An analytical model to predict axial load in grouted rock bolt for soft rock tunnelling

An analytical model is proposed to predict the axial force of grouted rock bolt in the tunnelling design. The interaction mechanism of the rock bolt and the soft rock mass has been described according to their consistent displacement. Coupling and decoupling behaviors of the rock bolt around a circular tunnel have been analyzed. According to case studies, the theoretical prediction of the axial force agrees well with the in situ measured results. The installing time and the length of the rock bolt, and the deformation modulus of the rock mass are taken as study parameters to analyze the supporting behavior of the rock bolt. According to the results of the theoretical analysis, there are some conclusions as followings: (1) a lower axial force is resulted because of the delay of installing rock bolt and its supporting effect may be reduced; (2) the larger displacement is caused by the lower deformation modulus of the rock mass, and a higher axial force is resulted in the rock bolt. If the shear strength of the rock mass is not enough, the decoupling behavior will take place interior the rock mass, and the performance of rock bolt may be reduced; (3) the position of a neutral point is related with the radius of tunnel, the physical properties of the rock bolt and the rock mass. It is found that the position of the neutral point and the maximum axial force of the rock bolt installed in the soft rock may tend to be constant when its length is long enough, which means that the supporting effect of the rock bolt can not be improved significantly only by increasing the length of the rock bolt. By using this model, a way is supplied to analyze the supporting behavior of the rock bolt, and a method is provided for the quantitative evaluation of its supporting effect in NATM tunnelling.     

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.     

3D Behaviour of bolted rock joints: experimental and numerical study

Rock bolting is the most effective and also the most economical means of supporting excavations in rock. Various types of bolts are used today, and an understanding of the way in which these bolts work is essential for an optimal, safe, and economical use. Fully grouted, untensioned bolts have been commonly used in rock mechanics (i.e., mines, rock fall stabilisation, underground works) for many years. In the 1980s a new type of bolt, called Swellex, was developed, becoming more and more widespread because of their easy and fast installation. However, regardless of the type, the mechanical behaviour of the bolted rock joint is not fully understood, and only the experience accumulated on rock bolting gives the know-how for the reinforcement calculation and execution. In this paper the different mechanical responses of full steel bars as opposed to the frictional Swellex are discussed. The study was done through experimental tests coupled with numerical simulations. The analysis of the results obtained both from finite element (FEM) modelling, and from large-scale (1:1) shear tests on rock joints, reinforced with fully grouted rods and Swellex bolts, clearly shows that the two bolt types deform in dissimilar ways, responding very differently to shear load.     

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.     

Dynamic testing of steel for a new type of energy absorbing rock bolt

Rock bolts of steel are the most frequently used components in systems for rock reinforcement. Special types are used as energy absorbing elements in systems exposed to dynamic loads and these must yield plastically under high loading velocities. A new type of energy absorbing rock bolt of soft steel has been suggested and tested. To investigate the strain rate effects on the yield stress and ultimate strength of the steel were bars dynamically loaded in a high speed testing machine. The observed tensile forces can be divided into one part from impact and another from quasi-static straining. The tests showed that there was a considerable strain rate effect on the yield stress of the steel. It is recommended that higher dynamic yield stresses are utilized in the dynamic design which has to incorporate the risk of high peak forces from impact loading.     

Development and application of an interface constitutive model for fully grouted rock-bolts and cable-bolts

This paper proposes a new interface constitutive model for fully grouted rock-bolts and cable-bolts based on pull-out test results.A database was created combining published experimental data with in-house tests.By means of a comprehensive framework,a Coulomb-type failure criterion accounting for friction mobilization was defined.During the elastic phase,in which the interface joint is not yet created,the proposed model provides zero radial displacement,and once the interface joint is created,interface dilatancy is modeled using a non-associated plastic potential inspired from the behavior of rock joints.The results predicted by the proposed model are in good agreement with experimental results.The model has been implemented in a finite element method(FEM) code and numerical simulations have been performed at the elementary and the structural scales.The results obtained provide confidence in the ability of the new model to assist in the design and optimization of bolting patterns.     

A new analytical solution to the mechanical behaviour of fully grouted rockbolts subjected to pull-out tests

A new analytical approach able to predict the mechanical behaviour of fully grouted rockbolts subjected to pull-out tests is proposed in this paper. Input parameters of such approach are: bolt radius, bolt’s Young modulus, displacement of the free end of the bolt and the constitutive law of the rockbolt–grout joint interface. The limited circumstances under which it is accurate to determine such constitutive law from pull-out tests are also presented. A solution for the load–displacement curve obtained during pull-out tests has been developed and is detailed in the case of a tri-linear bond-slip model. Comparison with experimental results obtained via in situ pull-out tests has led to the validation of this approach.     

Interaction of shotcrete with rock and rock bolts—A numerical study

The shotcrete–rock interaction is very complex and is influenced by a number of factors. The influence of the following factors was investigated by a series of numerical analyses: the surface roughness of the opening, the rock strength and Young's modulus, the discontinuities, the extent and properties of the excavated disturbed zone, the mechanical properties of the interface between shotcrete and rock, and the thickness of the shotcrete lining and the rock bolts. The study was carried out as a sensitivity analysis. The results showed that the rock strength and the surface roughness had significant impact on the number of failures at the rock–shotcrete interface and in the shotcrete lining. Furthermore, the behaviour of the lining is sensitive to small amplitudes of the surface roughness. In all the cases investigated, a high interface strength was favourable. The results indicate that if a thick shotcrete lining is dependent on the bond strength. The benefit of using a thicker lining can be doubtful. The analyses showed that for an uneven surface the extent of the EDZ had a minor effect on the behaviour of the shotcrete lining. Furthermore, if rock bolts were installed at the apex of the protrusion instead of at the depression, the number of failures decreased both at the interface and in the lining.     

Pull-out behaviour of aluminium screw port joints: experimental and numerical investigation

Special joint systems are obtained by means of mechanical fasteners located in semi-hollow parts of aluminium extruded shapes. The aluminium industry showed great interest on these joints due to many advantages: easiness, simplicity, rapid execution, material optimisation and machining reduction. A typical case of these special joints, namely screw port, consists of a screw engaged in a slot of an extruded profile. Screw port joints are used in several structural and non-structural applications such as façades, door and window frames. Nevertheless, no clear design rules are provided by aluminium structural codes. For this reason, an experimental campaign aimed at assessing the influence of different geometrical and mechanical parameters on the pull-out behaviour of screw port connections has been carried out at the University “Federico II” of Naples. In addition, on the basis of experimental results, non-linear finite element models have been developed and calibrated in ABAQUS environment.     

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.     

岩体锚固支护的数值流形方法模拟及其应用

利用数值流形方法理论 ,建立了岩体锚杆 (索 )加固的数值流形方法模型 ,该模型反映了裂隙岩体锚固的力学机理 ,可以仿真各种加固锚杆 (索 )的复杂几何布置及几何参数。数值算例表明 :计算模型能较好地反映锚杆 (索 )加固岩体的变形行为 ,具有广泛的工程实用前景。     

加锚层状岩体的本构模型

将加锚层状岩体作为等效连续介质推导了加锚层状岩体的本构方程 ,新的本构模型考虑了节理的剪胀扩容现象和锚杆作用 ,弥补了以往分析中的不足 ,该本构模型可对节理岩体中锚杆的加固作用做出合理的定量评价     

木结构钢板螺栓连接节点承载力计算分析及试验研究

为便于木结构工程设计,提出了钢板螺栓连接承载力计算式,并提出了受弯螺栓连接节点承载力上限和下限的计算方法。设计制作了钢填板螺栓连接节点试件,并进行了复合受力节点承载力试验研究。试验结果表明,各受力工况下试验节点的承载力平均值均介于所计算的承载力上、下限之间,且基本等于所计算的承载力上、下限的平均值。从而验证了所提出的钢板螺栓连接承载力计算式和连接节点承载力计算方法的正确性和适用性,为设计提供了可靠依据。     

Estimating the relation between surface roughness and mechanical properties of rock joints

Discontinuities in rock masses have an important influence on deformational behaviour of blocky rock systems. For a single rock joint, the roughness of its surface is of paramount importance to its mechanical and hydraulic properties, such as friction angle, shear strength, and dilatancy/aperture. Many methods have been used to characterize the surface roughness of rock joints, such as joint roughness coefficients (JRC), root mean square (RMS) value, structure function (SF) etc. However, most of these methods can only be used in the 2-D models. In this study, we carried out direct shear experiments on rock joints under both constant normal load (CNL) and constant normal stiffness (CNS) conditions, and measured the surfaces of rock joints before and after shearing, using a 3-D laser scanning profilometer system. By using a 3-D fractal evaluation method of roughness characterization, the projective covering method (PCM) and a direct shear apparatus of high accuracy, the relation between mechanical properties of rock joints under different boundary conditions and the change of their fractal dimensions in both 2-D and 3-D models have been examined, which gives a new approach to accurately evaluate the evolution of roughness of rock joint surfaces and its influence on the hydro-mechanical behaviours of rock joints.     

锚喷支护技术在滑坡治理中的应用

通过具体工程实例,详细介绍了锚喷支护技术在边坡滑坡地质灾害处理中的应用情况,监测结果说明锚喷支护技术作为煤矿煤泥堆放场地边坡的一种新的防护形式是可行的,从而推广锚喷支护技术的应用.     

An analytical solution of equivalent elastic modulus considering confining stress and its variables sensitivity analysis for fractured rock masses

The equivalent elastic modulus is a parameter for controlling the deformation behavior of fractured rock masses in the equivalent continuum approach. The confining stress, whose effect on the equivalent elastic modulus is of great importance, is the fundamental stress environment of natural rock masses. This paper employs an analytical approach to obtain the equivalent elastic modulus of fractured rock masses containing random discrete fractures (RDFs) or regular fracture sets (RFSs) while considering the confining stress. The proposed analytical solution considers not only the elastic properties of the intact rocks and fractures, but also the geometrical structure of the fractures and the confining stress. The performance of the analytical solution is verified by comparing it with the results of numerical tests obtained using the three-dimensional distinct element code (3DEC), leading to a reasonably good agreement. The analytical solution quantitatively demonstrates that the equivalent elastic modulus increases substantially with an increase in confining stress, i.e. it is characterized by stress-dependency. Further, a sensitivity analysis of the variables in the analytical solution is conducted using a global sensitivity analysis approach, i.e. the extended Fourier amplitude sensitivity test (EFAST). The variations in the sensitivity indices for different ranges and distribution types of the variables are investigated. The results provide an in-depth understanding of the influence of the variables on the equivalent elastic modulus from different perspectives.     

A review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering

The purpose of this review paper is to present the techniques, advances, problems and likely future developments in numerical modelling for rock mechanics. Such modelling is essential for studying the fundamental processes occurring in rocks and for rock engineering design. The review begins by explaining the special nature of rock masses and the consequential difficulties when attempting to model their inherent characteristics of discontinuousness, anisotropy, inhomogeneity and inelasticity. The rock engineering design backdrop to the review is also presented. The different types of numerical models are outlined in Section 2, together with a discussion on how to obtain the necessary parameters for the models. There is also discussion on the value that is obtained from the modelling, especially the enhanced understanding of those mechanisms initiated by engineering perturbations. In Section 3, the largest section, states-of-the-art and advances associated with the main methods are presented in detail. In many cases, for the model to adequately represent the rock reality, it is necessary to incorporate couplings between the thermal, hydraulic and mechanical processes. The physical processes and the equations characterizing the coupled behaviour are included in Section 4, with an illustrative example and discussion on the likely future development of coupled models. Finally, in Section 5, the advances and outstanding issues in the subject are listed and in Section 6 there are specific recommendations concerning quality control, enhancing confidence in the models, and the potential future developments.     

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.     

KT形RHS加强节点承载力试验研究与数值分析

结合呼和浩特火车东站站房钢结构工程,针对其高频度使用的矩管KT形RHS节点,从受力、工艺和经济性出发,综合阐述了节点局部构造加强方式的工艺力学性能差异。采用模型试验和数值计算结合的方法,分析弦杆外套和弦杆内加劲两种强化方式的静力力学行为差异,数值分析采用引入Low-Speed Dynamics Damping Factor的非线性N-R方法,考虑材料屈服与破裂失效,基于Adina的隐式算法模拟模型的显式破坏特征。对节点进行抽象简化处理,综合考虑工艺力学性能,设计3组不同构造的KT形RHS节点,进行1∶2缩尺比例静力加载试验。分析两种强化方式对其质量控制、承载能力、破坏形态的影响,并与无任何加强措施的构件结构进行对比,结果表明,套管强化方式能够明显提高节点的承载能力和刚度,工艺性较好,可结合实际工程条件,提供节点强化设计参考。