Laboratory pull-out tests on fully grouted rock bolts and cable bolts:Results and lessons learned

Laboratory pull-out tests were conducted on the following rock bolts and cable bolts:steel rebars,smooth steel bars,fiberglass reinforced polymer threaded bolts,flexible cable bolts,IR5/IN special cable bolts and Mini-cage cable bolts.The diameter of the tested bolts was between 16 mm and 26 mm.The bolts were grouted in a sandstone sample using resin or cement grouts.The tests were conducted under either constant radial stiffness or constant confining pressure boundary conditions applied on the outer surface of the rock sample.In most tests,the rate of displacement was about 0.02 mm/s.The tests were performed using a pull-out bench that allows testing a wide range of parameters.This paper provides an extensive database of laboratory pull-out test results and confirms the influence of the confining pressure and the embedment length on the pull-out response(rock bolts and cable bolts).It also highlights the sensitivity of the results to the operating conditions and to the behavior of the sample as a whole,which cannot be neglected when the test results are used to assess the bolt-grout or the grouterock interface.     

Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts

Rock bolts are one of the primary support systems utilized in underground excavations within the civil and mining engineering industries. Rock bolts support the weakened rock mass adjacent to the opening of an excavation by fastening to the more stable, undisturbed formations further from the excavation. The overall response of such a support element has been determined under varying loading conditions in the laboratory and in situ experiments in the past four decades; however, due to the limitations with conventional monitoring methods of capturing strain, there still exists a gap in knowledge associated with an understanding of the geomechanical responses of rock bolts at the microscale. In this paper, we try to address this current gap in scientific knowledge by utilizing a newly developed distributed optical strain sensing (DOS) technology that provides an exceptional spatial resolution of 0.65 mm to capture the strain along the rock bolt. This DOS technology utilizes Rayleigh optical frequency domain reflectometry (ROFDR) which provides unprecedented insight into various mechanisms associated with axially loaded rebar specimens of different embedment lengths, grouting materials, borehole annulus conditions, and borehole diameters. The embedment length of the specimens was found to be the factor that significantly affected the loading of the rebar. The critical embedment length for the fully grouted rock bolts (FGRBs) was systematically determined to be 430 mm. The results herein highlight the effects of the variation of these individual parameters on the geomechanical responses FGRBs.     

岩质反倾边坡局部锚杆加固分析方法研究

全长黏结型锚杆作为一种有效且经济的加固手段,在边坡工程中得到广泛的应用。基于最新提出的岩质反倾边坡弯曲倾倒破坏分析方法和极限平衡理论,建立锚杆局部加固后该类边坡的力学模型和稳定性分析方法,给出加固后的边坡安全系数计算公式,并将理论分析结果与离散元(UDEC)计算结果进行对比分析。研究结果表明:理论解与UDEC计算的数值解具有较高的一致性,两者相互得到了验证;用全长黏结型锚杆加固岩质反倾边坡时,最优加固位置位于叠合倾倒区内,具体位置与边坡的物理力学参数、锚杆锚固参数有关;减小锚杆与层面的夹角,能够充分发挥锚杆对层面的阻滑作用,进而提高加固效果。     

Effect of Poisson''s ratio on the normalized radial displacements occurring around the face of a circular tunnel

The distribution of displacements occurring around the advancing face of a tunnel is one of the widely discussed topics in tunnelling. Unavailability of a closed-form solution for this problem has directed the researchers to the utilization of numerical analyses mainly. In this study, a three-dimensional finite difference stress analysis program, i.e. FLAC^3D, has been employed for determining the variation of radial boundary displacements along the longitudinal direction of a circular tunnel located in a hydrostatic in situ stress field. According to the results for a linear elastic material behavior, the normalized radial displacements occurring in the vicinity of the face are affected by Poisson's ratio of the surrounding medium. In addition, it has been noted that, depending on Poisson's ratio, the value of elastic pre-deformation occurring at the face ranges between 20 and 30% of the final tunnel deformation occurring far from the face. Furthermore, expressions obtained by non-linear curve fitting are presented for the normalized radial displacements occurring ahead of and behind the excavation face of the tunnel.     

Back-analysis of viscoelastic displacements in a soft rock road tunnel

The rheological behavior of the rock proximate to excavations is of importance and practical interest for engineers in the design of soft rock tunnels. In situ rheology tests may be conducted to determine the appropriate parameters of the rock masses and the ground stress component; however, this can cost a great deal in both expense and time. Hence, it is useful to study back-analysis of viscoelastic displacements induced by the excavation of tunnels. Taking four rheological models, namely, Kelvin body, Maxwell body, Poynting–Thomson body and Kelvin–Voigt body as examples, this paper presents the basic finite element equations for such back-analysis, and the associated calculation methods for practical applications. A trial calculation method for the different models is adopted so as to obtain the most suitable rheological model and corresponding parameters. Furthermore, the authors have successfully applied the back-analysis method in a test section of a road tunnel excavated in soft rock. Compared with the direct in situ rheology tests, the back-analysis method presented by this paper is more concise and involves less money and time.     

Effects of frequency and grouted length on the behavior of guided ultrasonic waves in rock bolts

Experiments were conducted to study the behavior of guided waves in free and grouted rock bolts. Ultrasonic waves with frequencies from 25 to 100 kHz were used as excitation inputs. Tests were first conducted on free bolts to help understand the behavior of guided waves in non-grouted bolts. The effects of wave frequency and grouted length on the group velocity and attenuation of the guided ultrasonic waves were then evaluated. The test results indicated clear but different trends for the group velocity in the free and the grouted bolts. The attenuation in free bolts was not affected by bolt length and frequency. However, in grouted bolts it increased with frequency and grouted length. It was also found that the two main sources of attenuation are the setup energy loss, which has a fixed quantity for a specific type of test setup, and the dispersive and spreading energy loss which varies with frequency and bolt length.     

The analysis of radial displacements occurring near the face of a circular opening in weak rock mass

The effect of opening size, rock properties and stress conditions on the radial displacements ahead and behind of the advancing face of a circular opening is one of the most important topics in civil and mining engineering. In this paper we develop a prediction system for radial displacements around the face of an opening by combining rock mass rating classification system, a three dimensional finite difference code and multiple regression modeling. Predictive equations and response surfaces for rock mass response are proposed. By using these equations and response surfaces, it is possible to assess radial displacements around the face and to construct the longitudinal deformation profile of a circular opening with different diameters excavated in weak rock mass under different stress conditions. The predictive equations were validated by means of statistical tests and analytical solution. Proposed response surfaces and equations can provide the basis of guiding the design and construction of underground openings.     

Observations of brittle failure around a circular test tunnel

In 1987, AECL proposed several experiments for its Underground Research Laboratory. Among these was the Mine-by Experiment, the main focus of which was to investigate the formation and characteristics of the excavation disturbed zone adjacent to an underground opening. The extent and nature of this zone are important concerns for both the operational safety and the sealing of a vault to control the migration of radionuclides.The excavation response phase of the Mine-by Experiment, which involved the excavation, without explosives, of a 46-m-long, 3.5-m-diameter test tunnel in brittle unfractured granite, was carried out between January and July 1992. State-of-the-art instrumentation comprising both geomechanical and geophysical instruments was used to monitor the progressive failure process around the circular tunnel, and the in situ experiment was supplemented by extensive laboratory tests. This paper provides a detailed record of the brittle failure process that was observed during excavation of the tunnel.     

A theoretical approach for analysis of the interaction between grouted rockbolts and rock masses

Rock-support interaction concepts, which are applied to support systems such as concrete linings and ungrouted rockbolts, cannot be applied to grouted rockbolts, because of the nature of the structural interaction of the rock mass and grouted rockbolts. In this paper, with consideration of a non-linear strength criterion for rock mass and on the basis of two material behavior models and rock-support interaction concepts, corresponding relationships are derived. In this method, the equation of the ground response curve which has been reinforced with grouted rockbolts is also derived for a circular underground excavation under hydrostatic stress field. Because of algebraic complexity, it is not possible to obtain closed-form solutions; therefore a computer program was prepared for solving the problem on the basis of numerical methods.     

Simulation of failure around a circular opening in rock

A biaxial compression test was performed on a sandstone specimen with a circular opening to simulate a loading-type failure around an underground excavation in brittle rock. The axial force and displacements were monitored throughout the failure process, and microcracking was detected by the acoustic emission technique. To model the observed damage zone around the opening, the distinct element computer program, particle flow code (PFC^2D), was used. The numerical model consisted of several circular elements that can interact through contact stiffness, exhibit strength through contact bonds and particle friction, and develop damage through fracture of bonds. For the determination of micro-mechanical parameters needed in the calibration process of the computer program, only the macroscopic parameters of Young's modulus, Poisson's ratio and uniaxial compressive strength were used. It is shown that PFC^2D was capable of simulating the localization behavior of the rock and the numerical model was able to reproduce the damage zone observed in the laboratory test.     

Stability analysis of rock blocks around a tunnel using a statistical joint modeling technique

A three-dimensional statistical joint modeling technique was used to analyze the stability of rock blocks around a tunnel. The rock blocks generated from the joints and the tunnel were analyzed for their volume, height, perimeter, safety factor, and probability of occurrence. All these procedures were programmed into the computer program blocstab. The characteristics of rock block occurrence due to the variation of joint persistence (diameter), orientation and volumetric frequency were investigated. The rock block stability around a storage cavern of liquefied petroleum gas in Korea was analyzed as a case study. Here, the joints were assumed to follow the Baecher disk model for their shape and location, and their orientations were modeled by the Fisher distribution. The joint diameter distribution was estimated by using the window sampling method recently developed by Song and Lee.     

某电站模型洞围岩位移反分析及预测研究

张河湾抽水蓄能电站位于河北省井陉县测鱼镇甘陶河干流上。电站安装4台单级混流可逆式水泵水轮机组,总装机容量为1000 MW。地下厂房开挖尺寸(长×宽×高)为151.1 m×23.4 m×48.3 m。为进一步研究地下厂房围岩稳定性和优化设计参数,在地下厂房近旁进行了模型洞围岩观测试验研究,依据模型洞围岩收敛位移、钻孔多点位移、锚杆应力观测结果,进行了反分析,计算结果:水平应力2.52～2.88 MPa,应力比值0.7～0.8,弹性模量26.0 GPa。依据反分析的围岩初始地应力及变形参数,对主厂房及主变室围岩位移及应力分布进行了预测,结果表明,围岩变形均较小,且与地下厂房围岩位移实测值一致性较好。     

Effect of grout properties on the pull-out load capacity of fully grouted rock bolt

This paper represents the result of a project conducted with developing a safe, practical and economical support system for engineering workings. In rock engineering, untensioned, fully cement-grouted rock bolts have been used for many years. However, there is only limited information about the action and the pull-out load capacity of rock bolts, and the relationship between bolt–grout or grout–rock and the influence of the grout properties on the pull-out load capacity of a rock bolt. The effect of grout properties on the ultimate bolt load capacity in a pull-out test has been investigated in order to evaluate the support effect of rock bolts. Approximately 80 laboratory rock bolt pull-out tests in basalt blocks have been carried out in order to explain and develop the relations between the grouting materials and untensioned, fully grouted rock bolts. The effects of the mechanical properties of grouting materials on the pull-out load capacity of a fully grouted bolt have been qualified and a number of empirical formulae have been developed for the calculating of the pull-out load capacity of the fully cement-grouted bolts on the basis of the shear strength, the uniaxial compressive strength of the grouting material, the bolt length, the bolt diameter, the bonding area and the curing time of the grouting material.     

A new analytical solution for the displacement of fully grouted rock bolt in rock joints and experimental and numerical verifications

This study proposes a new analytical solution to predict displacement of a fully grouted rock bolt intersected by single rock joint. The main characteristics of the analytical model, consider the bolt profile and joint movement under pull test condition. The anchorage capacity of fully grouted bolts has been studied for many years; however, the bolt profile and its effect on bolt shear resistance are poorly understood. Investigations of load transfer between the bolt and grout indicate that the bolt profile shape and spacing play an important role in improving the shear strength between the bolt and the surrounding strata. Rock displacement is a sum of elastic part and a jump part due to the presence of joints planes. The performance of the proposed analytical model is validated by experimental method and comparison with numerical modeling. The results showed that there is a promising agreement between analytical and numerical methods. Studies indicate that the displacement rate between the bolt and the rock declines exponentially. Which is dependent on the bolt characteristics such as: rib height, rib spacing, rib width and thickness grout, material and joint properties.     

Back analysis of surrounding rock parameters of tunnel considering displacement loss and space effect

Bulletin of Engineering Geology and the Environment - Displacement back analysis is often performed to estimate rock parameters in tunnel construction. However, most researchers use the final...     

The plastic zones and displacements around underground openings in rock masses containing a fault

Faults are the commonly encountered large geological discontinuities in hard rock masses, most severe underground structure instability is found to be closely associated with the faults presence nearby. The parametric study carried out in this paper using numerical method (UDEC) has identified some fault parameters to be really critical for the underground structure stability. These fault parameters are fault dips, fault shear strength and fault locations relative to the underground structure. This numerical investigation revealed that faults affect the stability of underground structure by the tendency of increasing the plastic zones, displacements and causing both asymmetrically distributed in the rock masses adjacent to the excavation. The relationship of the induced plastic zones, maximum displacements varying with these fault parameters was established. The distribution of plastic zone and displacement was graphically presented and the mechanisms such effects were discussed. These results offer a guideline in support design.     

高速铁路隧道围岩参数空间变异性的力学响应分析

 针对大断面高速铁路隧道围岩的参数变异性及空间相关性等问题,在随机场理论的基本框架下,将非侵入式随机分析、地层结构模型、有限差分法和蒙特卡洛模拟有机结合,提出考虑围岩参数空间变异性的非侵入式随机有限差分分析模型。运用该模型对隧道开挖后围岩的力学响应进行分析,得到围岩参数空间变异性的2个基本要素：参数变异性和空间相关性,其特征参数的取值变化会直接影响到围岩变形特性、塑性区发展模式以及地表沉降形态。同时与确定性模型的计算结果进行比较,提炼出隧道开挖后围岩参数空间变异性的4种典型工程效应：(1) 围岩结构性低强度荷载占优效应;(2) 围岩结构空间相关各向异性效应;(3) 功能函数敏感性差异及非线性效应;(4) 功能函数转变分布类型效应。研究成果为拓展基于空间变异性的高速铁路隧道工程稳健性设计奠定了一定的基础。