# Review of an Improved Longwall Panel Layout for Ground Control in Longwall Mining

Authors: Z. M. Cui; P. F. Wang; Z. Li; Y. Yang
Journal: Shock and Vibration
 (2022)

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## Abstract

Conventional longwall layout (CLL) for mining coal, trona, etc. is still fraught with problems including low resource recovery, wavy ground surface subsidence, bumps or rockbursts, gateroad support and maintenance, poisonous air hazard, spontaneous combustion in coal mines, instability of equipment at inclined, or steeply inclined working face, etc. In order to improve longwall mining, a novel alternate longwall panel layout that has been employed for over 20 years in China, Australia, and Russia, etc. is proposed termed “split-level longwall layout” (SLL). Gateroads on either end of the same SLL panel are split level, i.e., placed in different levels. This paper provides a review of existing research literature and insights on the problems encountered in CLL and the improvement of the new approach. Theoretical, numerical analysis, and field observation results of the new strategy demonstrate that the problems mentioned above in CLL are mitigated, and the development entries using the improved longwall system have a better stress environment with associated reduction in large roadway convergence, coal bumps or outbursts, and many other issues on ground control. Based on the current development of SLL and the future requirement of mining industry, the prospects as well as its applications, limitations, and concerns for SLL are presented.


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## Body

## 1. Introduction

Longwall mining is a very important method for mining coal, trona, and many other mineral resources [1]. Commercial coal mining has been several hundred years. As a result, easily minable coal reserves are being depleted, and the coal deposits to be mined are less favorable. Many of the countries’ governments require that all possible deposits be mined to conserve the nation’s energy resources [2]. These factors and the large percentage of inclined seams and faults make mining very difficult and costly. What is worse, the population density and the heavy surface buildup cause additional expense in the form of payments for subsidence damage to surface structures. In addition, greater depth of coal seams results in higher ground pressure that would require extremely large coal pillars to maintain stability of the gate roads.The ultimate objective of a mining method is to produce coal as much as possible while ensuring health and safety. Frequently, it is inferred in the mining industry that safety and productivity are inconsistent with one another and that the mining system selected must represent a compromise between the two [3, 4]. Actually, the safest system will always prove to be the most productive and vice versa [5]. Longwall operations requiring the minimum number of entries and allowing maximum recovery of resources is the mining method almost exclusively practiced [6]. Longwall mining consists of driving one or more gates or entries approximately 100 to 350 m apart, providing an interconnection and then mining the rib of the interconnection on a longwall, hence the name longwall mining. The longwall can be mined either on retreat or on advance. The principal underground coal extraction method in China is longwall mining mostly that is of the retreating type, although room and pillar and some special methods are used in some sporadic areas [7]. The longwall mining system is the simplest among all the mining methods attributed to the continuous production, and full potential for automation, productivity, and personnel health and safety is also further improved consequently [8].Despite its advantages, however, according to the longwall mining practice in China, many problems are gaining more attention as coal seams are more adverse and complicated geologically such as low recovery, pillar problems, gate road support and maintenance, surface subsidence, coal bumps or outbursts, methane control, spontaneous combustion, and face equipment slide in inclined and steeply-inclined seams, etc. Therefore, there are many critical issues in longwall mining. Based on the development of longwall mining in China, this paper provides a review of existing research literature and insights on the problems encountered in CLL. The paper also serves to provide a review of existing research literature and insight into a new strategy for tackling or mitigating above problems.


## 2. Review of Main Problems in CLL

The main problems encountered in CLL are analyzed first in this section.Low recovery. As mentioned earlier, present-day coal reserves are more limited, so the precious resources have to be treasured. However, in order to avoid the influence of abutment pressure, commonly huge chain pillars or gate pillars have to be left unmined. Current pillar width in America is about 30 m that is huge and thus leading to low recovery. For today’s high production longwalls, every one foot reduction in pillar width is of significant attractive incentive [8]. Large amount resource loss due to large pillar is no exceptions in China, either. In depth and in areas of high stress concentration, pillar width is inevitably larger meaning larger amount of coal loss. A typical conventional longwall layout (CLL) with top coal caving in China is illustrated in Figure 1.Figure 1
Conventional longwall layout (CLL) with top coal caving.Pillar problem: chain pillars or gate pillars give rise to many problems [9, 10]. First of all, stress concentration occurs in pillars often results in slabbing and sloughing (sometimes even worse, bumps, or bursts) from coal pillars due to its own fragility, thus reducing the size of the pillar that in turn increases the weight on the size-reduced pillar. With the deterioration of pillars, instability and poor safety conditions are more likely to occur. The top coal above the gateroads eventually caves in becoming loose coal in the gob, as shown in Figure 1. Loose coal in the gob is costly and risky to clean up. Progressively smaller size of pillar results in stress transfer to gateroads that increases support expense. Constantly, larger size pillars have to be left unmined to avoid side abutment pressure thus leading to lower recovery rate. Coal beds are underlain by claystone very commonly, and it may be relatively soft. As shown in Figure 1, too much roof pressure concentrated on the pillar results in lateral flow outward and upward of the rock beneath the pillars into the gateroads resulting in floor heave or roadway convergence that greatly aggravates mining operations.Gateroad support and maintenance problem: when coal is extracted in slices as it is in the longwall mining, the roof over the gob area tends to deflect as a cantilever and eventually caves in. As a result, stress uniformly distributed in the coal seam in the longwall panel before excavation is readjusted until a new equilibrium is achieved, resulting in front abutment ahead of the longwall face, side abutments along the two gateroad ribs, and the rear abutment in the gob area bridging between the headgate and tailgate [11–13].Since abutment pressures are caused by the cantilevered roof beams, the length and thickness of the immediate roof that overhangs the gob determine their magnitudes. The longer and thicker the immediate roof, the larger the abutment pressures. This is true if a massive sandstone layer is immediately above the coal seam [14]. And a weak shale will break immediately behind the powered supports without any overhang. At the T-junctions of the mains and tailgates, front and side abutment superimposes to become peak abutment pressures and plays havoc with mining.As shown in Figure2, along the positive direction of x axis, the side abutment pressure increases exponentially with the distance to the origin and reaches peak value at a distance x0. In mining practice in China, the distance x0 generally ranges from 5 to 12 m [11]. Then it decreases exponentially with the distance to the origin. Solid coal on the right is divided into four zones: I—destressed yield zone, II—overstressed plastic zone, III—overstressed elastic zone, and IV—premining vertical stress zone. In some cases, a stress of five times the cover pressure can occur for the peak abutment pressure and causes a significant support and maintenance problems in gateroads, such as roof falls, floor heave, coal bumps, or bursts, especially in retreat mining when previous adjacent panel is mined out, the gateroads next to the mined-out panels will suffer concentrated pressure.Figure 2
Stress redistribution due to coal excavation using CLL [13].To avoid this, the gateroads are commonly driven in zone III leading to a huge coal pillar or IV leading to even lower recovery. While at the face, the vertical pressure reduces to far below the cover load and gradually increases toward the gob because the fragmented roof rocks become gradually compacted and eventually supports the main roof at a distance between 3/10 and 4/10 of the overburden thickness from the faceline where the maximum pressure is equal to the cover load and no further increase in pressure is observed [12].Sliding of face equipment along the true dip. For an inclined seam and steeply inclined seams, instability of face equipment along the true dip and disconnection of stage loader to the armored face conveyor (AFC) remain problems, and it is difficult for miners to keep balance as well due to gravity traction thus leading to poor stability and unsafe condition [14].Bumps and bursts: openings in “deep” mines in weak, ductile (or pseudo-ductile) rocks, such as salt rock, shale, or bituminous coal, are characterized by viscoplastic (or pseudo-viscoplastic) deformation of the surrounding rock. This behavior serves to mitigate the impact of high ground stresses. In strong, brittle rocks, however, strain energy resulting from excavation is not dissipated through viscous flow, but is stored in the rock until a limit is reached at which point failure occurs in an abrupt, violent, or explosive manner. Such explosive failures as bumps and bursts, like earthquakes, are accompanied by emission of large amount of acoustic energy. As mentioned earlier, pillars would be the source of bumps and bursts as stresses concentrate on them. But note that bumps and bursts may also occur at relatively shallow depths due to catastrophic failure of improperly designed pillars. It is especially serious in longwall mining as the number of pillars is less than that in room and pillar method, which means more concentrated stress transfers to chain pillars resulting in higher likelihood of bumps and bursts [15].Surface subsidence: CLL panels are separated by chain/gate pillars, and ground surface above the coal bed consequently cannot reach maximum subsidence in most cases, especially in depth [16]. The left pillars in CLL induce a deformation of a wavy pattern above multiple isolated panels [17]. As subsidence is not fully developed, any damage sustained by the structure during mining is impermanent and likely to grow as the underground pillars begin to deteriorate, crush, and fail sometime later. Therefore, the chain/gate pillars are the source of nonuniform deformations on the surface and artificially influence the continuity of extraction of coal bed as well as disaster of secondary subsidence years after coal extraction [18]. The deformation above chain gate pillar is shown in Figure 3. There are many proper protective measures being adopted to reduce the amount of deformation occurred on the surface and the structures including complete extraction, continuous extraction, selected extraction, limited thickness extraction, simultaneous extraction, backfilling or stowing, etc. [19]. However, they are either costly or of low recovery.Figure 3
Nonuniform deformation above a chain/gate pillar.Methane control: adequate dilution and removal of methane are required in the longwall mining to ensure a safe environment [20]. In China, many coal mines are confronted with methane control problem both in the working sites or gas-drainage entry. Faces with high coal production rates, newly developing mines, and mines where development headings are being driven in virgin coal are all potential candidates for excess methane emissions. Methane is a gas that is slightly lighter than air and is colorless tending to concentrate in the upper corners. To solve this problem, very often, an extra dedicated gas-drainage entry has to be driven along air-return gateway, as shown in Figure 1, which increases additional tunneling costs.Note that besides these main problems, there are also other problems in CLL. For instance, top coal at the both ends of a longwall face is left unmined in order to protect the gateroads thus forming the “T” shape pillar (Figure 1). However, the top coal caves in as loose coal after the panel is mined out, which may spontaneously combust within gob. In addition, spontaneous ignition frequently occurs in the high coal roof fractured areas above gateroads that are hazards for mine safety [17].


## 3. New Strategy

In conventional retreat longwall system, the gateroads on either end of the panel are located at the same elevation within a same coal seam, as shown in Figure1, which gives rise to the problems analyzed above. A novel strategy that is widely used in China is reviewed in this paper that mitigates those problems. The new method is termed longwall mining with split-level gateroads or split-level longwall layout” (SLL) in short [21, 22]. Figure 4 shows the system of a typical SLL in a flat coal seam.Figure 4
A typical SLL layout with top coal caving in a thick flat coal seam.As we can see, the gateroads are placed in different levels in an SLL panel. The headgate is along the floor, while the tailgate is along the roof. Therefore, a gradually elevating section on the right end of the panel is formed. The headgate of the active panel and the tailgate of the previous mined-out panel are overlapped or offset. By adjusting inclination of each pan and shield according to site conditions and requirements, the elevating part can be established. The headgate of future panel is driven along floor and under the gob edge of the current panel over 6 months after extraction of the current panel in the same manner as the headgate of the active is driven under the gob edge of the previous panel, thus forming an approximate triangular coal pillar with only less than half the height of the coal seam. Figure5 shows the three-dimensional views of one SLL panel.Figure 5
3-D views of one SLL panel [1].


## 4. Analysis of the New Method

This superficially simple improvement is actually of significance. The review of scientific advantages of the method is presented in this section.


### 4.1. Stress Environment for the Headgate under the Edge of SLL Gob

First of all, the headgate is driven under the gob edge of the previous panel, which is similar with that in multislice longwall mining where gateroads in the lower panel are located directly underneath the gob of the immediate super-adjacent panel, i.e., the destressed zone [12]. As we can see from Figure 4, since the roof of the headgate of the active panel is no more top coal, but caved rocks, known as pseudo- or regenerated or artificial roof, then the mechanical properties, mainly the bonding strength, depend on compaction and cementation degree as well as the bridging structures that main roof and overburden strata form after their fracturing. Support of the headgate of the active SLL panel next to an extracted SLL panel at Zhenchengdi Colliery [17] in China is shown in Figure 6.Figure 6
Support in headgate of SLL panel in Zhenchengdi colliery [17].De-stressed environment of headgate leads to less roof support requirement. Moreover, the bumps and bursts in the headgate are effectively avoided as no stress concentration occurs in this de-stressed area, as shown in Figure7. The headgate is located within Section 5, where the vertical stress is far lower than premining stress. The triangular coal loss is on negative axis, “negative coal pillar” is therefore used while in CLL the pillar is positive.Figure 7
Stress distribution due to extraction of one SLL panel.


### 4.2. Roof Condition of the Tailgate of SLL

The tailgate is now driven under the competent roof rock, rather than top coal, providing a better roof support condition. Figure8 shows roof bolting in tailgate in Zhenchengdi coal mine [17]. As we can see, the bolting result is satisfactory. Compared with CLL, the new strategy reduced a large amount of cost waste on roof support.Figure 8
Roof bolting in tailgate of SLL.


### 4.3. Spontaneous Combustion and Methane Drainage

Loose coal is substantially reduced in artificial-roof preventing spontaneous combustion both within the gateroad and in the gob effectively. Another advantage of SLL is that higher tailgate is beneficial to drainage methane generated in the working face. Thus, the dedicated gas-drainage entry is unnecessary resulting in lower tunneling cost.


### 4.4. Recovery Ratio

What is more, the coal loss is tremendously reduced as originally huge “T” shape pillar turns to small triangular pillar [22]. The comparison of recovery of two types of pillar is shown in Figure 9.Figure 9
Recovery comparison [23].Assume that the average coal seam height is 5 m, panel width is 180 m, and hence the cross section of the panel is 900 m2. Assume that the pillar width is 30 m, gateroad width is 6 m, the width of top coal left unmined above the gate road is 10 m wide, and according to most SLL practices, the number of pans and shields needed for the elevating section is about 12, as shown in Figure 5, thus the width of the triangular pillar is about 15 m. Therefore, the cross section of the “T” shape pillar is of the order of 175 m2, while that of triangular pillar is only about 18.75 m2, which means the recovery is increased by 17%. For a panel length in order of 2000 m, this is a very attractive incentive.


### 4.5. Surface Subsidence

What is more, the new strategy is very effective in ameliorating the surface deformations, reducing the damage to surface structures. According to the characteristics of the surface movement and deformation, the left pillar in CLL would induce a deformation of a wavy pattern above multiple isolated panels. While in SLL, the large pillars are eliminated, the small triangular coal pillars hardly have influence on surface deformation [24]. Compared with Figure 3, it can be seen that a continuous surface trough on the surface over multiple SLL panels is developed, as shown in Figure 10.Figure 10
Subsidence characteristics in SLL and CLL.Coupled with additional protective measures for structural elements, satisfactory results would obtain for surface structures protection. Especially in depth, the mined out area generally cannot reach the critical size, which means no maximum potential subsidence occurs on the surface, and second subsidence may occur years after the panel is mined out because large voids in gob have not closed even after several years of extraction of the coal seam [21]. In SLL, however, the isolated troughs are ultimately joined together due to the supercritical size is reached for multiple SLL panels (normally at least two). Most importantly, as the gob in SLL is more complete and sufficiently compacted thus avoiding the second subsidence and wavy deformations on the surface [23]. What is more, by using the continuous overburden grout injection, a better subsidence control result is obtained, with higher coal recovery ratio, less drilling work load, and less future potential secondary subsidence hazards, as shown in Figure 11 [25].Figure 11
Overburden grout injection [25]. (a) Independent overburden grout injection. (b) Continuous overburden grout injection.
(a)(b)


### 4.6. Seam Roof Influence

Zhiqiang Wang [26] used theory of elastic thin plate [27] and key strata theories [11] to analyze the stability characteristics of overlying strata in SLL. Deflection analytical solution and break interval formulae for overlying strata were derived; it is concluded that with no conventional rectangular pillars left between panels, the lowest intact key stratum is higher, corresponding models for height of caved zone was established. Both before and after breakage of the main roof of the first panel, its bending moment distributes symmetrically with the maximum occurring in the middle, and bending moment of the main roof after its breakage is larger than that before breakage. While in successive panel, despite maximum bending moment of the main roof before its breakage also occurs in the middle, its distribution, however, is not symmetrical. Bending moment adjacent to gob is apparently larger than that on the opposite solid side inby. After breakage of the main roof of the successive panel, the maximum occurs adjacent to the gob.Based on elastic thin plate theory and beam theory, it is observed that break intervalsL1, L2, L3, and L4 for the following four cases satisfy [28].Before breakage of the main roof of the first panel:(1)1−μ221+μλ1221+λ14h2qL12≤σ.after breakage of the main roof of the first panel:(2)L2=16L1.before breakage of the main roof of the successive panel:(3)1−μ224+3μλ3242+λ34h2qL32≤σ.after breakage of the main roof of the successive panel:(4)31−μ221+μλ4241+λ44h2qL42≤σ,where q—loading acting on the main roof, KPa; μ—Poisson’s ratio; h—height of the main roof, m; σ—tensile strength of main roof, MPa; λ1—ratio of suspending width of the main roof to face length before its breakage of the first panel, i.e., b1/a; λ3—ratio of suspending width of the main roof to face length before its breakage of the successive panel, i.e., b3/a; λ4—ratio of suspending width of the main roof to face length after its breakage of the successive panel, i.e., b3/a;Based on key strata theory, mechanical models were built, as shown in Figure12.Figure 12
Mechanical models of key strata above longwall panels: (a) Mechanical model of key strata above SLL panels; (b) mechanical model of key strata above conventional longwall mining panels.
(a)(b)There is no pillar left, and two key plates above the two panels become a unit plate with four anchored edges. According to literature [28]:(5)Mmax=7qλ2a2161+4/7λ2+λ4≤h26σ.It indicates that overburden strata would break at the point where maximum normal stress exceeds its tensile strength. As overburden strata over SLL panels are connected and behave like one, for the same size of overhanging plates, stable key strata in SLL develops higher, as shown in Figure13 [1].Figure 13
Stable strata development for the two methods: (a) SLL; (b) CLL.
(a)(b)Numerical modelling shows that a fracture ring is developed above the two ends of the first SLL panel (Figure14(a). When two SLL panels are mined out (Figure 14(b)), the vertical fracture ring above the two ends of the mining area is higher and more sufficient just like a supercritical longwall panel with great panel length. This is a strong scientific foundation for gas flow and drainage using gob wells or cross measure boreholes [29]. While in CLL, two independent rings are developed above two ends of each panel, and the height of the rings is roughly the same with that of the first single SLL panel (Figure 14(c)).Figure 14
Numerical modelling of Vertical fractures development: (a)after extraction of the first SLL panel; (b)after extraction of two SLL panels; (c)after extraction of two CLL panels.
(a)(b)(c)Lastly, in inclined and steeply inclined seams, SLL can solve the problem of instability of equipment at the working face, which can be seen from the applications of SLL in Tangshan [30] and Wangjiashan [31] Collieries in China, as shown in Figure 15. By adjusting each pan and shield one after one (Figure 15(d)), these mining equipment gradually becomes horizontal. The upper inclined shields are subjected to the resistance by the flat shields at the horizontal part, thus enhancing the stability of the whole set of shields in the working face.Figure 15
Inclined coal seam: (a) CLL; (b) SLL layout; (c) face overlapping; (d) shields layout and transition; (e) SLL in inclined coal seam in Wangjiashan coal mine [30, 31].
(a)(b)(c)(d)(e)SLL is flexible and diversified for engineers, designers, or researchers to choose from, except for the inner offset form, as shown in Figure4, there are also many other forms shown in Figure 16. Note that SLL is not only limited to thick, medium-thick, or ultra-thick coal seams, actually, rock tunnel can now be driven at relatively lower cost thanks to the development of tunneling technology, SLL can also achieve high yield and high efficiency in thin coal seams, as shown in Figure 17, the tailgate is excavated in the roof rock.Figure 16
Other commonly used forms of SLL [32].Figure 17
SLL tailgate excavated in roof rock in thin coal seams.For cases with positive coal pillar left in SLL, according to literature [32], the width of pillar is(6)B=x0+2m+x1,where B is the pillar width; x0 is the depth of yield of pillar from the gob edge; m is the mining height; and x1 is the depth of yield of pillar from next new roadway. And [16].(7)x0=m2ξtanϕlnKγH+CcotϕξCcotϕ,where ξ=1+sinφ/1−sinφ; φ is the angle of internal friction; K = stress concentration factor; γ is unit weight of the over burden strata; H is the cover depth; and C is cohesion.Since the mining height “m” reduces, then x0 reduces accordingly. Hence, “B” reduces considerably. So the size of coal pillar is much smaller than that in CLL.


### 4.7. Seam Floor Influence

Gob geometry of an SLL panel is unique, which has a curved section on one end of the panel, as a consequence, the floor behavior of SLL panel is different from CLL panel. For both flat coal seams and inclined coal seams, the failure behavior of floor of the SLL panel is similar to that of the CLL panel except for the curved section.The authors carried out a study on stress environment of entry driven along gob-side through numerical simulation incorporating the angle of break (AOB), as shown in Figures18–20 [34]. It indicated that the results show that a “/\ shape” shear failure zone develops around the gob, yielded area within the gob floor close to the gob edge is smaller, the stress around the god-side entry is much smaller than premining stress, and the area of intact rock mass at the elevating section is larger than conventional layout. This demonstrates the protective function of the AOB, and part of the gob-side entry is within this intact zone where the stress is favorable as it is far lower than premining stress. Field observation was carried out at the coal mine extracting an inclined coal seam with dip angle of 12 [35]. The result shows that 4.1 m of the borehole of the floor close to the gob is of poor integrity. The closer to the gob, the higher degree of damage of the floor coal. At the center of borehole, about 9.8 m is intact. Close to the tailgate, only around 1.1 m of coal is yielded, as shown in Figures 19 and 20.Figure 18
Modelling results of SLL [34, 35]. (a) Modelling results of SLL in flat coal seam. (b) Modelling results of SLL in inclined coal seam.
(a)(b)Figure 19
Speculated, numerical modelling and borehole camera field observational results comparison [35].
(a)(b)Figure 20
Enlarged borehole camera figure demonstrating rock failure depth [35].Overall, the floor behavior corresponds to the shape or geometry of working face. In another word, panel geometry plays a significant role in floor rock mass failure behavior. By using SLL, ground control problems, such as roof falls, bumps, and outbursts, are minimized.


## 4.1. Stress Environment for the Headgate under the Edge of SLL Gob

First of all, the headgate is driven under the gob edge of the previous panel, which is similar with that in multislice longwall mining where gateroads in the lower panel are located directly underneath the gob of the immediate super-adjacent panel, i.e., the destressed zone [12]. As we can see from Figure 4, since the roof of the headgate of the active panel is no more top coal, but caved rocks, known as pseudo- or regenerated or artificial roof, then the mechanical properties, mainly the bonding strength, depend on compaction and cementation degree as well as the bridging structures that main roof and overburden strata form after their fracturing. Support of the headgate of the active SLL panel next to an extracted SLL panel at Zhenchengdi Colliery [17] in China is shown in Figure 6.Figure 6
Support in headgate of SLL panel in Zhenchengdi colliery [17].De-stressed environment of headgate leads to less roof support requirement. Moreover, the bumps and bursts in the headgate are effectively avoided as no stress concentration occurs in this de-stressed area, as shown in Figure7. The headgate is located within Section 5, where the vertical stress is far lower than premining stress. The triangular coal loss is on negative axis, “negative coal pillar” is therefore used while in CLL the pillar is positive.Figure 7
Stress distribution due to extraction of one SLL panel.


## 4.2. Roof Condition of the Tailgate of SLL

The tailgate is now driven under the competent roof rock, rather than top coal, providing a better roof support condition. Figure8 shows roof bolting in tailgate in Zhenchengdi coal mine [17]. As we can see, the bolting result is satisfactory. Compared with CLL, the new strategy reduced a large amount of cost waste on roof support.Figure 8
Roof bolting in tailgate of SLL.


## 4.3. Spontaneous Combustion and Methane Drainage

Loose coal is substantially reduced in artificial-roof preventing spontaneous combustion both within the gateroad and in the gob effectively. Another advantage of SLL is that higher tailgate is beneficial to drainage methane generated in the working face. Thus, the dedicated gas-drainage entry is unnecessary resulting in lower tunneling cost.


## 4.4. Recovery Ratio

What is more, the coal loss is tremendously reduced as originally huge “T” shape pillar turns to small triangular pillar [22]. The comparison of recovery of two types of pillar is shown in Figure 9.Figure 9
Recovery comparison [23].Assume that the average coal seam height is 5 m, panel width is 180 m, and hence the cross section of the panel is 900 m2. Assume that the pillar width is 30 m, gateroad width is 6 m, the width of top coal left unmined above the gate road is 10 m wide, and according to most SLL practices, the number of pans and shields needed for the elevating section is about 12, as shown in Figure 5, thus the width of the triangular pillar is about 15 m. Therefore, the cross section of the “T” shape pillar is of the order of 175 m2, while that of triangular pillar is only about 18.75 m2, which means the recovery is increased by 17%. For a panel length in order of 2000 m, this is a very attractive incentive.


## 4.5. Surface Subsidence

What is more, the new strategy is very effective in ameliorating the surface deformations, reducing the damage to surface structures. According to the characteristics of the surface movement and deformation, the left pillar in CLL would induce a deformation of a wavy pattern above multiple isolated panels. While in SLL, the large pillars are eliminated, the small triangular coal pillars hardly have influence on surface deformation [24]. Compared with Figure 3, it can be seen that a continuous surface trough on the surface over multiple SLL panels is developed, as shown in Figure 10.Figure 10
Subsidence characteristics in SLL and CLL.Coupled with additional protective measures for structural elements, satisfactory results would obtain for surface structures protection. Especially in depth, the mined out area generally cannot reach the critical size, which means no maximum potential subsidence occurs on the surface, and second subsidence may occur years after the panel is mined out because large voids in gob have not closed even after several years of extraction of the coal seam [21]. In SLL, however, the isolated troughs are ultimately joined together due to the supercritical size is reached for multiple SLL panels (normally at least two). Most importantly, as the gob in SLL is more complete and sufficiently compacted thus avoiding the second subsidence and wavy deformations on the surface [23]. What is more, by using the continuous overburden grout injection, a better subsidence control result is obtained, with higher coal recovery ratio, less drilling work load, and less future potential secondary subsidence hazards, as shown in Figure 11 [25].Figure 11
Overburden grout injection [25]. (a) Independent overburden grout injection. (b) Continuous overburden grout injection.
(a)(b)


## 4.6. Seam Roof Influence

Zhiqiang Wang [26] used theory of elastic thin plate [27] and key strata theories [11] to analyze the stability characteristics of overlying strata in SLL. Deflection analytical solution and break interval formulae for overlying strata were derived; it is concluded that with no conventional rectangular pillars left between panels, the lowest intact key stratum is higher, corresponding models for height of caved zone was established. Both before and after breakage of the main roof of the first panel, its bending moment distributes symmetrically with the maximum occurring in the middle, and bending moment of the main roof after its breakage is larger than that before breakage. While in successive panel, despite maximum bending moment of the main roof before its breakage also occurs in the middle, its distribution, however, is not symmetrical. Bending moment adjacent to gob is apparently larger than that on the opposite solid side inby. After breakage of the main roof of the successive panel, the maximum occurs adjacent to the gob.Based on elastic thin plate theory and beam theory, it is observed that break intervalsL1, L2, L3, and L4 for the following four cases satisfy [28].Before breakage of the main roof of the first panel:(1)1−μ221+μλ1221+λ14h2qL12≤σ.after breakage of the main roof of the first panel:(2)L2=16L1.before breakage of the main roof of the successive panel:(3)1−μ224+3μλ3242+λ34h2qL32≤σ.after breakage of the main roof of the successive panel:(4)31−μ221+μλ4241+λ44h2qL42≤σ,where q—loading acting on the main roof, KPa; μ—Poisson’s ratio; h—height of the main roof, m; σ—tensile strength of main roof, MPa; λ1—ratio of suspending width of the main roof to face length before its breakage of the first panel, i.e., b1/a; λ3—ratio of suspending width of the main roof to face length before its breakage of the successive panel, i.e., b3/a; λ4—ratio of suspending width of the main roof to face length after its breakage of the successive panel, i.e., b3/a;Based on key strata theory, mechanical models were built, as shown in Figure12.Figure 12
Mechanical models of key strata above longwall panels: (a) Mechanical model of key strata above SLL panels; (b) mechanical model of key strata above conventional longwall mining panels.
(a)(b)There is no pillar left, and two key plates above the two panels become a unit plate with four anchored edges. According to literature [28]:(5)Mmax=7qλ2a2161+4/7λ2+λ4≤h26σ.It indicates that overburden strata would break at the point where maximum normal stress exceeds its tensile strength. As overburden strata over SLL panels are connected and behave like one, for the same size of overhanging plates, stable key strata in SLL develops higher, as shown in Figure13 [1].Figure 13
Stable strata development for the two methods: (a) SLL; (b) CLL.
(a)(b)Numerical modelling shows that a fracture ring is developed above the two ends of the first SLL panel (Figure14(a). When two SLL panels are mined out (Figure 14(b)), the vertical fracture ring above the two ends of the mining area is higher and more sufficient just like a supercritical longwall panel with great panel length. This is a strong scientific foundation for gas flow and drainage using gob wells or cross measure boreholes [29]. While in CLL, two independent rings are developed above two ends of each panel, and the height of the rings is roughly the same with that of the first single SLL panel (Figure 14(c)).Figure 14
Numerical modelling of Vertical fractures development: (a)after extraction of the first SLL panel; (b)after extraction of two SLL panels; (c)after extraction of two CLL panels.
(a)(b)(c)Lastly, in inclined and steeply inclined seams, SLL can solve the problem of instability of equipment at the working face, which can be seen from the applications of SLL in Tangshan [30] and Wangjiashan [31] Collieries in China, as shown in Figure 15. By adjusting each pan and shield one after one (Figure 15(d)), these mining equipment gradually becomes horizontal. The upper inclined shields are subjected to the resistance by the flat shields at the horizontal part, thus enhancing the stability of the whole set of shields in the working face.Figure 15
Inclined coal seam: (a) CLL; (b) SLL layout; (c) face overlapping; (d) shields layout and transition; (e) SLL in inclined coal seam in Wangjiashan coal mine [30, 31].
(a)(b)(c)(d)(e)SLL is flexible and diversified for engineers, designers, or researchers to choose from, except for the inner offset form, as shown in Figure4, there are also many other forms shown in Figure 16. Note that SLL is not only limited to thick, medium-thick, or ultra-thick coal seams, actually, rock tunnel can now be driven at relatively lower cost thanks to the development of tunneling technology, SLL can also achieve high yield and high efficiency in thin coal seams, as shown in Figure 17, the tailgate is excavated in the roof rock.Figure 16
Other commonly used forms of SLL [32].Figure 17
SLL tailgate excavated in roof rock in thin coal seams.For cases with positive coal pillar left in SLL, according to literature [32], the width of pillar is(6)B=x0+2m+x1,where B is the pillar width; x0 is the depth of yield of pillar from the gob edge; m is the mining height; and x1 is the depth of yield of pillar from next new roadway. And [16].(7)x0=m2ξtanϕlnKγH+CcotϕξCcotϕ,where ξ=1+sinφ/1−sinφ; φ is the angle of internal friction; K = stress concentration factor; γ is unit weight of the over burden strata; H is the cover depth; and C is cohesion.Since the mining height “m” reduces, then x0 reduces accordingly. Hence, “B” reduces considerably. So the size of coal pillar is much smaller than that in CLL.


## 4.7. Seam Floor Influence

Gob geometry of an SLL panel is unique, which has a curved section on one end of the panel, as a consequence, the floor behavior of SLL panel is different from CLL panel. For both flat coal seams and inclined coal seams, the failure behavior of floor of the SLL panel is similar to that of the CLL panel except for the curved section.The authors carried out a study on stress environment of entry driven along gob-side through numerical simulation incorporating the angle of break (AOB), as shown in Figures18–20 [34]. It indicated that the results show that a “/\ shape” shear failure zone develops around the gob, yielded area within the gob floor close to the gob edge is smaller, the stress around the god-side entry is much smaller than premining stress, and the area of intact rock mass at the elevating section is larger than conventional layout. This demonstrates the protective function of the AOB, and part of the gob-side entry is within this intact zone where the stress is favorable as it is far lower than premining stress. Field observation was carried out at the coal mine extracting an inclined coal seam with dip angle of 12 [35]. The result shows that 4.1 m of the borehole of the floor close to the gob is of poor integrity. The closer to the gob, the higher degree of damage of the floor coal. At the center of borehole, about 9.8 m is intact. Close to the tailgate, only around 1.1 m of coal is yielded, as shown in Figures 19 and 20.Figure 18
Modelling results of SLL [34, 35]. (a) Modelling results of SLL in flat coal seam. (b) Modelling results of SLL in inclined coal seam.
(a)(b)Figure 19
Speculated, numerical modelling and borehole camera field observational results comparison [35].
(a)(b)Figure 20
Enlarged borehole camera figure demonstrating rock failure depth [35].Overall, the floor behavior corresponds to the shape or geometry of working face. In another word, panel geometry plays a significant role in floor rock mass failure behavior. By using SLL, ground control problems, such as roof falls, bumps, and outbursts, are minimized.


## 5. Operations in SLL

Support strategies are also required to be specific and correspond to this spatial configuration. Some support designs have been put forward regarding the different SLL forms [35–37]. So the operations are more or less different from CLL.For headgate directly beneath the gob edge of previous SLL panel, a unique mining operations in SLL are called “Triple Sections Mining Operation” (TSMO) was proposed [35], see Figure 21.Figure 21
Triple sections mining operation (TSMO) of SLLSection “a” is right under the artificial roof. The roof is meshed with steel wire mesh and is constructed when mining the upper level of the previous panel. So the headgate is excavated under the mesh and a part of the section in the face is also extracted under it. The headgate is supported by steel set as shown in Figure6 to prevent falling of caved rock fragments [16]. Section “b” uses CLL top coal caving operations. Section “c” uses multislice longwall mining operations. Therefore, it requires that the shield here has wire mesh laying device on the top to form an artificial roof for the next section “a” of the next new panel. All workings of the three sections form a complete mining cycle for the panel.


## 6. Applications

SLL has been widely used in over twenty coal mines all over China for about 15 years including Zhenchengdi [7], Guandi [9], Huafeng [11], Dongxia [15], Xiegou [18], Tangshan [30], Wang jiashan [32], Hebi [38], Jingfang [39], Baijiazhuang [40], Gongsuwu [41], Hedong [42], and Guzhuang Collieries, etc. [43] and gained tremendous profit.Figure22 shows the SLL in Zhenchengdi mine. The offset of the two panels along the face advance direction is due to fault [23].Figure 22
Panel layout of SLL and transition part in Zhenchengdi coal mine: (a) plane view; (b) section view; (c) transition part (elevating part) [23].
(a)(b)(c)Table1 shows front abutment in two gateroads of 18111-1 panel of Zhenchengdi coal mine. As we can see, stress on 18111-1 intake entry is relatively low and stable without big changes due to the entry is driven under the artificial roof of the gob being subject only to some weight of caved rocks under caving line. While stress on gas-drainage entry is similar to that in CLL.Table 1
Front abutment in two gateroads of 18111-1 panel of Zhenchengdi coal mine.
Intake entryGas-drainage entryDistance to faceline (m)Average working resistance of props (MPa)Distance to faceline (m)Average working resistance of props (MPa)201.77207.50191.75186.91181.76177.03161.76167.19151.78158.23131.75148.05121.781210.72111.761113.0691.761014.0481.74912.0761.76811.0151.7269.8241.7649.1531.6938.2121.6727.5311.6717.06As another example, engineers designed several mining sequence schemes for Thangshan Colliery employing SLL, as shown in Figure23. The sequence is designed to leave enough time for gob rock piles to compact to form competent regenerated roof for future longwall panels.Figure 23
Mining sequence schemes for Tangshan Colliery: (a) three panels; (b) four panels; (c) five panels; (d) six panels.
(a)(b)(c)(d)As a large number of coal seams all over the world are inclined, the stability of the machine in inclined face is a world-wide problem, SLL is a novel strategy to solve these problems, as shown in Figure15. Another very typical case employing SLL in Dongxia Colliery is introduced in detail by Wang [15], as shown in Figure 24. He pointed out that the layout plays an effective role in improving overall stability of the shields in the steeply-inclined working face. For roof control of staggered or overlapped SLL layout for super thick beds, literature [36, 37] provided a good solutions by using rock bolts and cable bolts.Figure 24
SLL in Dongxia Coalmine, Huating coal field, Gansu province, China [15].
(a)(b)Overall, SLL is flexible and diversified to use. Engineers should consider appropriate patterns when facing varied geological backgrounds.


## 7. Concluding Remarks

SLL is a new novel approach for longwall mining. It broadens the lines of thought in mining field for researchers, engineers, and scholars, etc. It improves longwall mining conditions by ameliorating many problems existing in present-day longwall mining system such as low recovery, pillar problems, gateroad support and maintenance, coal bumps or outbursts, methane control, surface subsidence, spontaneous combustion, and instability of mining equipment at inclined and steeply-inclined faces, etc.Since gob needs time to settle, most of times this takes several months to one year to be compacted. Therefore, SLL panels have to be extracted sequentially but discontinuously. The authors have put forward several methods to solve this [36, 44–46]. Some further methods are to be proposed and hopefully that researchers all over the world can give more ideas to improve it.Some researchers are concerned about the gob-side entries problem such as air leakage and water inrush from previous mined panels. Through grout injection into the gob during the extraction of the active panel from gas-drainage entry, degree of consolidation is increased making sure that the leakage of air meets the requirement for ventilation. Grouting also decreases spontaneous heating of coal in the gob, which means fire preventing and extinguishing is also pretty good. In cases where there are accumulated water and gas in the gob, other forms shown in Figures16 and 17 should be considered. Once such case is adopted in mining of a super thick coal seam in Xiegou coal mine as demonstrated in Figure 25 [34].Figure 25
Schematic of surrounding rock mass after extraction of LTP 1 and LTP 2 panel (not strictly to scale).SLL is a preferential and flexible new strategy for engineers or designers to choose from based on geological, hydrogeological conditions, and other particular backgrounds or circumstances for longwall mines at present and in the foreseeable future. Still there are plenty of room to improve SLL such as dust control and air leakage to the gob so as to perfect it in practice. With the rapid development of intelligent mining technology, SLL is more applicable for various conditions in the future, especially for the precise working face ore cutting and control of shearer drums trajectory at the curved section.



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*Source: 7517116-2022-04-19.xml*