J. Cent. South Univ. (2021) 28: 572-581

DOI: https://doi.org/10.1007/s11771-021-4622-0

Anchoring effect and energy-absorbing support mechanism of large deformation bolt

ZHAO Tong-bin(赵同彬)^{1, 2}, XING Ming-lu(邢明录)^{1, 2}, GUO Wei-yao(郭伟耀)^{1, 2},WANG Cun-wen(王存文)³, WANG Bo(王博)⁴

1. College of Energy and Mining Engineering, Shandong University of Science and Technology,Qingdao 266590, China;

2. State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province

and the Ministry of Science and Technology, Shandong University of Science and Technology,Qingdao 266590, China;

3. Shandong Energy Group Co., Ltd., Jinan 250014, China;

4. Beijing Haohua Hongqingliang Co., Ltd., Ordos 014316, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: To research the anchoring effect of large deformation bolt, tensile and drawing models are established. Then, the evolution laws of drawing force, bolt axial force and interfacial shear stress are analyzed. Additionally, the influence of structure element position on the anchoring effect of large deformation bolt is discussed. At last, the energy-absorbing support mechanism is discussed. Results show that during the drawing process of normal bolt, drawing force, bolt axial force and interfacial shear stress all gradually increase as increasing the drawing displacement, but when the large deformation bolt enters the structural deformation stage, these three values will keep stable; when the structure element of large deformation bolt approaches the drawing end, the fluctuation range of drawing force decreases, the distributions of bolt axial force and interfacial shear stress of anchorage section are steady and the increasing rate of interfacial shear stress decreases, which are advantageous for keeping the stress stability of the anchorage body. During the working process of large deformation bolt, the strain of bolt body is small, the working resistance is stable and the distributions of bolt axial force and interfacial shear stress are steady. When a rock burst event occurs, the bolt and bonding interface cannot easily break, which weakens the dynamic disaster degree.

Key words: rock burst; large deformation bolt; numerical simulation; pull-out test; anchoring effect; energy-absorbing mechanism

Cite this article as: ZHAO Tong-bin, XING Ming-lu, GUO Wei-yao, WANG Cun-wen, WANG Bo. Anchoring effect and energy-absorbing support mechanism of large deformation bolt [J]. Journal of Central South University, 2021, 28(2): 572-581. DOI: https://doi.org/10.1007/s11771-021-4622-0.

1 Introduction

With the continuous increase of mining depth and intensity in China, rock burst becomes one of the major disasters threating the safe production of coal mine [1-3]. According to incomplete statistics, the number of coal mines threatened by rock burst has reached more than two hundred [4-6]. In aspect of rock burst prevention technique, large deformation bolt support is one of the essential technical methods except mining optimization, blasting, water injection, large diameter drilling method [7-9], etc. Aiming at the low elongation of normal bolts, scholars have developed different kinds of large deformation bolt. Generally, the large deformation bolts can be divided into two groups based on their working principles. One group is with extendable bolt body, which depends on the yield strength and percentage elongation of bolt material to provide support resistance and extension quantity, such as D-Bolts [10, 11]. The other group is with extendable structure element, which depends on the special mechanical structure to provide elongation and steady support resistance, such as He-Bolts [12, 13].

In the bolt support technology, the interaction between bolt and surrounding rock, namely the bolt anchoring effect, plays an essential role in the effectiveness of anchoring design and the stability of surrounding rock control. Lots of field investigations and laboratory tests have proven that the failure of anchorage body is mainly caused by the interface or bolt failure [14]. Thus, the key research points of anchoring effect are the stress distribution and evolution laws of anchorage body. PHILLIPS [15] pointed out that the stress distribution of interface between the bolt body and the cement paste shows a negatively exponential trend. WANG et al [16] proposed the neutral point theory of the full-length anchorage. YOU et al [17] obtained the elastic solution of stress distribution of full-length anchorage and the analytical solution of internal force distribution of bolt. ZHAO et al [18] and YIN et al [19] studied the stress distribution law of the anchorage section under different conditions through laboratory test and particle flow simulation. Additionally, ZHAN et al [20] used FLAC^3D to research the stress distribution of anchorage body and the influence of reinforced rock properties. HE et al [21] applied FINAL software to study the evolution laws of bolt axial force and interface shear stress. ZHOU et al [22] presented a numerical model to analyze the load-transfer mechanism of fully grouted rock bolts，which was based on the bi-exponential shear-slip model of the anchorage interface and the linear enhanced elasto-plastic constitutive model of the rock bolt. MA et al [23] obtained the stress distribution of the anchor rod body by using ANSYS software and the measured bolt shape parameters. ZHANG et al [24] developed the CABLE unit of FLAC^3D numerical simulation software by using fish language, and simulated the axial tensile mechanical behavior of a kind of large deformation bolt with high precision.

These literatures mainly focus on the stress distribution and evolution laws of normal anchorage body, but there are few researches related to the anchoring effect of large deformation bolt. The working process of large deformation bolt with extendable structure element includes three stages, i.e., elastic deformation stage, structural deformation stage and ultimate deformation stage [12]. The structure element means the extendable structure element which provides the elongation of large deformation bolt in this paper. This paper studies the anchoring effect of elastic deformation stage and structural deformation stage, because the elongation and energy-absorbing mainly depend on the structural deformation stage. First, bolt tensile and drawing models of bolts with extendable structure element are established using FLAC^3D simulation. Second, the evolution laws of drawing force, bolt axial force and interfacial shear stress are researched. Third, the influence of structure element position on the anchoring effect of large deformation bolt is discussed. At last, the energy- absorbing support mechanism is discussed.

2 Numerical tensile simulation of large deformation bolt

2.1 Establishment of numerical tensile models

This paper only studies the elastic deformation stage and structural deformation stage, whose deformation characteristics are similar to the ideal elastic-plastic body [25, 26]. The drawing force will keep steady when it reaches a certain value, as shown in Figure 1(a). Thus, the ideal elastic-plastic body is used to simulate the extendable structure element. Four kinds of bolt models are established, including one elastic bolt model MG0 and three large deformation bolt models MG1-MG3 (i.e., the position of structure element is different). The length and diameter are 220 and 20 mm, respectively. In z direction, the upper side is free and the bottom is constrained. The drawing rate is 0.001 mm/s, as shown in Figure 1(b). Large deformation bolt model is composed of elastic unit and Mohr-Coulomb unit. Mechanical parameters of tensile models are listed in Table 1.

Figure 1 Tensile property of large deformation bolt [10] (a) and bolt numerical tensile models (b)

Table 1 Mechanical parameters of tensile models

2.2 Results analysis

Figure 2 illustrates the drawing force-drawing displacement curve. As the drawing displacement increases, the drawing force of MG0 increases linearly. The drawing force of MG1-MG3 also shows a linear increasing trend before entering the structural deformation stage. In the structural deformation stage, the drawing force remains relatively steady. The drawing force of MG1 is constant at 78.5 kN, but MG2 and MG3 experience fluctuation at the range of 77-96 kN and 80-105 kN, respectively. The reason might be that when the structure element is close to the drawing end, the drawing force can easily keep steady. If a certain length of free section exists, the fluctuation is hard to be avoided.

Figure 2 Drawing force-drawing displacement curve

Figure 3 illustrates the evolution law of bolt axial force under different drawing displacements. The bolt axial force at different positions gradually increases during the drawing process of MG0, but the bolt axial force of MG1-MG3 tends to be stable. At the elastic deformation stage of MG1-MG3, the bolt axial force decreases with the increase of distance from the drawing side. When enters into structural deformation stage, the bolt axial force below the structure element fluctuates first and then stabilizes, but above the structure element generates a fluctuation phenomenon during the drawing process. The fluctuation range will be reinforced when the distance from the structure element to the drawing end increases. For instance, the fluctuation ranges of MG2 and MG3 are 78-94 kN and 78-102 kN, respectively. These results reveal that the normal bolt can easily break during the drawing process, but the large deformation bolt will not easily break due to the large elongation. Additionally, when the structure element approaches the drawing end, the fluctuation range of tensile force is small and the distribution of bolt axial force is steady.

Figure 3 Evolution law of bolt axial force under different drawing displacements:

3 Anchoring effect analysis of large deformation bolt

3.1 Establishment of drawing models of large deformation bolt

To study the anchoring effect of large deformation bolt, three kinds of drawing models of large deformation bolt are established, as shown in Figure 4. The model dimensions are 300 mm×300 mm. The borehole diameter is 40 mm. The length and diameter of bolt are 220 and 20 mm, and the anchoring length is 100 mm. The drawing rate is 0.001 mm/s applied to the bold end. Meanwhile, observation points of bolt axial force and interfacial shear stress are set at different positions. The upper side of z direction is free face, and the bottom, left side, right side, frond side and back side are constrained in the x-, y- and z-direction displacement, respectively. According to Ref. [20], Drucker-Prager model is used for binder and matrix. Parameters of drawing models are listed in Table 2. If the mechanical behavior of rock is researched, the internal friction angle and cohesion are two important mechanical parameters in the Mohr- Coulomb model. This paper tries to simulate the elastic deformation stage and structural deformation stage of large deformation bolt. The two controlling parameters are elastic modulus and tensile strength. Therefore, the internal friction angle and cohesion are not assigned.

3.2 Results and anchoring effect analysis

Figure 5 illustrates the drawing force-drawing displacement curve. As the drawing displacement increases, the drawing force of T1 increases linearly. The drawing force of T2 and T3 also shows a linear increasing trend before entering the structural deformation stage. When entering the structural deformation stage, the drawing force of T1 is constant at 78 kN, but T2 experiences fluctuation at the range of 74-142 kN.

Figure 4 Drawing models of large deformation bolts:

Table 2 Mechanical parameters of drawing models

Figure 5 Drawing force–drawing displacement curve

Figure 6 illustrates the evolution law of bolt axial force under different drawing displacements. The bolt axial force in the bolt free section is larger than that in the anchorage section. The distribution of bolt axial force in the anchorage section is uniform, which shows a nonlinearly decreasing trend. For T1 model, the bolt axial force increases with the increase of drawing displacement. For T2 and T3 models, the evolution law of bolt axial force in the elastic deformation stage is similar to T1 model. But in the structural deformation stage, the distribution of bolt axial force tends to be steady as increasing the drawing displacement. Further, the bolt axial stress in the free section varied in the range of 84-143 kN for T2 model, but for T3 model, this value keeps steady at 78 kN. This phenomenon indicates that when the structure element is close to the drawing end, it is advantageous for the stability of bolt axial force.

Figure 7 illustrates the evolution law of interfacial shear stress in the anchorage section under different drawing displacements. The distribution of interfacial shear stress is uniform, which increases first and then gradually decreases. As the drawing displacement increases, the interfacial shear stress increases at different positions for T1 model, but reaches steady for T2 and T3 models. Figure 8 gives the evolution law of the maximum interfacial shear stress in the anchorage section under different drawing displacements. With the continuous increase of drawing displacement, the maximum shear stress of T1 model shows a linearly increasing trend, while for T2 and T3 models, the maximum shear stress reaches 7.54 and 7.32 MPa, respectively, followed by a stable stage. Additionally, the increase of interfacial shear stress for T3 model is steadier than that for T2 model, whose corresponding increasing rate are 2.92 and 8.95 MPa/mm, respectively.

Figure 6 Evolution law of bolt axial force under different drawing displacements:

The results reveal two essential phenomena. One is that the drawing force, bolt axial force and interfacial shear stress all gradually increase with the increase of drawing displacement due to the small elongation of normal bolt, which indicates that the bolt and interface bonding will easily break. The other one is that the smaller the distance between the structure element and the drawing end is, the smaller the fluctuation ranges of drawing force and bolt axial force will be. Meanwhile, the increasing process of interfacial shear stress will be steadier. Thus, it is more advantageous for keeping stress stability of the anchorage body, and the anchoring effect is better under dynamic load.

Figure 7 Evolution law of interfacial shear stress under different drawing displacements:

Figure 8 Evolution law of maximum interfacial shear stress under different drawing displacements

4 Energy-absorbing support mechanism of large deformation bolt and case study

4.1 Energy-absorbing support mechanism

When the roadway is excavated, appropriate support must be taken to improve the stress state for stopping the large deformation and failure of surrounding rock. Then, a complete and stable bearing circle forms in the surrounding rock, maintaining the roadway stability. For shallow buried roadway, the in-situ stress is low. The loose surrounding rock can be reinforced by normal bolt. However, for deep buried roadway, the in-situ stress is high, rock burst can easily occur, causing the large deformation of roadway surrounding rock instantaneously. In this case, the drawing force will instantly exceed the ultimate load of normal bolt. The bolt will quickly break due to its small elongation, as illustrated in Figure 9(a). If the bolt failure does not occur, but the interfacial shear stress near the anchoring end exceeds the bonding strength, the peak shear stress area moves into the deep anchorage section until leading to the failure of bonding interface, as illustrated in Figure 9(b).

Large deformation bolt solves the issue that the normal bolt is not suitable for the large deformation of roadway surrounding rock. Large elongation can be generated under the joint action of structure element and bolt body. Meanwhile, the strain of bolt material is small. Thus, the bolt can not easily break, as illustrated in Figure 10(a). Additionally, when the working state translates from elastic deformation stage to structural deformation stage, the bolt axial force is steady, as illustrated in Figure 10(b). During this process, the interfacial shear stress varies from state 1 to 2 and then to 3, avoiding the bonding interface failure due to the rapid increase of interfacial shear stress or unstable stress condition. Therefore, not only the strain of bolt body is small, but also the working resistance, bolt axial stress and interfacial stress are in a steady condition. When rock burst occurs, the bolt and bonding interface cannot easily break. The strain energy released by the surrounding rock will be steadily absorbed, achieving the goal of rock burst mitigation and control.

4.2 Case study

The buried depth of middle-uphill roadway is 700-800 m in No. 10 coal seam of A Coal Mine. The region is close to fault group and synclinal axis. The No. 10 coal seam includes No. 10_up coal seam and No. 10_down coal seam. The average thicknesses of No. 10_up coal seam and No. 10_down coal are 1.82 m and 2.20 m, respectively. The immediate roof and main roof are siltstone and medium-fine sandstone, whose corresponding thicknesses are 3.93-6 m and 10-24 m. The immediate floor and main floor are both siltstone, whose corresponding thicknesses are 5-16 m and 5-8 m.

Figure 9 Sketch of anchor failure of normal bolt or cable:

Figure 10 Sketch of energy-absorbing support mechanism of large deformation bolt or cable:

A few rock bursts occurred after the roadway was excavated. When rock burst occurred, there was no mining activities. Characterized by high tectonic stress, time delay, spontaneity, randomness and other characteristics, classify it to be a typical tectonic-rheologic rock burst. Rock bursts occurring before October 2015 led to 400 mm rib and floor heave, causing the bolt and cable failure, as illustrated in Figure 11(a). The region of influence was 40 m. Then, large deformation cables were used for reinforcement support, as illustrated in Figure 11(b). The length and diameter were 7 m and 18.2 mm, respectively. The large deformation cable was arranged in a single row with 7 m spacing pattern. In November 2015, one rock burst occurred without obvious roadway damage. This rock burst event led to 150 mm floor heave. Part of the large deformation cables experienced 5-15 mm extension in length. This phenomenon indicates that the high energy accumulated in the surrounding rock was effectively released and the damage of rock burst was effectively reduced.

Figure 11 Normal bolt/cable failure and large deformation bolt support:

5 Conclusions

1) During the drawing process of normal bolt, drawing force, bolt axial force and interfacial stress all gradually increase with the increase of drawing displacement. The stress state of anchorage body is unstable. However, when the large deformation bolt enters structural deformation stage, drawing force, bolt axial force and interfacial stress keep relatively steady. In other words, the stress state is stable.

2) When the structure element of large deformation bolt is close to the drawing end, the fluctuation ranges of drawing force and bolt axial stress are small. Additionally, the distributions of bolt axial force and interfacial shear stress of anchorage section are steady and the increasing rate of interfacial shear stress is small. It is advantageous for keeping the stress stability of anchorage body.

3) In the working period of large deformation bolt, not only the strain is small, but also the working resistance is stable and the distributions of bolt axial force and interfacial shear stress are steady. When the surrounding rock experiences dynamic failure, the bolt and bonding interface will not easily break. The released strain energy of the surrounding rock can be effectively absorbed, obtaining the goal of rock burst mitigation.

4) Due to the frequent occurrence of rock burst in recent years, a few types of large deformation bolts have been developed. In the future, more numerical simulation, laboratory test and field study should be done to reveal the anchoring effect and support mechanism.

Contributors

ZHAO Tong-bin provided the concept and helped revising the draft of manuscript. GUO Wei-yao wrote and edited the draft of the manuscript. XING Ming-lu finished the numerical simulation. WANG Cun-wen helped revising the draft of manuscript. WANG Bo provided the field data.

Conflict of interest

ZHAO Tong-bin, XING Ming-lu, GUO Wei-yao, WANG Cun-wen and WANG Bo declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

大变形锚杆锚固效应及防冲吸能支护机理

摘要：大变形锚杆支护是冲击地压灾害防治的重要手段之一，为研究大变形锚杆拉伸及锚固特征，建立了大变形锚杆拉伸及拉拔数值模型，研究了拉拔力、锚杆轴力和锚固段界面剪应力演化规律，分析了结构元件设计位置对其锚固效应的影响，探讨了大变形锚杆防冲吸能支护机理。结果表明：普通锚杆在拉伸或拉拔过程中，拉拔力、锚杆轴力和锚固段界面剪应力均随着位移的增加而不断增大，但大变形锚杆进入结构变形阶段后，拉拔力、锚杆轴力和锚固段界面剪应力会处于相对稳定状态，尤其是锚固段锚杆轴力及界面剪应力分布会保持稳定；大变形锚杆的结构元件越靠近拉拔端，拉拔力波动幅度越小、自由段锚杆轴力分布越稳定、锚固段界面剪应力增速越小，更利于锚固体受力稳定。在大变形锚杆工作过程中，不仅锚杆本身应变很小和工作阻力相对稳定，而且锚杆轴力和锚固段界面剪应力分布相对稳定，当围岩发生冲击破坏时，锚杆不易被拉断或粘结界面不易破坏，可更有效地吸收围岩释放的弹性变形能，降低冲击地压灾害程度。

关键词：冲击地压；大变形锚杆；数值模拟；拉拔试验；锚固效应；吸能机理

Foundation item: Project(2019SDZY02) supported by the Major Scientific and Technological Innovation Project of Shandong Provincial Key Research Development Program, China; Project(51904165) supported by the National Natural Science Foundation of China; Project(ZR2019QEE026) supported by the Shandong Provincial Natural Science Foundation, China

Received date: 2020-07-16; Accepted date: 2020-10-12

Corresponding author: GUO Wei-yao, PhD, Lecturer; E-mail: 363216782@qq.com; gwy2018@sdust.edu.cn; ORCID: https://orcid.org/ 0000-0003-3589-4537