J. Cent. South Univ. (2017) 24: 2951-2960

DOI: https://doi.org/10.1007/s11771-017-3709-0

Assessment and analysis of strata movement with special reference to rock burst mechanism in island longwall panel

ZHU Guang-an(朱广安)^{1, 2}, DOU Lin-ming(窦林名)^{1, 2}, CAO An-ye(曹安业)^{1, 2}, CAI Wu(蔡武)^{1, 2},

WANG Chang-bin(王常彬)^{1, 2}, LIU Zhi-gang(刘志刚)^{1, 2}, LI Jing(李静)^{1, 2}

1. Key Laboratory of Deep Coal Resource of Ministry of Education, China University of Mining and Technology,Xuzhou 221116, China;

2. School of Mines, China University of Mining and Technology, Xuzhou 221116, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract: This study presents a novel approach using theoretical analysis to assess the risk of rock burst of an island longwall panel that accounts for the coupled behavior of stress distribution and overlying strata movement. The height of destressed zone (HDZ) above the mined panel was first determined based on the strain energy balance in an underground coal mining area. HDZ plays a vital role in accurately determining the amount of different loads being transferred towards the front abutment and panel sides. Subsequently, based on the load transfer mechanisms, a series of formulae were derived for the average static and dynamic stresses in the island pillar through theoretical analysis. Finally, the model was applied to determining the side abutment stress distribution of LW 3112 in the Chaoyang Coal Mine and the results of ground subsidence monitoring were used to verify the predicted model. It can be concluded that the proposed computational model can be successfully applied to determining the safety of mining in island longwall panels.

Key words: island longwall panel; overburden structure; height of destressed zone; stress distribution; rock burst risk

1 Introduction

The island longwall panel poses a serious threat to the production and safety of underground coal mining throughout the world due to early strip pillar mining or unreasonable mining layout. Most significantly, rock burst hazards occur more frequently in island longwall panels because of high stress concentrations and complex overburden structure. With the increase of mining depth and intensity, rock bursts in the island longwall panel have become a common safety problem for underground coal mines in China [1, 2]. A typical example of this is highlighted by rock burst induced by coal pillar destabilization and overlying strata movement that caused 6 deaths and trapped 2 people on 17 November 2012, during the track gate excavation of LW 3112 in the Chaoyang coal mine of the Shandong Province, China [3]. The rock burst in the Zhaolou Coal Mine in Yuncheng, Shandong Province, China on Jul 29th 2015, trapped 3 people at the early mining stage of LW 1305 which resulted in production being halted in the coal mine [4].

Numerous studies have been conducted to obtain a comprehensive understanding of rock burst over the past 50 years. Specifically, the foci of these studies have been the mechanism of rock burst [5, 6], overburden structure movement [7, 8], abutment stress distribution [9, 10] and assessment of rock burst risk [11–13]. Distribution of abutment stress has been a major topic in the study of rock bursts in island mining panels which determines rock burst risk and provides a theoretical basis for mining safety. JIANG et al [9] adopted FLAC^3D, a finite difference program, to explore the abutment pressure distribution characteristics of an island longwall panel and showed that abutment stress reaches a peak value at a distance of 7.5 m in front of the mining panel. The length of influence zone was shown to be approximately 30 m, consistent with those observed with field measurements. LI et al [14] used the static and dynamic stresses superposition theory to derive abutment stress expressions in an island pillar which showed that with the advancing mining panel, the maximum static stress within the coal mass rises gradually and comes close to the critical stress. The superposition of such high static and dynamic stresses reaches the critical stress level leading to frequent occurrences of rock burst. SUCHOWERSKA et al [15] used the Wilson equations to calculate vertical stress distribution. The key finding of the study shows that the abutment angle has a significantly greater effect on the magnitude of the relative changes in vertical stress than in the strata. WANG et al [16] analyzed the peak and declining areas of abutment pressure in an island longwall panel using ground radar.

In previous studies, less attention has been paid to determining the influence of the gob roof structure on abutment stress distribution, leading to a lack of theoretical basis of mine design for prevention of rock bursts. The roof structure of the gobs on both sides determines the load transfer mechanism and therefore affects the abutment stress distribution on the island panel. To date, the study of the rupture and movement characteristics have mainly focused on a single mining panel or a single overlying strata. However, the rupture and movement above an island panel are always affected by the geological conditions of the adjacent mining panels as the overlying strata will move and interact coordinately. The study of the overlying strata structure and abutment stress distribution is therefore of significant importance in terms of island pillar mining safety and productivity.

To determine the abutment stress of island longwall panels, this work first proposed a theoretical computational model based on analysis of load transfer mechanisms of overlying strata and used field monitoring techniques to verify the theoretical computational model using ground subsidence monitoring techniques. Finally, to evaluate rock burst risk, the abutment pressure calculation model was applied to a longwall mining design in the Chaoyang Coal Mine of the Shandong Province, China.

2 Classification and characteristics of overlying strata over island longwall panel

As the overburden above the gob has caved and ruptured, the overlying strata will move and interact coordinately during the excavation of the island longwall panel which may lead to high stress distribution and frequent occurrence of rock bursts. Based on the boundary conditions, the whole overburden structure resembles the letter “T”, which is called the “T” overburden spatial structure. The “T” overburden spatial structure can be grouped into three categories based on field observations and speculation: long-arm “T”, short-arm “T” and asymetric “T” [17].

Different structures show different strata behaviors. For the short-arm “T” structure, the primary key stratum (PKS) on both sides was broken, which means that the overburden is supported by the island pillar resulting in high stress concentration on the island panel. For the long-arm “T” structure, the key strata on both sides was unbroken, which means that the side abutment stress is higher than that of short-arm “T” structure. As a consequence, the maintainence of roadways is more difficult and mining-induced tremors will increase sharply. In addition, the PKS will be broken after the panel advances a certain distance which may induce strong tremors. For the third asymetric “T” structure, the strata behavior is similar to the former two structures during the early mining period. However, when the unbroken key strata begin to break, the strata behavior appears to be more violent. The main reason for this is that the fracture line on one side of the KS is located above the roadway and the middle fracture line close to the top of the roadway, therefore, the effect of strong tremors, triggered by the fracturing of KS, on the destruction of the roadway is greater. Figure 1 illustrates the “T” overlying strata spatial structure with four key strata above an island longwall panel.

3 Mechanism of rock bursts in an island longwall panel

3.1 Rock bursts induced by dynamic combined with static loads

A number of studies and field cases have shown that rock bursts are induced by tremors under concentrated stress, i.e., rock bursts induced by dynamic combined with static loads. It can be expressed as [11, 18]

                              (1)

where σ_j is the static stress in the coal and rock mass;σ_d is the dynamic stress induced by the tremor, and σ_bmin is the critical stress required for rock burst. Equation (1) indicates that the higher the superposition of static load in the coal and dynamic load induced by tremors, the higher the probability that rock bursts will occur.

3.2 Estimation of HDZ above mined panels

To obtain the static and dynamic stresses on the island pillar, a practical and reliable method to discriminate the destressed zones and estimate its scope was used. Numerous studies using both empirical and analytical methods have been conducted previously to obtain a comprehensive evaluation of the HDZ above the panel [19–22]. Previous studies have shown many effective parameters in determination of the HDZ such as extracted coal seam thickness, longwall panel width and the parameters of roof rock strata. However, none of these studies have considered all of the possible effective parameters into the investigation. In this work, an analytical model based on the strain energy balance [23, 24] is adopted to determine the HDZ.

Fig. 1 Sketches and classification of “T” overlying strata spatial structure showing long-arm “T” (a), short-arm “T” (b) and asymetric “T” (c)

3.2.1 Energy model

In longwall mining, with the excavation of the coal seam, the energy balance of the coal and rock system is changed which implies that the total stored strain energy in the mined coal is released and consumed in fracturing, caving, and distressing the panel roof rock strata.

In Fig. 2, L is the gob width, H is the cover depth, h is the thickness of coal seam, D is the island pillar width, H_d is the height of destressed zone, H_D is the distance of horizontal line across the center of gravity of the panel to the ground surface, V_m is the volume of gob panel, S_m is the surface of gob panel, V is the volume of the destressed zone above the gob and S is the surface of the destressed zone above the gob. Δσ₁ and Δσ₂ are the transferred stresses from all of the overlying strata over the gob area.

Based on the above descriptions, the stored strain energy in the mined coal should be equal to the stored strain energy in the caved roof rock strata within the destressed zone:

                                    (2)

where U_m is the stored strain energy in the mined coal and U_d is the stored strain energy in the caved roof rock strata within the destressed zone.

3.2.2 Calculation of U_m

To calculate the stored strain energy in the mined coal seam, SALAMON [23] derived the follow equation:

(3)

where is the stress or traction vector acting on a surface before the panel excavation;is the component of the displacement vector; X_i is the body force per unit volume in the overburden strata. The superscript “p” stands for “primitive” or the state “before the panel excavation”.

Fig. 2 Energy model used in mechanical analysis (after excavation of longwall panel)

Since there is no permanent support in longwall mining, the effect of body force can be ignored (X_i = 0). Therefore, Eq. (3) is modified as

                        (4)

It is assumed that the rock is homogeneous and isotropic with elastic modulus E and Poisson ratio ν, and the vertical and horizontal normal stresses are principal stresses. They are given by [25]

,    (5)

The components of the strain tensor e_ij can be calculated by

                          (6)

The only non-zero strain and displacement components are calculated based on Eqs. (5) and (6), and Hooke’s law is expressed as follows:

   (7)

          (8)

    (9)

where the part of is the moment of inertia of volume V_m with respect to the plane of the ground surface. Based on the parallel-axis theorem in statics, it can be written as

                              (10)

where I₀ represents the moment of inertia of V_m with respect to a horizontal plane across its centre of gravity, point O in Fig. 2.

With regard to the rectangular cross section of the gob panel, V_m and I₀ can be calculated as follows:

                                 (11)

                                  (12)

Then,

                            (13)

          (14)

According to Fig. 2:

                                (15)

Finally, the ultimate equation of U_m in longwall coal mining can be obtained as follows:

     (16)

3.2.3 Calculation of U_d

In general, the total stored strain energy in the rock mass within the destressed zone (U_d) is composed of the elastic strain energy (U_E) and the viscoplastic strain energy (U_V). In this work, the viscoplastic strain energy is ignored for the purpose of simplicity. Consequently, we obtain:

                                   (17)

The stored strain energy in the destressed zone can be calculated using the following equation:

        (18)

where σ is the uniaxial compressive strength of caved materials (σ_c); ε is the strain; H_d is the height of the destressed zone and A_d is the unit surface of the system.

To describe the stress strain behavior of the caved materials, the following equation can be used [23]:

                                (19)

where E₀ is the initial elastic modulus of the material, ε is the volumetric strain and ε_m is the maximum volumetric strain; ε_m can be obtained from the bulking factor (b) [26]:

                                 (20)

Then, the substitution of Eq. (20) into (19) leads to

                               (21)

Finally, by substituting Eqs. (21) into (18), the stored strain energy in the rock mass within the destressed zone (U_d) can be obtained as follows:

                           (22)

3.2.4 Calculation of H_d

The substitution of Eqs. (22) and (16) into Eq (2) leads to calculation of the HDZ as follows:

       (23)

For a specific coalface, the parameters L, H, h, E, b, γ, σ_c, and ν can be obtained by physical measurements.

3.2.5 Correction of H_d

To verify the proposed theoretical model, the obtained results were compared with the results of in-situ measurements reported in various studies. The results confirm a good correlation between the suggested model and in-situ measurements (refer to REZAEI et al [24] for more details). However, the obtained results are lower than the results of the in-situ measurements in all cases, particularly when a thick and hard overlying stratum exists above the longwall mining panel.

According to the key stratum hypothesis [27, 28], the overlying strata above a mining panel contain one KS or more key strata which control the overburden movement. When the KS is broken, its controlled strata will be broken simultaneously which plays an important role in calculation of the HDZ. Key strata can be identified by a computer programme named KSPB [29].

Considering the effect of KS,the above calculation of the HDZ should be corrected. The estimation procedure of the HDZ comprises the following steps:

Step 1: Collection of the coal and rock properties of the study panel and characterization of the strength level of overlying strata.

Step 2: Use of KSPB and Eq.(23) to identify KS and the HDZ in the overlying strata on the basis of borehole logs and rock properties.

Step 3: When the calculated ith KS is within the HDZ, if the KS is broken, the HDZ is equal to the height of ith KS plus its controlled strata. If the KS is unbroken, then the HDZ equals the height between the KS and coal seam.

3.3 Calculation of static stress

The side abutment pressure σ_s on the island pillar consists of gravity stress σ_g and the transferred stresses Δσ₁ and Δσ₂ from the overlying strata above the goaf area. Hence, the side abutment pressure can be written as

                         (24)

Among them, the gravity stress σ_g can be calculated as

                                   (25)

where γ is the average bulk density of the overlying strata.

For brevity, based on engineering experience, it is assumed that the stress transferred onto the coal seam from each KS block is distributed in a shape of an isosceles triangle from the coal face [30]. Thus, the stress transferred onto the coal seam from the ith KS block can be written as

(26)

where α is the fracture angle; H_d is the HDZ of the goaf area, and Δσ_max is the greatest stress on the coal seam generated by the goaf area.

For the three terms of “T” overburden spatial structure in Section 2, Δσ_max can be expressed as

short-arm “T”             (27)

long-arm “T”           (28)

or asymmetric “T”                        (29)

Through Eqs. (23)–(29), the side abutment stress distribution on the island pillars before mining can be determined.

3.4 Calculation of dynamic stress

3.4.1 Unbroken PKS

In Fig. 3, the ith KS was selected, A and B are fractured roof blocks, O is the contact point between A and B, L is the roof fracture length, H is the thickness of the roof, ρ is the density of the roof, p is the uniform stress load caused by the overlying strata, σ_T is the tensile stress distribution function of the fracture plane, τ is the uniform shear stress of the fracture plane, f(l) is the static stress distribution function of the support of the down overlying strata, T is the horizontal thrust between roof blocks A and B, and R is the friction between A and B.

The mechanical equilibrium of rock block A is calculated as

(30)

After roof block A is broken, the new mechanical equilibrium can be expressed as

     (31)

Hence, the dynamic stress increment △σ_d can be obtained

or                  (32)

The stress load on the elastic coal body will cause a dynamic load. Assuming the deformation is x and the coefficient of elasticity of coal is k, according to the law of conservation of energy, the equation can be written as

                                (33)

Then, the dynamic load σ_d can be expressed as

                             (34)

In general, the tensile strength of rock mass is smaller than the shear strength. In this study, it is assumed that roof strata present tensile failures.

3.4.2 PKS breaks

According to the actual engineering conditions, and the theory for KS fracture, it is assumed that the key strata of thick and hard layer are seen as rigid body. Based on the material mechanical and energy balance analysis, the dynamic load factor is significantly affected by the thickness and height of the block mass of the KS, as well as the fractured angle, the height of the coal seam and some safety factors (refer to FENG et al [31] for more details). Although some shortfalls exist, this study provides information for the guidance and practice to engineering processes.

4 Case study of static and dynamic stress computation for island longwall panel

4.1 Site description

The Chaoyang coal mine, owned and operated by Zhongtai Coal Group Company, is located in the south of Shandong Province, China. Currently, mining activity at the Chaoyang coal mine is carried out in LW 3112, as shown in Fig. 4. The panel 3112 is fairly deep at about 860 m underground and the fully-mechanized top coal caving method was used to retreat this panel. As of April 5th, 2015, LW 3112 had been developed by the excavation of track gate and belt gate, and had been retreated for about 0.5 m on June 19, 2015.

As demonstrated in Fig. 4, the west side of LW3112 neighbors on the gob of LW 3113, the east side is adjacent to the gobs of LW 3111, LW 3110, and LW 3109, the north and south sides are F2 fault and F3111-1 fault, respectively. The panel 3112 is 245 m long in the strike direction and 110 m wide in the dip direction. The coal seam thickness varies from 2.8 to 9.0 m with an average angle of 10°. The roof strata are mainly composed of medium sandstone with a thickness of 5–25 m.

Fig. 3 Model used in dynamic stress analysis of overlying strata (during excavation of the island longwall panel)

Fig. 4 Entry layout, burst source distribution, and disposition of surface subsidence stations around Panel 3112

4.2 Assessment of rock burst risk

4.2.1 Determination of HDZ

Using the experimental tests for the degree of bursting liabilities, the average geometrical and geomechanical parameters of the overlying strata over LW 3112 are given in Table 1. Substituting the above parameters into Eq.(23), the theoretically computed HDZ was estimated to be 497.29 m.

Table 1 Average values of geometrical and geomechanical parameters of overlying strata over LW 3112

The lithological information above LW 3112 is presented in Table 2. According to the key stratum hypothesis, there are a total of five key strata in the overlying strata above LW 3112, as shown in Table 2. To better understand the structure and movement of the overlying strata, a series of monitoring points were set to observe the surface subsidence. Figure 5 shows the observed surface subsidence curve in the dip direction (from stations S1 to S12, shown in Fig. 1) after the excavation of panels 3109, 3111, 3113 and 3110. The maximum values of the observed surface subsidence are 561 mm and 695 mm on May 17th, 2012 and Jun 28th, 2014, respectively. It can be seen from the figure that after two years, the surface subsidence is only 130 mm which infers that the PKS does not break and appears to be intact and the load of the overburden is transferred to the island pillar.

Table 2 Lithological information above LW 3112

Fig. 5 Curves of observed surface subsidence over island panel

Finally, considering the theoretically computed HDZ and the key stratum hypothesis, the theoretical HDZ should be corrected as 242.72 m.

4.2.2 Computational results of side abutment stress

Based on the above analyses and practical geological conditions of LW 3112, all of the parameters required for stress calculation are listed as follows: H₁=31.54 m, L₁=37.2 m, σ_t1=7.47 MPa; H₂=24.70 m, L₂=39.4 m, σ_t2=6.77 MPa; H₃=116.55 m, L₃=66.9 m, σ_t3=8.56 MPa; H₄=97.50 m, L₄=120 m, σ_t4=6.77 MPa; α=86°; γ=25.5 kN/m³.

Numerous studies show that the critical stress of rock bursts is related to the uniaxial compressive strength of coal [32] as shown in Fig. 6. When the uniaxial compressive strength is smaller than 16 MPa, the critical stress is 70 MPa, and when the uniaxial compressive strength is over 20 MPa, the critical stress reduces to 50 MPa. The mean compressive strength for the tests conducted for identifying coal burst tendency is about 10.356 MPa. Therefore, the critical stress of LW 3112 is 70 MPa.

Substituting these parameters into Eqs. (24), (25),(26), (28) and (32), the static and dynamic stress (see Table 3) distribution of LW 3112 can be obtained as illustrated in Fig. 7. It can be seen from Fig. 7 that the superposition of static and dynamic stress is approximately 60–90 MPa and the distribution of the stress presents a single peak curve. The influence range of the superposition stress exceeds 70 MPa which is about 80 m, therefore an elastic zone in the island pillar does not exist and is more likely to encounter rock burst hazards due to high stress concentration.

Fig. 6 Relationship between critical stress of rock burst and uniaxial compressive strength of coal [32]

Table 3 Calculated results of dynamic load

Fig. 7 Theoretically computed side abutment stress distribution over LW 3112 (KS 1 breaks)

In conclusion, the island panel 3112 cannot be mined. During the drivage and mining of the longwall panel, the panel frequently experienced strong seismic tremors and induced three rock bursts which caused great damage to the workface and roadways. In particular, the 3rd rock burst disaster on June 19th, 2015 led to a halt in production of the panel 3112.

5 Conclusions

In this work, a new theoretical model was developed to predict the static and dynamic stress distributions above the island panel which can provide a basis for mine design in underground coal mines. The main conclusions of the study are:

1) With the expansion of the goaf beside the island panel, the effect of mining on the overlying strata continuously increases. The roof structure can be divided into three forms: namely, long-arm “T” structure, short-arm “T” structure and asymetric “T” structure. The roof structure determines the load transfer mechanism.

2) To determine the total mining-induced stress distribution, the HDZ is obtained based on the strain energy balance in longwall coal mining which should also be corrected with the identified KS. The static and dynamic stresses over island pillar were then calculated based on the proposed model.

3) The HDZ over LW 3112 was obtained as 242.72 m indicating that the KS does not break which is in good agreement with the surface subsidence. Based on the computed stress distribution of LW 3112, the superposition of static and dynamic stresses is 60–90 MPa, which reaches and surpasses the critical stress (70 MPa). The results suggested that there is no elastic zone in the island pillar which has a high risk of rock burst.

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(Edited by HE Yun-bin)

Cite this article as: ZHU Guang-an, DOU Lin-ming, CAO An-ye, CAI Wu, WANG Chang-bin, LIU Zhi-gang, LI Jing. Assessment and analysis of strata movement with special reference to rock burst mechanism in island longwall panel [J]. Journal of Central South University, 2017, 24(12): 2951–2960. DOI:https://doi.org/10.1007/s11771-017-3709-0.

Foundation item: Project(2017CXNL01) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2016-03-23; Accepted date: 2016-07-06

Corresponding author: DOU Lin-ming, Professor; Tel: +86–13952261972; E-mail: dlm_burst@126.com