Review on Engineering Application Status of Gob-Side Entry Retaining Technology in China

Abstract

Gob-side entry retaining (GSER) technology has been widely used in underground coal mines. However, the applicable conditions of GSER technology in practical engineering still need further clarification to prevent the safety hazards and significant economic losses caused by its failure. In this study, 587 application cases of GSER in China from 2000 to 2024 were collected, and the relevant data, such as geological conditions and key technical parameters of GSER projects and their impact on engineering practices, were systematically analyzed. Considering the technical characteristics and the developing status of GSER technology, the current status of the application of GSER engineering in the mining areas of China was obtained, and then the applicable geological conditions and optimal technical parameters for the effective implementation of GSER technology were identified. Additionally, the existing technical challenges and further prospects of GSER technology were illustrated. This study provides a reference for reasonable applications and further research of GSER technology.

1. Introduction

During the implementation of pillar-free gob-side entry retaining (GSER) technology, an isolation body is constructed along the goaf’s edge in the upper section of the mining face, and corresponding measures for surrounding rock control are adopted, while the haulage roadway in the upper section is retained as the return airway for the lower section of the working face. This method eliminates the coal pillar in the interval of the working face and achieves continuous mining without coal pillars, which can effectively improve the coal resource recovery rate, reduce the excavation rate of the mine roadway, optimize the ventilation method of the working face, improve the working environment of the working face, and achieve the co-mining of coal and gas without coal pillars in low-permeability coal seams. GSER technology has been used in coal mines in China for decades [1,2,3,4,5,6]. Figure 1 shows the mechanized filling process of concrete using GSER technology. However, the surrounding rock of the retained entry experiences at least two impacts from working face mining, posing challenges to its stability maintenance. In recent years, with the gradual transition of China’s underground coal mining to deep mining, engineering conditions have become increasingly intricate. The application of GSER engineering is undergoing a transition from geological conditions characterized by shallow burial, low gas content, near-horizontal, gently inclined, and thin- and medium-thickness coal seam conditions to deep burial, high gas content, large inclination angle, and thick coal seam conditions. The application of GSER technology in complex deep mines faces some new technical difficulties and challenges. In the past few years, an inadequate comprehension of the applicable conditions and challenges of GSER technology has led to its imprudent implementation in coal mines under intricate geological conditions and even the implementation failure of GSER technology, leading to significant safety hazards and economic losses to the coal mine. Therefore, it is necessary to analyze the current application status of GSER technology in coal mines based on engineering conditions and its technical characteristics so as to promote the further development and promotion of GSER technology.

2. The Current Status of GSER Technology Development in China

After over 70 years of development, a relatively comprehensive theoretical and technical framework has been established for the application of GSER technology in China’s mining areas. The development process and research status of GSER technology are roughly as follows: In the 1950s, GSER technology was proposed in China and applied to thin coal seams (below 1.5 m) with simple engineering geological conditions. The construction method and roadway support were both in a simple passive form, with poor GSER effectiveness and high labor intensity. With the accumulation of engineering experience and further exploration, GSER technology has been applied to medium coal seams with a thickness of 1.5–2.5 m since the 1960s. The dense pillars, blocks, and gangue walls were gradually used to construct the entry-side structures. The I-beam support was added to the roadway support, and the implementation effectiveness of GSER technology was improved. Until the 1970s, the above stage can be referred to as the initial and exploratory phase of China’s GSER technology [7].

In the 1980s, Chinese researchers began to explore the mechanism of ground pressure appearance in the entry retaining, proposed the relationship between the roof subsidence of the retained entry and the mining thickness [8], and recognized that the “triangular hanging plate” structure at the end of the longwall working face could affect the stability of the surrounding rock of the retained entry [9]. Since then, mechanized entry-filling technology has been employed with quick-setting cement and hard gypsum as materials, with the development of metal yieldable support in the roadway. These advancements have significantly enhanced the implementation effectiveness of GSER technology under simplified engineering conditions. In the 1990s, mechanized coal mining technology for comprehensive working faces was vigorously boosted in China. The increase in mining height and the accelerated advancing speed in the working face have significantly augmented the difficulty of maintaining the surrounding rock of the retained entry. Although a set of high-water material filling was used in GSER technology, the low strength, slow resistance increase, susceptibility to weathering, and high cost of such materials were inadequate for accommodating the ground pressure in the retained entry. Moreover, the intricate nature and slow construction speed of GSER technology limited its further promotion and application in engineering. During this period, researchers proposed calculation methods for the support resistance and strength of entry-side structures under specific engineering conditions [10] and proposed a supporting principle of “controlling roof and yielding pressure” in the retained entry support [11]. At the same time, the position and time of the triangular hanging roof of the retained entry roof were predicted, providing an important basis for determining the timing of retained entry reinforcement support [12]. The above stage can be referred to as the sustained development stage of China’s GSER technology.

After 2000, China’s GSER technology entered a stage of rapid development and widespread application. The control theory, support technology, equipment machinery, and filling materials for retained entry surrounding rock have achieved significant advancements. The fundamental movement characteristics of retained entry rock layers were clarified, a mechanical model of the filling body and roof was established, and a preliminary calculation method for the resistance of retained entry support and the subsidence of the roof was formed [13]. It is also clearly pointed out that the relevant parameters of the filling body have a significant impact on the subsidence of the retained entry roof [14], and a calculation method for the compression amount of the filling body was proposed [15]. Based on the clarification of the stress transmission mechanism of the wedge-shaped area roof in the retained entry goaf, a reinforced support technology system for retained entry surrounding rock control based on high-prestressed anchor bolts and cables was formed [16,17,18,19,20,21,22]. With the gradual application of GSER technology to thick coal seams and even fully mechanized mining faces, the deformation and stress evolution characteristics of the surrounding rock of GSER in fully mechanized mining faces were investigated [23], and the calculation formula for the support resistance of entry-side filling body under different geological conditions was further improved. It was found that the caving of the roadway roof is directly related to the main roof [24,25]. Particularly in the 21st century, high-performance concrete [26], steel-reinforced concrete piers and columns [27], new ultra-high-water materials [28,29,30], flexible formwork concrete [31], and other construction materials have been continuously developed, as well as the rapid development of technologies related to roof-cutting and pressure relief in the roadway. Consequently, the maintenance effect of the retained entry surrounding rock has been significantly improved, and the study of the stress evolution characteristics of the retained entry surrounding rock has become more in-depth and detailed [32]. Different retained entry construction processes, supporting equipment, and support techniques have been formed according to different engineering geological conditions, and GSER technology has been widely promoted and applied. Engineering practice has also shown that the existing active and passive combined support technology has proven insufficient in effectively achieving stable control of the surrounding rock of retained entry under complex geological conditions. Only by using methods such as presplitting blasting and hydraulic fracturing to break the roof near the goaf and optimize the cantilever structure for roof-cutting can a favorable stress environment be created for the stable control of the surrounding rock of the retained entry [33,34,35]. Subsequently, roof-cutting and pressure relief in thin and medium coal seams and automatic tunneling without coal pillar technology were proposed, and supporting materials and equipment such as constant resistance anchor cables and seam drilling vehicles were developed [36,37,38,39,40]. Besides, GSER technology based on working face backfilling has also begun to be applied in engineering sites [41,42,43], greatly reducing the strong dynamic pressure impact caused by traditional GSER methods. This method provides new ideas for the development and application of GSER technology in China.

Based on the above, the main classification and application conditions of GSER technology systems in China are shown in Table 1. But their applicable conditions in practical engineering still need further study and clarification.

Table 1

Theoretical classification and application conditions of main GSER technology in China.

Classification by the Entry-Side Construction Method	Classification by Entry-Side Construction Materials	Technical Characteristics	Application Conditions	Development Trends
GSER with entry-side filling	GSER with entry-side filling of coal gangue: Gangue bags or the built gangue walls are used to fill the entry side.	Entry-side construction is the commonly used construction method for retained entry in the early days, which is characterized by simple craftsmanship, low cost, and reduced ground stacking of coal gangue. However, the support strength of manually constructed gangue filling walls in the entries is relatively low, resulting in poor control of the roof. The compactness of the filling body is low, and the labor intensity of workers is high, leading to severe air leakage in goaf areas	In thin coal seams with simple burial depth, no gas and spontaneous combustion	Combined with working face filling or roof-cutting.
	GSER with entry-side concrete filling: It includes filling support and concrete filling, concrete block filling, steel-reinforced concrete piers and columns, and flexible formwork concrete filling.	The filling material is mainly composed of cement, sand, gravel and additives. After the dry material is evenly mixed in the ground plant at the designed ratio, it is transported to the underground mine using manual or automatic belts to complete the feeding and mixing with water and transported to the filling point by concrete pump. New materials such as high-performance concrete and flexible formwork concrete have been developed, which have the characteristics of rapid setting, early strength, fast forming, and high pumpability. They exhibit good sealing effect for goaf and are suitable for different engineering conditions and mechanical performance indicators. The highest strength of these materials after the curing time of 28 d can reach 50 MPa.	Good adaptability to mining conditions, suitable for thin, medium, and thick coal seams, especially for deep roadway engineering. The implementation effectiveness of GSER technology is shown in Figure 2.	Research and development of low-cost, large-scale, high-performance concrete materials or their combination with roof-cutting
	GSER with entry-side filling of high-water and ultra-high-water materials	The material is mainly composed of A and B materials mixed in a certain ratio. A and B materials are first mixed with water and stirred separately to maintain nonsolidification for a long time. After being transported to the filling site through two pipelines and mixed, they can be solidified into high-moisture solids in a short time, with a maximum volumetric moisture content of 94%.	Coal seams with shallow burial depth and low ground pressure. The on-site application effect is shown in Figure 3.	Combined with filling mining in the working face or roof-cutting
	GSER with entry-side paste filling	The materials are mainly prepared by proportioning raw materials such as gangue, fly ash, solid waste, and gelling agents. The filling material is prepared into a paste-like thick material and pumped through a dedicated pipeline to the goaf side of the working face to seal the frame and construct a retained wall to support the roof. However, the strength of this material is not high after final solidification.	Thin coal seams with small burial depths and low ground pressures.	Combined with the paste-filling mining of the working face, as shown in Figure 4.
GSER with lateral roof-cutting	GSER with presplitting and roof-cutting self: 110 construction method and N00 construction method	The roof is automatically cut down along the entry-side wall due to the self-weight and overlying rock movement of the goaf-side roof, supplemented by blasting, hydraulic fracturing, etc., then a new entry-side wall can be formed. Consequently, supporting technologies and construction equipment such as advanced roof-cutting, constant resistance anchor cables, and cutting seam drilling vehicles are developed. However, the isolation effect of the goaf needs to be improved.	Coal seams with good geological conditions that are free of gas and spontaneous combustion. The implementation effectiveness of GSER technology is shown in Figure 5.	The sealing method of goaf, the prevention of coal spontaneous combustion, gas and other problems need to be solved.
	GSER with entry-side wooden stacks, dense pillars and roof-cutting	The early method of GSER mainly used passive support such as wooden stacks or dense individual hydraulic supports to support the roadway roof and to some extent achieve goaf-side roof-cutting. Compared with wooden stack support and cutting strength, dense individual supports have slightly better material consumption and labor intensity, and the effect of roof-cutting on goaf isolation is poor.	Thin coal seams with good geological conditions, shallow burial depth, no gas, and no spontaneous combustion	Currently, there are few applications available.

3. Parametric Study of GSER Application Status

Based on 587 application cases of GSER technology in coal mines of China published in relevant literature from 2000 to 2024, statistical and theoretical analyses were conducted on seven key parameters that directly affect the selection of GSER technology, including average burial depth, thickness, dip angle of coal seams, entry-side construction method, filling body width, and entry section shape.

3.1. Average Burial Depth of Coal Seams

Early GSER technology in China was mainly applied to shallow and simple coal seam mining conditions and gradually applied to medium-depth and deep, complex coal seam conditions with the continuous improvement of its technical system and changes in coal resource mining conditions. Among 587 application cases of GSER technology, a clear indication of the average burial depth of coal seams was found in 360 cases. The project with the shallowest average burial depth of coal seams is in the Jingu Coal Mine in Shanxi Province, with an average burial depth of only 65 m. The GSER along the fully mechanized mining face 14-31010 in No.12 Mine, Pingdingshan Coal Group Co. Ltd., located in Henan Province in China, demonstrates the deepest burial depth of coal seams among all engineering cases, with an average depth of 1112 m. Figure 5 shows the application of GSER technology under the average burial depth of different coal seams at a statistical interval of 100 m.

As shown in Figure 5, there are 34 application cases of GSER engineering projects with an average burial depth of less than 200 m in coal seams, 266 cases of GSER engineering projects in coal seams within a burial depth of 200–700 m (accounting for a total of 73.89%), and 60 cases of GSER engineering projects in coal seams buried with an average burial depth exceeding 700 m. As shown in Figure 6, when the average burial depth of 700 m is taken as the boundary for the medium burial depth of coal seams, the current GSER technology is mainly applied to the conditions of medium burial coal seams in China. When the burial depth exceeds 400 m, the number of engineering applications gradually decreases with the increasing burial depth. The main reason is explained as follows. With the increase in mining depth, the range of mining impact and mining stress intensity on the working face gradually increases, leading to increased difficulty in controlling the surrounding rock of the retained entry. Additionally, the mechanical performance requirements for the entry-side structures also increase accordingly, especially under the condition of burial depth above 700 m. The existing retained entry surrounding rock support technology cannot fully achieve efficient, safe, and stable control of the retained entry surrounding rock.

3.2. Average Thickness of Coal Seams

China’s GSER technology originated from thin coal seams and gradually spread to medium-thick coal seams. As the thickness of the coal seam in the working face increases, the activity range of the overburden in the mining area generally increases, resulting in a greater intensity and impact range of the corresponding ground pressure during the GSER period. In thick coal seams, GSER technology needs higher requirements for roadway support technology and entry-side structure performance. Among 587 cases of GSER engineering projects, 497 cases provide data on the average thickness of coal seams. The thinnest coal seam in the GSER engineering projects is found in the Bolin Coal Mine of Sichuan Coal Group, with a thickness of 0.48 m. The thickest coal seam in the GSER engineering projects is in the Wangjiashan Coal Mine of Gansu Jingmei Company (Baiyin, China), with an average thickness of 15.5 m. However, this working face used steeply inclined horizontal segmented fully mechanized mining, with a retained entry height of only 2.5 m. Using the average thickness of 0.5 m coal seam as the statistical interval, the application of GSER engineering with different average thicknesses of coal seams is shown in Figure 6.

As shown in Figure 6, there are 115 cases of GSER engineering projects in coal seams with an average thickness of less than 1.5 m and 239 cases in medium-thick coal seams with a thickness of 1.5–3.5 m, accounting for 48.10% of the total. Besides, there are 136 cases of coal seams with a thickness of 3.5–8.0 m and 7 cases of coal seams with a thickness greater than 8.0 m. Among them, the majority of the average coal seam thickness is 1.0–1.5 m in GSER engineering projects, accounting for 16.50% (82 cases) of the total. It can be seen that when the average thickness of the coal seam exceeds 2.5 m, the number of GSER engineering projects significantly decreases with the increase in the average thickness of the coal seam. At present, the average thickness of coal seams in China’s GSER engineering projects generally does not exceed 4 m. Even when the thickness of coal seams exceeds 4 m in GSER engineering projects, the mining parameters of the working face are adjusted to reduce the mining height of the coal seams at the position of the entry-side structures in order to optimize and control the size of the filling body and ensure the effectiveness of GSER technology and entry retaining speed. Therefore, the current GSER technology in China is more suitable for coal seams with medium thickness.

3.3. Average Dip Angle of Coal Seams

The change in the dip angle of coal seams affects the fracture structure of the goaf-side roof after the working face is mined, which in turn affects the stability control of the retained entry roof and entry-side structures. When the average dip angle of the coal seam is large, anti-toppling measures are usually required for entry-side filling, which increases the complexity of GSER construction to a certain extent. Among the 587 collected cases of GSER engineering projects, 427 cases provide data on the average dip angle of coal seams. The coal seam with the smallest average dip angle in GSER engineering projects is in Xin’an Coal Mine in Shuangyashan City, Heilongjiang Province, with an average dip angle of 1°. The coal seam with the largest dip angle is in the Boyucun Coal Mine in Liupanshui City, Guizhou Province, with an average dip angle of 67°. Figure 7 shows the coal seams with different average dip angles of coal seams at a statistical interval of 5° in GSER engineering projects.

As shown in Figure 7, there are 396 cases of GSER engineering projects in nearly horizontal and gently inclined coal seams with an average dip angle of less than 25°, accounting for 92.74%, and there are 33 cases of GSER engineering projects in inclined and steeply inclined coal seams with an average dip angle of greater than 25°, accounting for 7.26%. It indicates that the current GSER technology in China is mainly applied to near horizontal and gently inclined coal seams. As the dip angle of the coal seam increases, the number of GSER application projects gradually decreases. Therefore, the current GSER technology in China is widely used in nearly horizontal and gently inclined coal seams with an average dip angle of less than 25°.

3.4. Construction Method of Entry-Side Filling Wall

According to the actual engineering application on site, the construction methods of filling walls in China’s GSER application include wooden stack support, gangue filling, fly ash filling, concrete filling, (ultra) high-water material filling, paste material filling, roof-cutting, and entry retaining. The entry-side construction method of the filling body affects the selection of the overall process system for GSER and its effectiveness. The selection should consider the geological conditions of the GSER engineering projects and the structure and deformation characteristics of the surrounding rock of the retained entry. Among 587 cases of GSER engineering projects, 463 cases have identified the construction method of entry-side filling walls. Figure 8 shows the application of different entry-side filling structures in GSER engineering projects.

As shown in Figure 8, concrete filling beside the entry is the most common construction method applied in China’s GSER engineering projects, comprising a total of 160 cases and accounting for 34.56%. After 2010, with the proposal and development of GSER with roof-cutting, the application of GSER with presplitting and roof-cutting in on-site engineering has gradually increased. The statistical results of its engineering cases are 160 cases, accounting for 27.61%. The engineering cases of GSER with entry-side filling of high-water materials, gangue materials, and paste materials are 74, 59, and 28, respectively, accounting for 15.98%, 12.74%, and 6.05%, respectively. The construction methods of other main entry-side filling structures, such as GSER with wooden stack support and GSER with fly ash filling, are less commonly used due to their limited usage conditions. Based on the curve in Figure 9, it can be seen that GSER with concrete filling and GSER with presplitting and roof-cutting are the most commonly used methods. However, regardless of the construction method of the filling body, the combination of stress optimization technology in the retained entry in practical applications can improve the effectiveness of GSER to a certain extent.

3.5. Entry-Side Filling Width

In the application of GSER technology, the entry-side filling width is one of the key parameters that directly affect the stability of the surrounding rock and the entry retaining speed. If the filling width is too small, the bearing strength and stability of the filling body are difficult to adapt to the strong ground pressure during the GSER. Conversely, if the filling width is too large, it leads to higher construction material costs and slower construction speed, thereby impacting the mining efficiency of the working face. Therefore, the reasonable filling width should be scientifically determined based on key engineering parameters such as burial depth, thickness, and dip angle of coal seams. The mechanical properties of different materials used in the entry-side filling can also affect the determination of the filling width. Among 587 cases of GSER engineering projects, 149 cases provide the data on actual entry-side filling width. Figure 9 shows the application of GSER engineering projects with different filling widths at a statistical interval of 1 m.

As shown in Figure 9, the entry-side filling width does not exceed 5 m in 149 GSER projects. Among them, there are 84 cases using a 1–2 m filling width, accounting for 56.38%; 39 cases with a filling width of 2–3 m, accounting for 26.17%; 16 cases with a filling width of 0–1 m, accounting for 10.74%; and 8 cases with filling width of 3–4 m, accounting for 5.37%. From the curve distribution in the figure, it can be seen that the reasonable range of entry-side filling width in China’s GSER project is 1–3 m in order to meet the requirements of GSER and achieve better economic and technological benefits.

3.6. Section Shape of the Retained Entry

Due to the influence of the geological structure of coal seams, entry-side construction equipment and techniques, and supporting techniques, the rectangular section shape is generally used in coal mining roadways in China. However, when the rock self-stability of the roadway roof is poor, the coal seam inclination angle is large, or the horizontal stress perpendicular to the roadway axis is high, nonrectangular section shapes such as trapezoidal and semicircular can also be used. In 587 recorded cases of GSER engineering projects, 307 cases provide the data on cross-sectional shapes of the retained entry, mainly including rectangular, trapezoidal, inverted trapezoidal, oblique trapezoidal, arched, and other forms. Figure 10 shows the application of GSER engineering projects with different cross-sectional shapes.

Currently, rectangular sections are predominantly utilized in GSER engineering projects in China, encompassing 242 engineering cases and constituting 78.83% of the total. Choosing rectangular sections for entry retaining is also more conducive to the construction and stability of the entry-side filling body. The trapezoid sections are also used in GSER engineering projects, with 32 cases accounting for 10.42%. There are a total of 33 cases using other section shapes, accounting for 10.75% of the total sample size.

3.7. Length of the Retained Entry

The length of the retained entry is a key parameter in engineering applications for selecting GSER technology and controlling the retained entry surrounding rock. The influence of engineering geological conditions, such as the burial depth of the working face, thickness, and dip angle of coal seams, should be fully considered to determine a reasonable length of the retained entry. The longer the length of the retained entry, the longer the time for stable maintenance of the surrounding rock. Considering the strong ground pressure characteristics during the GSER implementation, achieving effective control over the surrounding rock of the retained entry becomes increasingly challenging. Therefore, the long-term stability requirements for the filling body and supporting structure of the retained entry surrounding rock are elevated. Among the counted 587 cases of GSER engineering projects, 66 cases clearly provide the length of the retained entry. The largest length of the retained entry is in the Xuehu Coal Mine of Henan Shenhuo Group, with a retained entry length of 2059 m. The high-water materials are used for the entry-side filling in the GSER engineering projects. Figure 11 illustrates the application of different lengths of the retained entry in GSER engineering projects with a statistical interval of 500 m.

It can be seen that the number of GSER projects with a retained entry length of 0–500 m is the highest, followed by those with a retained entry length of 500–1000 m. The above cases account for 77.27% (51 cases) of the total GSER projects. There are 12 cases with a retained entry length of 1000–2000 m and only 3 cases with a retained entry length exceeding 2000 m. As shown in Figure 11, the number of GSER projects in practical engineering gradually decreases with the increase in retained entry length. Currently, the general length of retained entries does not exceed 1000 m. The main reason is that the current surrounding rock control technology and the construction technology of entry-side filling cannot meet the needs of long-distance GSER engineering applications.

4. Discussion and Future Prospective

Based on the current status of GSER technology and the above analysis, the results show that current GSER technology is mainly applicable to the engineering conditions of the nearly horizontal and gently inclined coal seams with a thin and medium thickness, a buried depth of less than 700 m, and an average inclination of less than 25°. The concrete filling and presplitting roof-cutting techniques are mainly used in the entry-side construction model, and the width of the GSER filling body is generally 1–3 m. The retained entry section is predominantly rectangular in shape, with a length typically not exceeding 1000 m.

However, the promotion and application of GSER technology still encounter many new challenges, and they need to be further studied, such as insufficient adaptability control of surrounding rock, slow speed of GSER, and low intelligence levels. So, the authors identified the following four issues that need further research at present.

4.1. The Surrounding Rock Control Technology Adapted to Ground Pressure Appearance in GSER Technology

Based on the deformation and failure characteristics of the surrounding rock in the retained entry, a support technology system based on active support with high prestressed anchor bolts and cables + secondary strengthening support with the assistance of scaffolding has been formed in China. However, due to the impact of two mining operations on the retained entry surrounding rock, the commonly used primary support and roof-cutting and pressure relief techniques are still unable to fully control the large deformation of the retained entry surrounding rock. To meet the size requirements of the next mining face, the retained entry surrounding rock needs to be reinforced twice before mining, and wall expansion and floor heave [44,45,46] flattening need to be conducted after mining. Consequently, the support and labor costs are relatively high. With the increasingly complex mining conditions in China, higher requirements have been put forward for the adaptability of the existing retained entry surrounding rock support technology. Therefore, the characteristics of ground pressure appearance and the development trend of support technique under complex engineering conditions should be considered to further improve the retained entry surrounding rock control technique under complex engineering geological conditions.

4.2. Rapid Construction Technology and High-Quality Filling Materials for GSER Technology

After more than 70 years of continuous research and engineering application, China’s GSER technology has formed a GSER mode based on different construction methods of entry-side filling bodies according to different geological conditions and engineering needs. Among them, the filling systems of the entry-side concrete filling, paste filling, high-water material filling, and other material filling need to be established above and below the well. Moreover, the amount of filling materials used in the entry is large, leading to the high cost of entry-side filling, while its performance cannot fully meet the long-term strong effect of ground pressure of the retained entry. Engineering applications have shown that the employment of GSER technology inevitably increases the complexity of the mining process in the working face. In recent years, with the rapid increase in coal mining intensity and raw coal production in China, working face mining speed has been increasing day by day. Currently, the entry retaining speed cannot meet the requirements of rapid advancement of the working face. Therefore, it is necessary to further deepen research on fast GSER technology by combining the problem of the mismatch between entry retaining speed and rapid mining in the working face. Additionally, high-quality filling materials with low costs, large-scale mass production, and suitability for GSER implementation processes should be developed.

4.3. Intelligent GSER Equipment and Technical Systems

With the intelligent mining of coal mines becoming a strategic development trend in China’s coal industry, intelligence mining has become the fundamental guarantee for coal resources. With the help of technological progress and national and local policies, relevant research institutions and equipment manufacturing enterprises have initially formed relatively systematic intelligent mining equipment, system control platforms, and supporting mining processes through targeted technology research and development. However, compared with the development level of intelligent mining technology and equipment in coal mines, it is necessary to systematically research and develop intelligent equipment, intelligent control systems, and intelligent evaluation systems for GSER technology in China.

5. Conclusions

This work statistically summarized the current application status of GSER technology based on 587 practice cases in China, and then the applicable geological conditions and optional technical parameters of GSER technology were clearly presented. And the development status in China and the existing challenges of GSER technology were also systematically described. The following conclusions were drawn:

(1) After more than 70 years of continuous research and engineering application, China’s GSER technology has formed a relatively complete theoretical and technical system. GSER technology can be divided into two main modes: GSER with entry-side filling and GSER with cutting roofs. In recent years, there has been a growing inclination towards amalgamating and integrating these two modes of GSER.

(2) The application of GSER engineering projects and its technical characteristics indicate that this technology is currently mainly applicable on-site for near horizontal and gently inclined coal seam conditions with an average burial depth of less than 700 m, an average thickness of 1–3.5 m, and an average dip angle of less than 25°. The entry-side structure construction mainly adopts concrete filling and presplitting roof-cutting methods, and the entry-side filling width is generally 1–3 m. The cross-section of the retained entry is mainly rectangular, and the length of the retained entry generally does not exceed 1000 m. The relevant data provide a reference for the applicable geological conditions and optimal technical parameters for the effective implementation of GSER technology in coal mines and the prevention of safety hazards and significant economic losses caused by its failure.

(3) Although current GSER technology has been successful and widely applied in coal mines, it still encounters many challenges in dealing with complex engineering geological conditions. For instance, the insufficient adaptability of the retained entry surrounding rock control technology and the imbalance between the entry retaining speed and the advancement speed in working face need further research and solutions. In particular, an intelligent GSER technology system compatible with intelligent construction urgently needs to be innovated.

Author Contributions

Conceptualization and Methodology, Y.Z.; writing—original draft preparation and writing—review and editing, Y.Z., J.D. and Q.C.; visualization, Y.Z., J.D. and Q.C.; data curation, Y.Z. and J.D.; funding acquisition, Y.Z.; supervision, Y.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to express their appreciation to the authors who published the papers about 587 application cases of gob-side entry retaining in China from 2000 to 2024, without the data in their published papers, there would be no presentation of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. The schematic diagram of the concrete mechanized filling process of the GSER.

Figure 2. GSER with concrete filling of Guqiao coal mine in 780 m buried depth.

Figure 3. GSER with entry-side filling of high-water material of Liangshuijing coal mine in 130 m buried depth.

Figure 4. GSER with paste filling in filled working face of Daizhuang coal mine in 440 m buried depth.

Figure 5. Statistical chart of the GSER engineering projects at different buried depths.

Figure 6. Statistical chart of the GSER engineering projects with different coal seam thicknesses.

Figure 7. Statistical chart of the GSER engineering projects with different coal seam dip angles.

Figure 8. Statistical chart of the GSER engineering projects with different construction methods of the filling body.

Figure 9. Statistical chart of the GSER engineering with different filling widths.

Figure 10. Statistical chart of the GSER engineering projects application with different section shapes.

Figure 11. Statistical chart of the GSER engineering projects with different lengths of the retained entry.

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