Research on Pillarless Mining by Gob-Side Caving under Soft Rock Roof Conditions: A Case Study

Abstract

In China, soft rock roof makes up the majority of coal mine roof, yet it is easy to break due to low strength and poor integrity. As such, it is difficult for the traditional pillar-based roadway protection means and pillarless mining techniques to effectively control the roadway roof. In order to solve the problems with pillarless mining under soft rock roof conditions, using the 1510 working face of Xinyi Coal Mine as an example, a gob-side caving roadway forming (GSCRF) technique for broken immediate roof is developed. This paper discusses the adaptability and feasibility of this technology through theoretical modeling and on-site engineering testing. A roadway rock surrounding control scheme based on “cable + steel beams + yielding prop” is further designed, and field tests and monitoring are carried out. Field tests show that, during GSCRF of the 1510 working face, the maximum subsidence of the roof remains within 200 mm. The convergence of the two sides causes stabilization. The tension on the anchor cables is gradually becoming stable. The monitoring results show that the roadway has a good stress environment and the surrounding rock is effectively controlled. Compared with the traditional pillarless mining mode, this technology has the technical advantages of achieving complete elimination of coal pillars, reduced pressure on the roadway roof, and interference-free mining of the working face. The research outcome can provide useful reference for pillarless mining by GSCRF and a solution for pillarless mining under soft rock roof conditions.

1. Introduction

Coal resources are widely distributed in China [1]. The varying occurrence conditions of coal seams across different regions have resulted in complex and changeable roof and floor conditions. Among the roof conditions of coal mines in China, soft rock roofs account for a relatively high proportion [2]. As the rock of soft roof has poor mechanical properties, it can easily loosen and break under mining stress, making the roadway unstable and the roof uncontrollable [3,4]. With the increase of mining depth, deep stresses have caused severe deformation in the roof and floor and severe contraction in the sides during the service of the roadway [5]. At the same time, major roadway deformation has also led to capacity reduction or even failure of the support system [6,7]. Therefore, it is an urgent task to control the surrounding rock of roadways with soft rock roof.

The safety of mining roadways is the top priority of coal mine safety efforts. At present, most mines in China use big pillars to protect their mining roadways [8]. This places the mining roadway in a high stress area [9]. The stress environment is complex and harsh, and the surrounding rock is severely damaged, especially under soft rock roofs [10,11]; Figure 1 [12] shows an example of a soft rock roof roadway failure. In terms of soft rock roof support schemes, some studies have found that the combined supports from rock bolts and shotcrete are the most feasible method [13,14,15]. Pillarless mining is one of the effective ways to solve the problem of surrounding rock control for gob-side mining roadways with a soft rock roof. It also increases the coal recovery rate and reduces the waste of coal resources [16]. As a mining method, gob-side roadway forming technique allows for pillarless mining between working faces. As known by the classic mine pressure theory [17], on the one hand, in pillarless gob-side entry retaining mode, there is no stress concentration; on the other hand, the solid coal side has been damaged by compression, and the stress peak is deep inside the coal wall. The stress distribution is shown in Figure 2. Therefore, the mine pressure at the location of the roadway is at a low level and changes steadily. By reinforcing the roadway surrounding rock and building roadside support, it can bear the pressure of the roof and maintain a good roadway forming effect. This greatly reduces the dynamic hazards of the roadway and effectively increases the service life of the mine [18,19].

Researchers around the world have carried out a lot of research on pillarless mining by gob-side roadway forming, especially in traditional mining countries such as Britain, the former Soviet Union and Germany, having made great breakthroughs in both theory and field application [20,21,22]. In the 1930s, some coal mines in the former Soviet Union began to carry out pillarless mining experiments by reserving no pillars and building masonry filling zones. In the 1950s, Britain, German and Poland reported gob-side roadway forming research and experiment. In the 1970s, Britain developed high-water quick-setting materials and supporting pumping and filling systems [23]. German coal mines adopted a retreating method for working faces and traditional along-solid seam excavation for section haulage roadways. They used gypsum as the roadside filling support, which was finally kept as the gob-side forming roadway and used as the return airway of the next section. They also developed a gob-side roadway forming process system consisting of three major systems: excavation, pumping, and recovery [24].

At present, pillarless mining is basically popularized in foreign countries by implementing a gob-side roadway forming bulkhead with an inorganic-organic hybrid structure together with a steel structure skeleton [25]. In the 1950s, Chinese coal mining enterprises first carried out experiments and application of gob-side roadway forming techniques in thin coal seams. Gob-side roadway forming stayed at a low level during that period due to backward coal mine support materials and techniques, as well as low degree of mechanization. From the 1960s to the 1970s, the gob-side roadway forming technique for small coal pillars developed rapidly [26]. Pillars were reduced from 20 m to 2–3 m and were applied in many mining areas across China [27,28]. In the 1980s, with the development of backfill materials and backfill-related techniques and equipment, roadway-side-filled pillarless mining was more widely used, and the new technique of backfilling the goaf-side entry retaining for next sublevel was put forward. Traditional roadway-side-filled gob-side roadway forming uses roadside backfills such as concrete walls and gangue bags to protect the gob-side mining roadway [29,30]. But when the roof rock is soft, roof falling and net pocketing often occur in the roadway. Soft rock roof is an incurable “cancer” of roadway-side-filled pillarless mining.

In recent years, with the gradual increase of mining intensity in China, the shallow coal resources have been gradually depleted. The state vigorously advocates green and efficient mining. Recently, the continuous development of supporting materials and supporting theories and techniques has pushed the theoretical research and field application of gob-side roadway forming into a new stage of development [31,32,33]. Academician He Manchao proposed a roof cutting pressure relief method for pillarless mining. The core of this technique is to cut off the connection between the partial roof of the gob and the roof of the roadway by directional roof cutting so that the roof in a certain range above the roadway forms a short arm beam structure [34,35,36]. However, where the soft rock roof is unstable, easy to break, and falls off upon excavation, and without advance roof cutting to relieve the pressure, drill sticking and caving can often arise, which ultimately compromises the roadway forming quality and limits the development of pillarless mining. The problem of pillarless mining under soft rock roof conditions needs to be solved urgently.

Based on Academician Song Zhenqi’s “Practical Mine Pressure Theory System Centered on the Movement of Overlying Rock” [37,38,39], after field investigation, theoretical analysis and industrial tests using the mining roadway of Xinyi Coal Mine as the research object, we present a gob-side caving roadway forming (GSCRF) technique under soft rock roof conditions. Compared with the roof cutting technique, the process of this technique is simpler, and can well adapt to soft rock roof conditions. We also establish a mechanical model of GSCRF and the associated support scheme. Field experiments show that the new technique is very effective, inexpensive, quick and well able to maintain the stability of the mining roadway. It solves the problem of pillarless coal mining under soft rock roof conditions.

2. Project Overview

Xinyi Coal Mine is located in Yanzhou of Jining City, southwestern Shandong Province, as shown in Figure 3. The terrain in the mine field is flat, with ground elevation of 43.35–52.78 m. The mine is bounded by Ordovician limestone top interface outcrops in the east and west, the Yuncheng branch fault in the north and the Changgou fault in the south. The ground of the 1510 working face corresponds to the southwest of Xinyi Coal Mine Industrial Plaza. The elevation of the working face is −397–−368 m. The geological structure of this working face is mainly monoclinic and is located on the two flanks of syncline II. Existence of the Changgou branch fault in the east has also increased the slope of the coal sea, which has a certain impact on mining activities. The working face has actually exposed 6 faults, among which the 1510 track 3 faults has been exposed along the 1510 track gateway, with a fault drop of 3.4 m. The roadway studied is the mine roadway of the 1510 working face, which is a haulage track roadway located between the 1510 working face and the 1512 working face. The excavation project plan is shown in Figure 4.

The 1510 working face is mined by a long-wall retreating, one-through full-height fully mechanized coal mining method. The 3-up-1 coal being mined has stable occurrence. The Platts hardness is f = 1–2; the thickness $h_c$ is 0.8–1.6 m with an average $\overline{h_c}$ of 1.1 m. The 3-up-1 coal has a mudstone pseudo-roof $h_{pr}$ about 0.3 m thick. The immediate roof is silty mudstone; $h_{ir}$ = 2.5–3.5 m thick with an average $\overline{h_{ir}}$ of 3.0 m. The main roof is gray-white fine sandstone or medium fine sandstone; $h_{mr}$ = 1.5–9.0 m thick with an average $\overline{h_{mr}}$ of 4.5 m. The floor of the 3-1 coal, which is also the roof of the 3-up-2 coal, is an average $\overline{h_{mr}}$ of 3.0 m thick, consisting of sandy mudstone and fine sandstone. Medium sandstone is found in areas where the inter-layer spacing is large. The main water sources for the 1510 face are sandstone water and fault water on the roof and floor of the 3-up coal. The histogram of the rock formation is shown in Figure 5.

3. Feasibility Determination of GSCRF and Surrounding Rock Control Theory

3.1. Technical Principle of GSCRF

Under soft rock roof conditions, the immediate roof has low strength, poor stability, breaks easily, and falls off upon excavation. Under the traditional pillar-based roadway protection, where the mining roadway is in a high stress area, the stress environment is complex and harsh, and the surrounding rock is severely damaged. Pillarless mining by GSCRF makes use of the above-mentioned characteristics of the soft rock roof. After the working face has been advanced, under the action of mine pressure, the immediate roof rock is let fall and stacked in the gob. Using the gangue retaining function of along-strike props and metal nets, a gangue wall is produced on the edge to seal up the gob and to prevent the movement impact of the main roof and support the rock beam of the old roof. After the main roof has rotated to touch the gangue, a stable cross-roadway oblique beam structure is formed, which ensures that the mining roadway is well formed.

After the working face system is formed, bolts (cables) are used in the roadway to support the immediate roof in a given deformation state. Hydraulic props and gangue forming nets are set up along the gob side. While advancing the working face, the immediate roof is let fall under mine pressure to form a wall and serve as the roadway side. After the working face is has been advanced, the hydraulic props will support the main roof rock beam in a given deformation state. When the main roof sinks and touches the gangue, it gradually stabilizes. The characteristics of bolts (cables), steel belt and hydraulic props are described later in the article, in Section 4.2. Figure 6 shows the principle of the roadway as formed.

3.2. Feasibility Determination of GSCRF

As a new type of roadway forming, GSCRF can only be applied when the surrounding rock of the stope meets certain conditions and it is not necessarily suitable for all mines. Therefore, it is necessary to study the feasibility of this technique. According to the site conditions of the 1510 track gateway in Xinyi Coal Mine, the feasibility of GSCRF is determined.

3.2.1. Immediate Roof Cavability: Immediate Roof Can Fall Arbitrarily

To implement GSCRF, the immediate roof must be let fall upon excavation so that the immediate roof will not become a large overhanging roof after the working face has been advanced. The caving step distance of the immediate roof reflects the cavability of the immediate roof. For the convenience of expression, the caving step distance of the immediate roof can be approximated as the cantilever length when the cantilever beam reaches its failure limit, and compared with the critical overhanging distance for the cavability of the immediate roof, as shown in

Table 1. Determination of immediate roof cavability [40].

$L_c=\sqrt{\frac{{\sigma }_{Tr}{H}_{ir}}{3\rho g}}\le 0.3$	$L_c=\sqrt{\frac{{\sigma }_{Tr}{H}_{ir}}{3\rho g}}>0.3$
The immediate roof has low strength, accommodates cracks, and is soft and broken. The cantilever length at the failure limit of the cantilever beam is less than or equal to the critical overhanging distance for the cavability of the immediate roof (taken as 0.3 m).	The immediate roof has high strength, large thickness and good integrity. The cantilever length at the failure limit of the cantilever beam is greater than the critical overhanging distance for the cavability of the immediate roof (taken as 0.3 m).
After the working face has been advanced, the immediate roof will fall upon excavation, becoming a gangue wall at the gob side of the roadway to provide protection, as shown in Figure 7a.	After the working surface has been advanced, the immediate roof will become an overhanging roof at the gob side instead of caving into a wall. The roadway will lose its gangue protection at the gob side, as shown in Figure 7b.

$L_c$—overhanging distance of the immediate roof, m; ${\sigma }_{Tr}$—tensile strength of the immediate roof, MPa; $\rho$—density of immediate roof, kg/m³; $H_{ir}$—thickness of the immediate roof, m; $g$—gravitational acceleration, m/s².

. According to field observations and experience, the critical value is generally taken as 0.3 m. In Figure 7a, under the combined action of roof pressure and wire mesh, while playing a role of blocking rubble, the rubble in the collapse zone has very little effect on the hydraulic support.

3.2.2. Gangue Wall Formability: Gangue Height $H_g$ Should Be Greater Than Roadway Height $H$

After the immediate roof has caved in the gob, the gangue must be slightly higher than the roadway (according to numerous on-site observations, it can be set to $H+0.4 \mathrm{m}$ [40]). so as to become a wall and protect the roadway. Otherwise, air will flow between the roadway and the gob, and may cause impact. The determination method is shown in Table 2.

Table 2. Determination of the wall formability of gangue [40].

The thickness of the immediate roof is large; the fallen gangue is slightly higher than the roadway, as shown in Figure 8a.	The thickness of the immediate roof is relatively large; the fallen gangue is high enough to fill up the gob; the gangue is completely connected to the roof, as shown in Figure 8b.	The thickness of the immediate roof is small; the fallen gangue is lower than the roadway, as shown in Figure 8c.
$H_g=H_{ir}{K}_{\alpha }>H+0.4$	$H_g=H_{ir}{K}_{\alpha }=H+H_{ir}$	$H_g=H_{ir}{K}_{\alpha }<H+0.4$
The fallen gangue can seal up the gob and become a wall at the gob side of the roadway; the fallen gangue can effectively seal up the gob and the roadway.		The fallen gangue cannot seal up the gob and become a complete wall at the gob side of the roadway; air will easily flow between the roadway and the gob, affecting the ventilation of the roadway ventilation.

$H_{\mathrm{g}}$—the height of the immediate roof after it is completely fallen and expanded, m; $K_{\alpha }$—the rock dilatancy coefficient.

Table 2. Determination of the wall formability of gangue [40].

The thickness of the immediate roof is large; the fallen gangue is slightly higher than the roadway, as shown in Figure 8a.	The thickness of the immediate roof is relatively large; the fallen gangue is high enough to fill up the gob; the gangue is completely connected to the roof, as shown in Figure 8b.	The thickness of the immediate roof is small; the fallen gangue is lower than the roadway, as shown in Figure 8c.
$H_g=H_{ir}{K}_{\alpha }>H+0.4$	$H_g=H_{ir}{K}_{\alpha }=H+H_{ir}$	$H_g=H_{ir}{K}_{\alpha }<H+0.4$
The fallen gangue can seal up the gob and become a wall at the gob side of the roadway; the fallen gangue can effectively seal up the gob and the roadway.		The fallen gangue cannot seal up the gob and become a complete wall at the gob side of the roadway; air will easily flow between the roadway and the gob, affecting the ventilation of the roadway ventilation.

$H_{\mathrm{g}}$—the height of the immediate roof after it is completely fallen and expanded, m; $K_{\alpha }$—the rock dilatancy coefficient.

3.2.3. Impact-Free Main Roof Subsidence: Excessive Settlement Height Will Induce Strong Impact

The stable, impact-free subsidence of the main roof rock beam is an important guarantee for the safety of the roadway. When the gangue caves in, its height should not be too far away from the main roof rock beam (according to numerous field observations, the critical value here is set to 1 m). Otherwise, the subsidence of the main roof rock beam can impact the falling gangue and affect the safety of the roadway. The determination method is shown in Table 3.

Table 3. Determination of gangue impact [40].

The thickness of the immediate roof is large; the fallen gangue is less than 1 m from the main roof, as shown in Figure 9a.	The thickness of the immediate roof is too small; the fallen gangue is more than 1 m from the main roof, as shown in Figure 9b.
$\Delta H=H+H_{ir}-H_{ir}{K}_{\alpha }<1m$	$\Delta H=H+H_i-H_i{K}_{\alpha }>1m$
The gangue is not far from the main roof; the main roof rock beam bends and sinks to touch the gangue, which has no impact on the gangue and does not threaten the safety of the roadway.	The gangue is too far from the main roof; after the end and middle of the main roof are cracked, a large amount of bending elastic energy will be released; the gangue will also be greatly impacted, thus affecting the safety of the roadway.

Table 3. Determination of gangue impact [40].

The thickness of the immediate roof is large; the fallen gangue is less than 1 m from the main roof, as shown in Figure 9a.	The thickness of the immediate roof is too small; the fallen gangue is more than 1 m from the main roof, as shown in Figure 9b.
$\Delta H=H+H_{ir}-H_{ir}{K}_{\alpha }<1m$	$\Delta H=H+H_i-H_i{K}_{\alpha }>1m$
The gangue is not far from the main roof; the main roof rock beam bends and sinks to touch the gangue, which has no impact on the gangue and does not threaten the safety of the roadway.	The gangue is too far from the main roof; after the end and middle of the main roof are cracked, a large amount of bending elastic energy will be released; the gangue will also be greatly impacted, thus affecting the safety of the roadway.

3.3. Roadway Stress Environment and Surrounding Rock Damage

In the process of GSCRF, the mining roadway is formed in a unique way from traditional pillar-based roadway protection and gob-side roadway forming technique. The stress environment is also different. Ensuring roadway surrounding rock control during the mining of the first and next working faces is the key to pillarless mining by the GSCRF process.

Under GSCRRF, the stress environment of the mining roadway has the following characteristics:

(1)

The roadway is in the low stress area; the solid coal side has been crushed and damaged and lies in the internal stress field where the stress is low; at the gob side where the gangue is not anchored by bolts and cables, the stress is modest, too.

(2)

The immediate roof of the coal seam being mined is broken, has low strength and accommodates many cracks. Therefore, there may be considerable strata behavior in the roadway space.

(3)

The gangue in the gob can easily slip into the roadway to affect the internal space of the roadway.

(4)

There is no coal pillar at the gob side; the roof is not supported and can deform badly.

The instability of the mining roadway mainly results from the action of loosening pressure, deformation pressure and expansion pressure on the surrounding rock supporting structure. If the roadway work is not supported in time, the deformation pressure and expansion pressure will damage the surrounding rock and turn into loosening pressure, causing the surrounding rock to become unstable.

After the gob-side caving roadway is formed, under the action of secondary stress, the surrounding rock will be locally plastically, viscoelastically or visco-elastoplasticcally deformed. The force of the loosening rock mass will directly act on the roadway support, causing the surrounding rock stress to be redistributed. Part of the surrounding rock or its structural surface will detach from the parent rock and become a separate block and a bulk mass. Under the action of gravity, it will overcome minor resistance to produce caving and slump movements, causing the surrounding rock to become structurally unstable.

4. Field Application of GSCRF

4.1. Feasibility Determination of GSCRF to the 1510 Working Face

According to its actual production, Xinyi Coal Mine plans to implement a GSCRF test along the 1510 track gateway in the upper part of the fifth mine. After determining the feasibility of pillarless mining of the 1510 working face by GSCRF, through calculation and demonstration, the 1510 working face of Xinyi Coal Mine was found to have the following characteristics:

(1)

Immediate roof cavability

The immediate roof of the 1510 working face of Xinyi Coal Mine is composed of mudstone and argillaceous sandstone, with low strength and well-developed fissures, which caves in upon excavation. The immediate roof has a tensile strength ${\sigma }_{Tr}$ of 0.2 MPa, a thickness $H_{ir}$ of 3 m, a density $\rho$ of 2.5 t/m³, and a gravitational acceleration $g$ of 9.8 m/s². Accordingly, based on Table 1, the failure limit $L_c$ of the cantilever beam is:

$$\begin{array}{cc}{L}_c& =\sqrt{\frac{{\sigma }_{Tr}{H}_{ir}}{3\rho g}}\hfill \\ & =\sqrt{\frac{0.2 \mathrm{MPa} \times 3 \mathrm{m}}{3 \times \frac{2.5 \mathrm{t}}{{\mathrm{m}}^3} \times \frac{9.8 \mathrm{m}}{{\mathrm{s}}^2}}}\hfill \\ & =0.286 \mathrm{m}<0.3\hfill \end{array}$$

(1)

(2)

Gangue wall formability

The mining height of the 1510 working face of Xinyi Coal Mine is 1.5 m; the thickness of the immediate roof is 3 m; the rock dilatancy coefficient of the roof is 1.35 after the immediate roof has fallen, as can be calculated according to Table 2. The height of the fallen gangue is 4 m, which is much higher than the roadway. This way, a wall is formed at the gob side as a roadside.

(3)

Impact-free main roof subsidence

For the 1510 working face, according to Table 3.

$$\begin{array}{cc}\Delta H& =H+H_{ir}-H_{ir}{K}_{\alpha }\hfill \\ & =1.5 \mathrm{m} + 3 \mathrm{m} - 3 \mathrm{m} \times 1.35\hfill \\ & =0.5 \mathrm{m} < 1 \mathrm{m}\hfill \end{array}$$

(2)

After the immediate roof has fallen, the fallen gangue is already in contact with the main roof. While rotating and falling, the main roof will not release strong bending elastic energy. Its rotation, sinking and gangue touching will not impact the roadway. That is, it will make a “soft landing”.

The above results show that pillarless mining by GSCRF is feasible for the 1510 working face.

4.2. Surrounding Rock Control Scheme

4.2.1. Roof Support Scheme

The 1510 track gateway has an excavation width $W_e$ of 4.0 m and a net width $W_n$ of 3.8 m and is supported by anchor nets (cables). The roadway is supported by 8# cold-drawn wire woven metal nets (sized 60 × 60 mm) across the whole section, with 2100 mm long Φ20 mm resin anchor bolts for the roof, and 2100 mm long Φ18 mm full thread anchor bolts for the sides. Bolt plates sized 150 × 150 mm (length × width) are 10 mm thick steel plates pressed into an arc shape. The row spacing of the anchor bolts is 950 × 1000 mm for the roof (center-to-center) and 1000 × 1000 mm for the sides (center-to-center). The torque of the anchor bolts $t_{ar}$ is a minimum of 200 Nm for the roof and 150 Nm for the sides. The diameter of the anchor cable is 18 mm, the length is 6 m, the spacing is 1400 mm, the row spacing is 3 m. These cables are installed in groups of 2 and supported by 500–600 mm 11# I-steel cable beams pretensioned to the design value of 200 KN.

According to theoretical analysis and field investigation, with the advancement of the working face, the immediate roof rock will fall in time, and it can basically fill up the gob. Under such conditions, since there is almost no free space below when the main roof rock beam rotates and sinks, its movement is slow and gradual, and will not impact the stope support or the roadside support body of the gob-side retained roadway. Therefore, the pressure on the roadside support or the anchor-net-cable support is mainly from the immediate roof. That is, the main function of the roadside support or the anchor-net-cable support is to directly hold or hang the immediate roof, so that it is closely combined with the main roof rock and can collapse and sink together with the main roof rock beam until the main roof rock beam touches the gangue and gradually compacts the gangue in the gob.

According to the support principle model, the function of anchor cables for the roof is to suspend the immediate roof rock tightly on the main roof rock so that the immediate roof will not break away from the main roof and sink alone. The 1510 track gateway was originally supported by anchor nets (cables), and the strength of the roof metal nets cannot meet the requirements of roof protection during the roadway forming period. Thus, to ensure the safety of the roof, it was necessary to install new nets (rhombic metal nets) and additional anchor cables during roadway forming. After comprehensive consideration, we decided to reinforce the roof by “one steel tape + three cables” alternately with “one beam + three cables”.

In order to track the support effect, it is also necessary to analyze and check the force of M-tapes. The bottom view of the M-tape arrangement is shown in Figure 10.

As M-tapes are generally exposed to breakage failure, the tensile strength needs to be checked. Assume the spacing of M-tapes is 1 m and all the weight of the rock underlying the anchor point is carried by the M-tape, then the pressure of the M-tape in the vertical direction [40] is:

$$\begin{array}{cc}P& =\frac{{\rho }_t \times H \times D}{S}\hfill \\ & =\frac{2.5\frac{\mathrm{t}}{{\mathrm{m}}^3} \times 8 \mathrm{m} \times 1 \mathrm{m}}{0.137 \mathrm{m}}\hfill \\ & =1.46 \mathrm{MPa}\hfill \end{array}$$

(3)

where: $P$—the pressure of the M-tape in the vertical direction, $\mathrm{Pa}$; ${\rho }_t$—test weight, taken as $2.5 \mathrm{t}/{\mathrm{m}}^3$; $H$—height of the immediate roof, $\mathrm{m}$; $D$—tape spacing, $\mathrm{m}$; $S$—tape width, $\mathrm{m}$.

Then, the tension force on the M-tape [40] is:

$$\begin{array}{ll}F& =P\times S\times L\hfill \\ & =1.46 \mathrm{MPa} \times 3.2 \mathrm{m} \times 0.137 \mathrm{m}\\ & =640 \mathrm{kN}\hfill \end{array}$$

(4)

where: $F$—tension force, $\mathrm{kN}$; $L$—tape length, $\mathrm{m}$.

The breaking force of M-tape is generally 350 kN. Obviously, the breaking force of one layer of M-tapes is less than the load of 640 kN. When two layers are used, it can withstand the breaking force of 640 kN. However, it is necessary to prevent the M-tape from being broken one by one during on-site implementation. Considering the lack of experience at the beginning, steel beams and double-layer M-tapes are used alternately in the first 50 m of the roadway. It has been verified that double-layer M-tapes can meet the support requirements.

The overall support effect of the 1510 roadway is shown in Figure 11.

4.2.2. Gangue Retaining and Anti-Air Leakage Scheme

In order to prevent the gangue in the old gob from escaping into the roadway, suspended rhombic metal nets are installed and connected to the newly-installed roof nets in the roadway. These nets are overlapped for 60 mm, tied in double-strand double-breasted diamond form with 16# galvanized iron wire, with interlocking spacing of 120 mm, and used in conjunction with roof-cutting anchor cables, roof-cutting beams and hydraulic props, with the bottom at least 0.5 m deep into the roof-cutting anchor cables. A combination of suspended metal nets and additional metal props is used to retain gangue, as shown in Figure 12. The metal nets are rhombic wire nets; the metal props are hydraulic single props, which can be replaced with I-beam props after the roadway is stabilized. In order to prevent air flow between the old gob and the roadway, unused wind tube cloth is hung at the roadhead and within 10 m of the gob-side roadway, as shown in Figure 13.

5. Monitoring and Analysis of the Roadway Forming Effect

5.1. Monitoring Scheme of Roadway Forming Effect

In order to further verify the feasibility and safety of gob-side roadway forming, it is necessary to monitor the displacement of the roadway surface and the pressure on anchor cables in the gob-side roadway forming section.

(1)

Roadway displacement monitoring

When observing the relative displacement and deformation across the roadway section, starting from the beginning of roadway forming, one measuring point is set for every 10 anchor cable tapes. The measuring points are arranged in a “cross”, as shown in Figure 14. The measurement method is manual measurement. A total of 30 points are set. Each measuring point records the displacement of the roof and floor and the two sides as well as the advancement rate and the relative position to the working face once a day until the working face has been fully advanced.

(2)

Pressure monitoring of anchor cables

For the pressure of the anchor cable, we used the YAD400 mine bolt cable dynamometer to measure the tension of the anchor cable, as shown in Figure 15. The dynamometer is shown in the figure below: Starting from the completion of the roadway forming, the first group of mine cable dynamometers is installed on the fifth anchor cable tape (beam). After that, one group is installed for every 40 cable tapes. A total of four groups of dynamometers are installed. Observation is made once a day for the first month, and once a week afterwards. Observation is made for five consecutive months.

The arrangement of monitoring points is shown in Figure 16.

5.2. Monitoring Results and Analysis of Road Forming Effect

5.2.1. Monitoring Results and Analysis of Roadway Deformation

(1)

Monitoring Results of Roadway Surface Displacement

As shown in Figure 17, at the first test point of the first cable tape, the measured convergence of each position and the advance distance of the working face are as follows:

(2)

Analysis of monitoring results

As shown in Figure 17, for the left side of the roadway (gob side): with the advancement of the working face, the amount of deformation increases sharply. When the working face has been advanced for about 15 m, the deformation slows down as the broken rock in the gob is restrained by the side props and stabilizes after two cycles of pressure.

For the right side of the roadway (solid coal side): With the advancement of the working face, 0–80 m from the working face, the deformation increases at a speed $v_d$ of 4–25 mm/d; beyond 80 m from the working face, the convergence of the right side stabilizes.

For the roadway floor: The heave of the roadway floor increases continuously with the advancement of the working face without any slowdown. As the floor is quite soft, the floor heave is obvious, exceeding 600 mm at last. Further floor treatment will be needed.

For the roadway roof: With the advancement of the working face, after 80 m, the roof subsidence gradually decreases and stabilizes; the subsidence remains within 200 mm. By then, the fallen gangue in the gob is in contact with the main roof and has filled up the gob, helping support the main roof. This suggests that the roadway forming effect is good.

5.2.2. Monitoring Results and Analysis of Anchor Cable Force

(1)

Monitoring results of pressure distribution on the anchor cable

As shown in Figure 18, at the first test point of the fifth cable tape, the value measured by the cable dynamometer as a function of the advance distance of the working face is as follows:

(2)

Analysis of monitoring results

As shown in Figure 18, the pressure on the right and middle anchor cables of the roadway increases slowly with the advancement of the working face; the pressure on the anchor cable on the left side of the roadway (gob side) increases sharply during the first cycle of pressure, and then decreases sharply, suggesting that the roof rock is segregated and broken. After the second cycle of pressure, the pressure on the anchor cables smooths away. The impact of mining on the roof is gradually reduced. The mining disturbance of the roof is gradually stabilized. The surrounding rock of the roof is well controlled.

5.3. Overall Roadway Forming Effect

The 1510 working face of Xinyi Coal Mine has good strata behavior on the GSCRF site. Anchor cables and M-tapes are used to control the roadway in advance. The control is provided before the influence of the bearing pressure of the working face, about 30 m ahead of the working face. Judging from the site situation, the immediate roof is relatively stable; the support method adopted can control the movement form of the immediate roof in advance. A real photo is shown in Figure 19. Because the immediate roof is weak and broken, there is no need to cut or cave it. Instead, it is let fall on the gob side to fill up the main roof and become a wall. Judging from the on-site situation, the immediate roof can fall upon excavation to fill up the entire gob. A real photo is shown in Figure 20. The overall roadway-forming effect is good with minimal strata behavior. After the roadway is repaired, it can allow for ventilation and pedestrians. The overall effect is shown in Figure 21.

5.4. Discussion

In pillarless mining by GSCRF, in view of the stress environment of GSCRF, it is necessary to effectively control the roadway surrounding rock, including immediate roof control, main roof control, gob side gangue control, and air leakage design. The surrounding rock control process follows the procedure below:

(1)

Immediate roof can be pulled up

Since the immediate roof is relatively broken and its strength is low, in order to effectively control the immediate roof during roadway forming and throughout the mining of the working face, it is necessary to give full play to the anchoring effect of the anchor cables so that the main roof moves together with the immediate roof as a whole and will not segregate from the main roof rock beam.

(2)

Broken immediate roof can be held

As the immediate roof is broken, it is necessary to install auxiliary support devices such as anchor nets and anchor cable beams in the roadway to completely hold the broken immediate roof and prevent it from caving in and affecting the safety of the roadway.

(3)

Gangue can fill up the gob

The immediate roof outside the roadway is not supported so that it will be successfully caved after the working face has been advanced; the gangue will form a wall and no overhanging wall will appear.

(4)

Gangue can be kept out of the roadway

In order to prevent the gangue from falling directly into the roadway, a steel wire net must be put up along the gob side, and secured by dense props with sufficient strength.

(5)

The main roof sinks without impact

The fallen immediate roof must be high enough for the main roof rock to touch the gangue slowly to prevent the main roof from sinking suddenly into the gob and producing impact.

Compared with other methods for forming the mining roadway, the technical characteristics of GSCRF and these methods are listed in

Table 4. Technical Features Comparison Table.

Method	Mining Roadway with Large Coal Pillar	Gob-Side Excavation of Small Coal Pillars	Roadway-Side-Filled Pillarless Mining	GSCRF
Roadway surrounding rock pressure	Large	Relatively small	Relatively small	Small
Pillar existence	Exist	Exist	Exist	None
Pillar deformation	Large	Large	Large	None
Impact of working face mining	Exist	Exist	None	None
Impact of working face mining	None	None	Exist	None

.

6. Conclusions

The GSCRF technique is implemented on the soft roof working face of a mine in eastern China. Taking the haulage roadway of the 1510 working face in Xinyi Coal Mine as an example, through theoretical analysis and industrial experiments, the feasibility of this method is discussed, and the surrounding rock support scheme is proposed. This project has made a new exploration of pillarless coal mining under soft rock roof conditions.

The main conclusions of this analysis are as follows:

(1)

A pillarless mining technique by GSCRF under soft rock roof conditions is proposed and practiced. The technical principle of this roadway-forming method is described. This technique can effectively control the immediate roof in the roadway. It lets the immediate roof fall to form a wall outside the roadway. Compared with the traditional pillarless mining techniques, it does not need pillars for roadway protection and does not disturb the mining of the working face.

(2)

A method for determining the feasibility of GSCRF is proposed. A roof “cavability”, “wall formability” and “impact free” formula is established. Based on this, the feasibility determination criteria for pillarless mining by GSCRF are created. The proposed support scheme of “cable + steel beams + yielding prop” has a good control effect on the overlying strata.

(3)

The GSCRF effect of the 1510 working face is good with minimal strata behavior and no obvious dynamic pressure impact. The tension on the anchor cables is relatively stable as a whole with no breakage failure. The support parameters are reasonable. The convergence of the roadway is within the controllable range. After the roadway is repaired, it can allow for ventilation and transportation.