*Review* **Application of Pre-Splitting and Roof-Cutting Control Technology in Coal Mining: A Review of Technology**

**Shengrong Xie, Yiyi Wu, Fangfang Guo, Hang Zou, Dongdong Chen \*, Xiao Zhang, Xiang Ma, Ruipeng Liu and Chaowen Wu**

> School of Energy and Mining Engineering, China University of Mining and Technology, Beijing 100083, China **\*** Correspondence: chendongbcg@163.com

**Abstract:** According to the development requirements of green mining of coal resources, it is imperative to improve the extraction rate of coal and the application of safe and efficient mining technology. Pre-splitting and roof cutting technology is widely used in reducing residual coal pillars and safe pressure relief mining, which has become the crucial technology for pillar-free mining methods. Therefore, it is essential to review and discuss the research hotspots, cutting-edge methods, principles of action, and application areas of the development of this technology. Above all, the research data on pre-splitting and roof-cutting development in the past ten years are summarized and outlined. The research's hot spots are pressure relief technology and gob-side entry retaining technology. Then, the functional forms of pre-splitting and roof cutting technology are discussed and compared, including explosive blasting (directional energy gathering blasting, liquid explosive blasting, and composite blasting), hydraulic fracturing, liquid CO2 gas fracturing, and mechanized roof cutting (chain arm saw machine and directional cutting roof rig). Through the analysis of field application cases, the application field is divided into three major areas: non-coal pillar mining (gob-side entry driving with narrow coal pillar, gob-side entry retaining with the filling body, completely gob-side entry retaining, and "N00" construction method), pressure relief at working face (thick and hard main roof cracking and end area hard roof cracking), and pressure relief at roadway (gob-side roadway pressure relief and blasting pressure relief technology for roadways). By detailing the process of each application technology one by one, the principle and mode of pre-splitting in each technology are expounded. Finally, the development prospects of pre-splitting and roof cutting in new technical methods, deep pressure relief mining, intelligent unmanned mining, and green and efficient mining are prospected, providing references for similar projects.

**Keywords:** pre-splitting blasting; roof cutting technology; hydraulic fracturing; no-coal pillar mining; thick hard roof; pressure relief technology

#### **1. Introduction**

In Chinese coal mining, high-intensity mine pressure in the roadway and working face has been the critical factor that endangers the average coal production [1–4]. In the past 30 years, with the rapid development of computer science and sensor technology, the study of mine pressure in the coal mining process has become more profound and transparent [5–8]. Theoretical models of mine pressure in mining fields and roadways have been proposed and gradually recognized by field engineers, such as the structural model of "masonry beam" [9,10], the mechanical model of "plate structure" of the stope [11–15], the theory of slip line of roadway floor [16], and the theory of butterfly plastic zone [17,18]. Many scholars have profoundly and extensively studied the location and causes of stress concentration areas in the mining field and the roadway through pressure monitoring in the field and numerical calculations by computer [19–22]. Among them, the stability and movement of the roof after coal mining are closely related to the regional stress concentration condition of the mining field and the roadway. Especially for the coal mining

**Citation:** Xie, S.; Wu, Y.; Guo, F.; Zou, H.; Chen, D.; Zhang, X.; Ma, X.; Liu, R.; Wu, C. Application of Pre-Splitting and Roof-Cutting Control Technology in Coal Mining: A Review of Technology. *Energies* **2022**, *15*, 6489. https://doi.org/ 10.3390/en15176489

Academic Editor: Sheng-Qi Yang

Received: 1 August 2022 Accepted: 31 August 2022 Published: 5 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

face with a hard roof, composite roof, and thick roof, when it adopts usual mining methods, many problems such as support crushing, large deformation, and strong disturbance will occur [23–27]. Simultaneously, the increase of the buried depth makes the surrounding rock conditions of the roadway increasingly bad. Under the complex surrounding rock environment, it is difficult to maintain the stability of the roadway only by support, and the application of pressure relief technology is urgently needed [28–30]. Based on this, pre-split or cutting off the roof before working face mining is gradually being implemented to study pressure relief in mining fields and roadways.

Pre-splitting and roof-cutting control technology refers to the directional pre-fracturing or cutting off of the roof of the mining field and roadway by explosive blasting or highstrength hydraulic fracturing [31–33]. As shown in Figure 1, according to its application method, it can be classified into directional blasting roof cutting technology, directional hydraulic fracturing technology, and liquid CO2 gas fracturing technology [34–36]. According to its application object, it can be classified into pre-splitting control technology of mining field's roof and roadway's roof. According to its application scenario, it can be classified into the technology applied to roof cutting and pressure releasing of gob-side entry driving, roof cutting and pressure releasing of gob-side entry retaining, roof cutting for self-forming roadway technology, hard main roof cracking, and pressure relief on the surrounding rock of the roadway.

**Figure 1.** Category of pre-splitting and roof-cutting technology.

With the development of roof cutting and pressure relief theory, pre-split means, and supporting equipment, pre-splitting and roof-cutting technology has become essential to realize pressure relief and no pillar mining. Based on this technology, the recovery rate of coal mining increases and the roof disaster decreases, which is conducive to producing highquality and efficient green coal mining [37–40]. The research application of pre-splitting and roof-cutting technology in the coal mining field in the past five years is analyzed by VOSviewer, as shown in Figure 2.

By counting more than 300 papers on pre-splitting and roof-cutting in the global mining industry in the past five years, we can get that the research hotspots are: <sup>1</sup> Presplitting roof applied to the stability study of working face and roadway roof; <sup>2</sup> Analysis of the mechanical mechanism of pre-cracked roof effect, including mechanical model and pressure release principle; <sup>3</sup> Process design and optimization of support means of pre-fractured roof technology; <sup>4</sup> Numerical simulation analysis study of pre-fractured roof technology, including the evolution of its plastic zone, stress, and deformation; <sup>5</sup> Application of pre-fractured roof technology, including hard-roof cracking and pressure relief, no pillar mining (gob-side entry driving and gob-side entry retaining), and self-

forming roadway technology; <sup>6</sup> Research on gas and rock explosion in the process of the pre-fractured roof.

**Figure 2.** Research hotspot map of pre-splitting and roof-cutting technology.

China is the technological powerhouse country in coal underground mining. Pre-splitting and roof-cutting technology is extensively implemented in Chinese coal mines [2,32]. By searching the keywords "pre-splitting and roof-cutting" in CNKI (China National Knowledge Internet), 180 related papers were obtained in the past 10 years. As shown in Figure 3, the subject terms of these research papers are roof cutting and pressure relief application, gob-side entry retaining technology, pre-split blasting technology, working face pressure relief mining, technical parameter study, non-pillar mining, hard roof cracking, gob-side entry driving technology, and non-pillar self-forming roadway. Among them, the study of gob-side entry retaining through roof-cutting technology is the hot spot of research, which accounts for 56%. The deformation of the roadway along the gob is extensive and challenging to maintain [31,38]. By cutting the roof, the intense stress concentration of the roof can be released, thus significantly improving the stress environment of the roadway along the gob [34]. In addition, in the past four years, research on pre-splitting and roof-cutting has been at the forefront of the industry in the field of gob-side entry retaining and pressure relief mining at the working face. As the geological conditions of applied coal mine working faces become more diverse and complex, scholars have begun to improve the traditional presplitting and blasting technology, and the research on the effects of directional energy-gathered blasting has gradually increased.

With the more widespread application of pre-splitting and roof-cutting technology by Chinese and foreign scholars and engineers in coal mining sites, the technology has been comprehensively developed in the coal mining field. Furthermore, four major application scenarios have gradually been derived: roof cutting and pressure releasing for gob-side entry driving and gob-side entry retaining, non-pillar self-forming roadway, cracking of hard and thick layer roof, and pressure relief of working face and the roadway; this paper presents a comprehensive summary of this technology in coal mining in recent years and systematically describes the basic methods, technical principles, mechanical mechanisms, and application scenarios. Currently, underground coal mines advocate the development of safe and green mining technologies with high recovery rates; this technology is widely used to increase recovery rates by reducing or eliminating pillars. Therefore, this paper takes the pre-splitting and roof-cutting technology that helps coal achieve efficient and green mining

as the research object of the review and comprehensively analyzes the current situation and development prospect of the application of this technology; it provides reference suggestions for the project experts who need this technology in the mining field to be applied more effectively in green coal mining.

(**b**)

**Figure 3.** Category of pre-splitting and roof-cutting technology. (**a**) Nine research hotspots (**b**) Research trends in the past four years.

#### **2. Forms of Pre-Splitting Technology for the Roof**

The technology of pre-splitting and roof-cutting is to destroy the roof of the working face and roadway in advance to achieve the effect of releasing its stress concentration, and its core is to fracture the roof successfully [41]. In the field, there are four primary forms of this technology: blasting pre-split technology, hydraulic fracturing technology, liquid CO2 gas fracturing technology, and mechanical roof-cutting technology.

#### *2.1. Pre-Splitting Roof Technology of Explosive Blasting*

#### 2.1.1. Directional Concentrated Blasting Technology

Drill-hole blasting applied in roof-cutting is different from regular blasting. As shown in Figure 4, the blast wave and energy of ordinary blasting spread from the center of the hole to the surrounding area. Due to the anisotropy of the rock mass, the cracks around the hole are randomly extended after blasting. Although this is locally fractured in hardroof, the blast holes are not effectively connected and do not achieve the "cut-off" effect. Therefore, most field applications of bidirectional energy-gathered tensioning and forming blasting technology. The basic principle of this technology is to control the transmission of blasting shock and stress waves through the energy-gathering pipe, which generates high-pressure gas to stretch the fracture expansion directionally [42–45]. Each blasting hole is blasted simultaneously, and the rock layer is cut off by producing directional fissures.

**Figure 4.** Directional concentrated blasting technology.

In ordinary directional energy-gathered blasting, the explosive is in a dry explosive environment, and its shock wave propagates in the air medium during blasting. Applying this method in underground coal mines generates lots of dust and toxic gases, which is not conducive to the green mining of coal. In order to overcome this drawback, the energy-gathered charge hydrodynamic blasting technology is gradually applied [46]; it is the application of the water medium filled with explosives, the use of water to spread the explosive energy, which reduces the generation of coal dust and toxic gases while enhancing the blasting effect, and it becomes a green and safe method for directional roof-cutting.

#### 2.1.2. Directional Pre-Splitting Technology of Liquid Explosive

Ordinary solid explosives dry blasting drawbacks, improved for water-filled pressure blasting, which is effective in gas and dust control, but still shows the explosive explosion instant power and cracking range is not enough. Therefore, liquid explosives blasting technology gradually developed, the structure of the three as shown in Figure 5.

**Figure 5.** Charge structures of different blasting methods.

Liquid explosives to achieve the pumping method of filling the blast hole, the explosive blast energy in the fracture during the explosion is continuously supplied, the rock breaking load in the seam is robust and long duration, conducive to the full development and expansion of rock fractures. In addition, compared with hydrogel explosives and emulsion explosives, the fracture density of the borehole wall of the surrounding rock after blasting is large, and the fractures are evenly distributed [47–49]. Therefore, liquid explosives in coal mines with blasting the energy gathering pipe can achieve coupled charging in the pipe while playing the flow characteristics; its directional blasting fracturing effect and environmental protection are much better.

#### 2.1.3. Directional Seam-Making Technology of Composite Blasting

In the actual roof-cutting work, the problem of precise and continuous fracturing of the hard and thick roof is often encountered, which requires the composite means of multiple blasting methods. For this reason, the composite blasting directional fracturing technology, which includes energy-gathered injection and high-pressure splitting, is gradually being applied in the field. The core device of this technology is a high-energy perforating gun, which needs to be installed at a certain depth of the blast hole, to be sealed after the excitation of the detonating cord to detonate the perforating bullet; its explosion also stimulates the rapid detonation of the composite propellant to produce high-pressure gas secondary splitting [50]. The result is a radially continuous fracture surface with the fusion of holes and seams to achieve precise control of rock collapse of the roof.

#### *2.2. Hydraulic Fracturing Pre-Splitting Roof Technology*

#### 2.2.1. Hydraulic Fracturing Principle

The roadway roof is drilled according to a certain elevation angle, and the radial cutting groove is pre-fabricated on the inner wall of the borehole. After that, the hole is sealed, and the high-pressure water pump is used for water injection fracturing. The roof rock layer cracks and expands, and the crack generated by other boreholes is connected to achieve the effect of cutting off the roof; its core is that the surrounding target rock is pre-cut and fractured by high-pressure pre-water injection, thus weakening the overall strength [51,52].

As shown in Figure 6, for the pre-splitting surrounding rock, the pre-cutting groove is drilled first, and then the two sides of the groove are sealed by the packer, and then the high-pressure water is injected into the sealing fracturing section to realize fracturing of the surrounding rock. Concerning the fracture initiation mechanism, through the force analysis of the coin-shaped crack section, the initial conditions of the coin-shaped fracture in the water-saturated and natural state are obtained:

$$\begin{cases} \begin{array}{l} P\_W = 0.506 K\_{ICW} \sqrt{\frac{\pi}{a}} + \frac{\sigma\_\upsilon + \sigma\_h}{2} - \frac{\sigma\_\upsilon - \sigma\_h}{2} \left( \cos 2\alpha + (-1)^k \frac{2.418}{2 - \mu\_W} \sin 2\alpha \right) \\\ P\_N = 0.505 K\_{ICN} \sqrt{\frac{\pi}{a}} + \frac{\sigma\_\upsilon + \sigma\_h}{2} - \frac{\sigma\_\upsilon - \sigma\_h}{2} \left( \cos 2\alpha + (-1)^k \frac{3.056}{2 - \mu\_N} \sin 2\alpha \right) \end{array} \tag{1}$$

where *a* is the radius of coin-type fracture, mm; *σ<sup>V</sup>* and *σ<sup>h</sup>* are vertical stress and minimum horizontal principal stress, respectively, MPa; *α* is the elevation angle of the borehole, *α*∈(0◦~90◦); *KICW* and *KICN* are the type-I fracture toughness values under full water and natural state; *PW* and *PN* are the water pressure of groove initiation, MPa; *μ<sup>W</sup>* and *μ<sup>N</sup>* are Poisson's ratio; for parameter *k*, *σ<sup>V</sup>* > *σ<sup>h</sup>* takes an even number, and vice versa takes an odd number. As a result, the trench inclination or borehole elevation angle *α*, which is most favorable for fracture initiation in hydraulic fracturing, can be calculated under different surrounding rock and stress environments [53,54].

#### 2.2.2. Application of Hydraulic Fracturing Technology

The hydraulic fracturing technology includes fracture hole drilling, hole trenching, hole sealing, roof hydraulic fracturing, and effect monitoring [55,56]. As shown in Figure 7, the 2129 working face has a thick and hard main roof rock layer, designed to be hydraulically fractured to the full extent of the top slab prior to its mining to attenuate the impact of solid rock pressure from the hard roof.

**Figure 6.** Principle of hydraulic fracturing rock.

**Figure 7.** Hydraulic fracturing technology for roof-cutting.

The straightforward steps of the hydraulic fracturing process: firstly, drilling is carried out with a geological drilling rig, and after the drilling is completed, the drilling team will proceed with the next drilling operation. Then a sealer is installed in the completed hole, and the hole is sealed with a manual pump and an energy reservoir. Finally, a high-pressure pump is connected for fracturing. The fracturing of the coal seam to be mined and the hard roof above the roadway is achieved, after which the working face can be advanced and mined [57]; this method is safe, does not produce toxic gas and dust, and is gradually being more widely accepted at coal mine sites.

#### *2.3. Liquid CO2 Gas Pre-Splitting Roof Technology*

CO2 gas phase pre-fracturing refers to the rapid conversion of CO2 from liquid to gas by a high-pressure detonation head and its instantaneous volume expansion pressure to fracture rock along the fracture face.

As shown in Figure 8, liquid CO2 is stored in a storage tank, which requires an environment with *p* > 7.35 MPa and T < 31 ◦C to maintain the liquid form. The volume of liquid CO2 in the fracture expands 600 times in 20 ms under the action of the heating device, creating an expansion pressure of 80–270 MPa to fracture the rock. Quick steps of liquid CO2 blasting: After the heating device is activated, the liquid CO2 in the storage tank expands and destroys the fixed pressure shear sheet. CO2 is released sharply and generates low temperature and high-pressure gas to act on the pre-fractured part to realize pre-fracture blasting [58]; this method has excellent continuity, does not quickly produce a lot of dust and sparks, has apparent safety and environmental advantages, and is currently being applied in complicated rock fracturing in the excavation and recovery workings.

**Figure 8.** Liquid CO2 storage tank and cracker.

#### *2.4. Mechanized Directional Roof-Cutting Technology*

#### 2.4.1. Continuous Cutting Technology of Chain Arm Saw Machine

The continuous cutting machine with a chain arm saw cuts the rock in a specific direction and inclination with low-speed chipping; it considers both ductile and brittle cutting methods, and the cutting head can make extrusion and shear damage to the rock body, thus forming a continuous through-cutting slit and achieving precise directional cutting of the top slab [59].

This method has a high degree of mechanization and can directly cut the roof by remote directional control, and continuous operation reduces the time of cutting the roof. The low-speed chipping way avoids spark generation and does not consume much water; the mechanical cutting is highly accurate, and the cut surface is smooth and flat [60]. The development of mechanized roof control is an essential step to intelligent mining in coal mines, and the continuous cutting technology of chain arm saw gives an application idea.

#### 2.4.2. Application of Directional Cutting Roof Rig

The directional roof cutting drilling rig is the mechanized roof cutting equipment developed by Academician He Manchao's "N00" construction method [32]; it has the crucial technology of simultaneous drilling in multiple holes and dynamic adjustment in multiple directions, which realizes multi-hole with the same surface and efficient cutting [61]. According to the field test, it effectively cuts off the stress transfer between the goaf's roof and the roadway's roof so that the roof of the roadway is in a specific range to form a short arm beam structure. The method is being gradually improved, popularized, and applied.

#### **3. Pre-Splitting and Roof-Cutting Technology for Non-Pillar Mining**

The method of non-pillar mining can significantly improve the coal recovery rate and reduce the waste of coal resources, which is in line with the concept of high-efficiency and green mining and has been extensively applied in the field [62,63].

#### *3.1. Application in Gob-Side Entry Driving*

Gob-side entry driving means that a narrow coal pillar (3~8 m) is set on one side of the last working face to drive the roadway of the next working face.

As shown in Figure 9a, the main roof break appears "O-X" shape in the plane when the working face is mined. The roadway and narrow coal pillar are under the "triangle" key block in gob-side entry driving [64,65]. According to the support practice of gob-side entry on-site, the gob-side entry is disturbed strongly by the superimposed stress, and

its maintenance is quite tricky. The key influencing factor of mining pressure behavior in gob-side entry driving is the fracture position of the "triangle" key block. The unfavorable fracture position directly leads to large deformation of the roadway and roof collapse. Therefore, it is necessary to actively control the fracture location of critical blocks to alleviate the concentrated stress in the roadway, as shown in Figure 9b.

**Figure 9.** The schematic diagram of roof-cutting for gob-side entry driving. (**a**) Roof structure without roof cutting (**b**) Roof structure after roof cutting (**c**) Cutting roof and unloading the pressure along the gob-side entry.

As shown in Figure 9c, in the mining face, the subsequent mining face side of the roadway driving along the goaf to carry out the pre-splitting operation, the location of roof cutting is in the narrow coal pillar side of the roof. When a section of the roadway roof cutting operation is completed, and the roof collapse in the goaf is stable simultaneously, the roadway digging operation along the goaf. Attention should be paid to the replacement relationship between the mining operation of the previous working face and the tunneling operation of the next working face to ensure that the safety distance is maintained [66,67]. According to the field practice, the stress concentration degree along the empty roadway of this method is small, and the roadway support effect is practical, ensuring the roadway roof's safety.

#### *3.2. Application in Gob-Side Entry Retaining*

The gob-side entry retaining technology realizes the succession of the completely non-coal pillar between working faces, an efficient and green mining technology that is booming and applying [68].

#### 3.2.1. Gob-Side Entry Retaining with the Filling Body

This gob-side entry retaining method uses an artificial filling body as a narrow coal pillar in gob-side entry driving. The filling bodies include high water material, paste material, and concrete [69].

#### (1) Delayed pre-splitting and cutting roof

As shown in Figure 10a, the force on the filling body is similar to that on the narrow coal pillar in the gob-side entry driving. At this moment, the roof structure and fracture state of the gob-side entry is similar to that of gob-side entry driving, which still leads to roadway maintenance difficulties. In order to reduce the pressure of the filling body and make the roof entirely collapse, pressure relief blasting technology with circular shallow holes is used, as shown in Figure 10b. After mining, a row of round holes is drilled towards the goaf next to the filling body about 20 cm to carry out energy-accumulating blasting [70,71]. As a result, the roof above the mining area collapses, falls, and then compacts and continues to support the overlying rock layer, as shown in Figure 10c.

**Figure 10.** Delayed pre-splitting and cutting roof for gob-side entry retaining. (**a**) Gob-side entry retaining with backfill (**b**) Cutting roof on goaf side (**c**) Working methods of roof-cutting and gob-side entry retaining with backfill.

The disadvantage of this method is that it interferes with the parallel operation of mining and filling of the working face, which cannot realize the fast and high-intensity operation of the comprehensive mining face.

#### (2) Advance pre-splitting and cutting roof

The method of roof-cutting along the side of the roadway to be mined is the most commonly used in the field before mining the working face. As shown in Figure 11a,b, setting the filling body in the roadway for gob-side entry retaining, the main roof structure above the roadway and the filling body is more integral and stable after cutting the roof in advance [72–74]. According to the size of roadway and backfill, ample ventilation, and laneway equipment size, roadway expansion measures are often taken to ensure that the roadway size is reasonable, as shown in Figure 11c; this method can be applied in thick coal seams, soft rock, and deep high-stress environment.

**Figure 11.** Advance pre-splitting and cutting roof for gob-side entry retaining. (**a**) Gob-side entry retaining with backfill (**b**) Roof cutting operation in advance (**c**) Working methods of roof cutting in advance and gob-side entry retaining with backfill.

#### 3.2.2. Completely Gob-Side Entry Retaining

The method of fully gob-side entry retaining means not leaving the roadside filling body, using a specific prop or single pillar to support the roof of the goaf side, and setting up gangue protection and Air leakage prevention device [75,76]. Compared with cutting the roof of retaining roadway leaving narrow coal pillar and roadside filling body, this method is more straightforward; it fully uses mining pressure and rock mass fragmentation to realize automatic lane formation.

#### (1) Cutting roof for retaining roadway with advance pre-splitting

During the working face mining period, pre-fracturing and roof-cutting are overrun along the solid coal side in the roadway to weaken or release the physical-mechanical connection between the roadway's roof and the gob's roof, as shown in Figure 12a,b; it is supported by a temporary prop (portal prop) with a single pillar to provide upward support resistance. The roof of the gob area is cut down as a whole by using mining pressure and artificially created roof-cutting cracks in advance. The roadway is under a complete and stable cantilevered structure, as shown in Figure 12c. According to the mechanism of roof-cutting and roadway-retaining, the site is usually divided into three operational phases: the pre-cracking and roof-cutting stage, the reinforcement support stage, and the stabilization stage of the left roadway [77–80]. The three stages correspond to different locations, so parallel operations are realized between the processes, which will significantly improve the efficiency of the left roadway. Since this method does not require the setting of a filling body, this has little effect on the advancing speed of the working face; this method is being applied under the conditions of different coal seam thickness, burial depth, dip angle, and roof conditions, and it has become a hot spot in the research of no pillar mining methods.

**Figure 12.** Gob-side entry retaining technology without coal pillar and filling body. (**a**) Pre-splitting roof in advance of the working face (**b**) Cutting roof to form roadway automatically (**c**) Working methods of roof cutting in advance and gob-side entry retaining with no-pillar.

(2) Combined support for cutting roof retaining roadway

For special roof conditions (e.g., thick direct roof), the pre-fracturing and roof-cutting can be re-optimized. For the working face with a thick immediate roof, the roof breaks down and collapses after working face mining. Due to the fragmentation of rocks, the collapsed rock can adequately fill the goaf. At this time, the combined support can be used to achieve roof cutting and roadway retaining, that is, to strengthen the support of the roadway to be retained in advance; it mainly includes the firm anchoring of the deep hole of the high-strength bolt cable on the roof and the support on the goaf side (π-shaped steel, the cross-hinged roof beam, and the single pillar of the gob-side entry retaining); they provide robust support resistance and use the mining pressure of the working face to realize the cutting of the immediate roof of the goaf side and fill the goaf to complete the roadway retaining operation. The overall approach covers four technologies: combined support roof-cutting technology, advanced-area reinforcement support technology, prevention, control technology of gangue in goaf, and beam-column auxiliary support technology of roadway [81,82].

This method directly eliminates the process of over-advance pre-fracturing and roofcutting; meantime, it avoids the impact on the working face advancement efficiency due to blasting or hydraulic fracturing, which realizes the efficient retention of the roadway with the rapid advancement of the working face. Although its application conditions are still limited at present, its method indicates the future need to develop a mechanized way of cutting the roof and leaving the roadway without blasting.

#### *3.3. "N00" Construction Method*

The "N00" coal mining method efficiently combines the working face mining and roadway tunneling to realize the mining of N working faces, the tunneling of 0 roadways, and the retaining of 0 district sublevel coal pillars, as shown in Figure 13 [32]. In this method, the working face is mined to form the next working face's roadway, and the roadway's roof along the goaf is treated by pre-splitting and roof-cutting. The directional

cutting method, such as energy-accumulating blasting, was successful at the initial test stage; however, due to the large-scale blasting operation behind the working face, there are hidden dangers to safety. After that, the team researched and developed the supporting equipment for the "N00" method and developed a directional roof-cutting rig, which is located on the side of the left-over roadway and follows the hydraulic support of the working face to push forward the roof-cutting operation [83,84].

**Figure 13.** Cutting roof and retaining roadway technology in the "N00" construction method. (**a**) Plane diagram of "N00" construction method (**b**) The diagram of the retained roadway.

In addition, with the roof-cutting prop to support the roof on the side of the mining area of the roadway, to achieve automatic mechanical connection of coal mining, roadway formation, roof-cutting and pressure relief, support, and other processes of the working face; this method is being gradually improved and promoted in the field, providing a complete set of solutions to achieve coal pillar-free automated mining in new mining areas ultimately.

#### **4. Pre-Splitting and Roof-Cutting Technology for Pressure Relief Mining of Working Face**

The hard roof is often encountered when the long-wall working face is mined. The hard roof is difficult to collapse, and the support of the working face continues to bear the high load, which requires pre-cracking and unloading the roof [85,86].

#### *4.1. Fracturing Operation of Thick and Hard Main Roof*

The physical and mechanical properties of the main roof above the coal seam directly affect the smooth mining of the longwall working face. With the increase of mining depth, the stress level during coal seam mining gradually increases. For the coal seam with a thick and hard main roof, the weighting step and pressure of the main roof increase significantly when the working face is mined. The intense pressure threatens the safety of equipment and workers, such as hydraulic support in the working face [87–89]. Therefore, the pre-splitting operation is usually carried out before the mining face for the thick and hard main roof.

As shown in Figure 14, the main roof rock is pre-cracked by hydraulic fracturing, and the crack location includes the roof of the coal body to be mined, the roof of the roadway on both sides, and the roof of the working face's open-off cut. After the main roof's pre-splitting, the working face's coal wall pressure is slight during the weighting period. The shrinkage of the pillar is minor, and the roof collapse is closely followed by the support, indicating that the main roof activity is not intense during the fracturing [90–92]; this method has been widely used in pressure relief mining of working faces with the thick and hard main roof.

**Figure 14.** Fracturing operation of the thick and hard main roof. (**a**) Plane diagram of pre-splitting drilling hole (**b**) Side view of boreholes in roadway (**c**) Side view of boreholes in the open-off cut.

#### *4.2. Fracturing Operation of Hard Roof at the End of the Working Face*

For the broken roof of the longwall working face, there is a "triangle" key block structure in the end area [93,94]. When the roof of the working face is hard, it is often challenging to collapse on both sides of the roof, resulting in an excessive hanging roof area. Improper treatment will cause air leakage in the working face, gas accumulation in the goaf, and sudden caving of the sagging roof. Therefore, the hard roof at the end side should be cracked in time, and the borehole layout is shown in Figure 15. Due to the goaf behind the end support, there is a risk of explosive blasting, and hydraulic fracturing is generally used to crack the roof [95,96]. According to the field practice, in the subsequent mining process, the end top plate, after cracking, is no longer suspended but gradually collapses with the advance of the end support, which dramatically reduces the pressure of the support and achieves a good pressure relief effect.

**Figure 15.** Fracturing operation of a hard roof at the end of the working face.

#### **5. Pre-Splitting and Roof-Cutting Technology for Roadway's Pressure Relief**

For gob-side entry, tunneling entry, and deep roadway with a high-stress environment, standard support methods are often tricky to work, and active pressure relief is needed for the position of a continuous deformation [97].

#### *5.1. Large Deformation of Roadway along Goaf*

When one side of the working face is being mined, the phenomenon of large deformation occurs in the local area of the roadway along the goaf of the adjacent digging face, especially in the roadway section within the influence of the working face mining, which rate is speedy and challenging to control [98,99]. In order to reduce the influence of mining-induced stress on gob-side entry as much as possible, pre-splitting and roof-cutting operations are designed in the adjacent roadways of this working face. The borehole layout is shown in Figure 16. By cutting the roof in advance, the continuous structure between the working face and the roof of the gob-side entry is severed, hindering mining-induced stress transmission. Thus, the surrounding rock stress environment of the roadway along the goaf is improved, and the stability of the roadway can be successfully maintained by reinforcing the support means [100–103]. Pre-splitting and cutting the roadway's roof in the overhead section of the working face to unload the pressure for the roadway along the goaf is commonly implemented in the field, which has become an important method to control the large deformation of the roadway along the goaf.

**Figure 16.** Pressure relief technology of roof-cutting for large deformation of roadway along goaf.

#### *5.2. Blasting Pressure Relief Technology for Roadways*

The deformation characteristics have apparent continuity and persistence for the deep high ground stress and robust rheological environment of the roadways. The general reinforcement support method cannot prevent the rheological characteristics of the surrounding rock, and the roadway needs to be expanded and repaired many times. At this point, the key to controlling deformation is to relieve pressure and cut off the transmission path of continuous high stress [104,105]. As shown in Figure 17, illustrates three blasting methods to relieve pressure to cope with substantial deformation of the roof, gangs, and floor. Blasting to relieve pressure means creating cracks by blasting to destroy the integrity and continuity of the rock to release the concentrated stress and cut off the stress transmission path to control the rheology of the surrounding rock in the roadway.

**Figure 17.** Blasting pressure relief technology for roadways.

For the pressure relief of the roadway under the complex high-stress environment, concentrated drilling is often carried out in the surrounding rock of the complex high-stress area. The small-scale pressure relief and fracture area are generated through the deformation and collapse of the drilling hole. At present, directional pre-splitting and roof-cutting pressure relief technologies such as concentrated energy blasting and hydraulic fracturing have been widely applied [28]. According to different surrounding rock environments and the mechanical properties of rock mass, a reasonable pressure relief method is selected to better realize the stability of the roadway under complex conditions.

With the development of the method, the internal pressure relief method and hydraulic pressure relief method of roadway surrounding rock are gradually evolved [106], which have achieved outstanding field application effect in controlling deep continuous deformation roadway.

#### **6. Discussion**

This paper summarizes the four forms of application of pre-splitting and roof cutting technology (explosive blasting, hydraulic fracturing, liquid CO2 gas fracturing, and mechanized roof cutting) and elaborates its process in three major fields (non-pillar mining, working face's pressure relief, and roadway's pressure relief). The following conclusions and prospects are obtained:


**Author Contributions:** Conceptualization, S.X., D.C. and Y.W.; methodology, D.C., Y.W. and F.G.; software, Y.W. and H.Z.; formal analysis, Y.W., H.Z. and F.G.; investigation, Y.W., H.Z. and X.Z.; data curation, Y.W., H.Z. and X.M.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W.; supervision, R.L. and C.W.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (Grant No. 52074296), the Fundamental Research Funds for the Central Universities (Grant No. 2022YJSNY18), the National Natural Science Foundation of China (Grant No. 52004286), the Fundamental Research Funds for the Central Universities (Grant No. 2022XJNY02), the China Postdoctoral Science Foundation (Grant No. 2020T130701, 2019M650895), all of which were gratefully acknowledged.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** No conflict of interest exists in the submission of this manuscript, and the manuscript is approved by all authors for publication.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Yang Liu 1,2,3, Tong Zhang 2,3,\*, Jun Wu 1,2, Zhengyang Song 1,4 and Fei Wang 1,2,\***


**Abstract:** Mining-induced hydromechanical behavior of sandstone is critical to mining safety and disaster prevention. To investigate the evolution behavior of the mechanical and permeability properties of sandstone, mining-induced stress was imitated by increasing axial stress and decreasing confining stress, and a set of hydromechanical experiments were further performed, incorporating the effect of in situ stress, pore pressure, and mining stress. The results show the similar variation tendencies of the deformation and permeability of sandstone under different loading paths of in situ stress and pore pressure. Most sandstone samples maintain a compression state for the peak stress condition. The failure mode evolved from shear failure to shear–tension failure with the increase in in situ stress. The stress-relief effect significantly effects the permeability, since the permeability of sandstone increases exponentially with decreasing effective confining stress. The growth rate of permeability in Stage II is significantly greater than that in Stage I. One order of magnitude of permeability was presented at the peak stress situation. A fitting exponential model based on the alteration of effective confining stress was proposed to describe the permeability evolution dominated by the stress-relief effect, and the discovered permeability model can accurately describe the experimental results. The research results provide significant guidance for understanding the hydromechanical behavior and water hazard prevention for underground coal mines.

**Keywords:** sandstone; deformation behavior; permeability evolution; mining stress path

#### **1. Introduction**

The mechanical properties and seepage characteristics of sandstone, a typical sedimentary rock, have been widely investigated in underground rock mass engineering, such as carbon geological storage, coal mining, water hazard prevention of mines, and so on. As underground activities move towards the deeper crust, the stress–fracture–seepage field behavior in the rock becomes more complex [1–3]. Therefore, the research on rock deformation behavior and permeability evolution under hydromechanical coupling is an urgent basic topic in the field of rock mechanics, which has extreme scientific significance and application prospects for deep underground activities.

Existing related studies mainly concentrated on the mechanical property of the triaxial compression test, and the seepage law in this loading process has been widely and deeply discussed. At present, many researchers have carried out triaxial total stress–strain and cyclic loading–unloading seepage tests on different types of rocks, such as sandstone, limestone, and granite, including intact and fractured rocks [4–9]; analyzed the mechanical and seepage characteristics with rock brittleness–ductility under different confining pressures and osmotic pressures [10–15]; and obtained the variation relationship between permeability and axial stress before and after peak strength [16]. Liu et al. [17] studied

**Citation:** Liu, Y.; Zhang, T.; Wu, J.; Song, Z.; Wang, F. Experimental Study on Deformation Behavior and Permeability Evolution of Sandstone Responding to Mining Stress. *Energies* **2022**, *15*, 7030. https:// doi.org/10.3390/en15197030

Academic Editor: Manoj Khandelwal

Received: 5 August 2022 Accepted: 13 September 2022 Published: 25 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the hydromechanical behavior of sandstone with different physical properties, and modeled the damage variables and permeability evolution of the Weibull random distribution. Xiao et al. [18] investigated the hydromechanical properties of red sandstone, collected in Ganzhou, with triaxial compression seepage tests, and established a piecewise model for stress-dependent permeability to discuss the impact of osmotic pressure on the strength and stiffness of red sandstone before the peak strength. Li et al. [19] carried out triaxial compression tests of sandstone to research the effect of different confining pressures and seepage pressures on the failure mechanism and permeability evolution of reservoir sandstone, and established COMSOL numerical model to accurately inverse the laboratory experimental results. Moreover, CT scanning technology, as a hot visualization technology, is widely used in this research field [20,21].

The current rock mechanics theory is mostly based on the total stress–strain test obtained from the triaxial rock mechanics standardization test [22–24]. However, when analyzing engineering disturbance and disaster, it can be found that this standardized test is separated from engineering activities and does not reflect the influence of in situ stress state and mining disturbance path. Therefore, Xie et al. [25,26] took the lead in proposing the mining stress path of coal under different mining layouts, i.e., protective coal-seam mining, top-coal caving mining, and non-pillar mining, which established a bridge between laboratory experiment and field practice. In addition, corresponding mining-induced mechanical behavior, fracture distribution, and coal permeability evolution were analyzed. Zuo et al. [27] investigated the mechanical behavior of limestone under different stress paths, including the complete stress–strain process and mining stress path. Xia et al. [28] conducted loading and unloading triaxial mechanical tests with real-time ultrasonic monitoring for single fissure sandstone and studied the deformation characteristics, the failure form, and the crack evolution of single fissure sandstone with specific angles and lengths under the fully mechanized top-coal caving mining stress path.

As stated above, most of the existing work focused on the mechanical behaviors and permeability evolution for coal in the mining process. However, there is still a lack of quantitative analysis on the mining-induced mechanical behavior and seepage property of sandstone. Moreover, it would be of great significance to investigate the differences in deformation and permeability responding to the change of the mining-induced underground environment. Therefore, in this study, the influence of in situ stress, pore pressure, and mining-induced stress on the deformation and transport behavior of sandstone was investigated. The damage deformation at different mining-induced stress states was dynamically monitored, and the failure mode under the peak stress condition was analyzed. Besides, the effective stress-based permeability evolution was discussed.

#### **2. Experimental Section**

#### *2.1. Sample Preparation*

A gray sandstone block taken from a coal mine in the Ordos Basin was used in this study. Some 50 mm diameter cylindrical samples were drilled from the sandstone block in the same direction by a core-drilling machine. Then, the cylindrical samples were cut to about 50 mm in diameter and 100 mm in length by a grinding machine with the highprecision standard proposed by the ISRM. The X-ray diffraction (XRD) results show that the sandstone is mainly composed of quartz (59.2%), clinochlore (12.3%), albite (10.1%), and other minerals (18.4%) (Figure 1a). The pore spaces of sandstone are completely dominated by intergranular pore, intercrystalline pore, and microfracture based on the scanning electron microscope (SEM) results (Figure 1b). Moreover, the average density and porosity of the sandstone are 2.44 g/cm3 and 11.2%, respectively.

**Figure 1.** The (**a**) XRD and (**b**) SEM test results.

#### *2.2. Experimental Apparatus*

The hydromechanical tests were carried out using a triaxial servo-controlled permeation experimental apparatus. The apparatus, comprising six parts, a servo loading system, a triaxial cell, a pressure control system, a liquid/gas testing system, a data collection system, and an auxiliary system, can supply an axial and confining stress applied to the samples with a stable temperature during the measurement of permeability in this work. The experimental apparatus is shown in Figure 2, and the equipment parameters were described in detail by a previous study [29].

**Figure 2.** Triaxial servo-controlled mechanical–seepage coupling experimental apparatus [29].

#### *2.3. Experimental Procedure*

Nitrogen with a purity of 99.999% was used as the testing gas, and different in situ stresses (15 MPa, 20 MPa, and 25 MPa) and gas pressures (1 MPa, 1.5 MPa, and 2 MPa) were considered in the experiments. Nine sandstone samples were prepared, and each test corresponded to one sample under one stress state. All the experiments were carried out at 20 ◦C. Moreover, the information on the sandstone samples with designated experimental conditions was summarized in Table 1. The specific mining stress path is described below (Figure 3):



**Table 1.** Information on sandstone samples.

In this study, the permeability of sandstone was obtained by using Darcy's law:

$$k = \frac{2qP\_0\mu L}{\pi \left(\frac{\phi}{2}\right)^2 \left(P\_1^2 - P\_2^2\right)}\tag{1}$$

where *k* is the permeability of the sample, m2; *q* is the gas flow rate, m3/s; *P*<sup>0</sup> is the atmospheric pressure, Pa; *μ* is the gas viscosity, Pa·s; *L* is the length of the cylindrical

sample, m; *φ* is the diameter of the cylindrical sample, m; *P*<sup>1</sup> is the inlet gas pressure Pa; and *P*<sup>2</sup> is the outlet gas pressure, Pa.

**Figure 3.** Loading schemes of different sandstone samples ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

#### **3. Results and Discussion**

*3.1. Deformation Behavior of Sandstone under Mining Stress*

Taking the Biot coefficient remain constant at 1, the effective stress can be expressed as [30]:

$$
\sigma\_{\mathfrak{e}} = \frac{1}{3}(\sigma\_1 + 2\sigma\_3) - P \tag{2}
$$

where *σ<sup>e</sup>* is the effective stress, MPa; *σ*<sup>1</sup> is the axial stress, MPa; *σ*<sup>3</sup> is the confining stress, MPa; and *P* is the pore pressure, MPa. Besides, ignoring high-order small quantities, the volume strain can be approximately expressed as:

$$
\varepsilon\_{\upsilon} = \varepsilon\_1 + 2\varepsilon\_3 \tag{3}
$$

where *ε<sup>v</sup>* is the volumetric strain, *ε*<sup>1</sup> is the axial strain, and *ε<sup>3</sup>* is the radial strain. The mechanical deformation characteristics of the sandstone samples under the mining stress path at Stages I and II are shown in Figures 4–6. As shown in Figures 4–6, the variation tendency of the axial strain, the radial strain, and the volumetric strain of sandstone under different load conditions of the axial stress, confining stress, and effective stress are approximately similar, respectively. The axial strain of the sandstone shows a linear growth trend with the increase in the axial stress, and the growth rates of Stage I and Stage II remain unchanged (Figure 4). For the radial strain, it presents an exponential function growth trend with the unloading of the confining stress, and the growth rate of Stage II is significantly higher than that of Stage I (Figure 5). As for the volumetric strain of sandstone, its trend with the variation of effective stress roughly depicts a transverse "V" symmetrical distribution, which corresponds to the reduction in the effective stress at Stage I and the increase in the effective stress at Stage II (Figure 6: the arrow in the figure indicates the stress path of the effective stress). In detail, at the end of the loading process in Stage II, the volumetric deformation of sandstone changes from compression to expansion with the increase in initial confining pressure. Under the same initial confining pressure, the influence of mechanical deformation in the sandstone is mainly affected by mining stress compared with pore pressure.

**Figure 4.** Axial strain variation of sandstone under different initial confining pressures ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

**Figure 5.** Radial strain variation of sandstone under different initial confining pressures ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

**Figure 6.** Volumetric strain variation of sandstone under different initial confining pressures ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

The peak parameters of sandstone, including deformation and permeability parameters, at the peak stress state in Stage III, are listed in Table 2. To intuitively summarize the corresponding effects of initial confining pressure and pore pressure, Figure 7 is drawn by using the data in Table 2. Under the same initial confining pressure, the peak stress of sandstone reduces with the increase in pore pressure, while the peak stress of sandstone increases with the incremental confining pressure under the same pore pressure. The absolute value of both the axial strain and the radial strain increases with the incremental confining pressure. Under the same confining pressure conditions, the volumetric strain at the peak stress state gradually decreases due to the increase in pore pressure. Most sandstone samples still maintain compression at the peak stress state, except for samples B5 and B8, which was significantly different from the coal samples [26]. The mineral composition and pore–fracture structure are the main reasons for this distinction. The sandstone is mainly composed of brittle minerals and is dense, with fewer pores and microcracks resulting in it being hard to deform. Conversely, the coal is composed of ductile minerals and has more microcracks and cleats, so it becomes more easily deformed at low confining stress. Note that the individual difference in samples leads to a slight deviation between the test results and the overall test law.


**Table 2.** Information of peak parameters of sandy mudstone sample.

**Figure 7.** Peak stress and corresponding deformation of sandstone ((**a**–**d**) are the peak strength, corresponding axial, radial, and volumetric strain, respectively).

#### *3.2. Failure Mode of Sandstone under Mining Stress Conditions*

The failure characteristics of the sandstone at Stage III are shown in Figure 8. Overall, after the sandstone undergoes a simulated mining stress loading path, its failure mode at Stage III is mainly a shear failure, because the low confining stress makes the sandstone more brittle, which is manifested as follows: the macroscopic shear fracture pierces through

the top and bottom end surfaces of the entire sample, so a part of the samples is in shear– tension failure mode, accompanied by some secondary tensile cracks and shear cracks. Under the same pore pressure, the final fracture morphology of the sandstone tended to be complicated with the increase in initial confining pressure. The overall performance is the transition from shear failure to shear–tension failure, which corresponded to the transformation from a single shear fracture to multiple positive and negative shear fractures, which tend to form an approximate "X"-shaped conjugate shear fracture (samples B5 and B8) or to the main shear fracture followed by a secondary shear–tension fracture (samples B7 and B9). At the same time, the number of fractures developed by the final failure form of the sandstone has a good correspondence with the peak permeability under the same initial confining stress, which represents the greater number of fractures, the better connectivity of the pore–fracture system of the sandstone, the increase in the seepage channel of the fluid, and the improved permeation property, which caused the peak permeability to be larger (samples B3, B5, and B8), indicating that the number of fractures has a significant impact on the permeability of the sandstone.

**Figure 8.** Mining-induced failure characteristics of sandstone.

#### *3.3. Permeability Evolution of Sandstone under Mining Stress*

Under the different initial confining stress and pore pressure, the variation characteristics of the permeability of sandstone samples in the mining stress path are shown in Figure 9. It can be seen from Figure 9 that the permeability of sandstone samples, under

the same initial confining pressure and pore pressure, presents a gradual increasing trend in the form of exponential function with the axial loading and radial unloading, which are mainly induced by the confining stress relief, also called the stress-relief effect. In addition, the growth rate in Stage II is significantly greater than that in Stage I. This phenomenon suggests that under the mining activities, the impact of radial unloading on the permeability of the sandstone (the seepage direction is axial) is dramatically higher than that of axial loading; the main reason for this is that under the action of simulated mining stress, the unloading of the confining stress leads to the aperture broaden of the seepage channels, which are mainly composed of pores and throats, in the sandstone. At the same time, the loading of the axial stress also contributes to the unloading of the confining stress, which can be also proven by the obvious increase in the permeability of the sandstone in Stage II, further enlarging the radii of the pores and the throats and increasing the flow capacity of the seepage path in the sandstone, which results in a gradual increase in the permeability of the sandstone.

**Figure 9.** Permeability variation of sandstone under different initial confining pressures ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

To represent the variation of the permeability of sandstone samples at different loading stages intuitively, the permeability growth rate *η* is introduced as follows:

$$
\eta = \frac{k\_i - k\_{ini}}{k\_{ini}} \times 100\% = \left(\frac{k\_i}{k\_{ini}} - 1\right) \times 100\% \tag{4}
$$

where *η* is the permeability growth rate; *ki* is the permeability of sandstone, m2; and *kini* is the initial permeability of sandstone, m2. Under different initial confining pressures and pore pressures, the *η* of the sandstone samples changed with the mining stress, which is presented in Figure 10. The *η* of the sandstone samples in Stage I range from 0% to 20%, and the maximum value is 19.92%, indicating that the mining stress in Stage I has a slight impact on the permeability of the sandstone. Once reaching Stage II, the *η* increases markedly, ranging from 20% to 680%, and the maximum value is 678.81%. With the increasing of the in situ stress, the *η* of the sandstone samples changes more obviously. When the axial stress was loaded to 2.5 times of the in situ stress, and the confining stress was unloaded to 0.2 times of the in situ stress, the *η* of the sandstone samples is the largest; this indicates that at the end of Stage II, the sandstone enters the deformation stage of the stable expansion of the cracks, and a large number of new microfractures in the sample germinate and nucleate, connecting and converging with the original seepage channels, so the enlargement of the radii of the original pores and throats finally leads to a sharp increase in the *η* of the sandstone under this stress state.

**Figure 10.** Permeability change ratio variation of sandstone under different initial confining pressures ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

The relationships between the permeability and the effective stress under the different initial confining pressure and pore pressure conditions are shown in Figure 11. Under the action of mining stress, the permeability of the sandstone samples increases linearly with the decrease in the effective stress in Stage I and increases exponentially with the increase in the effective stress in Stage II. It can be concluded that selecting the confining stress can better reflect its relationship with the permeability, compared with the axial stress and the effective stress.

**Figure 11.** Permeability variation of sandstone under different effective stresses ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).

The peak permeability of the sandstone samples in the peak stress state at Stage III is shown in Figure 12. As seen in Figure 12, on the whole, under the same initial confining pressure, the permeability of the sandstone samples at peak stress gradually increased with the increasing pore pressure, while, under the same pore pressure, the permeability of the sandstone samples at peak stress decreased with the increasing initial confining pressure. The permeability of all sandstone samples at peak stress increased by up to one order of magnitude. The influence of the initial confining pressure on the mining-induced seepage behavior of sandstone is stronger than that of the pore pressure.

**Figure 12.** Peak stress and corresponding permeability of sandstone.

#### *3.4. Permeability Fitting Model of Sandstone under Mining Stress*

The permeability of the rock is a function of the characteristics of rock media, which describes an average property of the ability to transport fluid. As the unloading confining stress is the dominant factor on the permeability change of sandstone in the mining stress path, the permeability is related to the effective confining stress. In Stages I and II of the mining stress path, the sandstone permeability is mainly contributed to by the variation in pore radius because of the change of the effective confining stress. The exponential relationship between the permeability and the effective confining stress is discovered through regression and analysis using the experimental data, which is expressed as Equation (5).

$$k\_i = k\_0 + Ae^{-B(\sigma\_\varepsilon - \sigma\_{i0})} \tag{5}$$

where *ki* is the sandstone permeability, m2; *k*<sup>0</sup> is the initial sandstone permeability, m2; *A* and *B* are fitting parameters; *σ<sup>c</sup>* is the effective confining stress, MPa; and *σc*<sup>0</sup> is the initial effective confining stress, MPa.

In this study, furthermore, the effective confining stress *σ<sup>c</sup>* can be calculated by the following equation:

$$
\sigma\_{\mathfrak{c}} = \sigma\_{\mathfrak{3}} - \nu(\sigma\_1 + \sigma\_{\mathfrak{3}}) + (2\nu - 1)P \tag{6}
$$

where *ν* is the Poisson's ratio of sandstone, with a value of 0.02 obtained by the mechanical tests.

To verify the mentioned permeability fitting model, the theoretical permeabilities acquired using Equation (5) are compared with the testing permeability data for the sandstone in the simulated mining stress path (see Figure 13). In addition, the corresponding fitting parameters used in the model are listed in Table 3. Figure 13 shows that the permeability of sandstone increases slightly with the decrease in effective confining stress in Stage I of mining stress. In Stage II of mining stress, the permeability grows rapidly with the decrease in effective confining stress. It can be clearly obtained from Figure 13 that the analytical permeability results attained from the exponential function fit well with experimental results based on the original experimental permeability data of sandstone, with the R-square value exceeding 0.98, which indicates that the fitting model can fully capture the permeability evolution for sandstone under the mining stress state. Table 3 illustrates that the permeability model coefficient *<sup>A</sup>* has a large variation span, varying from 1.8 × <sup>10</sup>−<sup>6</sup> to 0.21, while that of *B* ranges from 0.23 to 1.32. The fitting accuracy increases with increasing

in situ stress and pore pressure. Notably, the discovered fitting permeability model is validated with data from sandstone samples under the mining stress state only. Whether this model can be employed on other rocks requires further investigation. Moreover, its physical significance also needs further derivation.

**Figure 13.** *Cont*.

**Figure 13.** Permeability of the sandstone samples under different effective confining stresses and pore pressures, and modeling results from the proposed model in this study ((**a**–**c**) are the initial confining pressure of 15 MPa, 20 MPa, and 25 MPa, respectively).


**Table 3.** Parameters used for data fitting.

#### **4. Conclusions**

In this study, a series of mechanical–seepage coupling experiments were conducted, considering the mining stress paths. The response of permeability and mechanical properties was observed. A fitting exponential permeability model was proposed for sandstone. The main conclusions are given as follows:


incremental confining pressure. Under the same in situ stress, the volumetric strain at the peak stress gradually decreases, which is caused by the increased pore pressure. The failure mode of sandstone evolved from shear failure to shear–tension failure with the increase in in situ stress, which corresponds to the transformation from a single shear fracture to form an approximate "X"-shaped conjugate shear fracture or to the main shear fracture followed by a secondary shear–tension fracture.

(3) The sandstone permeability, under the same in situ stress and pore pressure, increases exponentially with the mining stress simulated by axial loading and radial unloading. The permeability under a mining stress state was well-characterized by the stressrelief effect.

**Author Contributions:** Y.L.: Conceptualization, formal analysis, investigation, data curation, writing original draft. T.Z.: Conceptualization, writing—review and editing, funding acquisition, supervision. J.W.: writing—original draft, visualization. Z.S.: writing—review and editing, supervision. F.W.: visualization, supervision. The manuscript was written through the contributions of all authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper was supported by the Institute of Energy, Hefei Comprehensive National Science Center (Grant No. 21KZS216); the National Youth Science Foundation (No. 51904011); the Anhui Provincial Natural Science Foundation (No. 1908085QE183); the Open Fund of State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines (Grant No. SKLMRDPC19ZZ05); and the Collaborative Innovation Project of Colleges and Universities in Anhui Province (Grant No. GXXT-2021-019). The APC was funded by the Institute of Energy, Hefei Comprehensive National Science Center (Grant No. 21KZZ508).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

