stability control effect of the roadway after hydraulic fracturing, two deep base point monitoring stations are set up at 42107AHR (100 m, 125 m, and 150 m in front of the working face). Deep base point separators are installed in the roof, left rib (on the side of the coal pillar), and right rib (on the side of the working face) of each station, with base point installation depths of 3 m and 8 m. The measured value at No. 3 is the largest, and its monitoring data are plotted as a curve, as shown in Figure 10.

From Figure 10, t it can be seen that the measured deformation of the left rib is greater than that of the right rib. During repeated mining of 42107AHR, significant deformation began to occur approximately 100 m in front of the working face. The closer the working face is to the monitoring point, the more deformation occurs, and the deformation rate is faster and faster. When the measuring point is 40 m away from the working face, the deformation of the left rib at the 8 m base point of the deep surrounding rock is about 200 mm, and the deformation of the right rib is about 180 mm. The roadway 40 m in front of the working face is reinforced with an advanced hydraulic bracket, which damages the roof monitoring points. The rib monitoring points are supported by hydraulic support ribs to prevent continuous deformation of the ribs. In general, compared to the large deformation of surrounding rock before hydraulic fracturing, the variation of roadway has been effectively controlled at the present stage. The failure depth of surrounding rock is mainly concentrated within 3 m of the surrounding rock, accounting for more than 80% of the total deformation.

5.2. Monitoring of Periodic Weighting Parameters

To monitor the control effect of hydraulic fracturing on the stope strata behaviors, we analyzed the hydraulic bracket data in the mining process of panel 42106 and panel 42107. The monitoring data hydraulic bracket is shown in Figure 11 and Figure 12.

From Figure 11 and Figure 12, the following conclusions can be drawn. The average step distance of periodic weighting of panel 42106 is approximately 21.6 m, with an average duration of 7.5 m. The normal load of the hydraulic bracket (LHS) is about 289 bar when the working face is advancing normally, and the average final working resistance of the hydraulic bracket (WRHB) is 15,207 kN. During the periodic weighting process, the average LHS is approximately 395 bar, and the average final WRHB is 22,535 kN. The average step distance of periodic weighting of panel 42107 is about 16.9 m, with an average duration of 4.9 m. The normal LHS is about 265 bar when the working face is advancing normally, and the average final WRHB is 15,146 kN. The average LHS is about 354 bar during the periodic weighting, and the average final WRHB is 19,599 kN, which is less than the rated resistance of the hydraulic bracket of 21,000 kN.

Based on the above analysis, it can be inferred that after implementing hydraulic fracturing measures in panel 42107, the roof fracture line is close to the coal pillar. Hydraulic fracturing can achieve stress unloading on both sides of the working face, promoting the transfer of high stress in the surrounding rock of the roadway. The use of hydraulic fracturing technology promotes timely collapse of the roof, avoids high concentration of stress, and effectively controls the behavior of strong rock layers in the working face.

6. Conclusions

(1) The characteristics of strong strata behaviors of AHR were obtained by field observation. The specific manifestations of the strong strata behaviors include a large mining impact range, large roadway deformation, severe anchor cable damage, large inclination angle of roadway hydraulic support, and early warning of hydraulic support resistance in the working face.

(2) Based on various factors, the mechanism of strong strata behaviors in the roadway of the Burtai 4-2 coal seam has been obtained. The thicker and harder the roof strata, the greater the elastic energy accumulated before the roof fractures. The reserved roadway was greatly affected by the repeated mining of the working face when the double roadway was arranged, and the stress superposition was obvious. The coal pillar in the goaf of the overlying 2-2 coal seam promotes stress concentration in the 4-2 coal seam, resulting in more complex regional stress field conditions.

(3) Based on the theoretical analysis, the implementation scope of hydraulic fracturing was determined. Combined with the site situation, the pressure relief control technical scheme of hydraulic fracturing in the 42107 working face is proposed. After adopting a reasonable hydraulic fracturing scheme, the average step distance and load of periodic weighting were significantly reduced, the deformation and failure of the roadway were weakened, and high stress transmission in the working face area was achieved.