Isolated Overburden Grout Injection Technology Mining and Grouting Parameters Discussion and Redesign

doi:10.21203/rs.3.rs-3604906/v1

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Isolated Overburden Grout Injection Technology Mining and Grouting Parameters Discussion and Redesign

https://doi.org/10.21203/rs.3.rs-3604906/v1

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The "Three down" coal resource mining, ground subsidence management, and fly ash waste treatment are carried out using the Isolated Overburden Grout Injection Technology, which embodies the idea of green mining. The concurrent mining and grouting of bed-separation voids beneath key strata are intricately intertwined and subject to space and time constraints. Isolated Overburden Grout Injection Technology may be successfully analyzed using the large-deflection inclined thin plate mixed with the slurry model. Nevertheless, the diversity in stratigraphic parameters, including main roof and principal key strata (PKS), across various mines, as well as variations in design parameters such as working face dimensions, mining height, coal seam angle of inclination, mining speed, and grouting parameters like the number of grouting holes, start and finish times, and slurry consistency, ultimately contribute to discrepancies in outcomes such as PKS deflection and Bed-separation development, as well as the maximum subsidence and extent of the ground subsidence basin. To ensure the precise and efficient protection of village structures, the extraction of coal resources, and the proper disposal of fly ash waste, it is imperative to conduct a comprehensive analysis of the impact of each parameter of the model. This analysis will help elucidate the mining and grouting process and facilitate the rational design of the relevant parameters. This paper categorizes and discusses the impact of each parameter on the outcomes, using the case study of the 7221 work face grouting operations in Huaibei, Anhui Province, China. In order to fulfill the protection needs of the Gaochangying and Houlou Gaojia villages, modifications were made to the mining and grouting sections' parameters. These modifications evaluated the potential benefits of increasing the original design of coal seam resources and implementing fly ash backfill.

Key Strata Theory

Isolated Overburden Grout Injection Technology

Large deflection inclined thin plate

Ground subsidence control

Parameters redesign

Isolated Overburden Grout Injection Technology Mining and Grouting
PKS deflection and Bed-separation development
Ground subsidence control and building protection
Parameter redesign of the grouting work face
Coal resource extraction and fly ash backfill

After an extended period of mining activities in China, it has been observed that coal resources are predominantly concentrated in the "Three down" areas, including buildings, water bodies, and railroads[1, 2]. By the principles of sustainable mining practices, filling mining technology has gained significant traction in the scientific acquisition of coal resources categorized as the "three down" [3]. Implementing advanced mining technology guarantees the comprehensive extraction of coal resources while facilitating the reintegration of gangue trash back into the mining site. The mitigation of ground deformation and the proper disposal of waste material during mining operations can significantly decrease environmental pollution and facilitate the implementation of environmentally friendly and sustainable coal mining practices [4, 5]. The mining process can be classified into two categories based on the location of waste recharges: extraction area filling mining [6] and bed-separation filling mining. The extraction area filling mining mostly involves the deposition of waste materials within the designated extraction area after the extraction of coal seams. The classification of filling materials is based on their state, which includes water-sand filling [7], solid filling [5, 7], paste filling [8, 9], and slurry filling [10]. Bed-separation filling mining refers to introducing filler material into a cavity, primarily via the Isolated Overburden Grout Injection Technology. Extraction area filling offers several benefits, including simplified operational procedures, proficient management of waste materials, and the utilization of well-established technological advancements. Implementing the Isolated Overburden Grout Injection Technology operations has been shown to have minimal impact on the routine mining activities of the work face. This technique also allows for improved management of ground subsidence, hence reducing potential damage to neighboring structures [11, 12].

The underground mining-induced structural degradation of overloaded rock strata and the progression of bed separation is a multifaceted phenomenon influenced by spatial and temporal factors [13, 14]. The Chinese scholar Qian Minggao's key strata theory provides the foundation for developing the Bed-separation [15, 16]. The deposition of rock strata with differing levels of softness and hardness resulted in the formation of strata. Among these, a distinct type of stratum is known as the key strata, characterized by its hard composition and notable flexural and tensile strengths. These key strata serve a crucial role in maintaining the deflection of the overall rock strata. Specifically, the deflection of the thicker rock layer, which possesses a high Young's modulus and tensile strength, is comparatively lower than that of the underlying rock layer. This discrepancy leads to the formation of cavities between the two layers, known as bed-separation cavities.

The Isolated Overburden Grout Injection Technology involves the creation of boreholes in the earth and the subsequent injection of a high-pressure filling slurry into cavities generated during rock formations' movement. This process utilizes the tension released by the slurry to support the important layers located above [17]. Soviet scientists initially introduced grouting as a means to mitigate sinking. Subsequently, tests investigating this technique were conducted in Poland throughout the 1980s [18]. The exploration of this technology in Australia commenced in 2000 [19, 20]. In 1985, the pioneering engineering experiment on the Isolated Overburden Grout Injection Technology was carried out by Chinese scholar Fan Xueli at the Tiger Terrace Mine in Fushun [21]. Nevertheless, the current subsidence mitigation rate remains modest, resulting in a significant variation in ground sinking that fails to provide the necessary safeguards for above-ground structures. In the year 2000, Xu Jialin introduced the method known as "Isolated Overburden Grout Injection" to ensure that the ground surface deformation remains below the specified threshold as stipulated in the building protection standard [22]. The abovementioned technique was used in the Huaibei mining region in Anhui Province, China. Its application resulted in the mitigation of ground deformation during work-face mining. As a result, the objective of coal mining without the need for village relocation was effectively achieved [23, 24].

Following the proposal of the Isolated Overburden Grout Injection technology, scholars predominantly employed a method wherein they estimated the spatial volume change by subtracting the volume of the extraction area from the volume of the injected slurry. Subsequently, the researchers incorporated the Probability Integration Method (PIM) into their analysis to compute the magnitude of ground sinking [22, 25]. The present methodology is limited to estimating the magnitude of ground deformation resulting from slurry-filling mining activities. The current model cannot adequately consider the complex interplay between coal seam mining, injection activities, and the geological layers. This paper presents the author's theoretical framework for analyzing large deflection-inclined thin plates, which draws upon established ideas in plate theory, fracture mechanics, and the energy method. This study analyses the mechanical aspects of mining operations involving inclined coal seams, specifically focusing on the deflection process, dynamic development of bed separation, and ground deformation of the underlying critical strata after mining activities. Subsequently, the slurry was introduced into the inclined Bed-separation cavities, where the principles of non-Newtonian hydrodynamics and slurry rheology were employed to investigate the impact of stress release on the crucial strata. Additionally, a model was developed to analyze the behavior of large-deflection inclined thin plates when subjected to the influence of the slurry. The model was effectively utilized in the 7221 work face of a mine in Anhui, China. This application not only provided a dynamic demonstration of the mechanical interaction between coal seam mining, grouting activities, and the ground but also quantitatively assessed the actual reduction in subsidence achieved through the implementation of the "Isolated Overburden Grout Injection" technology in the mining area.

Once the model for a large-deflection inclined thin plate subjected to the effects of slurry is formulated and implemented in the practical mining of various work faces in different mines, a set of variables needs to be resolved inside the model. The variables under consideration encompass three primary domains: the physical characteristics of the overlaying strata, the design of the mining operation at the working face, and the parameters related to the design of the slurry. The overarching strata predominantly encompass physical parameters, such as the spatial arrangement of the primary key strata, their thickness, and the lithologic characteristics of the strata. The primary factors considered in work face design are the dimensions of the mining area, including length, width, and height, as well as the degree of inclination of the coal seam. Additionally, aspects such as mining speed are also taken into account. The slurry design parameters encompass various factors, such as the number of holes, commencement and conclusion time, slurry viscosity, and additional physical parameters. The design of the "Isolated Overburden Grout Injection" technology and the resulting PKS deflection and ground subsidence are influenced by several physical characteristics. Consequently, utilizing the previously established large-deflection inclined thin plate model influenced by slurry, the present study categorizes and examines the impact of stratigraphic parameters, work face, and slurry design parameters on the deflection of the PKS and ground subsidence. Furthermore, a comprehensive set of parameters has been acquired in the context of a mining operation at the 7221 work face in Anhui Province, China. These parameters encompass rational assumptions and have facilitated the re-design of the mining and grouting processes at the abovementioned work. In order to assess the model's universality and stability, enhance its applicability to different mining regions, and establish a more robust theoretical foundation and practical guidelines for the "Isolated Overburden Grout Injection" technology, it is imperative to conduct further evaluation.

The subsequent sections of the paper are organized in the following manner: The subsequent section of the study describes the effects of slurry and the results of measurements of stratigraphic physical parameters, mining parameters, and slurry parameters specific to the 7221 work face. The fourth section of the study classifies and examines the impact of individual parameters in the design of the work face on both the height of the Bed-separation development and the volume of the cavity. The final section of the study classifies and examines the contributory influence of individual parameters of the injected Bed-separation cavity slurry on the deflection of the PKS. The sixth section of this study integrates the impacts of various parameters. It presents a revised parameter design scheme for slurry filling mining, tailored to meet the mining demands of work face 7221. Furthermore, a comprehensive comparison and analysis between the proposed scheme and the mining outcomes are conducted. The seventh section summarizes the model's universality and assesses its practical application impact.

2.1 General overview of the study area

This study selected 7221 work face of a mine in Huaibei, Anhui Province, China, with mining start and end time from December 6, 2017, to June 3, 2018. The 7221 work face has a strike length of 588m, an inclination length of 100m, a striking dip of 6.8°, an inclination dip of 8.2°, and a coal seam mining thickness of 4.6m. The corner point of the cutting line of the working face and the upper mountain boundary is -351m, the corner point of the closing line and the lower mountain boundary line is -433.4m, and the ground elevation is 28m. The thickness of the bedrock is 220.58m, and the thickness of the thick loose layer ranges from 158.42m to 240.82m. The spatial distribution and geological structure of the working face are shown in Fig. 1. Due to the inclination angle of the coal seam, the bedrock above the opening and closing line is loaded by a loose layer with a thickness difference of 82.4m after mining the coal seam with a longer mining length.

2.2 The engineering geology

Analyze the rock distribution and physical properties of rock strata in a drill hole data above the working face, and determine the distribution of key strata of bedrock above the working face. The information of each rock strata number, name, thickness, depth from the surface, rock strata classification schematic, and key strata distribution were obtained from drilling, as shown in Fig. 2.

Concerning the physical properties of each rock strata in the actual borehole measured by the mine and related studies, the parameters of each rock stratum in the above borehole, such as density, tensile modulus, shear modulus, elastic modulus, cohesion, internal friction angle, and Poisson's ratio, were obtained as shown in Table 1.

Table 1

Reference table of physical properties of each rock strata in drill holes
Lithology	Density (kg/m³)	Tensile strength (MPa)	Elastic modulus (GPa)	Shear modulus (GPa)	Cohesion (MPa)	Angle of internal friction (°)	Poisson's ratio
Loose layer	2200	0.1	0.3	0.115	1.5	20	0.3
Mudstone	2600	1.2	13	5.28	1.73	31.3	0.23
Siltstone	2700	4.41	22.9	9.7	4.74	31.4	0.18
Coarse sandstone	2700	4.02	34.2	14.37	5.17	37.5	0.19
Fine sandstone	2700	4.34	34.1	14.96	3.15	35.9	0.14
Coal	1300	0.3	5.6	2.15	0.5	25	0.3

2.3 Grouting parameters for 7221 work face

The 7221 work face mining method incorporates the utilization of Bed-separation grouting as a technique to manage waste material and mitigate ground subsidence. Five grouting holes were strategically positioned to facilitate the grouting process within the internal cavity of the Bed-separation layer above the 7221 work face. Table 2 displays the quantity of grouting holes, spatial coordinates, drilling depth, and the initiation and cessation time of grouting within the grouting holes.

Table 2

Parameters of grouting holes of 7221 work face
Grouting holes number	Grouting holes x-axis coordinates(m)	Grouting holes y-axis coordinates(m)	Grouting holes depth(m)	Grouting start time	Grouting finish time
No.1 grouting hole	3709975	451047	-295.7	January 8, 2018	March 12, 2018
No.2 grouting hole	3710032	451055	-303.4	January 29, 2018	July 14, 2018
No.3 grouting hole	3710183	451064	-325.2	March 3, 2018	May 22, 2018
No.4 grouting hole	3710232	451072	-331.2	March 17, 2018	July 14, 2018
No.5 grouting hole	3710305	451079	-329.2	April 17, 2018	July 14, 2018

In identifying the presence of coal seams, it is important to conduct drilling operations to ascertain the thickness of each successive rock stratum and extract core samples to measure diverse physical characteristics. Each rock stratum above the 7221 work face consists mostly of two critical strata that will directly influence the process of coal seam excavation and slurry filling mining. Upon excavation of the 7221 work face, it is anticipated that the initial uppermost strata, known as the main roof, will experience fracturing and subsequent collapse. The fracture trace angle of the main roof thin plate and the initial and periodic pressure steps are directly influenced by the physical parameters of the main roof strata. The grouting slurry is situated within the Bed-separation cavity beneath the PKS. The physical characteristics of the PKS directly influence the progression of Separation and, therefore, affect the parameter design of the Bed-separation grouting. This section focuses on the selection of thicknesses and physical qualities of the PKS in the primary roof and curvature zone located above the coal seam. The objective is to investigate stratigraphic parameters' impact on the PKS deflection and the Separation.

3.1 Influence of main roof thickness and lithologic parameters on the top plate

The initial stratum within the cave zone, known as the key strata, experienced breaking, resulting in increased strain on the roof above the active mining area. The main roof thin plate's fracture trace angle and breakage distance are determined by applying the large-deflection inclined thin plate model. These calculations serve as a theoretical foundation for predicting the initial and periodic incoming pressure steps. In general, an increase in the thickness of the main roof is associated with a decrease in the angle between the initial fracture trace and the working face. Simultaneously, there is an increase in the magnitude of the initial incoming pressure step. However, it is important to note that this increase in incoming pressure results in a more pronounced impact.

The actual detection of the major roof above the 7221 work face is siltstone with a thickness of 13.15 m. The fracture trace angle and incoming pressure stepover data acquired by computation are presented in the initial row of Table 3. This study assumes a thickness of 8.15m and 18.15m for the main roof. The rock layers are replaced from siltstone to fine sandstone for computation. The corresponding results can be found in the remaining rows of Table 3. The practical computations of the model of a thin plate with large deflection and inclination exhibit similarities to the corresponding theoretical framework. Specifically, when the primary roofs are thicker, the fracture trace angle decreases while the first incoming pressure step increases.

Table 3

Results of the effect of different parameters of the main roof on the fracture trace and the step of incoming pressure
Main roof thickness and lithotype	Initial breakage distance (m)	Initial fracture trace angle (°)	Periodic breakage distance (m)	Periodic fracture trace angle (°)
13.15m siltstone	36	57.2	17	54.3
8.15m siltstone	25	60.7	16	54.8
18.15m siltstone	46	55.4	17	54.3
13.15m fine sandstone	36	57.1	17	54.3

3.2 Effect of PKS thickness and lithologic parameters on PKS deflection and Bed-separation development

The primary location for the grouting process, Isolated Overburden Grout Injection, is the Bed-separation cavity under the PKS within the collapsed zone. Hence, the physical characteristics of the PKS influence the bending of slender plates and hold significance in forming Separation. The PKS was acquired as a finely-grained sandstone with a thickness of 18m, as determined by the drilling parameters. The first row of Table 4 presents the outcomes of the large-deflection inclined thin plate model, which was utilized to compute the maximum deflection height of the PKS and the development of bed separation. Subsequently, this study posits three different thicknesses for the PKS layer, namely 8m, 13m, and 21m. The PKS rock layer is then substituted with siltstone, and calculations are performed to determine the initiation and termination positions of PKS deflection, the magnitude of PKS deflection, the volume of the resultant void space, and the maximum height of Bed-separation development. The findings are presented in further rows of Table 4. As the thickness of the PKS decreases, the deflection value of the 7221 work face increases, accompanied by an increase in the amount of space resulting from the deflection. Simultaneously, due to the heightened PKS deflection value, the vertical expansion of the Bed-separation development is constricted, reducing the volume of the Bed-separation cavity and the injected slurry. The reduced thickness of PKS results in a decrease in waste disposal volume and an increase in the complexity of ground subsidence control. When building an Isolated Overburden Grout Injection face, it is important to consider the thickness of the PKS.

Table 4

Effect of different parameters of PKS on the development of PKS deflection and Bed-separation
PKS thickness and lithotype	PKS deflection start position(m)	PKS deflection end position(m)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Maximum height of Bed-separation(mm)
18m fine sandstone	29	549	1868	25638	1725.2
13m fine sandstone	29	549	2068	30400	1525.2
8m fine sandstone	29	549	2885.6	36721	707.6
21m fine sandstone	29	549	1638	20456	1955.2
18m siltstone	31	548	1691.6	27954	1900.4

The initial step involves acquiring the drilling data and afterward analyzing the depth of the coal seam, as well as identifying the composition and thickness of each rock stratum above it. Following this assessment, the subsequent phase entails formulating the coal seam mining design parameters. The primary factors considered in work face design are the dimensions of the mining area, including length, width, and height, as well as the degree of inclination of the coal seam. The design parameters of the work face have a dual impact on the subsidence basin and the development of the Separation below the PKS. Firstly, they cause the PKS to deflect, affecting the expansion of the subsidence basin at the ground level. Secondly, these parameters also influence the design considerations for subsequent grouting of the Bed-separation slurry filling mining. Hence, in implementing Isolated Overburden Grout Injection Technology, it becomes imperative to develop a rigorous and precise mechanical model, appropriately determine the mining parameters of the work face, and ensure compatibility with the parameter design of the slurry filling.

4.1 Design length of 7221 work face

Given the hypothetical presence of an infinitely long coal seam, it is observed that the current mining length of work face 7221 is 588m. However, for this discussion, it is now postulated that the mining length of work face 7221 is 3000m. This study aims to conduct a graphical analysis of various parameters related to unpressurized grouting in mining operations. Specifically, we will examine the change in PKS deflection value, the volume of deflection space, the maximum subsidence value of the ground subsidence basin, the maximum height of the Bed-separation development, the volume of the Bed-separation cavity, and the injection ratio. These analyses will be conducted on the length of mining after extending the 7221 mining length.

4.1.1 7221 work face length effects on PKS deflection and ground subsidence

Upon assuming that the mining length of the 7221 work face is 3000m, graphical representations were constructed to determine the PKS deflection value and the volume of deflection space. The outcomes of these calculations are presented in Fig. 3. The PKS deflection value gradually increases as the mining of 7221 work face progresses. This trend continues until a mining distance of 1455m is reached; at this point, the PKS deflection value reaches its maximum value of 2214.8mm. Subsequently, the PKS deflection value decreases as the mining distance increases. Specifically, when the mining distance reaches 3000m, the PKS deflection value decreases to 2031mm. As the mining progress advances in the 7221 work face, the volume of the PKS flexure space undergoes a corresponding increase. Upon reaching a mining distance of 3000m, the flexure space attains its maximum volume, measuring 145712m³.

Once the PKS flexure space volume is determined, the ground subsidence basin's inscribed function is formed based on the concept of equal volume. The calculation of the maximum subsidence value of the basin has been performed, and the outcomes are presented in Fig. 4. The mining width of 100m for the 7221 work face is determined based on the strong flexural strength of the rock formation and the thickness of PKS, which is 18m and has a high Young's modulus. However, this mining width is considered insufficient and may result in the maximum sinking value of the ground hole. When the mining distance of the 7221 work face reaches 1455 meters, the deflection value of the PKS achieves its maximum value of 2214.8 millimeters. However, the maximum sinking value of the earth is only 826 millimeters. In the interim, it has been observed that the forward mining of the 7221 work face has resulted in the expansion of the ground subsidence basin. Consequently, the volume of the deflection space for the PKS has been progressively increasing. Furthermore, it has been noted that once the maximum subsidence value is attained, the ground's subsidence value does not exhibit a decreasing trend as expected.

4.1.2 7221 work face length impact on grouting design

Given a mining length of 7221 work face, we can determine the maximum height of the Bed-separation development, the volume of the Bed-separation cavity, and the grouting injection ratio under no pressure. These results are depicted in Fig. 5. The grouting under pressure injection ratio is hypothesized to apply minimal pressure to the grouting hole, facilitating the smooth flow of slurry into the Bed-separation cavity. The grouting volume is determined by dividing the result by the volume of the extraction zone, yielding a percentage. As the mining operation progresses in the 7221 work face, the height of the Bed-separation development gradually grows. It reaches its peak value of 2037 mm when the mining distance reaches 1455 m. Subsequently, as the mining of the 7221 work face progresses, the PKS undergoes deformation, and the Separation gradually diminishes. The maximum height of the Bed-separation development reaches 1659 mm at a mining distance of 3000 m, smaller than the Bed-separation observed when the work face was mined at 588 m. The magnitude of the Separation is comparatively reduced to the dimensions seen during the mining of the work face. The highest vertical distance between two points is measured to be 1659mm, significantly smaller than the distance of 588m seen during the mining of the work face. As the mining process progresses in the 7221 work face, the cavity volume in the Separation area experiences a continuous increase. The volume of the cavity does not exhibit an increase and is better suited for the mining method involving the filling of bed-separation slurry. Hence, including the grouting injection ratio parameter in the absence of pressure demonstrates that as the mining face progresses, the actual injection ratio exhibits a pattern of initial increase followed by a subsequent decrease. This finding supports the argument that infinitely extending the advancing distance of the working face is inappropriate for Bed-separation grouting and mining.

4.2 Design width of 7221 work face

The design width of the work face is a critical factor in the Isolated Overburden Grout Injection design. It is necessary for the design width to adequately accommodate the mining process in order to ensure the formation of the Bed-separation cavity. The designated width of the 7221 work face is 100 meters, indicating that it falls under the category of non-sufficient mining. Various assumptions have been made regarding mining widths, including 60m, 140m, 180m, and 220m. These assumptions regarding the width of the work face contribute to the transition of the Bed-separation 7221 work face from non-sufficient mining to sufficient mining. The purpose of this study is to visually analyze the relationship between the width of the work face design and several factors, including PKS deflection, deflection space volume, maximum ground subsidence value, height of Bed-separation development, and Bed-separation cavity volume. The objective is to see how these factors change as the seam width increases.

4.2.1 7221 work face width effects on PKS deflection and ground subsidence

Given the hypothetical scenario of an infinitely expandable coal seam in terms of strike and inclination, it is permissible to consider that the 7221 work face has been designed with varying mining widths. Figure 6 presents the outcomes on PKS deflection and deflection space volume for mining widths of 60m, 100m, 140m, 180m, and 220m. The deflection value of PKS exhibits a quick increase with the expansion of mining breadth, assuming adequate mining conditions while maintaining a constant distance. At a mining distance exceeding 200 m, the PKS deflection value and the volume of the deflection space are seen to be more than double compared to a mining width of 100 m. The work face design in question (7221) has a propensity towards adequate mining. It has been observed that the rate of growth in the deflection value of the PKS and its maximum value are not significantly influenced by an increase in width. Nevertheless, both factors exhibit a significant magnitude, which poses a disadvantageous situation for employing mining techniques to mitigate ground sinking.

The PKS flexure space volume with 7221 work face mining is analyzed in this study. The analysis considers various mining widths and thicknesses, assuming an increase in these parameters. The concept of equal volume is employed to calculate the inscribed function of the ground subsidence basin. Additionally, the maximum value of ground subsidence is determined. The results are presented in Fig. 7. Due to different mining widths, the ground subsidence exhibits significant variation in the absence of Bed-separation slurry filling mining. When the mining width is 60 meters, the ground surface experiences a maximum subsidence value of around 100 millimeters. When the mining width is 100 meters, the ground experiences a maximum subsidence value of 640 millimeters. When the mining breadth of the working face reaches 140m, it can be said to have reached the stage of sufficient mining. There is a substantial alteration in the regulation governing the fluctuation of subsidence values. The subsidence value reaches a maximum of over 1400mm. Subsequently, as the mining breadth of the working face expands to 180m and 220m, the subsidence pattern of the ground surface reemerges. The subsidence value is twice the mining width of 100m. Subsequently, as the working face width expanded to 180m and 220m, the ground subsidence pattern was reoccurrence, accompanied by a doubling of the subsidence magnitude compared to the mining width of 100m.

Table 5 presents the statistical outcomes of basin expansion length, width, maximum ground subsidence value, basin volume, and coordinates of the center point of the subsidence basin in the ground subsidence basin after the design of various mining widths. Based on the statistical findings, it is evident that, while disregarding the slurry parameter design for Isolated Overburden Grout Injection, opting for Longwall mining and designing a mining width of 180m is a more favorable alternative for mining the 7221 work face.

Table 5

Deformation results of ground subsidence basin with different mining widths for assuming 7221 work face
Design width of work face(m)	Length of subsidence basin(m)	Width of subsidence basin(m)	Ground maximum subsidence value(mm)	volume of subsidence basin(m³)	Coordinates of center point of ground subsidence basin
Width of work face 60m	791	333	82	3272	(308.7,56)
Width of work face 100m	795	376	640	30300	(310.5,78)
Width of work face 140m	799	420	1457	76935	(312.6,100)
Width of work face 180m	803	464	1079	63575	(314.7,122)
Width of work face 220m	807	518	1308	85194	(316.3,144)

4.2.2 7221 work face width impact on grouting design

The parameters, including the height of Bed-separation development, the volume of the Bed-separation cavity, and the non-pressure slurry filling ratio, were measured for various mining widths in the 7221 work face, where Bed-separation slurry filling was employed. The findings are presented in Fig. 8. When the width of design mining reaches a point where it is considered sufficient for mining purposes, there is a noticeable increase in the height of Bed-separation development. Nevertheless, in the context of the mining operation where the working face is oriented in a forward direction, it is seen that the height of the Bed-separation development tends to diminish. Specifically, when the mining width measures 220m, the Separation is entirely sealed off after a mining distance of 550m. As the mining operation progresses to a distance of 588m, it is observed that the volume of Bed-separation cavities in mining widths of 140m and 180m is greater compared to the mining width of 100m.

Additionally, the non-pressure grouting injection ratio exceeds 6%. Nevertheless, the progress of Separation exhibits more swiftness throughout its first stages. Consequently, when implementing Bed-separation slurry filling mining, it becomes imperative to elevate both the slurry injection rate and quantity. In order to design the suitable mining width for the 7221 work face, it is imperative to meticulously consider the integration of grouting holes and grouting equipment while also considering the input of grouting cost.

Table 6 presents comprehensive data on various aspects of the 7221 work face, including the precise locations of initial and final deflection, the maximum deflection values observed in the PKS, the volume of deflection space, the maximum height of bed-separation development, the volume of the bed-separation cavity, and the outcomes of non-pressure grouting injection ratios. These statistics are provided for different mining widths, offering a detailed analysis of the abovementioned parameters. The visualization and comparative analysis of PKS deflection and Separation development may be achieved when considering the mining width of the proposed working face within the spectrum of adequate and inadequate mining conditions. Additionally, it confirms that the Isolated Overburden Grout Injection design necessitates that the mining width of the work face falls within the realm of insufficient mining.

Table 6

Results of PKS deflection and Bed-separation development assuming different mining widths for 7221 work face
Design width of work face(m)	PKS deflection start position(m)	PKS deflection end position(m)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Maximum height of Bed-separation(mm)	Maximum volume of Bed-separation cavity (m³)	Maximum Bed-separation injection/production ratio (%)
Width of work face 60m	33	548	353.1	2768	347.8	2738	1.68
Width of work face 100m	30	548	1868	25638	1725.2	22053	8.29
Width of work face 140m	27	548	3999.6	65098	2234.6	34840	9.17
Width of work face 180m	24	548	3661.6	53794	2232.1	27242	8.28
Width of work face 220m	22	548	4903.9	72087	2234.4	18418	8.25

4.3 Mining height of the 7221 work face

The mining height of the 7221 work face refers to the thickness of the coal seam and gangue that is intended to be cut during the mining process. The mining height directly impacts the overall quantity of coal resources that can be removed, as well as the size of the mining extraction area. The closure of the Separation is achieved by progressively reconnecting the PKS flexure with the underlying strata as the mining operations proceed, facilitated by a limited mining height. The PKS deflection and bed separation are only reconciled when the mining height reaches 1.6m, and the mining distance exceeds 360m. Consequently, the Bed-separation height experiences a quick reduction to zero. In all other mining height scenarios, bed separation occurs beneath the PKS. The deflection of the PKS follows the same curve as a thin plate that is fully supported without any foundation support. Consequently, the deflection of the PKS at various mining heights is not currently subjected to detailed analysis.

Figure 9 displays the outcomes of determining the maximum height of Bed-separation development, the volume of Bed-separation cavities, and the non-pressure grouting injection ratio for the mining heights of 1.6m, 2.6m, 3.6m, 4.6m, and 5.6m at the 7221 work face. Given that the underlying strata do not provide any support for PKS deflection and bed separation beyond a mining height of 3.6m, it can be observed that the maximum height of the off-separation development and the volume of the bed-separation cavity yield identical outcomes across all three scenarios, with the curves coinciding. As the 7221 work face progresses in mining, the mining height becomes smaller, increasing PKS deflection. Consequently, the PKS increasingly reconnects with the underlying strata, rapidly compressing the Bed-separation space. While the injection ratio of non-pressure grouting is larger in situations with less mining height, the closure of bed separation occurs at a later stage, resulting in a smaller overall volume of the bed separation cavity. The parameter design of the mining height of the working face is a critical factor in the extraction volume of coal resources and the economic returns when implementing Bed-separation slurry-filled mining.

4.4 Coal seam strike angle of 7221 work face

During the coal seam exploration process, determining the inclination angle between the strata and the coal seam is crucial. The working face strike angle can also be considered a stratigraphic parameter. Given that the model proposed in this study is a large-deflection inclined thin plate model, the authors aim to exploit the potential of the 7221 work face. It is assumed that the work face and the outer gangue are relatively thicker, allowing for mining at various strike angles throughout the operation. This paper examines the practical distinctions in the formation of PKS deflection and bed separation resulting from the analysis of inclined coal seam mining under varying strike angles of the seam.

4.4.1 7221 work face strike angle inclination effects on PKS deflection and ground subsidence

As the inclined seam is progressively mined, the depth of the working face mining line gradually rises, resulting in a linear increase in the overburden load above. The steady increase in the inclination angle of the coal seam leads to a corresponding increase in both the lateral load and mid-plane force, resulting in a gradual increase in the deflection value and deflection space volume of the PKS. Figure 10 illustrates the outcomes obtained from mining the 7221 work face at various strike angles, specifically to the PKS deflection values and the volume of deflection space. The fixed inclination angle of the coal seam is 8.2°. When the coal seam strike angle is 1.8°, it is commonly considered suitable for horizontal seam mining. When the coal seam strike angle is 21.8°, the deflection value of PKS is 3.5 times greater than that of horizontal seam mining, and the volume of the deflection space is estimated to be around three times larger. The inclination of the coal seam significantly influences the deflection of PKS. The conventional thin plate model exhibits several inaccuracies, which does not consider the coal seam inclination. These inaccuracies highlight the practical application value and significance of the large-deflection inclined thin plate model proposed by the authors of this research paper.

The impact of varying coal seam strike angles on the volume of PKS flexure space was determined by calculations. Subsequently, the effect on the ground subsidence basin was assessed using the concept of isovolumetric behavior. The outcomes of this analysis are presented in Fig. 11. The maximum ground subsidence resulting from mining the coal seam with a strike angle of 21.8° is 1.6 times more than that of the coal seam with a strike angle of 1.8°, although it is still less than three times the value of the PKS flexural volume. The expansion of the ground-moving basin strike length occurs as the depth of coal seam mining grows. This phenomenon reduces the maximum ground sinking value while simultaneously exerting a broader influence on the surrounding terrain.

The study involves the statistical mapping of several coal seams' strike inclination following the dynamic alteration of the maximum subsidence value of the ground throughout the progression of the working face. The characteristics measured in Table 7 include the length, width, basin volume, and coordinates of the subsidence basin's center point resulting from the coal seam's strike angles. The inclination of the working face strike has been observed to impact the length of the ground subsidence basin directly. Specifically, it has been noted that as the strike inclination of the working face increases, the length of the subsidence basin experiences a significant increase beyond a mining distance of 588m. This increase in length is accompanied by a simultaneous increase in both the maximum value of ground subsidence and the basin volume. The displacement of the central point of ground subsidence basins is influenced by the varying strike angles of coal seams, causing it to deviate from the center of the horizontal coal seams.

Table 7

Deformation results of ground subsidence basin with different strike angles for assuming 7221 work face
Strike angle of work face(°)	Length of subsidence basin(m)	Width of subsidence basin(m)	Ground maximum subsidence value(mm)	volume of subsidence basin(m³)	Coordinates of center point of ground subsidence basin
Strike angle of 1.8°	765	340	499	20067	(300.3,75)
Strike angle of 6.8°	795	346	640	30300	(310.5,78)
Strike angle of 11.8°	821	352	742	39375	(329.9,81)
Strike angle of 16.8°	857	356	664	48140	(350.9,83)
Strike angle of 21.8°	901	362	863	56407	(379.4,86)

4.4.2 7221 work face strike angle impact on grouting design

The variation in the inclination of the coal seam results in a proportional rise in the lateral load and mid-plane force exerted on the PKS thin plate. Consequently, this leads to an escalation in the deflection of the PKS and the value of Bed-separation, thus impeding the progression of Bed-separation. The maximum height of the Bed-separation development, the volume of Bed-separation cavities, and the non-pressure grouting injection ratio were calculated using the large-deflection inclined thin plate model for various coal seams with different strike inclinations. The outcomes of these calculations are depicted in Fig. 12. As the inclination of the working face increases, there is a corresponding drop in the deflection value of the PKS, resulting in a reduction in both the height of the Bed-separation development and the volume of the Bed-separation cavities. When the mining distance is short, the slope of the coal seam strike leads to a greater development of Separation compared to the horizontal coal seam.

Given that the strike angle of a coal seam is a geological feature, it is imperative to thoroughly account for the impact of both the strike and inclination angles when determining the appropriate dimensions for the work face during design. Simultaneously, it is imperative to consider the impact of coal seam inclination when establishing the grouting holes. Consequently, it is recommended to position the grouting holes within the Bed-separation area beneath the PKS. The change in the inclination angle affects the stress release of the slurry, thereby altering the supporting action area of the PKS. This leads to deflection and bulge formation in the PKS thin plate. The precise outcomes of this phenomenon will be further elaborated upon in the subsequent section.

Table 8 provides a comprehensive statistical analysis of various parameters influenced by the strike inclination, including the start and end positions of PKS deflection, the maximum PKS deflection value and deflection space volume, the maximum height of Bed-separation development, the volume of Bed-separation cavities, and the non-pressure grouting injection ratio. The 7221 work face is assumed to be mined at a constant distance.

Table 8

Hypothetical PKS deflection and Bed-separation development results for different strike angles of 7221 work face
Strike angle of work face(°)	PKS deflection start position(m)	PKS deflection end position(m)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Maximum height of Bed-separation(mm)	Maximum volume of Bed-separation cavity (m³)	Maximum Bed-separation injection/production ratio (%)
Strike angle of 1.8°	29	542	1313	17056	1220.9	15861	5.84
Strike angle of 6.8°	30	548	1868	25638	1725.2	22053	8.29
Strike angle of 11.8°	31	553	2530	33494	2268.6	28006	10.97
Strike angle of 16.8°	31	557	3076	40952	2434.3	23005	11.44
Strike angle of 21.8°	31	560	3583	47983	2509.9	19167.4	11.51

4.5 Design mining speed of 7221 work face

After the determination of the length, width, and height of the mining face in work 7221, it is observed that the mining rate does not have an impact on the final values of PKS deflection, deflection space volume, height of Bed-separation development, and Bed-separation cavity volume. Once the length has been established, the mining rate alone impacts the timing of these outcomes; specifically, the commencement of PKS deflection and Separation only occurs once they have reached their maximum values.

4.5.1 7221 work face mining speed effects on PKS deflection and ground subsidence

Figure 13 displays the results obtained from calculating the average mining speed of the 7221 work face based on its designated start and finish time. The mining speeds of 3m/day, 5m/day, 6m/day, and 7m/day were assumed, and the corresponding PKS deflection value and deflection space volume were calculated. The average mining speed of the 7221 work face was determined to be 4m/day. The work face designated as 7221 has been specifically developed for mining operations spanning a distance of 588 meters. A direct relationship exists between the speed of mining and the duration required to complete the mining process. Hence, the PKS deflection value and the volume of deflection space exhibit an earlier attainment of their maximum values, resulting in an escalation of the ground subsidence rate. The speed at which mining occurs directly impacts the speed at which Isolated Overburden Grout Injection must be performed. Failure to match the mining speed with the grouting speed can hinder the ability to manage the ground effectively.

4.4.2 7221 work face mining speed impact on grouting design

The variation in mining speeds for the 7221 work face does not impact the alteration of the maximum height of the Bed-separation development and the maximum value of the Bed-separation cavity volume. Nevertheless, its impact is confined solely to the rate at which the Bed-separation progresses. Figure 14 displays the outcomes of varying speeds in the design of the 7221 work face, resulting in dynamic changes in the development height of the Bed-separation and the volume of the Bed-separation cavity with the mining date. The rate at which mining occurs directly influences the pace of Bed-separation development and the duration of the reserved grouting interval. The mining speed's influence on the speed of Isolated Overburden Grout Injection (IOGI) grouting and the start and end time of grouting necessitates considering all three factors collectively for effective management of ground subsidence.

The excavation of 7221 work face results in varying degrees of flexure and the creation of cavities in the above layers. The design parameters for the Isolated Overburden Grout Injection Technology included determining slurry injection holes, slurry injection start and stop time, and slurry concentration. The study employed a combination of the large-deflection inclined thin plate model and the slurry model to conduct a quantitative analysis of the impact of various grouting parameters on the deflection of PKS structures and ground subsidence. It is postulated that the quantity of slurry injected into the Bed-separation cavities did not surpass the capacity of the said cavities, neither in augmentation nor reduction. The completion date of slurry injection for all slurry holes was not chosen as July 14, 2018. Instead, June 3, 2018, the completion date of mining at the 7221 work face, was selected for comparison.

5.1 Design number of grouting holes

When the grouting efficiency of each grouting hole is uniform, and the concentration of injected slurry remains constant, incorporating additional grouting holes is to augment the grout injected concurrently. The grouting holes were positioned above the inclined centerline of the 7221 work face. Once the location of the first grouting hole was established, subsequent holes were placed consecutively at regular intervals of 50 meters. The grouting holes were positioned along the centerline of the 7221 work face. In this study, the focus is on the design of slurry holes for mining purposes, specifically for the filling of work faces with slurry. The research assumes five slurry holes and aims to assess the impact of varying the number of slurry holes (specifically, 3, 4, 6, and 7) on the deflection value of PKS. The obtained results are presented in Fig. 15. The efficacy of PKS deflection control is compromised when the quantity of grouting holes is inadequate for properly managing PKS deflection. The attainment of the grouting threshold for Bed-separation cavities is expedited upon augmenting the quantity of grouting apertures. Furthermore, any elevation in grouting pressure or reduction in grouting efficacy will increase mining expenses. Hence, it is imperative to integrate the growth rate of Bed-separation cavities and the efficacy of grouting to determine an appropriate quantity of grouting orifices, ensuring optimal exploitation of each grouting orifice.

The completion date of mining at work face 7221, namely June 3, 2018, was determined. Additionally, calculations were conducted to assess the PKS lower KS2 deflection, PKS deflection value, volume of deflection space, slurry injection volume, and maximum ground subsidence value. The lateral force of the formation and the downward squeezing impact of the slurry are subject to the deflection of KS2. This results in constructing a cavity with varying degrees of deflection when the grouting area intersects with PKS. The volume of deflection space in the PKS directly impacts the volume of the ground subsidence basin. The maximum value of ground subsidence can be estimated by identifying the dimensions and shape of the ground subsidence basin. Table 9 illustrates the impact of employing varying quantities of grouting holes on the deflection of PKS and the maximum values of ground sinking. The correlation between the number of injection holes and the volume of slurry injected into the Bed-separation cavity is positively associated with the effectiveness of PKS support. The decrease in PKS deflection value is accompanied by a reduction in the volume of the PKS deflection space, resulting in a decrease in the maximum value of ground sinking. This demonstrates effective control over ground subsidence.

Table 9

Effect of grouting with different number of grouting holes on PKS deflection and ground subsidence
Number of grouting holes	KS2 maximum deflection value(mm)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Slurry injection volume(m³)	Ground maximum subsidence value(mm)
Three grouting holes	1993	1360	20008	9285.6	-498
Four grouting holes	2040	1160	17078	12380	-428
Five grouting holes	2055	1020	15253	15476	-354
Six grouting holes	2153	888	13075	18571	-324
Seven grouting holes	2332	840	12620	21666	-301

5.2 Starting and stopping time of grouting

The implementation of slurry filling mining for the 7221 work face commenced on January 8, 2018, when the mining of the 7221 work face had reached a distance of 147m. During a previous period, the mining distance of work face 7221 was observed to be relatively limited. Utilizing the large-deflection inclined thin plate model calculation; it was determined that deflection in the form of PKS commenced after the mining of the work face reached a distance of 71 meters. Hence, the designated dates for grouting were established as January 29, 2018, February 13, 2018, March 3, 2018, and March 17, 2018, to align with the practical requirements of the mining operation. The 3D deflection function of PKS is determined by calculating the deflection at 7221 work face for various injection dates. The obtained findings are presented in Fig. 16. Due to the inclined nature of the seam mining in work face 7221, the slurry injection cannot adequately support the adjacent PKS along the open tangent line. Consequently, a bulge will form on the left side of the PKS. The smaller the volume of slurry injected into the Bed-separation cavity, the less effective it would be in providing support to the PKS, therefore necessitating a later start for the slurry injection. As of the commencement of slurry injection on March 17, 2018, it was observed that the slurry did not provide adequate support for the PKS until the completion of mining activities at the 7221 work face. The process of injecting grout into isolated overburden. Selecting an optimum date for slurry filling is crucial in properly managing ground sinking. Insufficient delay in the grouting process results in the inability of the Bed-separation cavity to retain the slurry, leading to the suspension of grouting instruments and subsequent escalation of grouting expenses. If the injection time is delayed, it is important to note that the ground may have already undergone significant sinking. Consequently, the injected slurry may not adequately mitigate the occurrence of ground subsidence.

Table 10 presents a comparative analysis of the impact of varying grouting start and finish times on PKS deflection and ground subsidence. The table includes calculations of KS2 and PKS deflection, together with corresponding values for the volume of space and maximum ground subsidence. According to the theoretical framework of the large-deflection inclined thin plate model, the calculation postulates that the Longwall mining operation conducted on the 7221 work face results in a maximum subsidence value of -640 mm at ground level. Compared to the assumed initiation time of slurry, slurry filling mining plays a significant role in the management of ground subsidence. In the pursuit of efficient mining waste disposal and implementing environmentally friendly mining practices, the crucial factor in slurry design lies in carefully selecting optimal commencement and completion dates for grouting activities.

Table 10

Effect of different grouting start and finish time selection on PKS deflection and maximum subsidence values
Grouting start date	Days to start mining(day)	KS2 maximum deflection value(mm)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Slurry injection volume(m³)	Ground maximum subsidence value(mm)
January 8, 2018	33	2055	1020	15253	15476	-354
January 29, 2018	54	2088	1148	16862	13826	-438
February 13, 2018	69	2130	1288	18965	12340	-486
March 3, 2018	86	2170	1465	21380	10523	-553
March 17, 2018	100	2196	1552	22800	9473	-591

5.3 Grouting slurry consistency

Various concentrations of slurry are introduced into bed-separation cavities. Firstly, variations in concentration give rise to distinct physical characteristics, such as density in non-Newtonian fluids. Consequently, this necessitates the application of varying injection pressures within the injection holes, as well as differential impacts on the squeezing effect of KS2 and the supporting effect on PKS. Upon the injection of the slurry into the Bed-separation cavity, there is a subsequent loss of water from the surrounding formation. The quantity of fly ash injected directly impacts the waste treatment capacity. The Isolated Overburden Grout Injection technique involves injecting a slurry with an average concentration of 78%-80%. In this paper, we assume different gradients of slurry concentration, namely 40%, 50%, 60%, 70%, and 90%. However, we do not delve into the specific pressure requirements that different slurry concentrations impose on the casing of the slurry hole. Our focus is solely on calculating the impact of the injected slurry on the extrusion effect of the KS2 and the support provided to the PKS during the Bed-separation process. Given that all other mining parameters remain constant and the slurry injection date remains unchanged, the outcomes on the three-dimensional deflection of PKS on June 3, 2018, specifically when mining ceased at work face 7221, have been computed and are visually presented in Fig. 17. The figure illustrates that various concentrations of slurry yield comparable levels of support for PKS deflection, with equal maximum deflection values observed.

The densities of various concentrations of slurry were calculated, assuming that the deflection values of KS2 and PKS, the amount of injected slurry, and the maximum ground subsidence values were obtained after the injection of slurry with varied concentrations into the Bed-separation cavity. The table illustrates that the impact of various concentration slurries on PKS deflection and ground subsidence is similar, with the primary distinction being observed in the recycling approach employed for fly ash.

Table 11

Effect of different slurry consistency on PKS deflection and maximum ground subsidence values
Percentage of slurry concentrations (%)	Slurry density(kg/m³)	PKS maximum deflection value(mm)	PKS deflection space volume(m³)	Slurry injection volume(m³)	Injected fly ash weight(t)	Ground maximum subsidence value(mm)
40	0.92	1096	16111	14130	4521.6	-407
50	0.9	1072	15771	14440	5776	-395
60	0.88	1048	15438	14773	7091	-385
70	0.86	1032	15095	15116	8465	-375
80	0.84	1020	15253	15476	9905	-354
90	0.82	976	14357	15854	11415	-348

The mining of work face 7221, the grouting of the Bed-separation cavity under PKS, and the control of ground subsidence are influenced by various parameters. These parameters can be categorized into three main aspects: stratigraphic parameters, design parameters of the work face, and parameters of the Bed-separation grouting slurry. The calculation of large-deflection inclined thin plates combined with the slurry model is used to analyze these parameters. The stratigraphic parameters encompass the PKS site's thickness, the main roof's thickness, and the lithology characteristics associated with stratigraphy. These parameters were established by the examination of the workings during the exploration process. The design parameters for the working face encompass various factors such as the length and width of the working face, the mining height, the inclination of the coal seam, and the mining speed. The inclination of the coal seam is typically determined during the exploration phase of the working face. On the other hand, determining the mining height requires considering the impact of the coal mining machine and the roof support. It is important to note that these two parameters will remain unchanged in the context of this paper. The parameters for bed-separation grouting slurry encompass the quantity of grouting holes, the initiation and completion time of grouting, and the concentration of the slurry. The design of slurry concentration should be based on the grouting pump power, the yield value of slurry in the grouting pipe, and parameters such as pressure drop and flow rate. It is important to note that the content of this paper will remain unaltered.

In brief, the redesign parameters on the 7221 work face for grouting encompass the length, width, and mining speed within the design limits of the work face. Additionally, the Bed-separation grouting slurry parameters encompass the number of grouting holes and the grouting start and end times. The primary aim of implementing slurry filling and mining activities at the 7221 work face is to effectively manage ground subsidence and ensure that the subsidence value within the protection zone of the neighboring villages remains below − 10mm. This objective is crucial due to the two villages close to the work as mentioned above face. Another objective of Isolated Overburden Grout Injection is to maximize the extraction of coal resources while simultaneously slurring the fly ash to facilitate trash filling and recycling, hence achieving environmentally sustainable mining practices.

6.1 Mining parameter redesign for 7221 work face

In order to ascertain the extension distance of the mining length and width of the work face, the spatial vertical distance between the 7221 work face and the two villages was re-measured. In order to maintain building subsidence values below − 10 mm in the two villages, it is necessary to forecast the ground subsidence basin size accurately. This will ensure that the village protection zones align with the tangent of the basin's inscribed function line. Figure 18 depicts the modified dimensions of the working face mining length and width and the associated ground subsidence basin. The current configuration of the 7221 work face entails a mining length of 588m and a mining width of 100m.

Additionally, the ground surface subsidence basin is characterized by a length of 794m and a width of 376m. A solid red line in the accompanying image visually represents the boundaries of this subsidence basin. To optimize the extraction of coal reserves and safeguard the integrity of the village structures, the operational front of the mining site was reconfigured, taking into account the dimensions of the subsidence basin, as indicated by the black dashed line in the diagram. According to the measurements, the distance between the opening cut line of work face 7221, and Gaochangying village is determined to be 963 meters. Additionally, it has been observed that the nearest point of the work face to the protection zone of Houlou Gaojia village is around 170 meters.

Based on the observations depicted in Fig. 6 to Fig. 8, it becomes evident that the complete extraction of the working face occurs when the mining width exceeds 140m. Additionally, managing PKS deflection and Bed-separation filling mining, which is influenced by the PKS deflection and Bed-separation, becomes challenging when employing Bed-separation slurry filling mining. Consequently, limiting the work face mining width to less than 140m is recommended to mitigate these difficulties. The objective is to modify the mining breadth of work face 7221 to increase its width downward while ensuring the subsidence basin engraved line does not encroach upon the protected area of Houluo Gaojia Village. The design of the ground subsidence basin includes an engraved line that extends tangentially along the strike expansion of the protection zone in Gaochangying village. This engraved line determines the mining length of the 7221 work face, which is designed to be 795m. In Fig. 6 to Fig. 8, the mining breadth of work face 7221 is observed to have been expanded by an additional 40m.

Consequently, the work face has reached its maximum mining capacity at a width of 140 meters. In this study, we have recalculated the PKS deflection, Bed-separation development height, Bed-separation cavity volume, and maximum ground subsidence values for a work face with mining widths of 110m, 120m, 130m, and 140m. The recalculated results are presented in Fig. 19. The figure illustrates the relationship between mining length, width, PKS deflection, and bed-separation development. When the mining length is 795m, and the mining width is 100m, the PKS deflection and bed-separation development measure 1900mm. Similarly, when the mining width is increased to 110m, the PKS deflection and bed-separation development increase to 2800mm, with the maximum height of bed-separation development reaching 2100mm. Finally, when the mining width is further increased to 120m, the PKS deflection and bed-separation development measure 3800mm, with the bed-separation development height remaining 2100mm. The bed-separation cavities exhibit a volume of approximately 4.2×104m³ for widths ranging from 110m to 120m, whereas a narrower width of 100m results in a reduced volume of 3.4×104m³. The observed ground subsidence values exhibited significant variation, with maximum measurements of 740mm, 1077mm, and 1369mm corresponding to mining widths of 100m, 110m, and 120m, respectively. The PKS deflection and Bed-separation value exhibit significant changes when the mining width expands to 130m and 140m. These changes reach their peak value of 4000mm when the mining length ranges from approximately 250m to 400m. Additionally, the maximum subsidence value of the ground exceeds 1000mm, posing a substantial challenge for Bed-separation slurry filling mining. The height of the Bed-separation development and cavity volume do not exhibit substantial differences under mining widths of 130m and 140m compared to other design widths. Therefore, the design mining widths of 100m, 110m, and 120m are chosen. Various grouting parameters were devised to assess the deflection values of PKS and the maximum ground subsidence values resulting from different mining widths. These parameters were then comprehensively compared.

After determining the mining length of work face 7221, several mining widths were combined to design different mining speeds. This design considered the PKS deflection, Bed-separation development height, cavity volume, and maximum ground subsidence value. The initial operational face was engineered to achieve a mean excavation rate of 4 meters per day, covering a total distance of 588 meters over a mining duration of 147 days. Due to the expanded dimensions of mining in terms of length and width, the 7221 work face has been reconfigured to accommodate mining speeds of 3m/day and 4m/day. The outcomes of a comparative analysis, which involved assessing the deflection caused by mining-induced PKS, the development of bed separation, and ground subsidence, were graphically presented in Fig. 20. The disparity in mining velocity solely impacts the mining process's temporal aspect and does not influence the ultimate outcomes. When comparing the rate of change for each deformation value, it is observed that for mining widths of 110m and 120m, the mining speed is 3m/day. It is noted that the ultimate deformation value is bigger for these widths, but the pre-deformation speed is smaller compared to a mining width of 100m, which has a speed of 4m/day. Using specific mining parameters can effectively mitigate the challenges associated with controlling the PKS phenomenon and ground subsidence deformation during the initial stages of mining operations. By increasing the mining width and reducing the mining speed, it is possible to achieve the dual benefits of enhanced coal resource extraction and improved capacity for waste disposal through an increase in the volume of bed-separation cavities. Enlarging the mining breadth led to an augmentation in the magnitudes of the last deflection of the PKS and ground subsidence. Additionally, additional analysis and examination are required to thoroughly examine the outcomes of the final deformation after mining while using the technique of Bed-separation slurry filling.

6.2 Design of grouting parameters of the 7221 work face

Once the length, width, and mining speed dimensions for the 7221 work face have been established, the Isolated Overburden Grout Injection method is employed to define the parameters for slurry filling in the work face. This includes determining the number of slurry holes and the specific starting and finishing times for the slurry filling process. A linear relationship exists between the quantity of injection holes and the amount of slurry injected into the Bed-separation cavity. This relationship directly impacts the ultimate deflection of the PKS and the ground subsidence in the Bed-separation area. The initiation and termination of the grouting process are contingent upon the mining velocity. The grouting procedure will commence once the Bed-separation cavity has been sufficiently developed to satisfy the grouting criteria. Subsequently, the grouting holes will be terminated once the volume of the grouting slurry aligns with the predetermined design specifications.

The initial design of the mining length for the work face is 7221 meters, with a width of 100 meters. Additionally, there are a total of five grouting holes in the design. The current dimensions of the mining area are 795m in length, with varying widths of 100m, 110m, and 120m. The design of the volume of slurry injected into the Bed-separation cavity and the location of the slurry injection holes are based on the control requirements for ground subsidence and the volume of the Bed-separation cavity. The dimensions of the ground subsidence basin have been determined as follows: the length is 1059m, and the width is 390m. These measurements are based on the assumption that the maximum sinking value of the control ground is -350mm. Consequently, the predicted volume of the ground subsidence basin amounts to 22906m³.

The isovolumetric concept determines that converting to the PKS flexural space volume amounts to 19,382m³. In order to meet the specifications for ground subsidence management, the grouting process must ensure a minimum volume of 19,382m³ for the Bed-separation cavity. The number of grouting holes can be determined by comparing the calculated volume of the Bed-separation cavity under various mining design parameters with the desired volume. Based on the grouting parameter statement, it was seen that the mean grouting volume for each grouting hole in the 7221 work face amounted to about 30m³/day. Furthermore, the grouting operation was successfully concluded 42 days after ceasing mining activities in the work mentioned above. According to the parameter report, the commencement of the grouting operation coincides with the arrival of the mining line at the initial grouting hole location.

Based on the theoretical framework of the large-deflection inclined thin plate combined with grouting model, it can be shown that when the pace of expansion of the Bed-separation cavity resulting from daily mining operations surpasses the rate of grouting, the slurry has a propensity to infiltrate the Bed-separation cavity readily. Consequently, this presents an opportune moment to initiate the grouting process. To the growth rate of Separation with varying widths and mining rates, as depicted in Fig. 17, it is observed that distinct mining velocities yield diverse initiation and completion periods for grouting. These outcomes are presented in Table 12. It is postulated that the initiation of mining at the 7221 work face coincided with the commencement of the original design on December 6, 2017. The completion of mining as per the original design occurred on June 3, 2018, while the grouting activities were concluded on July 14, 2018. The redesign of the 7221 work face will involve the integration of various mining parameters, such as the number of grouting holes, grouting start and finish time, and corresponding grouting volume. These parameters will be systematically organized in a table format to meet the requirements of ground subsidence control. Consequently, the deflection values of the PKS will also be recorded and presented in Table 9.

Table 12

Design of the number of grouting holes and grouting start and finish time with different mining parameters
Mining width, speed	Bed-separation cavity volume(m³)	Number of grouting holes	Grouting start time	Grouting finish time	Grouting volume(m³)	PKS deflection value(mm)
100m, 3m/day	33989	7	January 25, 2018	August 29, 2018	47793	976
100m, 4m/day	33989	8	January 8, 2018	June 30, 2018	47793	976
110m, 3m/day	41946	11	January 20, 2018	August 28, 2018	79553	892
110m, 4m/day	41946	13	January 6, 2018	July 1, 2018	79553	892
120m, 3m/day	41273	13	January 17, 2018	August 31, 2018	97608	816
120m, 4m/day	41273	16	January 2, 2018	July 10, 2018	97608	816

6.3 Comprehensive Mining and Slurrying Parameters 7221 work face slurry filling mining plan selection

The implementation of Isolated Overburden Grout Injection was conducted in the 7221 work face to enhance the management of ground subsidence and safeguard the structural integrity of village structures. In order to maximize the acquisition of coal resources, the parameters for mining and grouting were reconfigured to align with the prevailing mining circumstances. To save the architectural structures of Gaochangying and Houlou Gaojia village, a comprehensive assessment of the 7221 work face's maximum mining length was conducted. Subsequently, many alternative work face widths were devised as part of the redesign process. The selection of the working face width was based on the requirement for the Bed-separation cavity volume to align with the fly ash waste handling capacity. Additionally, the deflection of PKS and the maximum deformation rate of Separation could be effectively managed by implementing grouting techniques. To ensure the timely and efficient execution of grouting activities, the design of various mining speeds is carefully coordinated to establish six distinct combinations of mining parameter design schemes. Upon consulting the initial design scheme, an estimation was made about the expected completion time for the grouting process. Additionally, the number of grouting holes and the designated start time for grouting were reconfigured by integrating the volume of the Bed-separation cavity and the typical rate of grouting per grouting hole.

Table 13 displays the mining and grouting parameter schemes that have been ultimately determined. A total of six schemes have been created. The variations in design widths result in differences in the PKS deflection value, deflection space volume, and Bed-separation cavity volume when the maximum subsidence of the control ground subsidence basin is -350mm. Consequently, the handling capacity of fly ash waste and mined coal resources also differs. Variations in mining speeds primarily dictate disparities in the durations of mining and grouting processes and the number of grout holes that need to be installed. When considering the protection requirements of ground buildings and the primary objectives of coal resource extraction and fly ash waste management, it is advisable to choose a mining program with a bigger width among the options of 100m, 110m, and 120m. The mining rate significantly impacts the duration of mining and grouting operations, and an extension of these activities results in higher mining and grouting expenses. The acceleration of mining operations leads to a quick increase in PKS deflection and ground subsidence values. To effectively regulate the thin plate, it is necessary to either increase the number of grouting holes and perform simultaneous grouting or enhance the power of the grouting pump. However, these measures also result in an escalated cost of grouting.

The initial dimensions of the work face 7221 are 588m in length and 100m in width. The average speed of progress is 4m per day. Additionally, there are a total of 5 grouting holes. These parameters may be found in Table 2 and Fig. 4. Table 10 compares the percentage increase in coal resource mining, mining, and grouting duration, volume of injected Bed-separation slurry, and the number of grouting holes for the original 7221 work face and the six mining design schemes mentioned above. Comparing the pros and disadvantages of each design scheme with the original design scheme can enhance the intuitive understanding, hence facilitating a more informed selection of the most suitable scheme for mining purposes.

Table 13

Comparison of different mining and grouting scheme designs for 7221 work face with the original design
Mining width, speed	Percentage increase of coal resource extraction (%)	Duration of mining(day)	Duration of grouting(day)	Percentage increase of grouting volume (%)	Number of grouting holes
100m, 3m/day	135.2	265	216	308.8	7
100m, 4m/day	135.2	198	173	308.8	8
110m, 3m/day	148.7	265	220	514	11
110m, 4m/day	148.7	198	176	514	13
120m, 3m/day	162.2	265	226	630.7	13
120m, 4m/day	162.2	198	189	630.7	16

Depending on the design alternatives for large-deflection inclined thin plate and grouting model, which consider various combinations of mining width and mining speed, each program exhibits distinct advantages and disadvantages. Consequently, a judicious selection should be made depending on the mining conditions. The completion of the 7221 work face in mining serves as the basis for the theoretical design presented in this study. The design is focused on comparing the effects of various mining and grouting parameters on the decision-making process of mining programs, considering the developed mechanical model. By formulating a more rational mining and grouting strategy, the objective is to achieve increased coal production while simultaneously implementing environmentally sustainable practices, minimizing the adverse effects on the surrounding ecosystem.

This work presents a comparative analysis of the impacts of each parameter of the large-deflection inclined thin plate and slurry models on the deflection of PKS and ground subsidence. The authors build upon their previously provided models and approach the analysis from a mechanistic perspective. Given the premise of keeping other factors constant, we shall make rational assumptions regarding the parameter variation. Subsequently, we will quantitatively compare the outcomes of each parameter's influence and draw the following conclusions:

The stratigraphic characteristics of the coal seam have been established during the first exploration phase, and it is not feasible to adjust or alter these parameters during the subsequent mining operations. The lithologic characteristics of the primary roof directly impact the fracture trace angle of the critical strata within the primary roof, as well as the displacement of the overlying strata when subjected to pressure. The lithologic properties of PKS are significant factors in influencing the deflection of PKS, the development of bed separation, and the extension of the ground-moving basin. The analysis of lithological parameters for these two significant strata is crucial in designing mining and grouting operations at the work face. It is essential to develop appropriate mining and grouting parameters based on these parameters.

This research focuses on the design parameters of the work face, specifically examining the impact of mining length, mining width, mining height, coal seam angle of inclination, and mining speed characteristics. Determining the angle of inclination of a coal seam is a crucial component in the coal mining field. The parameters of mining length, mining breadth, and mining height directly impact PKS deflection, Bed-separation development, and ground subsidence. Therefore, it is necessary to carefully design these parameters to balance efficient coal resource extraction and effective control of ground subsidence. The maximum value of Bed-separation development and ground sinking remains unaffected by mining speed. However, mining speed does influence the timing of Bed-separation emergence and existence, as well as the rate of ground subsidence. Consequently, when designing slurry filling mining, it is essential to consider the mining speed concurrently.

The primary factors to consider in Isolated Overburden Grout Injection design are the number of slurry injection holes, the start and end times of slurry injection, and the parameters related to slurry concentration. The quantity and velocity of slurry injected into the Bed-separation hollow directly correlate with the number of injection holes. The timing of grouting initiation and completion is critical in managing ground subsidence. If grouting is initiated too early, the formation of the Bed-separation cavity may not be adequately established. Conversely, if grouting is completed too late, the ground may experience significant subsidence. The impact of slurry consistency on PKS deflection and ground subsidence was insignificant, primarily attributed to variations in the capacity to manage fly ash waste. Using a highly concentrated injected slurry, rapid pressure drop and sluggish flow rate within the casing necessitates an elevated gear setting for the grouting pump. Consequently, this results in an augmented expenditure of electrical energy. The report lacks a detailed discussion on the impacts of grouting speed and casing pressure on the grouting process due to the absence of obtained parameters for the grouting pump.

The objective is to evaluate and design a more rational mining combination for the 7221 work face by summarizing the stratigraphic, work face design, and slurry design parameters. Maintaining the integrity of the stratigraphic parameters while recombining the design parameters and grouting parameters of the work face is important. To mitigate building subsidence in rural areas, this approach aims to augment the overall coal reserves, reduce mining expenses through enhanced fly ash disposal, and achieve environmentally sustainable mining practices.

Acknowledgements The authors are grateful to anonymous reviewers for their valuable suggestions.

Funding This research was supported by the Entrusted Project of Huaibei Mining Co., Ltd. (2023-129); the National Natural Science Foundation of China [41971401].

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Isolated Overburden Grout Injection Technology Mining and Grouting Parameters Discussion and Redesign

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Abstract

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Highlights

1 Introduction

2 The study area and the engineering geology

2.1 General overview of the study area

2.2 The engineering geology

2.3 Grouting parameters for 7221 work face

3 Stratigraphic parameter effects on PKS deflection and Bed-separation development

3.1 Influence of main roof thickness and lithologic parameters on the top plate

3.2 Effect of PKS thickness and lithologic parameters on PKS deflection and Bed-separation development

4 Effect of work face design parameters on PKS deflection, Bed-separation development and ground subsidence

4.1 Design length of 7221 work face

4.1.1 7221 work face length effects on PKS deflection and ground subsidence

4.1.2 7221 work face length impact on grouting design

4.2 Design width of 7221 work face

4.2.1 7221 work face width effects on PKS deflection and ground subsidence

4.2.2 7221 work face width impact on grouting design

4.3 Mining height of the 7221 work face

4.4 Coal seam strike angle of 7221 work face

4.4.1 7221 work face strike angle inclination effects on PKS deflection and ground subsidence

4.4.2 7221 work face strike angle impact on grouting design

4.5 Design mining speed of 7221 work face

4.5.1 7221 work face mining speed effects on PKS deflection and ground subsidence

4.4.2 7221 work face mining speed impact on grouting design

5 Effect of slurry filling parameters on PKS deflection and Bed-separation development and ground subsidence

5.1 Design number of grouting holes

5.2 Starting and stopping time of grouting

5.3 Grouting slurry consistency

6 7221 work face Bed-separation grouting mining parameters redesigned

6.1 Mining parameter redesign for 7221 work face

6.2 Design of grouting parameters of the 7221 work face

6.3 Comprehensive Mining and Slurrying Parameters 7221 work face slurry filling mining plan selection

7 Conclusions

Declarations

References

Additional Declarations

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