J. Cent. South Univ. (2018) 25: 448-460

DOI: https://doi.org/10.1007/s11771-018-3749-0

Prediction of upper limit position of bedding separation overlying a coal roadway within an extra-thick coal seam

YAN Hong(严红)¹, ZHANG Ji-xiong(张吉雄)¹, LI Lin-yue(李林玥)²,FENG Rui-min(冯锐敏)³, LI Tian-tong(李天彤)¹

1. State Key Laboratory of Coal Resource and Safe Mining, Key Laboratory of Deep Coal Resource Mining of Ministry of Education of China (China University of Mining & Technology), Xuzhou 221116, China;

2. Guizhou Xinlian Blasting Engineering Group Co., Ltd, Guiyang 550002, China;

3. Department of Chemical and Petroleum Engineering, University of Calgary, Calgary T2N 1N4, Canada

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Failure of the surrounding rock around a roadway induced by roof separation is one major type of underground roof-fall accidents. This failure can especially be commonly-seen in a bottom-driven roadway within an extra-thick coal seam (“bottom-driven roadway” is used throughout for ease of reference), containing weak partings in their roof coal seams. To determine the upper limit position of the roof interlayer separation is the primary premise for roof control. In this study, a mechanical model for predicting the interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam was established and used to deduce the vertical stress, and length, of the elastic, and plastic zones in the rock strata above the wall of the roadway as well as the formulae for calculating the deflection in different regions of rock strata under bearing stress. Also, an approach was proposed, calculating the stratum load, deflection, and limiting span of the upper limit position of the interlayer separation in a thick coal seam. Based on the key strata control theory and its influence of bedding separation, a set of methods judging the upper limit position of the roof interlayer separation were constructed. In addition, the theoretical prediction and field monitoring for the upper limit position of interlayer separation were conducted in a typical roadway. The results obtained by these two methods are consistent, indicating that the methods proposed are conducive to improving roof control in a thick coal seam.

Key words: extra-thick coal seam; bedding separation; coal roadway; roof fall; mechanical model

Cite this article as: YAN Hong, ZHANG Ji-xiong, LI Lin-yue, FENG Rui-min, LI Tian-tong. Prediction of upper limit position of bedding separation overlying a coal roadway within an extra-thick coal seam [J]. Journal of Central South University, 2018, 25(2): 448–460. DOI: https://doi.org/10.1007/s11771-018-3749-0.

1 Introduction

In underground coal mines, it is necessary to excavate a mining roadway for access, ventilation, coal conveying, etc. before the formation of a working face. The stability of a mining roadway, including its roof, two ribs, and floor, directly influences mine production and, more importantly, affects the safety of underground operators. For example, the frequent sudden roof collapse is one kind of disaster caused by unstable surrounding rocks [1–4]. By virtue of the accident inquiry system of the Chinese State Administration of Work Safety, it is statistically found that roof accidents are respectively 8 and 49 times, which are more likely than water in-rush, and fire accidents in Chinese coal mines from 2001 to 2010. In addition, as reported by the Mine Safety and Health Administration (MSHA), in the USA, the death toll from roof accidents accounts for approximately 70% of all underground deaths in the USA [5]. Similarly, in India, about 61.1% of accidents in coal mines are due to roof caving: roof accidents and casualties respectively accounted for 43.6% and 41.9% of the totals from 1995 to 2000 [6, 7].

In China, the reserve of thick/ultra-thick coal seams accounts for 45% of total recoverable amount of coal. These coal seams are mainly distributed in Shanxi, Shaanxi, Mongolia, Ningxia, Gansu, Xinjiang etc [8, 9]. Among the roof accidents induced by mining roadways with complex characteristics, such as compound roof, soft coal seam, and high stresses, the accidents resulting from roof separation-instability due to driving a bottom coal roadway in extra-thick coal seams have been one of the most frequently occurred underground disasters. These accidents have shown the worst casualty rates due to their complexity. For instance, the excavation section of the roadway is large; the roof is made up of a thick coal seam and thin, soft, weak partings; there are widely-developed joint fissures in the coal seam [10–12], as shown in Figure 1. Thereby, effective control of the roof separation of the roadway in extra-thick coal seams can prevent the development of roof failures on the premise that the upper limiting position of interlayer separation can be reasonably predicted in advance.

Figure 1 Layout of coal roadway within an extra-thick coal seam

Roof separation is a kind of separation phenomenon appearing in roof strata or adjacent rock strata during roadway excavation, as shown in Figure 2. Except for the physico-mechanical properties of roof coal-rock strata, the separation is also greatly affected by other factors, such as roadway section, burial depth, and geological conditions [13–15]. Many researchers have investigated the distribution of roof separation from different perspectives, including mining science, coal field geology, engineering mechanics, etc [16–18]. Some scholars proposed that roof separation arises when the lower stratum is separated from the upper strata, so that the strength thereof is gradually reduced from bottom to top until the strata are bent. This is owing to the effect of gravity stress, the normal tensile stress on the coal seam, and that the weak parting exceeds the normal tensile strength of the bedding surface of the strata [19, 20]. With the development of research into roadway stability, the concepts of temporary roof separation [21] and periodic separation have been introduced [22]. In recent years, monitoring devices related to roof separation have been developed, such as the mechanical roof separation indicator, on-line monitoring systems for separation, stratum probing radar, and acoustic emission and reception device technologies [23, 24].

Figure 2 Schematic diagram of roof separation in coal roadways with extra thick coal seam

According to the non-linear contact theory, ZHANG et al [25] simulated the characteristics of roof separation in layered roadways by ANSYS software. QIAN et al [26] established a mechanical model for the stability of roof strata to derive the instability criterion for roof separation by combining macroscopic damage mechanical methods. WU et al [27] studied the stability and separation of a complex roof in a deep roadway and considered that, initially, the plastic separation plays the main role, while the later stage is mainly based on interlayer separation. KWON et al [28] proved that the bedding separation exerts an important influence on roof stability of underground roadways. HEBBLEWHITE et al [13] undertook detailed field observations and monitoring of roof separation values during different mining activities on a typical roadway with laminated and weak roof strata. The aforementioned studies mainly focused on the following aspects: the characteristics of roof separation in compound roadways, separation monitoring devices, stress distribution, etc. However, bottom-driven roadways within an extra- thick coal seam have rarely been investigated. The authors determined the upper limit position of the interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam by establishing a mechanical model for the roof interlayer separation therein to provide guarantees of reasonable control technologies for roadways in thick coal seams.

2 Mechanical model for predicting bedding separation of a roadway roof in an extra-thick coal seam

The failure region of the coal bottom-driven roadway roof within an extra-thick coal seam mainly consists of the coal seam and soft and weak partings. After excavating the roadway, part of the interfaces between the coal and rock in the roof is separated owing to the differences in the physical and mechanical properties of the roof strata and coal seams. When the measured data used for predicting separations in some regions are greater than certain limit values, roof strata gradually become unstable, and even fall down. To determine the upper limit position of roof interlayer separation, a mechanical model representing the interlayer separation of overlying a bottom-driven roadway within an extra-thick coal seam was constructed as shown in Figure 3.

It is assumed that there are n layers above the roof, namely, S₁, S₂, S₃, …, S_i, …, S_n in sequence, and the uniform loading on the overlying strata is q.

Combined with key strata theory [29], the strata below the above the roof are included and then the load borne by the stratum is:

            (1)

Considering the stratum, the load carried by the stratum is:

Figure 3 Mechanical model for interlayer separation of a roadway roof in an extra-thick coal seam

          (2)

The shear stress τ_xy is produced by roadway excavation, and the surrounding rock stress is redistributed. The vertical stress generated by the strata above the coal rib increases to form abutment pressure areas. Figure 4 shows the stress distribution on the stratum, where k_m is a stress concentration coefficient. It can be seen that the greater the distance between the rock stratum and the surface of the roadway roof, the smaller the value of k_m, where k_m≥1.

Figure 4 Stress distribution on stratum above wall of roadway

To calculate the vertical stresses in the plastic, and elastic zones, namely, σ_y(m₁) and σ_y(m₂), and their length x_m1 and x_m2, the following fundamental assumptions are given:

1) The lateral stresses in plastic, and elastic zones σ_x(m₁) and σ_x(m₂) are uniformly distributed, and they are independent of the thickness of the stratum.

2) The relationship between the vertical stresses, σ_y(m₁) and σ_y(m₂) and the corresponding lateral stresses σ_x(m₁) and σ_x(m₂) in the plastic, and elastic zones are as follows:

a) In the plastic zone:

                        (3)

b) In the elastic zone: when, and only when,

x=x_m1 or x=x_m1+x_m2, σ_x(m₂)=λ_mσ_y(m₂)          (4)

3) The vertical stresses in both plastic, and elastic zones, namely σ_y(m₁) and σ_y(m₂) are the correlation function of x. Moreover, the relationship with the shear stress τ_xy(m₁) and τ_xy(m₂) on the interface meets the following conditions:

              (5)

4) Owing to the vertical stress in the elastic zone σ_y(m₂) reducing successively with a negative exponential relationship, its function can be approximated as a quadratic:

                (6)

where is the minimum load distributed vertically in the plastic zone; h_m is the thickness of the stratum; λ_m is the lateral stress coefficient of the stratum; and are the internal friction angle and cohesion of the interface between the upper adjacent seams of the stratum respectively; H_m is the vertical distance between the ground surface and thestratum; and are coefficients.

To calculate the vertical stresses in the plastic zone σ_y(m₁), as shown in Figure 5, a finite element is selected as follows:

   (7)

Substituting Eqs. (3) and (4) into Eq. (7), it can be found that:

      (8)

Then it can be solved by using Eq. (8) to give:

               (9)

Figure 5 Stresses on a finite element in stratum

At the boundary of the roadway, namely x=0, σ_y(m₁)–q′_m, we have:

                     (10)

When x=x_m1, σ_y(m₁)=k_mγH_m, it can be seen, through Eqs. (9) and (10), that:

     (11)

To calculate the vertical stresses in the elastic zone σ_y(m₂), the shear stress τ_xy(m₂) and the length of the elastic zone x_m2, it can be seen from elastic mechanics that the Airy stress function f should meet the general solution and compatibility equation of the homogeneous differential equations, namely:

                  (12)

(a) When x=x_m1, σ_y(m₂)=k_mγH_m;

(b) When x=x_m1+x_m2, σ_y(m₂)=γH_m;

(c).

Combining conditions (a), (b), (c) and Eq. (6), gives:

(13)

Substituting Eq. (13) into Eq. (6) gives:

(14)

Substituting Eq. (12) into Eq. (14) gives:

             (15)

Fundamental Assumption 1 indicates that σ_x(m₂) is uniformly distributed, which is independent of the thickness of the stratum, so functions f₁(x) and f₂(x) can be expressed as follows:

                  (16)

Substituting Eq. (16) into Eq. (15) gives:

               (17)

The shear stress in the elastic zone on the interface of the upper surface of the stratum and the adjacent strata is:

(18)

At the junction of the limit equilibrium, and elastic zones, namely x=x_m1, we have τ_xy(m₁)–τ_xy(m₂). Thereby, the length of the elastic zone can be obtained by substituting τ_xy(m₁) in Eq. (11) into Eq. (18), namely:

           (19)

The stratum was selected for stress analysis. The lower stratum in the elastic zone acted as a ground model, where is the foundation coefficient; l_m is the length of the stratum under the effect of uniform load.

Owing to the fact that the load on the rock beam of the stratum is bilaterally symmetrical, the authors only need to analyze half of the model (see Figure 6).

Figure 6 Stress on one side of rock beam in stratum

The differential equation for the deflection in different areas of thestratum is:

  (20)

After calculation, the deflections in different areas are as follows:

(21)

Based on the boundary conditions at and the continuity conditions between different areas, it can be found that:

                      (22)

The specific values of the deflection parameters in each area of the stratum, including A_m1, B_m1, C_m1, D_m1, A_m2, B_m2, C_m2, D_m2, C_m3 and D_m3 can be solved by using Eq. (22), and then these values are substituted into Eq. (21) to obtain the concrete form of the deflection function in all areas of the stratum.

The equation for σ_y(m₁), σ_y(m₂), x_m1, x_m2, y₁(x), y₂(x), and y₃(x) can be obtained according to the aforementioned results, which is conducive to determining the rock strata deformation and separation values of the mining roadway under a thick coal roof. However, due to the fact that the parameters are influenced by many complex factors, there are some limitations of determining the upper limit position of roof interlayer separation. In accordance with a large number of field monitoring results, after excavation of the roadway, the lateral stress on the roof strata is much lower than the corresponding value on the adjacent working face during mining. As a result, the stress has a limited effect on roof interlayer separation. On this basis, the stratum can be further simplified as a simply-supported beam with unit width (see Figure 7).

During analysis of the force on the stratum, the forces on the stratum and the stratum are calculated. The loading condition for generating separation in roof strata is the corresponding load borne by the stratum and (q_n)_m>(q_n+1)_m.

The differential equation for the deflection in the stratum can be further obtained as follows:

Figure 7 Model for stratum within roof above a coal roadway

             (23)

Owing to the maximum deflection of simply supported beam occurring at its midpoint, the maximum value is:

              (24)

Then the second condition for roof separation is: as for the adjacent strata, the maximum deflection of the lower rock stratum is greater than that of the stratum. Taking the stratum as an example,

Evidently, the normal stress of an arbitrary point in the stratum σ is:

                  (25)

where y is the distance between the point and the neutral axis; I_m is the moment of inertia about the symmetric neutral axis; h_m is the thickness of the stratum; and M is the bending moment at the section in which the point is located.

The maximum bending moment of simply supported beam is at the midpoint of the beam, namely and then the maximum tension stress at this point σ_max is:

       (26)

The maximum tensile stress is regarded as the basis for strata fracture. Then when σ_max reaches the ultimate tensile strength [σ_ms], the beam fractures at its mid-point, and the limiting span is:

                       (27)

Comparing the limiting span of a rock beam and the width of the roadway, the third condition for separation in roof coal and rock seams is that the limiting span of the rock stratum containing the separation is greater than the width of the roadway, i.e., l_mT>L.

In addition, based on the field monitoring results, the main zone influenced by the roof interlayer separation during mining on a bottom- driven roadway within an extra-thick coal seam was up to 6 m away, and the maximum influence distance was h₀. Therefore, when the key stratum is beyond the scope influenced by the separation, namely h_s>h₀, the interlayer separation produced between the key stratum and its adjacent strata can be ignored in theoretical calculations. On the contrary, if the key stratum is within the scope influenced by the separation, i.e., h_s>h₀, it is necessary to consider the interlayer separation generated between the key stratum and its adjacent strata.

In the light of this analysis, it can be concluded that the method of judging the upper limit of the interlayer separation overlying the bottom-driven roadway within an extra-thick coal seam (Figure 8) is applicable.

3 Case study

3.1 Geological and production conditions

The analysis was conducted on the 29211 working face in the No. 9 coal seam of a mine in Shanxi, China. The thickness of the coal seam is 9.82 to 14.29 m, with a mean thickness of 12.39 m, as shown in Figure 9. In the working face, the fully mechanised top-coal caving mining technology was adopted and an all-collapse method was applied to roof management. Besides, the return airway of the working face is tunnelled along the floor of this coal seam, which is rectangular cross-section with dimensions: 5 m in width and 3.5 m in height, and the average mining depth of the roadway is 220 m. The coal seam including multilayer weak partings (see Figure 10), and the physico-mechanical parameters of the core samples collected by drilling from the surface of the roof to the rock stratum of the roadway are obtained by experimental tests, as shown in Table 1.

Figure 8 Method of determining upper limit of interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam

Figure 9 Layout of return airway No. 29211

Figure 10 Coal seam and rock strata columnar section

Table 1 Physico-mechanical parameters

3.2 Theoretical determination of upper limiting position of roof interlayer separation

Using the method presented in Figure 7, namely the method for determining the upper limit of interlayer separation overlying the bottom-driven roadway within an extra-thick coal seam, the upper limit is determined in the following steps:

Step 1: Determining load

The load on the stratum q₁ is:

Taking the stratum into consideration, the load borne by the stratum is:

Taking the andstrata into consideration, as shown in Figure 11, the strata S₁, S₂, S₃ and S₄ affect the load carried by the stratum, while the load on the stratum is unaffected by the stratum. Thus, the load borne by the stratum is (q₄)₁, and (q₅)₁<(q₄)₁, which meets the loading condition required by the separation between the stratum and the stratum of the roof.

Figure 11 Calculated bearing load from S₁ to S₈

In accordance with the same method, the load on the rock strata above the stratum is calculated. It can be seen that the and strata affect the load carried by the stratum, while there is no influence of the stratum on the load borne by the stratum. Hence, the load borne by the coal seam is (q₇)₅ and (q₈)₅<(q₇)₅, which satisfies the loading condition for the separation between the stratum and the stratum of the roof.

Step 2: Determining deflection

Owing to the fart that the load borne by the stratum is (q₄)₁, the deflection equation of the stratum can be acquired by substituting it into Eq. (23), namely:

          (28)

Through Eq. (24), when x=l_m/2, the maximum subsidence of the stratum w(x)_max1 is:

        (29)

Then from Eq. (23), the deflection curve of the stratum is:

          (30)

Moreover, Eq. (24) shows that the maximum subsidence of the stratum w(x)_max2 is:

        (31)

Owing to the maximum clearance between them is:

                  (32)

The stratum is made up of fine sandstones, which are characterized by great thickness and strength, so it is regarded as the key stratum of the roadway roof. Likewise, it can be found that

The deflection curves for the and strata, and the and strata are shown in Figure 12. It can be seen that the interlayer deflection close to the surface of the roadway roof is much greater than that far from the surface of the roadway.

Figure 12 Deflection curves between adjacent strata in roadway roof in an extra-thick coal seam:

Thereby, both the and strata, and the and strata meet the deflection conditions for roof separation.

Step 3: Determining limiting span

According to the principle of composite beams, the deflection curves for all strata are consistent. In consequence, the deflection curves for strata S₁, S₂, S₃ and S₄ are w₁(x). Through Eq. (26), it can be seen that the relationship between the maximum tensile stress and the length of the stratum is:

               (33)

From Eq. (27), the limiting span calculated from the maximum tensile stress is:

Considering the effect of the stratum on the stratum, the load borne by the stratum is:

According to Eq. (26), the relationship between the maximum tensile stress and the width of the roadway of the stratum is:

               (34)

Then through Eq. (27), the limiting span is:

The limiting spans of strata S₃–S₇ can be successively computed and compared with the excavated width of the roadway in the same way, as shown in Figure 13. It can be found that the limiting spans of strata S₁–S₇ are greater than the width of the roadway. As for the stratum, as the key stratum, it is obvious that the limiting span is much greater than the width of the roadway. Thus, all of them can meet the limiting span conditions required for separation.

The relationship between the maximum tensile stress on each stratum and the length of the roadway is shown in Figure 14. The maximum tensile stress on each stratum increases theoretically with increasing roadway span.

Whereas, as for the roadway with a span of 5 m, the maximum tensile stresses on all strata are less than that required for the corresponding breaking span.

Step 4: Relationship between the position of key stratum and zone of influence of roof separation

The calculated results indicate that the distance between the stratum, namely the key stratum, and the surface of the roadway roof is 11.62 m, which is much greater than the zone of influence of the separation of the roadway roof in its extra-thick coal seam, i.e., h₁>h₀. Therefore, combined with Figure 6, it is evident that the interlayer separation between the stratum on the stratum would not happen in field, and thus the separation should not be considered.

Figure 13 Limiting span for different rock strata overlying bottom-driven roadway within an extra thick coal seam

Figure 14 Theoretical relationship between maximum tensile stress and roadway width

3.3 Field monitoring results

The roof separation of a continuous region in the roadway was monitored with an on-line monitoring device. The distributed technology and intelligent integrated sensor were used in this device. The mining monitoring substation for displacement was connected with the upper main station to transmit the monitoring data to the ground monitoring server. Based on the huge amount of data from roof monitoring above the coal roadways, it was found that bedding separation often occurs in the roof of two times height of the roadway. In order to further investigate the damage and influence range of bedding separation above the coal seams and to exam the situation of bedding separation beyond the range of anchorage zone, 8 monitoring points were set up in the vertical direction from the first point 1.8 m far away from the top of the roadway to the 8th point 8.8 m in steps of 1 m, and the interval between adjacent sensors was 1.5 m, from which six groups of typical monitoring results (Figure 15) were selected.

The bedding separations mainly focus on thickness of 3.8 m and 6.8 m above the roof. Due to the micro-differences in the thickness of the occurrence of the field coal-rock, the roof interlayer separations are mainly at the and strata. The separation at the thickness of 6.8 m mainly lies in the thick coal seam, which is associated with the separation produced by the extension of joint fissures in the coal seam. By comparing the predicted value and field monitoring, the results for the upper limiting position of the separation between the roof coal seams and rock strata are shown in Table 2. It was found that the upper limiting position of the roof interlayer separation obtained using the two different methods is consistent, which proved the validity of the theoretical method.

4 Conclusions

1) A mechanical model for the interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam was established to deduce the vertical stress and the length of the lateral plastic and elastic zones in roadway roof strata, as well as the deflection equations for different areas.

2) A set of methods were used for determining the upper limiting position of the interlayer separation of the bottom-driven roadway roof within an extra-thick coal seam. Besides, combined with the mechanical calculation method, the conditions required for locating the upper limiting position of the interlayer separation were determined, including the loading condition, limiting span, deflection condition, and a method for judging the position of the key roof stratum and the zone of influence of the separation.

Figure 15 Monitoring results for roof separation at a measuring point in roadway of an extra-thick coal seam:

Table 2 Comparison between predicted and monitored results for upper limiting position of interlayer separation overlying a bottom-driven roadway within an extra-thick coal seam

3) A typical bottom-driven roadway within an extra-thick coal seam was selected for the analysis and the field monitoring of its upper limiting value of interlayer separation. The computed and monitoring results demonstrated that the upper limiting position of separation determined using the two methods is consistent, which verified the validity of the proposed method.

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(Edited by YANG Hua)

中文导读

特厚煤层沿底巷道顶板层间离层上限位置的判定方法

摘要：顶板离层致巷道围岩失稳破坏是煤矿井下冒顶灾变事故的主要类型之一，其中以顶板煤层中含夹矸层的特厚煤层沿底巷道尤为突出，而开展此类巷道顶板稳定性控制进而防止冒顶事故重要前提是合理确定顶板层间离层的上限位置。本文以特厚煤层沿底巷道为研究对象，针对顶板层间离层的上限位置合理确定问题，综合运用力学计算、理论分析、现场监测等方法，建立了特厚煤层巷道顶板层间离层数学模型，理论推导得出了巷道顶板岩层侧向塑性区和弹性区的垂直应力、长度及不同区域挠度方程式。提出了一套判定特厚煤层巷道顶板层间离层上限位置的方法，并结合数学力学计算方法推导得出层间离层上限位置确定所需要满足的荷载条件、极限跨度、挠度条件及顶板关键层位置与离层位置范围的判定式。选择典型特厚煤层巷道进行层间离层上限值的理论分析和现场监测，计算推导和数据监测结果显示两者确定的离层上限位置是一致的，证明了提出的判定方法是正确的。

关键词：特厚煤层；顶板离层；煤巷；冒顶；力学模型

Foundation item: Project(2017XKQY012) supported by the Fundamental Research Funds for the Central Universities, China; Project(PAPD) supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions, China

Received date: 2016-12-12; Accepted date: 2017-12-10

Corresponding author: YAN Hong, PhD, Associate Professor; Tel: +86–516–83593019; E-mail: linodex@163.com: ORCID: 0000- 0001-6784-5962