J. Cent. South Univ. (2017) 24: 1121-1132

DOI: 10.1007/s11771-017-3515-8

Stability and control of room mining coal pillars—taking room mining coal pillars of solid backfill recovery as an example

ZHANG Ji-xiong(张吉雄)¹, HUANG Peng(黄鹏)², ZHANG Qiang(张强)¹, LI Meng(李猛)¹, CHEN Zhi-wei(陈志维)¹

1. School of Mines, China University of Mining & Technology, Xuzhou 221116, China;

2. The State Key Laboratory for Geo Mechanics and Deep Underground Engineering,

China University of Mining & Technology, Xuzhou 221116, China;

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The stability of room mining coal pillars during their secondary mining for recovering coal was analyzed. An analysis was performed for the damage and instability mechanism of coal pillars recovered by the caving mining method. During the damage progression of a single room coal pillar, the shape of the stress distribution in the pillar transformed from the initial stable saddle shape to the final arch-shaped distribution of critical instability. By combining the shapes of stress distribution in the coal pillars with the ultimate strength theory, the safe-stress value of coal pillar was obtained as 11.8 MPa. The mechanism of instability of coal pillar groups recovered by the caving mining method was explained by the domino effect. Since the room coal pillars mined and recovered by the traditional caving mining method were significantly influenced by the secondary mining during recovery, the coal pillars would go through a chain-type instability failure. Because of this limitation, the method of solid backfilling was proposed for mining and recovering room coal pillars, thus changing the transfer mechanism of stress caused by the secondary mining (recovery) of coal pillars. The mechanical model of the stope in the case of backfilling and recovering room coal pillars was built. The peak stress values inside coal pillars varied with the variance of backfilling ratio when the working face was advanced by 150 m. Furthermore, when the critical backfilling ratio was 80.6%, the instability failure of coal pillars would not occur during the solid backfill mining process. By taking Bandingliang Coal Mine as an example, the coal pillars’ stability of stope under this backfilling ratio was studied, and a project scheme was designed.

Key words: room mining; stability of coal pillars; coal mining of solid backfill; ultimate strength; instability failure

1 Introduction

During the early stage of solid coal mining process, room mining system is commonly adopted in Shendong and Yuheng mining areas in Western China mainly due to economic reasons. In this system, the overlying strata are supported by unexploited coal pillars. It results in a low mining rate of coal resources and a series of mine security problems, such as, spontaneous combustion of coal seams and mine earthquake [1, 2]. According to statistics [3-5], in Yulin area of northern Shaanxi province, China, 121 collapse earthquakes with magnitude greater than 2.0 occurred from 2004 to 2015. A total of 19 collapse earthquakes with magnitude greater than 2.0 occurred in 2012 alone. The instability of unexploited coal pillars in a large area can simultaneously cause problems such as environmental problems, damage to the existing topography, damage to the underground water resources, collapse of earth’s surface and deterioration of vegetation, as shown in Fig. 1. Considering that an improvement in coal mining technology can not only recover coal resources but also eliminate the potential safety hazards which may be caused by the coal pillars retained in the room mining goaf, this work proposed an improvement in the room mining technique of coal pillars through solid backfilling recovery. The coal mining technique of solid backfill [6-8] uses a mechanized way to directly backfill the goaf by a solid mass to act as a permanent support system bearing the overlying strata. It fundamentally changes the movement characteristics of overlying strata in the stope of working face and the mechanism of mine pressure, and replaces the coal resources which cannot otherwise be mined by the traditional mining methods. Meanwhile, the technique provides a good condition for recovering the room retained coal pillars.

At present, relevant theoretical studies on the

instability of coal pillars mainly focus on the cusp catastrophic theory [9] and the fold catastrophe model [10]. Similarly, the studies on the coal mining of solid backfill can be mainly summarized as follows. MIAO et al [8, 11, 12] proposed a coal mining system along with its principles, methods, and equipment for a comprehensive mechanized solid backfilling method. By systematically studying the correlations between the compaction of waste backfill and the time associated with it, the mechanical characteristics of the main roof’s movement were systematically analyzed, and an equivalent mining thickness model was proposed to analyze the strata-pressure relationship in the waste backfill mining [8, 13, 14]. ZHA et al [15-17] analyzed the relationships among the test reliability of waste rock compression, the compressive properties and the particle size distribution. The nonlinear deformation characteristics of waste rock were explained based on the science of mining subsidence. However, very few studies have focused on the recovery of coal pillars through solid backfilling.

Based on the theory of ultimate strength and the evolution of stress in a single coal pillar, this work concludes that a chain-type instability failure of multiple coal pillars can be caused by the caving mining method. This work proposes a method of mining and recovering room coal pillars through solid backfilling and develops a mechanical model of the backfill inside the room retained coal pillars. Meanwhile, a theoretical analysis is performed on the critical backfilling ratio under which instability of coal pillars would not occur in the room coal pillars having solid recovery. Aiming at Bandingliang Coal Mine, a specific project scheme was designed for the recovery of coal pillars through solid backfilling. The outcomes of this work are expected to have engineering significance for the room mining method of recovering left coal pillars, for the stability of left coal pillars and for the further improvement in the rate of mining coal.

2 Engineering background and safe stress of coal pillars

2.1 Engineering background

Bandingliang coal mine is located in the southwest of Haiwan mine field in Shenfu coal mine of Jurassic coalfield in northern Shaanxi Province, China. The area of mine field is 1.41 km², and the recoverable coal reserve is 4.92 million tons. Because room mining has been practiced for a long time, the mining rate of coal resources in the mine is only 50.2%, and the retained idle coal pillars amount to 2.544 million tons. In order to further prolong the service life of the mine and improve the mining rate of coal resources, the recovery of coal from the room mining coal pillars is needed.

The thickness of coal seam in the mining area is 4.00-6.63 m (in average 5.87 m). The size of a prearranged coal pillar is 7-12 m, and the size of a coal room is 6-10 m. The designed height of mining is 4.5 m, and a coal layer of 1.3 m thickness has been reserved as a roof coal layer. The structure of coal seam is simple with one layer of dirt band. The thickness of dirt band is

0.55-0.70 m. Meanwhile, the burial depth of this coal seam is 83.21-134.48 m (in average 120 m). It is a relatively stable coal seam.

Fig. 1 Environmental destructions caused by room mining:

According to the engineering geological conditions of Bangdinglian Coal Mine and the basic mechanical test of coal and rock mass, the specific physical and mechanical parameters of each rock stratum are obtained, as shown in Table 1.

2.2 Failure modes of coal pillars

The stability of room mining coal pillars is the key to ensure that the violent movement and subsidence of overlying strata will not occur, and that the mine security problem and environmental destruction will not be caused. Site surveys indicate that the possible failure modes of coal pillars in Bandingliang Coal Mine include six modes, such as lateral spalling and compression- shear failure [18], as shown in Fig. 2.

A closer examination at the site reveals that the lateral exploited failure mode of coal pillars was mainly manifested as denudation, necking and external surface spalling of coal pillars after room mining. Site surveys show that this is the most common mode of failure. Photographs of the lateral exploited instability anddamage of coal pillars in Bandingliang Coal Mine are shown in Fig. 3. Hence, this work focuses on this mode of failure of the coal pillars.

2.3 Safe stress calculation of coal pillars

Ultimate strength for the coal pillars was performed according to the formulas of the four strength theories of coal pillars that exist in Refs. [19-21], i.e.Holland- Gaddy, Bieniawaki, Obert-Duvall and Salamaon-Munro. The above-mentioned engineering geological parameters of Dingbanliang Coal Mine were used in the calculation, which provided the ultimate strengths of coal pillars, as shown in Table 2.

According to the ultimate strength theory [22], if the applied load reaches the ultimate strength of coal pillars, the carrying capacity of coal pillars will be reduced to zero, and coal pillars will be damaged. Thus, the failure criterion of coal pillar is given as

                                   (1)

where σ is the stress acting on the coal pillar; F is the safety coefficient, normally taken as 2-3; and σ_p is the ultimate strength of coal pillars.

The safety coefficient was taken as 2.2 in this work,

and the ultimate strength of coal pillar in Bandingliang Coal Mine was substituted into Eq. (1) to determine the safe stress. The results show that when the safety coefficient of coal pillars is 11.79 MPa, instability of coal pillars will not occur during the process of recovering room coal pillars through solid backfilling. Thus, it is viable to safely recover the coal pillars.

Table 1 Physical and mechanical parameters of each rock stratum

Fig. 2 Main failure modes of coal pillars:

Fig. 3 Site photographs of lateral exploited failure mode in coal pillar:

Table 2 Ultimate strengths of coal pillars by four formulas [19-21]

3 Analysis of mechanism of instability of mining room coal pillars by using caving mining method

As discussed above, the predominant mode of damage and instability of coal pillars in the site was the lateral exploited mode. By analyzing the stress evolution in a single coal pillar under this mode of failure, the instability and damage mechanism of coal pillar groups during the mining and recovering process were analyzed by using the caving mining method and by taking the values of safe stresses as a reference.

3.1 Evolution of shape of stress distribution during instability process of single coal pillars

Flac^3D numerical simulation software was used to investigate the loads applied on the top of a single coal pillar. The software provides a theoretical basis for the caving mining method and the recovery of room coal pillars through solid backfilling by predicting the evolution of distribution shape of coal pillars’ inner stress and the instability and damage mechanisms. For convenience, a numerical model was built based on the size of normal coal pillars in a room and a pillar stope. The dimensions of coal pillars in the model were 10 m× 10 m×4 m and the roof and floor were both 20 m×20 m× 1 m, as shown in Fig.4. The compensated uniform stress at the top of the model’s roof was 2.5 MPa. Mohr- Coulomb model was adopted as a failure criterion. The model consisted of 147200 elements and 156729 nodes. Table 1 shows the physical and mechanical parameters used in the model.

By writing the language code of a fish embedded in the FLAC^3D software, the damage degree of coal pillar was obtained for each calculated time step, as shown in Table 3. Figure 4 shows the damage degree of coal pillars and the evolution of inner stress distribution for the coal pillars under different calculated time steps.

Figure 4 and Table 3 indicate that when loads are applied on the top of a single coal pillar, the shape of the stress distribution is transformed from the stable saddle shape in the beginning (the overall damage of coal pillars is 5.9%) into a platform shape (the overall damage of coal pillars is 62.2%), with the transfer of stress. By this stage, the fractures of coal pillars have fully developed, and a large number of coal pillars have peeled off laterally. In the end, the shape of stress distribution of coal pillars evolves into an arch shape of critical

instability (the overall damage of coal pillars is 80.1%).

Table 3 Pillar damage in each time step

Fig. 4 Coal pillar damage in different time steps:

By combining the evolution of the shape of coal pillars’ stress distribution with the ultimate strength theory during the process of coal pillars’ instability, the peak value of safe-stress for the coal pillars is obtained as 11.79 MPa. This value can be taken as the criterion to determine the stability of coal pillars.

3.2 Domino effect during instability process of coal pillar groups

3.2.1 Numerical model and solution

The numerical simulation software, FLAC^3D, was used to study the stability of coal pillar groups during the recovering process of room retained coal pillars by using the caving mining method as well. A typical numerical model was built as shown in Fig. 5. The horizontal displacement was restrained on both sides of the model, while the perpendicular displacement was restrained at the bottom of model. The Mohr-Coulomb model was adopted as the failure criterion. A uniform load of 1.1 MPa was applied on the top of the model. The model size was 256 m×239.5 m×51.5 m (length×width×height). There were nine columns of coal pillars in the model, and there were eight coal pillars in each column. The coal pillar size was 9 m×9 m×4.5 m, and the width of coal room was 7.5 m. Boundary protective coal pillars of 50 m were reserved. Comprehensively considering the calculation accuracy and computation time, mesh refinement was carried out for the strata around coal seams. The model was meshed into a total of 327888 units and 347738 nodes.

In order to study the degree of damage of the coal pillars by the mining and recovering of room coal pillars using the caving mining method, six groups of modeling schemes are set as: the lengths of working face are separated from row 1 (24 m) to row 6 of coal pillars (106.5 m), and the advanced distances of working face are nine columns (156 m) of coal pillars, while one column of coal pillars (24 m) is recovered for every time till all the nine columns of coal pillars were recovered.

3.2.2 Stability analysis of coal pillars influenced by mining

Figure 6 presents the curves showing the variation in stress peak during the process of recovering coal pillars using the caving mining method. The figure also shows the 3D stress distribution diagram of stope corresponding to the peak values.

Figure 6 shows that for the three cases (when advancing nine columns of coal pillars by three-line mining or advancing seven columns of coal pillars by four-line mining, or advancing six columns of coal pillars by five and six-line mining), the stress peak of

coal pillars around the stope exceeds the safe stress peak of coal pillars and the stress distribution shows an arch shape. It can be known from the stability criterion of coal pillars that the coal pillar groups would go through instability failure.

Figure 7 presents the plastic zone of coal pillars around the stope of room mining and recovering retained coal pillars by using the caving mining method. As shown in Fig. 7, when advancing nine columns of coal pillars by three-line mining, 12 plastic zones of coal pillars have fully developed close to both sides of stope. Similarly, the plastic zones in the second row of coal pillars have developed for a distance of approximately 4.2 m on both sides of the stope. The coal pillars are damaged seriously. When advancing seven columns of coal pillars by four-line mining, or advancing five columns of coal pillars by five-line mining, four or five basic plastic zones of coal pillars in the first column of working face have well developed, while the plastic zones of the second column of coal pillars close to one side of stope have developed for a distance of approximately 2.5 m and 2.7 m. Meanwhile, twelve and eight plastic zones of coal pillars close to both sides of stope have fully developed, and the coal pillars are damaged seriously. The specific degrees of damage are shown in Table 4.

Fig. 5 Numerical model of room mining coal pillars (Unit: m)

Fig. 6 Stress peak variation curves and 3D stress distribution of stope for mined coal pillar groups using caving mining method

Fig. 7 Propagation of coal pillars’ plastic zone under influence of mining:

Table 4 Degree of damage of coal pillar groups under influence of mining

The following understanding can be made according to the distributions of stress and the plastic zones from the above mentioned caving mining method in recovering room coal pillars in a stope. When using the caving mining method to recover coal pillars, the outside of coal pillars is gradually exploited and the central elastic area is gradually reduced. Consequently, the lateral bearing capacity is lowered down, which is further accompanied by the transfer of stress to the inside of coal pillars. When the superimposed stress produced by the mining exceeds the safe stress peak of coal pillars, the overall instability damage of coal pillars will occur in the adjacent stope. Once there is any instability of one coal pillar of a stope, the stress of the stope will be immediately transferred to the adjacent coal pillars. When the stress of the adjacent coal pillars exceeds the safe stress, instability damage will, as well, occur in the pillars. This failure mode of coal pillar is just like Domino effect. Once there is any instability of one coal pillar, it will cause a chain reaction and result in the instability of other coal pillars. Owing to this limitation of the caving mining method, the method of mining and recovering coal pillars through solid backfilling is proposed. By supporting the overlying strata with the backfill body of goaf, some stress of a stope applied on the coal pillars will be shared, so as to reduce the coal pillars’ stress and to prevent instability damage to coal pillars.

4 Critical backfilling ratio of recovering and mining room coal pillars through solid backfilling method

During the process of recovering room coal pillars through solid backfilling, the stability of coal pillars mainly depends on the backfilling ratio of the backfill body in the goaf [23, 24]. Only when the backfilling ratio has reached a certain value, the roof can be effectively supported and some overlying loads on the coal pillars can be effectively shared to ensure the stability of coal pillars in the stope. The stability of coal pillars will also ensure that no large-scale safety accident will occur in the whole stope.

4.1 Mechanical model of recovering retained room coal pillars through solid backfilling

In the stope of solid backfilling method of recovering room coal pillars, the roof is taken as the main object of study. The original point is set up at the junction between the region of room coal pillars and the area of backfilling body in the goaf. The top of roof is subjected to a load q of overlying strata, while the floor of roof is subjected to the doubly-clamped beam supported together by the coal pillar with a length of l_p and an equivalent elastic foundation coefficient of k_p and the backfill body with a length of l_b and an equivalent elastic foundation coefficient of k_b. Meanwhile, the displacement function of ω(x) is taken as an unknown quantity in the coordinate system as shown in Fig. 8.

Fig. 8 A simplified model of backfilling material, pillars and roof:

4.2 Solution for bending deformation of roof

According to the Winkler assumption [6, 25], the deflection equations of roof over the coal pillars and the backfill body in the stope of recovering room coal pillars are, respectively:

      (2)

where E_I is the bending stiffness of beam cross-section, k_p is the foundation coefficient of coal pillars, while k_b is the foundation coefficient of backfill body.

The characteristic coefficients are taken as and Solving Eq. (2), the general solution will be:

      (3)

The relationship between the foundation coefficient of backfill body and the backfilling ratio is

                                (4)

where σ₀ is the stress of primary rock, φ is the backfilling ratio, and h is the height of mining.

The boundary conditions of the fixed end are

                               (5)

The continuity condition including the deflections, bending moments, rotation angles and shear forces are equal at the original point.

The parameters d₁, d₂, d₃, … and d₈ can be solved when the boundary and continuity conditions are substituted in Eq. (2).

4.3 Load calculation of coal pillars

Since the region of coal pillars is regarded as continuously distributed Winkler elastic foundation, the stress at any point in the area of coal pillars influenced by the mining follows the following relationship:

                                (6)

Substituting Eq. (3) into Eq. (6), we can obtain the stress of coal pillars in the regional domain as

            (7)

Assuming that the coordinate range in the load bearing area of a single coal pillar is [x₁, x₂], consisting of the length of coal pillar and the width of coal room, the stress in the single coal pillar will be the integration for the stress in this area range as

                                 (8)

By integrating Eq. (8), the stress in the single coal pillar is obtained as

                       (9)

4.4 Design of critical backfilling ratio of recovering room coal pillars through solid backfilling

Let us substitute the specific engineering parameters of Table 1 into Eq. (9). Considering the mining influence of stope, the advanced distance is taken as 150 m, and the stress values of the coal pillars under different backfilling ratios are calculated, as shown in Fig. 9.

Fig. 9 Curve-fitting between stress of coal pillars and backfilling ratio

The linear regression is carried out for the stress peak values of coal pillars under different backfilling ratios, and the obtained best-fit equation is provided below:

          (11)

where σ is the stress in the coal pillars, MPa; φ is the backfilling ratio; R² is the coefficient of determination.

By substituting the safe stress value of coal pillars 11.79 MPa into the best-fit equation of the stress (Eq. (11)), it can be obtained that, when the backfilling ratio is 80.6%, the safe recovery of coal pillars can be ensured during the process of recovering room coal pillars by solid backfilling method.

5 Engineering design of recovering room mining coal pillars through solid backfilling

5.1 Numerical analysis of stability on recovering room coal pillars through solid backfilling

By combining the backfilling technology with the conditions of coal pillars on the site of Bandingliang Coal Mine, two lines of coal pillars (39.5 m) were simultaneously backfilled and mined. The specific numerical simulation model and parameters are provided in Fig. 4 and Table 2. Combined with the analysis in Section 3, when the backfilling ratio is 80.6% (the basic parameters of backfill body can be determined according to the stress-strain curve in the compaction test of backfill material) and for the case of nine columns of coal pillars (156 m) being advanced, the 3D stress field of a stope and the plastic zone distribution of coal pillars are provided in Fig. 10.

Figure 10 indicates that for the backfilling ratio of 80.6%, the stress distribution of lateral coal pillars in the stope basically presents a saddle shape. The plastic zone of coal pillars has developed to a distance of approximately 0.6 m on both sides of the stope. Meanwhile, the lateral coal pillars of the stope are less influenced by the mining, and the stability of coal pillars is improved. Consequently, there will not be any large-scale instability damage.

5.2 Project design of recovering room coal pillars through solid backfilling

A mechanized system was designed for recovering room coal pillars by the solid backfilling method. In the working face, the height of mining was set as 4.5 m, and a roof coal of 1.3 m was reserved to prevent any accidental roof collapse. The first mined working face was designed to recover two rows of coal pillars, while the length of an open-off cut was 30 m and the advanced length of a strike was 111 m. The blasting room coal drop method was adopted for the working face. The caved coal was carried to the trackless tyred vehicles by the mining wheel type of anti-explosion loaders, and then transported to the ground by the trackless tyred vehicles. The backfill material was transported via a belt conveyor, and then, backfilled to the goaf through a high speed power material thrower. In the end, one bulldozer was used to compact it, so that the goal of supporting the roof and sharing of the stress of coal pillars could be realized. The specific backfilling process is described below:

1) After the blast holes of the working face were exploded, the fallen coal was loaded onto trackless tyred vehicles via the loader and then transported to the ground. The backfilling process began after the floating coal of working face was cleaned up;

2) First, belt conveyors started for the material delivery on the ground and the working face. A high- speed power material thrower started to transport the backfilling material. The backfilling material transported to the working face was thrown to the left hand side (Fig. 11) of goaf in the working face by the high speed power material thrower. Meanwhile, it was ensured that the backfill material was fully in contact with the roof.

3) After the left hand side was backfilled, the high speed power material thrower was rotated to throw the material into the right hand side of goaf. The middle area of goaf was backfilled in the end.

5.3 Control measures for backfill ratio

On the backfilled working face, attention can be paid to the following points to improve the backfilling

ratio and thus to ensure the stability of coal pillars:

Fig. 10 3D stress field of a stope and plastic zone distribution of coal pillars:

Fig. 11 Backfilling technology designed for recovering room coal pillars through solid backfilling

1) The driving support force is provided as far as possible by the backfill hydraulic support, thus to limit the early subsidence of the roof;

2) According to the compaction test of backfill material, above 70% of strain occurs within 2 MPa. So, it is ensured that the maximum compacting force of compacting mechanism can reach 2 MPa every time.

3) Enhance the backfilling process management, and strictly uphold the principles of one mining one backfill, simultaneous backfilling and mining, determination of mining by backfill and compaction for multiple turns.

6 Conclusions

1) During the process of instability of room coal pillars during recovering by the caving mining method, the shape of stress distribution of single coal pillars is divided into three phases. The original form is a stress distribution of stable saddle shape. It is then changed to a platform shape distribution due to the stress transfer during recovery. The shape was ultimately transformed to an arch-shaped distribution of critical instability. The three stress distribution shapes of coal pillars can be taken as one of the criteria to distinguish the stability of room mining coal pillars.

2) Based on the strength theory, the safe stress peak of coal pillars was obtained as 11.79 MPa and the shapes of coal pillars’ stress distribution are obtained. When using the caving mining method to mine coal pillar groups, they were subjected to a violent secondary mining influence. Once there is instability in any coal pillar, a chain-reaction would be caused. Meanwhile, instability damage would be caused to other coal pillars around the stope, and the Domino effect type of damage would be caused to coal pillar groups.

3) By building the stope mechanical model of recovering room retained coal pillars through solid backfilling, the stress peak values of coal pillars under different backfilling ratios are solved when the advanced distance of working face is 150 m. Furthermore, based on the safe stress value of coal pillars, when the critical backfilling ratio is 80.6%, damages to room retained coal pillars recovered through solid backfilling would be prevented.

4) Based on the geological conditions of Bandingliang Coal Mine, this work investigates the stability of coal pillars when the backfilling ratio of recovering room mining coal pillars through solid backfilling is 80.6%. A project scheme is designed in which two rows of coal pillars are mined at one time, and the blast holes are drilled to explode and fall the coal while the backfill body is thrown by a material thrower. Furthermore, this work proposes the measures of controlling backfilling ratio. This design of an engineering scheme can provide a theoretical basis and a technical reference for recovering room mining coal pillars through solid backfilling.

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(Edited by FANG Jing-hua)

Cite this article as: ZHANG Ji-xiong, HUANG Peng, ZHANG Qiang, LI Meng, CHEN Zhi-wei. Stability and control of room mining coal pillars—taking room mining coal pillars of solid backfill recovery as an example [J]. Journal of Central South University, 2017, 24(5): 1121-1132. DOI: 10.1007/s11771-017-3515-8.

Foundation item: Project(2014ZDPY02) supported by the Fundamental Research Funds for the Central Universities; Project supported by Qing Lan project, China

Received date: 2015-09-27; Accepted date: 2016-03-22

Corresponding author: HUANG Peng, PhD Candidate; Tel/Fax: +86-18796280203; E-mail:cumt_hp@126.com