Failure Characteristics of Thick Hard Roof Stratum under Hydraulic Pre-splitting and Its Application in A Coal Mine, Dongsheng Mining Area

Due to the presence of the hard suspended roof, it cannot collapse naturally after being mined in a large-scale goaf. If the roof suddenly destroys without human intervention, it would cause serious disasters. In this paper, we discuss the mechanisms and applications of hydraulic fracturing in alleviating the potential for catastrophic disasters. After the fracturing crack propagation principle, we investigate the failure and stress characteristics of overburden with and without fracturing, taking a mine in Dongsheng mining area. The results show that the regulated roof suffers severe damage after fracturing, and the initial rupture distance of hard roof is reduced to 40 m or so. Additionally, the development height of water-conducting fissure zone is approximately 138.18 m. The fracturing effect can be preliminarily speculated according to fluid pressure curves. It is inferred that hydraulic fracturing point 3 has the best damage effect on the hard suspended roof. On the other hand, a common trait of overburden stress is easily observed by monitoring points, namely that the overburden stress after fracturing shows a relatively smaller value. The study provides theoretical support for the safety management of thick hard roofs, especially in the coal mines seriously affected by roof accidents.


Introduction
Coal resources have been indispensable in the power and steel industries across the world (Liu et al., 2021;Wang et al., 2019).Longwall fully-mechanized mining has been widely acknowledged as an effective and productive mining technique for thick and ultra-thick coal seams (Kang et al., 2019;Peng et al., 2019;Sun et al., 2021).However, the effect of complicated geological conditions such as the thick hard roof and top coal on the mining operation has somewhat restricted the successful implementation of this technique (Gu et al., 2022).Mine disasters induced by the excavation of thick coal seams tend to occur due to the presence of a thick hard roof with high strength and elasticity strain energy overlying the excavation of coal seams (Li et al., 2023;Sepehri et al., 2020).As a working face continues to advance, roof strata without naturally caving form a large-scale suspended roof, resulting in a vacant area without caved rock blocks behind the coalface.What if the entire suspended roof breaks down violently, consequently leading to a series of geodynamic phenomena such as rock bursts, wind-blasts, coal-rock composition outbursts, etc (Cui et al., 2019;Mazaira and Konicek, 2015;Stacey, 2016;Zheng et al., 2015).
Against this backdrop, the technique of weakening suspended roof strata was born at this moment.The existing methods including borehole pre-split blasting and directional hydraulic fracturing are verified by some actual engineering practices (Hu et al., 2014;Zheng et al., 2021;Luo et al., 2022;Sun et al., 2021).However, it is unsuitable to use a destress blasting method in highgassy mines, which may cause gas explosion accidents.Furthermore, large amounts of CO and other harmful gases resulting from blasting can be emitted into the working space, which is harmful to personal health (Huang et al., 2018;Kang et al., 2023).Additionally, the construction costs of deep borehole blasting are relatively higher.Lastly, the safety management measures of blasting are relatively complex and the transport and daily management risks of explosives and detonators used for blasting must be also in consideration (Guo et al., 2020;Wang et al., 2020).In contrast to traditional pre-splitting blasting, hydraulic fracturing technology has shown a potential advantage as a simpler, controllable, and low-cost method to destroy the integrity of the hard roof, with broad application prospects in a variety of operation conditions.Hydraulic pre-split technology can change the distribution field of surrounding rocks' stress, release the elastic energy accumulation stored in the hard roof, and further shorten the length of a suspended roof overlying the goaf, increasing the permeability of coal seams (Kang et al., 2018;Liu et al., 2021).
Many scholars around the world have been conducting related studies and exploring into the action mechanism and effectiveness assessment of hydraulic fracturing and its prospects for application.The determination of the target formation for high-pressure liquid injection was a primary critical step."Masonry beam structure and slab" was proposed for the roof falling, and the thick hard rock layer denoted as the key stratum played a controlling role in the overburden movement (Qian et al, 1995).Theoretical analysis preliminarily determined the design scheme of hydraulic fracturing boreholes based on the mining height and the average bulking factor of the roof (Huang et al., 2018).Moreover, a combination approach of numerical simulation with microseismic events for overburden stress release further confirmed and validated the location of the fracturing stratum (key stratum).Hydraulic fracturing, which weakened the properties of a thick hard roof around the working face, was performed in an underground mine during the initial excavation.It was demonstrated that the roof collapsed sufficiently to fill the gob space behind the brackets when the accumulative advancement of the working face was at the appropriate distance (Huang et al., 2018).A surface hydraulic fracturing field experiment was conducted in 8101working face of Tashan coal mine, Shanxi province for weakening the far-field hard roofs.The findings were that the low-position hard roof strata should be considered first for hydraulic fracturing, and that abutment pressure and the hydraulic brackets deformation were significantly reduced after hydraulic fracturing of key stratum in the far field (Long et al., 2022;Lu et al., 2019).Directional hydraulic fracturing for hard roofs in different simulation conditions had been discussed by XFEM (extended finite element method).Besides, the differences between conventional drill holes and directional drill holes, transverse perforation and axial perforation leading to the expected results of fracture propagation were analyzed (Kamal et al., 2020;Sun et al., 2021;Zhang et al., 2022).A multi-stage fracturing prevention method for the hard roof was proposed to weaken the strength of low-level roofs, reduce the potential occurrence of large-scale suspended layers, and alleviate energy accumulation in overlying hard strata (Zheng et al., 2021).To enhance the caving ability of top coal and recovery rate in the excavation of ultra-thick coal seams where thick hard gangue layers developed, hydraulic fracturing was utilized in the high-level thick hard gangue layer (Luo et al., 2022).The amount of fractured gangue behind the brackets increased and the size of the top coal blocks decreased.The initial pressure step and periodic pressure step were reduced accordingly.Hydraulic penetration fracturing technology has also been applied to improve the low permeability of tight coal seams for the productive extraction of coalbed methane (Jiang et al., 2018;Xu et al., 2019Xu et al., , 2017) ) Hydraulic pre-splitting technology has been discussed in detail for control and prevention for geodynamic hazards and enhanced permeability of tight coal seams.However, the influence of hydraulic fractured stratum on the development height of water-conducting fracture has not been fully understood.In this paper, we investigate the development characteristic of hydraulic pre-cracks, overlying strata movement, and the height of water-conducting fracture through a numerical simulation.This study has significant implications for the management and control of mine water hazards to some extent (Cao et al., 2021).

Effect of hydraulic pre-splitting on fracture propagation
Underground hydraulic fracturing often constrains the application of high-performance fracturing equipment and limited fracturing scale due to narrow workspace and complicated working conditions, but it has the advantage of rapidly relieving and dispersing concentrated stress in the thick hard layers above goaf, with the opposite effect in surface hydraulic fracturing engineering.Therefore, a combination technology of underground and surface hydraulic fracturing is proposed within the mining area as depicted in Fig. 1.

Fig.1 Schematic diagram of underground and surface hydraulic fracturing
The effect of hydraulic fracturing depends on the expansion of man-made fracture in the target formation, so the research on evolution characteristics of fracture propagation is crucial in the different geological conditions.Prior experimental studies have revealed the characteristic of main hydraulic fracture would always propagate along the direction of maximum principal stress at the macro scale (Zhang et al., 2022;Zheng et al., 2021).Branching hydraulic fracture, which is laterally initiated around the main hydraulic fracture, may extend along the activated interface between the bedding planes and cleats.Influenced by the bedding planes, the branching hydraulic fracture would have three typical propagating behaviors of crossing, offsetting and diverting along the bedding planes, on the other hand, it is rather than controlled by bedding planes.The microfracture trajectory network is determined by μ-CT and acoustic emission (Cai et al., 2023;López-Comino et al., 2021).
Underground surrounding rocks are usually encountered by a three-dimensional stress field, that is, maximum horizontal principal stress-σH, minimum horizontal principal stress-σh, and vertical principal stress-σV, the fracture propagation patterns can be broadly divided into three types.When the confining pressure circumstance is σH>σv>σh, a series of ellipsoid crack networks appear along the fracturing sections as shown in Fig. 2a.Hard rock segmentation from this fractured type destructs the integrity of thick hard layers, shortens the step of initial and periodic weighting, and significantly weakens the accumulated elastic energy within the suspended roof and above.
When the principal stress directions in the surrounding rocks around the stope differ, the threedimensional stress field condition is determined as σH>σv>σh.It can be seen from Fig. 2b that horizontal crack networks are formed in the parallel direction to σh after fracturing.Thick hard overburden and top coal may be sliced into numerous thin layers by crack networks when there are amounts of horizontal crack networks developing within them.The caving ability of thick hard coal and rock composition has been effectively improved, avoiding the situation of suspended strata for a long time.
Only if the stress field of rock layers is confirmed to σv>σH>σh, that is, the vertical stress is the largest among them, and the crack propagation pattern within thick hard rocks is shown in Fig. 2c.It is demonstrated that vertical fracture plates dominated by σv extensively come into being, and the fracturing target stratum has been cut into some rock segmentations.The result is that the initial and periodic weighting steps induced by the excavation of the coal seam under the thick hard roof will be naturally shorter than in other areas without hydraulic fracturing measurement.The results of this study are in general agreement with those of Cai and Huang et al (Cai et al., 2023)in their experiments on the crack trajectory of rock samples under hydraulic fracturing.After the first hydraulic fracturing, the rock sample surface occurs the main hydraulic fracture along the direction of σv as maximum principal stress in Fig. 3b.Meanwhile, all the fractures except the main hydraulic fracture are asymmetrically distributed on a single side of the main hydraulic fracture, while the propagation path of the main hydraulic fracture is almost vertically straight.In the triaxial stress state, a hydraulic main fracture is more prone to direct expansion, indicating that there is almost no crack interference caused by adjacent cracks during the formation of the main hydraulic fracture.It is further inferred that the main hydraulic fracture is formed earlier than the branch fractures.
Hydraulic crack propagation in overlying strata proceeds a complex process.Except for the stress field, hydraulic crack propagation and the spatial distribution scale of cracks are affected by other factors, including the mechanical properties of coal and rock masses, joints, bedding, natural cracks, etc.In in-situ engineering practice, microseismic events and acoustic emission as widely used approaches can roughly locate the propagation range of hydraulic fracturing.

Engineering application
Considering the obvious advantages of hydraulic fracturing technology on the safe and effective management of thick hard roofs in coal mines, this paper selects a typical coal mine of the Dongsheng mining area in western China as an engineering application case, we not only discuss the stress characteristics of overlying strata but also investigate the influence of thick hard roofs reconstruction after fracturing on the development height of water-conducting fissure zone through numerical simulation for achieving coal excavation safety with water conservation in water-scarce areas.

Geological settings
The study area lies in Yijinhoro Banner, Ordos City, Inner Mongolia Autonomous Region as illustrated in Fig. 4a.Its geographical location is 109°51'E~110°46'E and 38°52'N~39°41'N, with an average altitude of +1200 m.The topography is characterized by dunes, sandy land, hills, cultivated land, and river valleys, most of which are aeolian sand accumulation landforms, undeveloped surface water systems, sparse vegetation, strong erosion and cutting, criss-cross gully where bare bedrock appear, river erosion landforms locally develop.The overall structural form is a monoclinic structure inclined to the southwest.The study area in the arid semi-desert plateau belongs to a semiarid continental monsoon climate, with long winters and short summers, and dramatic changes in temperature of day and night.
It can be seen from Fig. 4b that the Dongsheng uplift, as a typical geological structure, results in the strata elevation of the study area, while there are some large depressions around it.The regional geology is relatively simple, with only small-scale faults are locally surveyed.The strata in the area include the Upper Triassic Yanchang Formation (T3y), the Middle and Lower Jurassic Yan'an Formation (J1-2y), the Middle Jurassic Zhiluo Formation (J2z), the Lower Cretaceous Zhidan Group (K1zh), the Tertiary Pliocene (N2) and the Quaternary (Q).The main mining coal seams are distributed in Jurassic Yan'an Formation, which contains five groups, that is, 1-5# coal seam, 2-2# coal seam, 3-1# coal seam, 3-2# coal seam, 4-2# coal seam, and 5-1# coal seam.Confined aquifers of the Zhidan Group, Zhiluo Formation, and Yan'an Formation are regarded as the source of main mine outflow water.Overlying strata above coal seams mostly consist of siltstone and fine sandstone and sandy mudstone, and are of good mechanical properties.
Due to the fact that the scarcity of local water resources and the fragile ecological environment, it is particularly prominent to coordinate the relationship between coal resource exploitation and local ecological environment protection to achieve green and safe production of coal mines.

Materials and Methodology
The in-situ testing was conducted by drilling into the rock layers above the 4-2# coal seam of the operating coal mine in the Dongsheng mining area.Meanwhile, core samples were sent to the laboratory and processed to a size that satisfies the testing requirements.A series of mechanical experiments were then performed using the high-precision servo pressure device as illustrated in Fig. 5.The obtained basic mechanical properties of the rock serve as valuable data for further research.The test outcomes are documented in Table 1.According to the strata structure information from the geological drilling to penetrate the different strata above the working face, we have obtained a rough understanding of the thickness and lithology of each stratum.
Block dyna analytic module as one of the GDEM (Graphics discontinuous element method), developed by the Institute of Mechanics, Chinese Academy of Sciences, is an explicit dynamic and efficient numerical simulation program.Based on the continuous-discontinuous element method (CDEM), the block parts are used to characterize continuous materials, while the discontinuous materials are defined by the interface between the blocks.The simulation of the progressive failure process of materials can be achieved by means of the block boundaries and cracks inside the blocks.
In geotechnical engineering, oil and gas, water conservancy, machinery, and other fields, it is generally applied to realize multi-physics couplings such as seepage field, temperature field, and solid field (STS) (Li et al., 2021;Na et al., 2022;Pei at al., 2021).Considering the inherent advantages of this software, we employ it to establish a twodimensional geological model based on the detailed investigations of geologic conditions in the study area as illustrated in Fig. 6.The size of the geological model is 800 m×377 m and encompasses 22 rock layers from the ground down to the immediate floor of 4-2# coal seam.The model consists of 16960 nodes and 32662 elements following the meshing procedure.The lithology of the strata is predominantly characterized by sandy mudstone, various type sandstone, coal seams, and topsoil.To mitigate the impact of mining boundaries, a rational coal pillars width is reserved at the left and right boundaries during the excavation of 4-2 coal seam, the resulting in a working face length of 400 m with a step length of 20 m.This study focuses on a low-level thick hard rock layer over 4-2# coal seam to discuss the influences of hydraulic fracturing on it.The target layer, as a part of the main roof with good mechanical properties, is situated approximately 31 m away from the roof of the working face, where it is likely that the water-conducting zone will develop.Six hydraulic fracturing points (HFPs) are set in the above-mentioned layer and implemented sequentially at equal intervals.The flow rate of fracturing fluid is set to 25.0 m 3 /h.The coordinate positions and parameters of the fracturing points are given in Table 2.

Fig.5 Servo pressure tester and experimental process
Furthermore, to highlight the effect of hydraulic fracturing on overburden failure during 4-2 coal seam mining, we first choose the engineering model without any fracturing measurements as an initial comparison case to investigate the interrelation between hydraulic fracturing and thick tight overlying layer in the next stage.

Overburden failure characteristics without hydraulic fracturing
When the hard and thick stratum covering the working face is not subjected to hydraulic fracturing measures, disturbed excavation of coal seam has a limited effect on it.The overall bending deformation of the rock layer does not break and collapse, and only damaged cracks appear in local areas.
In this study, the integrity of the overlying roof of 4-2# coal seam was not destroyed until 60 m before the coal seam was extracted.The immediate roof gradually began to sink and separate from the adjacent stratum after 80 m of the working face advance as shown in Fig. 7a.At this moment the internal integrity of the rock layer was comparatively better.As the working face continued to advance up to 120 m, we can easily see from Fig. 7b that the magnitude of rock fragmentation above the coal seams was intensified, and there was a tendency to collapse towards the empty space.The bent immediate roof occurred to break down and numerous rock block fragments from the immediate roof fell to fill the vacant area behind hydraulic mechanized supports as the working face was excavated by 160 m in Fig. 7(c).Since then, it was concluded that the caving area of the immediate roof increased with the enlargement of goaf space scale from Fig. g.Excavation 320m h.Excavation 400m Fig. 7 Overburden deformation and failure characteristics during mining stages Nevertheless, there was no apparent plastic damage to the thick hard roof, except for the immediate roof breaking down throughout the mining period.Thick hard stratum and above layers were mainly characterized by elastic bending deformation, with occasionally vertical cracks in local areas.Due to large-area suspended and hard-to-cave roofs over the open goaf, the predictable result is that the risk of coal mine dynamic disasters occurrence, such as rock or coal bursts and windblasts, is likely to increase during the process of coal seam excavation.This demonstrated the importance of hydraulic fracturing measurement in roof safety management by comparing it with normal mining areas where no hydraulic fracturing engineering was performed.

Dynamic progress of hydraulic fracturing
HFPs were set in the low-level thick hard roof over the working face, and the sequence of fracturing operation was implemented following the advancing direction of the working face.The initial diffusion of pressure fluid flowed into stratum cracks from the fracturing point as the center of radial diffusion around as shown in Fig. 8.The closer the zone was to HFP, the higher the saturation of the zone.Then, the universal phenomenon at each fracturing stage was that the horizontal cracks developed faster than the vertical ones, so that the fracturing fluid traveled relatively farther without more resistance along the horizontal bedding and cracks.
The area above HFPs was relatively smaller compared with the area below it, which was affected by the in-situ stress of the surrounding rock and fluid dead weight.In addition, the area between HFPs did not suffer from pressure fluid during the special period when the first stage was completed and the second stage had just launched.As time went by, residual pressure fluid in the previous stage and the fluid currently being injected together contributed to expansion of the fractured area in the target layer.Meanwhile, we found that secondary fractures gradually formed in and around the fracturing zone from the first fracturing period.

Fig.8 Zone saturation cloud map at the initial second stage
As previously mentioned, the affected areas around hydraulic fracturing points were interconnected in a stable condition when all the fracturing operations were finished.Each influenced zone was similar in shape, with the exception of the third fracturing zone.Continuous fracturing zone facilitated the destruction of the target rock formations.It can be easily seen from Fig. 9 that hydraulic fracturing created some vertical cracks and long horizontal fractures around HFPs. Horizontal fractures extended for about 400 m, approximately the length of the working face excavated.A thick and intact rock layer was separated into many thin rock layers of lower mechanical strength.As soon as the suspended roof of pre-splitting is disturbed under extraction of coal seam, it will naturally fall to the goaf.

Overburden failure characteristics after hydraulic fracturing
After the implementation of the previous hydraulic fracturing measures, some pre-cracks appeared inside the hard and thick roof stratum.These pre-cracks may be discontinuous with respect to each other, however, concentrated stress resulting from mining activities would intensify the expansion of previous fracturing cracks and the development of the mining-induced cracks.The interconnections of different types of cracks caused the rock layer to be cut into many rock fragments of varying sizes.The physical and mechanical strength of the hard rock formations became worse and the damaged rock formations do not accumulate more elastic energy, allowing concentrated stress to be released.
As can be demonstrated in Fig. 12, the immediate roof started to collapse first as the working face advanced.During the collapse of the immediate roof, the base of the hard basic roof formed upward cracks.Meanwhile, the redistribution stress induced by mining coal seam was transferred upward, which severely damaged the rock strata above the hard rock stratum.When the goaf space gradually became larger, the range of stress propagation was further away.In addition, the cracks in the stress-affected zone were relatively developed due to the self-weight of the rock mass.The fracture development area eventually formed a stable 'two zones'-a caving zone and a waterconducting fissure zone.Above the two zones is the bending subsidence zone where vertical cracks developed locally.Based on the dynamic development characteristics of cracks, the initial breaking distance of the hard roof was about 40 m, which was approximately the location of the first fracturing point.Since then, mining activity had entered into the fractured area, particularly in nearby HFP2 and HFP3, with breaking distance sharply shortened due to the good fracturing effect in this area.

Fig.12
Mining overburden failure cracks after hydraulic fracturing It can be seen from Fig. 13 that the area was also the place with the largest subsidence displacement of the overlying strata.After passing through HFP3, the fracture distance of thick and hard rock strata was 25 m or so.We found that even though the separation layer was reduced after deformation, the phenomenon of the separation layer attributed to the differential subsidence between strata still existed as shown in Fig. 13.The development height of the water-conducting fissure zone was roughly 138.18 m, giving a ratio of 23.03 between the fissure and mining height.The ground surface above the working face also had been subject to varying degrees of subsidence displacement.

Pressure dynamic characteristic of fracturing fluid
In the process of hydraulic fracturing, we monitor the dynamic changes in fluid pressure at each fracturing point.As can be seen in Fig. 14, all fracturing points have highly common dynamic changes in fluid pressure.That is, the pressure increases sharply to its maximum within just a few minutes of the initial crack.After that, it decays to almost zero at approximately the same rate as in the pressure rise stage with several fluctuations during this period, but the overall change of trend is not significant.The pressure fluctuation lasts about 15 minutes.The fluid pressure at this stage roughly decreases by three to four orders of magnitude.Ten minutes before the termination of the

Separation layer
Water-conducting fissure zone fluid injection, the fluid pressure rises slowly with fluctuation.After completion of the fracturing, the fluid pressure gradually dissipates and remains at a relatively low value for a short time.

Fig.14 Dynamic change characteristics of fluid pressure at HFPs
It is worth noting that the fluid pressure at HFP3 slightly recovers after a sharp drop in the pressure compared to the pressure changes at the other fracturing point.The main possible cause is the presence of many man-made cracks around the fracturing points, where the fracturing fluid does not accumulate here for a short period of time.Based on the characteristics of fluid pressure recovery, the extent to which cracks develop in the fracturing zone can be preliminarily speculated.The large recovery of the fluid pressure values may indicate that the fracturing effect does not work well.From this, we deduce that HFP3 has the best damage effect on the hard-suspended roof.The saturation distribution of the fracturing fluid and the failure characteristics of the overburden in the simulated mining conditions can also be served as supportive evidence for the conclusions.During the excavation of the coal seam, the damage to the overlying rock around HFP3 is the most severe compared to other fractured areas with the rock layer being cut into numerous irregular mass fragmentations.This is likely due to the increased water storage capacity of the fractured rock layers, and the fact that the fluid is mainly enriched in the severely broken area.Consequently, the diffusion scale of the fracturing fluid is relatively small.

Stress characteristics of pre-splitting hard roof during mining
In this study, we set up 15 stress monitoring points, denoted as M1~M15, spaced 50 m apart on the roof of the pre-splitting layer to obtain the stress changes during the advancement of the working face.The impact of hydraulic fracturing on the hard roof is determined by comparing the stress variations under two conditions, no pre-splitting measure and pre-splitting measure.The relative position relationship between the stress monitoring points and the mining area is shown in Fig. 15.We select the stress monitoring data from M3 to M11 to discuss the differences in the stress characteristics with and without fracturing.Fig. 15 Design the location of stress monitoring points over the target layer The overburden monitored is essentially in a compacted state during most of the mining stages as shown in Fig. 16.In the early stage of excavation, due to the fact that the overlying strata above the monitoring layer bearing more pressure from rock layers weight are relatively integrity and tensile stress resulting from mining activity gradually begins to occur in the monitoring layer, the stress of the monitoring layer has been decreased.However, as the concentrated stress propagates, the damage range of the rock layer is enlarged, and the compressive stress is slightly increased for the monitored layer After the excavation is completed, tensile stress is detected at the local monitoring points, such as M7 and M10.
Meanwhile, we take notice of a general phenomenon by comparing the stress of overburden in two working conditions.More specifically, the overburden stress after fracturing at each stage of mining is shown to be relatively smaller than that without fracturing.The fractured layer bearing stress may reflect the degree of concentrated stress action on the hard strata above the working face.Clearly, hydraulic fracturing measures improve the stress environment in which thick suspended roofs threatening mining safety, the target roof that withstands lower external forces naturally breaks down, and the rock collapse releases the non-catastrophic energy that the mine can bear and control.Nevertheless, if no intervention, such as hydraulic fracturing is applied, the overlying strata with better mechanical properties can remain unbreakable for a long time under the higher energy accumulation circumstance.
In terms of the fluctuating variation of the stress at the monitoring points, it can also reflect the state of the overlying strata after excavation.Once the compressive stress exhibits an upward trend, the tensile stress promotes the possibility of the rock mass to fall.The compressive stress, on the other hand, shows a downtrend, indicating that the rock layer above the monitoring points may bend or break down to exert a force on the monitored layer.Moreover, the height of the water-conducting fissure zone is almost the same for both assumed conditions.

Conclusion
This research selects a coal mine in Dongsheng mining area as an engineering case to illustrate the advantages of hydraulic fracturing technology in the treatment of a thick hard roof using the numerical simulation method.We investigate the differences in the failure characteristics of overburden rock and stress variation in the fractured layer under non-fracturing and fracturing conditions.The results show that hydraulic fracturing plays a significant role in shortening the scale of the suspended roof and alleviating the stress concentration effect.Through the application in a coal mine, the following conclusions are drawn: (1) Under the non-fracturing condition, no obvious plastic damage in the thick hard roof is observed except for the immediate roof breaking down after mining.Thick-hard stratum is mainly characterized by elastic bending deformation, with occasionally vertical cracks in local areas.While the regulated roof suffers severe damage after implementing fracturing, with the initial rupture distance of the hard roof being about 40 m.Besides, the developed height of the water-conducting fissure zone is approximately 138.18 m, and the ratio of fissure to mining height is about 23.03.
(2) The residual fluid pressure around the HFP3 is the smallest of six hydraulic fracturing points.The probable cause is the rapid decay of fluid pressure due to the development of more cracks in the rock mass.Meanwhile, HFP3 slightly recovers after a sharp drop in pressure according to the fluid pressure dynamic curve, and the extent of cracks development in the fracturing zone can be preliminarily speculated.It is inferred that HFP3 has the best damage results for the hard-suspended roof.
(3) Through the stress changes of 15 monitoring points on the roof of the pre-splitting layer, a common trait is easily observed in two working conditions, that is, the overburden stress after fracturing shows relatively smaller than that without fracturing.The overlying strata remain compacted, although the compressive stress in the monitoring layer is reduced during the initial stage of excavation.The stress magnitude of the layers reflects the degree of concentration of stress action.
Hydraulic fracturing has played a positive role in improving the stress environment in which thick overhanging roofs threatened mining safety, promoting the fall of hard roofs in a relatively lower-stress circumstance.Meanwhile, the rock collapse releases non-catastrophic energy.The study serves as theoretical support for the roof safety management of mine, especially in the coal mines in western China, which have been severely affected by roof disaster accidents.

Fig. 3
Fig.3Morphology of main hydraulic fracture on the surface of a rock sample, which is listed as (a) before hydraulic fracturing; (b) after first hydraulic fracturing(Cai et al., 2023)

Fig. 6
Fig.6 Planar geological conceptual model and positions of hydraulic fracturing points

Fig. 9
Fig.9 Zone saturation cloud map after the final stage Meanwhile, the fluid pressure distribution in the fractured areas was also determined in this study.As shown in Fig.10, fluid within FHP1 had still a certain pressure when the second fracturing began to implement.In other words, even if the injection of fracturing fluid was stopped, the residual pressure fluid still has a fracturing effect on the rock mass to some extent.Under the combined action of fluids at different pressure values, the range of area influenced by hydraulic fracturing became large.

Fig. 10
Fig.10 Fluid pressure cloud map at the initial second stage When all fracturing tasks were completed, the residual fluid pressure around the HFP3 is minimal compared to the others as presented in Fig.11.The probable cause was the rapid decay of fluid pressure in the tight rock stratum around the HFP3.Although the fluid pressure decayed fast, more failure cracks around HFP3 may be created.It can be inferred from Fig.9 and Fig.10 that the fluid with pressure was concentrated around the HFP3 to counteract the energy of rock mass destruction.The results show that the greater the scale of the high saturation area, the smaller the possible damage to rock layers caused by fracturing.Generally, fluid pressure in the rock layers does not dissipate immediately after fracturing.

Fig. 11
Fig.11 Fluid pressure cloud map after the final stage

Fig. 16
Fig.16 Dynamic change of monitoring stress in the pre-splitting layer during mining

Table 1
Mechanical parameters of coal and rock mass for the simulated test Density(g/cm 3 )

Table 2
Hydraulic fracturing points settings