Study on Mine Pressure Control by Ground Fracturing High-level Hard Rock Strata

During extra-thick coal seam mining, the high-level thick and hard strata are the main reason for the presence of a strong ground pressure in the working face; however, there is no active and effective control technology for high-level hard strata. This paper proposes the concept of ground fracturing hard roofs, and the physical simulation was used to study the control effect of ground fracturing on the strata breaking structure and energy release. The results showed that the ground fracturing changed the structural characteristics of the strata and reduced the energy release intensity and overburden movement spatial extent, which had a signicant control effect on the ground pressure. The Datong mining area was selected as the engineering background, a ground horizontal well fracturing engineering test was conducted on site, and a 20-m-thick hard rock layer, which was 110 m vertically away from the coal seam, was determined as the fracturing target layer. On-site microseismic monitoring showed that the crack propagation length was up to 216 m, and the height was up to 50 m. On-site mine pressure monitoring showed that the roadway deformation was reduced to 100 mm, the periodic weighting characteristics of the hydraulic supports were not obvious, the ground pressure in the working face was signicantly controlled, and the ground fracturing was successful. Ground fracturing changed the occurrence characteristics of the high-level hard strata, which is benecial to ameliorate the stress environment of a working face and provide a new approach of hard rock control. is mainly used. Owing

The #3-5 extra-thick coal seam is mainly mined in the Tashan coal mine of the Datong mining area. The coal seam thickness is 14 m to 20 m, and the method of top-coal caving mining is adopted. The coal seam is buried at a depth of 400-800 m, and there are multiple hard roofs in the overburden with a compressive strength of 60-120 MPa. In the mining process of the working face, a strong ground pressure occurs periodically, and the characteristics are mainly re ected in the following aspects: (1) The rst roof weighting pace of the working face is 46 m to 55 m. The periodic weighting of the working face is re ected in the aspects of a short and long intension period and an extremely strong intension period.
(2) Short intension period: the roof weighting step is about 12 m to 26 m, which is mainly manifested in the increase of 20 to 30 supports resistance of the working face. There is an obvious coal wall rib spalling, and the top coal can be easily caved.
(3) Long intension period: the roof weighting step is about 30 m to 52 m, the resistance of more than 40 supports of the working face increases sharply, and the safety valves are opened substantially. The coal wall rib spalling is signi cant, and the top coal is liable to leakage. The roadway deformation exceeds 2000 mm.
(4) Extremely strong intension period: There is no obvious weighting step, and the occurrence frequency is low. During the period, the supports on the working face are frequently crushed, the minimum size of the roadway is only 1000×1000mm, and the advanced single hydraulic props are severely bended and split.
To explore the in uence of hard roofs failure and instability at different levels on the pressure appearance on a stope, a research study have been performed based on the method of eld measurement (Lan et al. 2018). In this case, the thickness of the coal seam was 19 m, and the method of top-coal caving mining was adopted. The strata movement measurement points were arranged in the key strata at different levels (22,51, and 104 m away from the coal seam). The thicknesses of the three key strata were 12, 9.8, and 23 m from the bottom up. Concurrently, the resistance characteristics of the working face supports were recorded in real time. The monitoring results showed that the support resistance in the working face increased with the breakage of two key strata that were 22 and 51 m away from the coal seam at a low level, and the dynamic load coe cient of the supports was 1.15 and 1.34, respectively. The pressure duration was 7 and 16 h, respectively, and there were no obvious indications of a strong ground pressure in the working face. When a 23-m-thick key layer (which was 104 m from the coal seam) broke, the #35-95 supports in the working face were crushed, and the dynamic load coe cient of the support reached 1.54. The pressure duration reached 43 h, and the ratio of the distance between the highest key layer and coal seam to the coal seam thickness was 5.47. It can be seen that in the mining of extra-thick coal seam, when there exists a thick and hard roof at a high level, its failure and instability will easily result in a strong impact strength, which is the main factor inducing the strong ground pressure in the working face, as shown in Fig. 1.

Technology of ground hydraulic fracturing
The presence of hard roofs at a high level is the main factor that causes the strong ground pressure in the working face. Furthermore, a large breaking step and an entire block rotation are the internal inducements of the strong ground pressure. Therefore, using a reasonable technology to weaken the hard strata in far elds and changing their physical and mechanical properties, structural occurrence, and fracture characteristics, thus reducing the breaking strength of the hard roofs and modifying the stress concentration in the stope, is an effective approach to achieve strong ground pressure prevention.
After years of research and practice, a set of hard roof control technologies based on the underground hydraulic fracturing and blasting technologies has been formed Ge et al. 2015;Yu and Duan, 2014;, which can effectively control the hard roofs at a low level from underground. However, owing to the limitations of the underground hydraulic fracturing equipment, technology, and construction conditions, it is generally only possible to weaken the hard roofs within 50 m away from the coal seam, and it is impossible to control those at a high level.
The ground drilling fracturing technology is widely used in the oil and gas exploitation process. The principle of the above technology is to manufacture cracks of a certain geometric size and conductivity arti cially by a hydraulic pump. Therefore, referring to the ground fracturing technology, the team of the author innovatively proposed the concept of ground fracturing hard roofs in a coal mining area. Moreover, it used ground fracturing equipment to weaken high-level hard rock formation to reduce the integrity and strength of the hard roofs and achieve the purpose of ground pressure control (Yu et al. 2019;Gao 2018). The schematic is shown in Fig. 2. 3 Simulation study on ground pressure control by ground fracturing In the process of ground fracturing hard roofs, in the fracturing, water is mainly used. The direction of the fracture propagation is directly related to the distribution of the three-direction stress eld in rock deformation. After fracturing, the fracture distribution in the strata can be divided into three situations: (1) the formation of horizontal fracture, such that the occurrence of fracture reduces the effective thickness of the strata and the complete rock layer is divided into two or several layers; (2) the formation of vertical fracture, which cuts off the thick and hard roof into two independent structures; (3) the formation of numerous disordered fractures, which destroy the integrity of the thick and hard rock strata, dividing them into several sections. In this paper, owing to the space limitation, only the in uence of vertical fracture occurrence on the characteristics of the overburden structure and ground pressure is discussed, and the occurrence of other fractures is further studied. By performing a physical similarity simulation, a vertical fracture is arti cially generated in the high-level hard roof of the model. By comparison and analysis with a non-fracturing model, the structural characteristics of the overburden and variation rule of the ground pressure are studied.

Physical Model
Taking the 8101 working face in the Tashan coal mine as the reference to build the physical model, and two models are made in total. Model 1 is built according to the geological conditions, and model 2 is based on model 1 with a vertical fracture arti cially created in the high-level hard roofs. Carboniferous #3-5 coal seam is mainly mined in the 8101 working face, and the average thickness, buried depth, and inclination angle of the coal seam are 20 m, 470 m, and 1-3°, respectively. The working face length is 230 m, and the continuous mining length is about 1500 m. The coal seam is covered with multiple layers of hard roofs. The geometric size of the physical similarity model is 2.5 × 0.2 × 1.47 m (length × width × height). The similarity ratio of the model is 150:1, and the simulated laying height is 220 m.
Sand, calcium carbonate, and gypsum were used in the laboratory for modeling, and the designed bulk density ratio is 1.667:1. The motion time similarity ratio, stress similarity ratio, and dynamic similarity ratio are calculated to be 12.25:1, 250:1 and 5.63×10 6 :1, respectively. The model is excavated continuously to the boundary every 30 min, and 5 cm is excavated each time. The thickness of the overlying unarranged rock layer is 272.65 m, and the compensation stress added to the upper part of the model is calculated to be 0.027265 MPa. The matching parameters of the rock layers obtained in the model are listed in Table 1. In Table 1 ,No.32,No.27,No.22,No.16,and No.9 rock formations are the key strata, which are recorded as KS1, KS2, KS3, KS4, and KS5 respectively. In this experiment, KS5 at a high level is mainly studied. In model 2, a crack is set arti cially in KS5 at a position of 90 m horizontally from an open-off cut by a thin iron piece to reduce the integrity of the KS5. A prefabricated crack is formed in the model by placing a 0.2-mm-thick piece of iron in the fractured target layer. Because the model is 20 cm wide, two 10 × 10 cm thin iron sheets are used. The model and fracture setting scheme are shown in Fig. 3 (a). Fig. 3(b) shows the monitoring scheme of the overburden displacement. The Vic-2D non-contact strain monitoring system is used, black speckles are sprayed on the surface of the model, and the displacement of the overlying strata is obtained by monitoring the displacement of each scattered black point.

Result Analysis
(1) Characteristics of overburden structure after ground fracturing The structural characteristics of the high-level KS4 and KS5 are analyzed. The structural characteristics of KS4 and KS5 before and after KS5 is fractured are shown in Fig. 4.
As shown in Figures 4(a) and (c), the high-level KS5 is not fractured, the breaking span of KS4 and KS5 are 170m and 175m respectively. The breakage of KS4 and KS5 causes an unstable and a synchronous rotation of the lower strata, and the range of the motion space in the overburden is wide. The vertical displacement of the cantilever beam structure close to the working face caused by the breakage of KS5 and KS4 is 0.48 m and 2.5 m, respectively.
After KS5 is fractured, the rst breaking span of the high-level KS4 is reduced to 142 m, of which the length of the broken block A is only 47 m, and the displacement of the immediate roof at the working surface is 0.45 m, as shown in Fig. 4(b). Compared to the KS5 unfractured model, the KS4 breaking span is reduced after KS5 is fractured; this is mainly because the bearing capacity of KS5 is reduced after fracturing. Moreover, the weight of KS5 and its overlying strata is transferred to KS4, increasing the load above KS4, thus causing the breaking span to decrease. It also can be seen from Fig. 4(b) that after KS4 is broken, KS5 also undergoes a large degree of de ection and rotation by the fracturing action, but it does not rotate synchronously with KS4 and still maintains a certain structural stability.
As the working face continues mining, KS5 continues to rotate. Because of its relatively slow rotation, its breaking and unstable energy release intensities are low and have no impact on the underlying strata. The underlying key layer structure can be kept stable. The rotation of the KS5 breaking block does not generate a ground pressure, as shown in Fig. 4(d).
(2) Strata movement control effect of ground fracturing Taking KS4 and KS5 as the research objects, their vertical displacement changes are obtained when they are broken under the conditions of occurrence and non-occurrence of KS5 fractured, as shown in Fig. 5. The abscissa is the position of the measurement point along the model mining direction.
As shown in Fig. 5(a), in the absence of KS5 fractured, when KS4 breaks, the maximum vertical displacement of KS5 is 1.2 m, and the KS5 structure remains stable. After the KS5 fractured, when KS4 breaks, the KS5 structural integrity is reduced, being affected by the weakening of the ground fracturing, and a large bending subsidence of 1.73 m occurs in KS5. However, the KS5 structure does not sink with KS4 synchronously, and it can maintain the structural stability and a certain separation space with the underlying rock formation. As the working face continues mining, KS5 slowly settles to stability.
It can be seen from Fig. 5(b) that when KS5 is not subjected to ground fracturing, its integrity is strong, and the breaking span is 175 m. The large rotary movement and high strength during KS5 breakage causes a synchronous movement of the underlying strata. Taking KS4 as an example, the KS5 breakage causes the vertical displacement of KS4 to reach 1.68 m. When KS5 is fractured, the rotation of KS5 is no longer a transient, high-speed, high-intensity movement process but a slow de ection and sinking one. The strength and energy release during the KS5 breakage is low, which has a weak impact on the underlying rock formation. The rotation of KS5 after the ground fractured will barely cause a synchronous movement of the underlying rock formation, the vertical displacement of KS4 hardly changes, and the KS4 structure remains stable. It can be seen that the ground fracturing signi cantly reduces the movement strength of the hard strata.
In addition, after KS5 is fractured, owing to its reduced bearing capacity, the part weight of the overburden is carried by KS4, which increases the load strength above KS4, causing the KS4 breaking span to decrease. It is found in the experiment that the KS4 breaking span reduces to 142 m, which is reduced by 28 m compared to the model of KS5 unfractured. Moreover, the degree of the strong mine pressure of KS4 breaking is reduced to a certain extent.
The vertical displacement of the immediate roof next to the supports during the breakage of KS4 and KS5 in the absence and presence of KS5 fractured is shown in Fig. 6. It can be seen from Fig. 6 that when the ground fracturing of high-level KS5 is not implemented, the breakage of KS4 results in a maximum sinking of 2.5 m in the immediate roof, which will probably cause the supports to crash on the working face. After KS5 is fractured from the ground, the vertical displacement of the immediate roof caused by the breakage of KS4 is signi cantly reduced to 0.45 m, and the KS5 rotation process barely causes the motion of the underlying strata. The ground fracturing of the high-level hard strata has a signi cant effect on reducing the strong mine pressure in the working face. (

3) Control effects of overburden movement space
The experimental research shows that the ground fracturing of high-level KS5 is bene cial for reducing the breaking span of KS5 and the low key strata, thereby reducing the in uence range during the breakage of high-level thick and hard rock strata and avoiding the formation of excessive structures in the overlying large space. The degree of the strong ground pressure in the working face is reduced, and the spatial structure of the overlying rock before and after KS5 fractured are shown in Fig. 7.
It can be seen from Figures 7(a) and (c) that when KS5 is not fractured, the large structural dimensions in the overburden after high-level key layers KS4 and KS5 break are 210 × 165 m and 225 × 165 m respectively, and after KS5 is fractured, the structural size is reduced to 210 × 135 m and 225 × 120m, respectively. The ground fracturing action reduces the spatial in uence range of the high-level hard strata breakage. The area of the overlying strata that acts on the working face supports is denoted as S. According to the statistics, the area, S, after the breakage of KS4 and KS5 before and after KS5 is fractured is given in Table 2. It is easy to see from Table 2 that the ground fracturing causes the area, S, to decrease in KS4 and KS5 breakage, and the area, S, of the KS4 breakage is reduced to 81.3%. The ground fracturing has the most signi cant control effect on the mine pressure caused by KS5, and the area, S, of KS5 breakage is signi cantly reduced to 16.01%, which highly alleviates the strength of the mining pressure.
In summary, it can be seen that after KS5 is ground fractured, the broken structure of KS5 is changed, and the stable rock stratum structure of KS5 is no longer present. The rotation process of the KS5 structure is sluggish, which signi cantly reducing the pressure effect of KS5. In addition, after KS5 is fractured, the load strength of the underlying strata is increased, resulting in a decrease in the breaking span. However, KS5 does not rotate synchronously with the underlying strata, and so, it does not increase the pressure strength of the underlying strata. Contrastingly, the pressure strength of the underlying hard strata A horizontal well can realize multistage fracturing, for which the control range is wide and the fracturing effect is good. In view of the need to achieve highe ciency control of high-level thick and hard rock layers, a horizontal well is adopted in this experiment.

Fracturing Process and Equipment
The The area around the target fractured strata was perforated by drilling numerous small holes on the walls of the fracturing well. These allow the fracturing uid to expand, thereby achieving the fracturing purpose. The target strata were designed to be fractured by three-stages in the horizontal section. To ensure the fracturing effect, the perforation density in the fractured zone is designed to be as high as 16 per meter. When the ground fracturing is performed, ve pump trucks, one sand mixer, one instrument vehicle, ve liquid tank trucks, and one sand tank truck are used. The reserve fracturing water is 2000 m 3 . The ground fracturing site construction is shown in Fig. 9.
The maximum bursting pressure in the rst stage is 12.46 MPa, and the total liquid volume is 470.9 m 3 . The maximum bursting pressure in the second stage is 10MPa, and the total liquid is 549 m 3 . The maximum bursting pressure in the third stage is 10.33 MPa, and the total liquid is 576.9 m 3 .

Crack propagation monitoring
The detectors were placed on the ground to monitor the microseismic wave signal during the fracturing process to describe the law of hydraulic crack propagation. Taking the fracturing well as the center, the detectors were arranged around the fracturing well, and the arrangement is shown in Fig. 10.
The positions of the detectors are listed in Table 3. The detectors were accurately positioned with high-precision GPS (GPS accuracy is not more than 3.0 m), and the ground depth of the detectors was not less than 30 cm. After the rst stage of fracturing, the crack propagation pattern was monitored, as shown in Fig. 11(a). As can be seen from Fig. 11(a), the crack propagation direction is NE90°, and the expansion lengths are 134 m and 62 m, respectively. After the second stage of fracturing, the crack propagation was monitored, as shown in Fig. 11(b). It can be seen from Fig. 11(b) that the crack propagation direction is NE55°, the crack spreads in two opposite directions, and the expansion lengths are 98 m and 118 m, respectively. The total length of the crack is 216 m. After the third stage of fracturing, the crack propagation was monitored, as shown in Fig. 11(c). It can be seen from the gure that the crack propagation direction is NE50°, and the cracks extend in two opposite directions. The expansion lengths are 118 m and 98 m, respectively, and the total crack length is 216 m.
The morphological characteristics of the cracks after fracturing thrice in the horizontal section are listed in Table 4. The fracturing crack extending in the horizontal direction is 216 m, which is longer than the working face length. The expansion direction is approximately perpendicular to the horizontal section of the fracturing well. The crack extending in the vertical direction is 50 m, and the crack expansion range is wide, completely covering the thickness range of the fractured target layer. To monitor the deformation of the working face after the ground fracturing in the range of crack expansion, the hydraulic supports in the middle of the working face were selected to note the resistance of the working face in the normal mining section and fractured crack extension range. The roadway deformation monitoring points were also arranged at intervals of 10 m in the normal mining section and fractured crack extension area in the roadway, and were recorded as #1-#6, respectively, to monitor the deformation of the roadway, as shown in Fig. 12.
The roadway deformation and support resistance before and after the working face enters the ground fracturing control zone are shown in Fig. 13. The deformation characteristics of the roadway ahead of the working face at 20 m of each measuring point are shown in Fig. 13(a). The roadway deformation at #1 measuring point and #3 measuring point is large, which is mainly owing to the large mining thickness of the coal seam. The roadway deformation is signi cant in the advanced 20 m of the working face, and the extent of the roof-to-oor and two-side convergence is more than 1500mm. The bending of the single pillar is prominent. When the working face enters the ground fracturing control zone, the roadway deformation in advance is highly controlled. Taking #5 measuring point as an example, the roadway deformation is less than 300 mm, the single props do not exhibit any bending phenomenon, and the roadway maintenance is in excellent condition.
The characteristics of the support resistance before and after the working face enters the ground fracturing control zone are shown in Fig. 13(b). It can be clearly seen from the gure that the periodic roof weighting pace of the working face is in the range from 35 m to 55 m before the working face enters the ground fracturing control zone. Affected by the breakage and instability of the hard roofs, the compressive strength is up to 43 MPa, which has an obvious in uence on the working face. When the working face is mined into the ground fracturing control zone, there are no obvious weighting step characteristics.
The maximum strength of the supports is reduced to 30 MPa.
In summary, after the ground fracturing of the high-level hard and thick strata, the integrity and structural characteristics of the hard strata are destroyed, which reduces the energy intensity of the strata breakage. The surrounding rock failure and compressive strength of the supports are signi cantly controlled. It can be seen that the ground fracturing has a positive effect on ameliorating the stress environment of the stope, controlling the deformation of the surrounding rock, and reducing the strength of the mine pressure. This is a new and powerful approach to control the hard roofs in coal mining areas.

Conclusions
(1) The research shows that in the condition of extra-thick coal seam mining, the breaking and instability of thick and hard strata at a vertical distance of 100 m above the coal seam are the main reasons causing the presence of a strong ground pressure. The paper proposes to use the method of ground fracturing to weaken high-level hard roof and only employing clear water, which are economical and environmentally friendly and have a strong operability.
(2) The physical simulation studies show that the ground-fractured high-level hard strata action reduces the breakage steps and changes the structural characteristics of the strata. The breakage of the high-level hard strata is no longer a transient, high-speed, high-intensity process but a process of slow rotation, which reduces the strata breaking strength and mining pressure. The control effect of ground fracturing is remarkable.
(3) A horizontal fracturing well is used for the fracturing test on the site. The fracturing target layer is 20 m thick and 110 m vertically away from the coal seam. The fracturing process is performed in three stages. The microseismic monitoring shows that the horizontal extension length of the fracturing crack reaches 216 m, the height reaches 50 m, and the expansion range is wide. The ground fracturing releases the strata stress concentration and changes the strata structure and its mining pressure action. After the ground fracturing, the roadway deformation and support resistance of the 8218 working face are highly controlled, and the ground fracturing control effect is remarkable.
(4) The successful test of the horizontal fracturing well shows that the control range and fracturing effect of the ground fracturing are remarkable, which provides a new approach for the control of the high-level hard and thick strata in coal mines and is of tremendous signi cance. In addition, the horizontal fracturing well can achieve multistage fracturing with a strong controllability and operability and has broad application prospects.

Declarations
Availability of data and materials The data and material are transparency in the paper.

Competing interests
None. Figure 1 Strata structure and corresponding ground pressure effect The characteristics of overburden breaking Vertical displacement caused by the KS5 fracturing Comparison of the displacement of the immediate roof during KS4 and KS5 breaking Overburden space structure of the breakage of KS4 and KS5 Figure 8 Structural representation of the hydraulic fractured well Construction site layout of the ground fracturing Figure 10 Position layout of the detectors Figure 11 The expansion of crack in the three times fracturing Figure 12 Ground pressure monitor layout Figure 13 Roadway deformation and supports resistance in uenced by the ground fracturing