Bolt-grout Interface Research Articles

Exploring the axial performance of protective sheathed rock bolts through large-scale testing

Understanding the axial load transfer mechanism of rock bolts under diverse conditions is essential for optimizing reinforcement in rock structures, advancing our comprehension of rock support, and facilitating the design of robust engineering solutions. This paper reports the outcomes of an extensive experimental investigation, focusing on the axial behavior of protective sheathed rock bolts employed in corrosive environments, assessed through pullout tests. Three distinct testing setups were designed to evaluate comprehensively the performance of these rock bolts in various scenarios. The results indicated that the failure characteristics and axial behaviors of sheathed rock bolts differ significantly from conventional counterparts. The findings revealed two primary failure modes in sheathed rock bolts: bolt rupture and slip at the grout-sheath interface, based on the testing arrangement and encapsulation length. The lack of adhesion and interlocking at the grout-sheath interface prevents shear stress at the bolt-grout interface from reaching its maximum potential strength, resulting in grout damage manifesting as circumferential cracks. This, in turn, initiates crack formation, reducing the system’s bond strength. Additionally, it causes slip at the grout-sheath interface to occur at lower pullout loads. It can be inferred that the inner surface of the plastic sheath lacks the necessary structural integrity to withstand high loads, significantly impacting bond stress distribution and failure modes. The results demonstrate that the protective sheath remains intact up to an axial displacement of 28 mm, irrespective of the testing configuration. Additionally, it was observed that the maximum bond stress at the bolt-grout interface falls within the range of 6–8.7 MPa, which is below the shear strength of the grout. Consequently, achieving failure at the bolt-grout interface is not feasible.

Study on Mechanical Characteristics and Durability of Prestressed Anchorage Structure of Rock and Soil under Fatigue-Corrosion Coupling Action

This paper presents a comprehensive approach encompassing indoor experiments, theoretical analysis, and numerical simulations to investigate the durability of prestressed anchorage structures subjected to fatigue loads and corrosion. The study addresses the critical issue of gradual aging and damage caused by cumulative loads and corrosion, which ultimately leads to a decrement in structural durability. Through a rigorous analysis of the effects of fatigue load and corrosion on the performance of steel bars, numerical simulations were conducted to elucidate the failure mechanisms and variation patterns within the internal anchoring section. After subjecting steel bars to fatigue and corrosion tests for a defined duration, they were systematically categorized and exposed to varying fatigue tensile cycles in diverse acidic and alkaline environments. Employing the PFC2D program, a numerical model of the prestressed anchorage structure under the coupled effects of fatigue load, corrosion, and fatigue load was developed. This model allowed for a comparative analysis of the evolution of shear stress, axial stress, and displacement fields at the bolt-grout interface under two distinct conditions. The findings reveal the microscopic mechanisms underlying bond degradation at the bolt-grout interface under the synergistic impact of fatigue load and corrosion. The proposed methodology and experimental results demonstrate that geotechnical anchoring technology can effectively reinforce up to 70% of geotechnical structures, significantly reducing soil loss by approximately 80%. This research provides valuable insights into the durability of prestressed anchorage structures, paving the way for future improvements and optimizations.

A novel constitutive law of confined grouted rock bolt under pull-out load based on a fully nonlinear bond-slip model

Rock bolts are widely applied in deep underground excavations for rock reinforcement. Here, a constitutive model that quantifies the pull-out load-displacement relationship of the grouted rock bolt has been developed though theoretically analyzing the load transfer behaviour along the bolt-grout interface. An improved nonlinear continuously yielding criteria was employed as a bond-slip model where the role of confining pressure playing in slip was accounted. The performance of the proposed constitutive model was demonstrated by comparing with three sets of laboratory pull-out tests on the fully grouted rock bolts under different confining pressure levels (including a short-encapsulated pull-out test designed in our laboratory). The input parameters in the model were determinable within the laboratory environment. The analytical simulation results of pull-out load-displacement performance, the axial stress in the rock bolt and shear stress along the bolt-grout interface agreed well with the laboratory data. We also found that the confining pressure significantly affects the critical embedment length and the higher the confining pressure, the shorter the critical embedment length. The findings enrich the understanding on the mechanical properties of grouted rock bolts and hence aid the rock engineers in rock reinforcement design in underground excavations.

Shear Mechanical Properties of Bolt-Grout Interface under Different Bolt Surface Profiles

The shear behavior of the Bolt-Grout interface has a significant effect on the stability of a bolting system. In this paper, a series of shear tests were conducted on Bolt-Grout interfaces, and the effects of rib spacing, rib angles, and normal stress on the shear characteristics and failure modes of the Bolt-Grout interface were investigated. The results showed that the shear strength varied nonlinearly with an increase in rib spacing and angle, and also that it increased linearly with an increase in normal stress. With smaller rib spacings, the effect of rib spacing on peak shear strength was more apparent. The failure modes of the interface can be categorized as shear-slip failure, shear-break failure, and composite failure. The proportion of shear-slip failure and shear-break failure mainly depends on the rib spacing, rib face angle and normal stress.

Mechanical behaviour of fiber-reinforced grout in rock bolt reinforcement

Grouted rock bolts subject to axial loading in the field exhibit various failure modes, among which the most predominant one is the bolt-grout interface failure. Thus, mechanical characterization of the grout is essential for understanding its performance in ground support. To date, few studies have been conducted to characterize the mechanical behaviour of fiber-reinforced grout (FRG) in rock bolt reinforcement. Here we experimentally studied the mechanical behaviour of FRG under uniaxial compression, indirect tension, and direct shear loading conditions. We also conducted a series of pullout tests of rebar bolt encapsulated with different grouts including conventional cementitious grout and FRG. FRG was developed using 15% silica fume (SF) replacement of cement (by weight) and steel fiber to achieve high-strength and crack-resistance to overcome drawbacks of the conventional grout. Two types of steel fibers including straight and wavy steel fibers were further added to enhance the grout quality. The effect of fiber shape and fiber volume proportion on the grout mechanical properties were examined. Our experimental results showed that the addition of SF and steel fiber by 1.5% fiber volume proportion could lead to the highest compressive, tensile, and shear strengths of the grout. The minimum volume of fiber that could improve the mechanical properties of grout was found at 0.5%. The scanning electron microscopy (SEM) analysis demonstrated that steel fibers act as an excellent bridge to prevent the cracks from propagating at the interfacial region and hence to aid in maintaining the integrity of the cementitious grout. Our laboratory pullout tests further confirmed that FRG could prevent the cylindrical grout annulus from radial crack and hence improve the rebar's load carrying capacity. Therefore, FRG has a potential to be utilized in civil and mining applications where high-strength and crack-resistance support is required.

A numerical model for fully grouted rock bolts considering the nonlinearities of the bolt and the bolt-grout interface

Fully grouted rock bolts are widely used in underground engineering, both the bolt and bolt-grout interface will exhibit apparent nonlinear behaviors when subjected to excessive load. Few of the existing studies simultaneously consider the nonlinear deformation characteristics both for the bolt itself and the bolt-grout interface. Therefore, according to the local deformation theory and finite difference method, a nonlinear numerical model for fully grouted rock bolts was developed. This numerical model can simulate the bolt’s yielding and hardening behaviors as well as the bonding, softening, and decoupling behaviors of the bolt-grout interface, and its rationality was validated by comparisons between its predicted results and the experimental data and analytical solutions from published papers. Finally, this numerical model was used to simulate fully grouted rock bolts with or without faceplates installed in the field, and their mechanical performances and failure modes were investigated. Some conclusions were concluded: 1) The main failure mode of the rock bolt without a faceplate is the shear damage near the excavation surface, while the bolt pull-off adjacent to the excavation surface is its dominant failure mode after a faceplate is installed. 2) Ignoring the nonlinearities of the bolt and the bolt-grout interface overestimates its bearing capacity.

Numerical simulation of fully grouted rock bolts with or without faceplates based on the tri-linear bond-slip model

A difference scheme of the governing equation of the bolt-grout interface of the fully grouted rock bolt was derived based on the tri-linear bond-slip model, local equilibrium condition, and finite difference method. The corresponding solution formats of the fully grouted rock bolts with or without faceplates were proposed according to their different boundary conditions. Consequently, a numerical model of the fully grouted rock bolts with or without faceplates was established. This model is able to simulate the bonding, softening, and decoupling characteristics of the bolt-grout interface of the fully grouted rock bolts with or without faceplates subjected to the pullout load or rock deformation. The prediction results of this model agree well with the pullout test results, the calculation results of related scholars, and the analytical solutions, which proves its correctness. Then, the formulas of the support reaction force of the rock bolts with or without faceplates were derived according to their support mechanisms, and the support reaction force was applied during the iteration to simulate the bolt-rock interaction. This method is convenient for engineering applications owing to that it does not need to add additional degrees of freedom for rock bolts. The abovementioned model and method were incorporated into a self-developed FEM program and the effect of the initial deformation of the surrounding rock before bolt installation was considered, resulting in a numerical simulation program for fully grouted rock bolts with or without faceplates in underground caverns. Finally, this program was applied in a circular tunnel and the influences of the faceplate, bolt installation timing, bolt length, and bolt radius on the support performance of rock bolts were investigated. The following conclusions were obtained. 1) The faceplate can greatly improve the support effect. 2) The main failure mode of the rock bolts without faceplates is the shear damage at the bolt-grout interface, while the bolt pull-off is the dominant failure mode of the rock bolts with faceplates. 3) There is a critical value of bolt length. Increasing the bolt length within the critical value can significantly enhance the support effect, whereas increasing the bolt length beyond the critical value has little effect on the support effect. 4) Increasing the bolt radius can effectively enhance the support effect.

An analytical analysis for the mechanical performance of fully-grouted rockbolts based on the exponential softening model

This paper presented a novel bond-slip model to better reveal the mechanical behavior of the bolt-grout interface for fully-grouted rockbolts under tensile loads by considering the non-linear response in the softening stage. The exponential decay function is adopted for describing the non-linear response in the softening stage. Based on the improved bond-slip model, the corresponding analytical solutions for the interfacial shear stress and the axial force of the bolt under different loading stages are solved. Then, the validity of this proposed model was verified by comparing with the experimental results. The results show that compared with the linear softening model, the proposed model is more suitable for predicting the mechanical performance of fully-grouted rockbolts. Finally, a series of parametric studies are conducted to explore the effect of model parameters on the mechanical properties of fully-grouted rockbolts. The results indicate that compared with the anchor length, the bolt diameter and the bond strength of the bolt-grout interface have a significance influence on the ultimate load of bolt, especially for the elastic and softening stage. Moreover, it can be found that using the linear softening model maybe overestimates the supporting performance of grouted bolt, resulting in an unsafe design for bolt.

Experimental investigation on the shear behavior of the bolt-grout interface under CNL and CNS conditions considering realistic bolt profiles

Experimental investigation on the shear behavior of the bolt-grout interface under CNL and CNS conditions considering realistic bolt profiles

Experimental Study on Anchorage Performance of Resin Grout with Steel Segment

With the advantages of large anchoring force and fast anchoring speed, resin cartridge has become the main anchoring means of geotechnical engineering and underground space engineering support. Based on the theoretical analysis, it is clear that adding aggregate can improve the mechanical properties of grout and the bolt-grout interface stress state; the mechanical properties of aggregate are positively correlated with its improvement effect on anchorage performance. By using the numerical simulation method, it is concluded that the addition of steel segments into the resin grout can improve the stiffness of the anchorage system and enhance the energy absorption and antifailure ability of the anchorage system. Relying on the self-developed anchorage mixing device, the effects of steel segment diameter and addition amount on the anchoring force were studied experimentally, and the optimal addition amount of different types of steel segment to improve the maximum anchoring force was determined.

A quantitative analysis of the effect of end plate of fully-grouted bolts on the global stability of tunnel

Among the analyses of fully-grouted rock bolt in the rock mass, the effect of the end plate on the global stability of the tunnel has not been comprehensively reported. In this paper, a simplified numerical procedure for the interaction between the fully-grouted bolt and rock mass with consideration of the end plate is proposed. The rock-bolt interaction is considered at the initial and final states. At the initial state, immediately after the bolt is installed, a finite difference solution of the stresses and strains in the rock mass is proposed. At the final state, when the fictitious support pressure is negligible, another finite difference solution of the stresses along bolt as well as displacements in the rock mass is presented. To obtain the final results, a simplified iterative process is performed by repeating the finite difference solution at the final stage. The proposed procedure is verified by comparing with the commercially available finite difference software and field results. In the parametric studies, the effects of the end plate for the geological conditions with variation in rock mass quality, in-situ stress, shearing stiffness at the bolt-grout interface, and end plate stiffness are simultaneously investigated. The results indicate that, under the squeezing conditions, the failure mode of the bolt is influenced by the end plate stiffness; an optimum design for the shearing stiffness and end plate stiffness can be implemented in order to achieve an excellent supporting performance of the rock bolt.

Numerical simulation of pull-out test on cable bolts

Due to presence of external loads, the bolt providing stability to the structure which on failure may create a hazardous situation in case of mining and construction activities. The failure may be due to the failure of bolt itself or the failure of grout (in case of grouted bolt) along the bolt grout interface or the grout rock interface. It is thus necessary to study the effect of variation of a number of factors on the pull-out resistance and hence the failure of bolt. These factors comprise of the kind of bolt used, kind of grout material used, dimensions of bolt used etc. The objective of this research is to simulate the pull-out behaviour of cable bolt using 3DEC software and study the influence of various factors for the cases of grouted and non-grouted bolt. These factors are diameter of the bolt, length of the bolt, rate of extraction of the bolt, confining pressure, thickness of grout material and angle of friction of the grout material.

Laboratory investigation into effect of bolt profiles on shear behaviors of bolt-grout interface under constant normal stiffness (CNS) conditions

Abstract Rock bolts have been widely used for stabilizing rock mass in geotechnical engineering. It is acknowledged that the bolt profiles have a sound influence on the support effect of the rock bolting system. Previous studies have proposed some optimal rib parameters (e.g. rib spacing); unfortunately, the interface shear behaviors are generally ignored. Therefore, determination of radial stress and radial displacement on the bolt-grout interface using traditional pull-out tests is not possible. The load-bearing capacity and deformation capacity vary as bolt profiles differ, suggesting that the support effect of the bolting system can be enhanced by optimizing bolt profiles. The aim of this study is to investigate the effects of bolt profiles (with/without ribs, rib spacing, and rib height) on the shear behaviors between the rock bolt and grout material using direct shear tests. Thereby, systematic interfacial shear tests with different bolt profiles were performed under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. The results suggested that rib spacing has a more marked influence on the interface shear behavior than rib height does, in particular at the post-yield stage. The results could facilitate our understanding of bolt-grout interface shear behavior under CNS conditions, and optimize selection of rock bolts under in situ rock conditions.

Shear Effects on the Anchorage Interfaces and Seismic Responses of a Rock Slope Containing a Weak Layer under Seismic Action

The shear effects on the anchorage interfaces under seismic action is a key problem requiring urgent investigation in the field of rock and soil anchorages. In this paper, the model of rock slope with a weak layer was constructed by pouring, and the large-scale shaking table model test was completed. The shear strain on the anchorage interfaces and the acceleration of the slope were collected using built measurement systems. The shear effects on the two anchorage interfaces (a bolt-grout interface and a grout-rock interface) and seismic responses of the slope under seismic action were investigated. The distribution laws of the shear stress on the two anchorage interfaces along the axial direction of the bolt under seismic action were gained. The variations of the peak acceleration amplification coefficient on the slope surface, the magnitude, and the growth rate of peak shear stress on the anchorage interfaces under seismic action with different excitation directions and intensities were obtained. Furthermore, the positive relationship between the shear effect on the anchorage interfaces and the seismic response of slope was revealed. This study provides support for theoretical research, numerical simulation analysis, and aseismic design of rock and soil anchorages under dynamic conditions.

DEM investigation of rock/bolt mechanical behaviour in pull-out tests

Rock bolt anchorage performance is crucial for tunnel support safety. We investigate the mechanical behaviour of reinforced rock and the bolts that reinforce it from the micro-scale to the macro-scale. Bolt pull-out tests were performed on soft rock using the distinct element method, in which a new contact model that considers bond size, is employed to constrain the main rock mechanical behaviour. The minimum sample width and height values for which the boundary effect can be neglected are first proposed through numerous tests on the influence of sample size on peak load and bond breakage. The influence of sample width is substantially greater than that of sample height. We then select an appropriate sample size to study the influence of bolt embedment length and confining pressure on the mechanical behaviours of the rock and bolt. The results show that increased rock bolt embedment length and confining pressure can increase the peak load; however, the bolt length effect is limited when exceeding the critical anchorage length. In cases without confining pressure, bond breakage occurs in the rock around the grout-rock interface and the breakage zone is rectangular, whereas in cases under confining pressure, the breakage zone presents an inverted cone shape. We use our results to discuss the influence of bond strength at the bolt–grout interface on the peak load and failure mode. The failure mode changes gradually from complex failure to single failure along the bolt–grout interface with decreasing interfacial bond strength.

Analytically Determining Bond Shear Strength of Fully Grouted Rock Bolt Based on Pullout Test Results

Usually, in a fully grouted rock bolt pullout test the load-displacement curve of the rock bolt head is recorded. This paper presents an analytical method to use this curve for determining the bond (bolt-grout and grout-rock interface) shear strength parameters. For this purpose, the fully grouted rock bolt interaction with grout and surrounding rock in the pullout test is investigated and the load-displacement curve of the bolt head (beginning of the bonded section) is obtained analytically. For modeling the bolt-grout interface behavior a distribution of the shear stress along the fully grouted rock bolt by consideration of bolt shank failure is used. In this regard, different stages including complete bonding, partial decoupling, decoupling with the residual shear strength and complete decoupling are considered. With increasing the applied load, two possible cases involving the rock bolt complete pullout and bolt shank yielding are taken into account. Based on the presented analytical method, the obtained bolt head load-displacement curve can be compared with the one recorded in the pullout test. With this, the relevance of selected shear strength parameters compared to real parameters can be assessed. A flowchart for determining the bolt bond shear strength parameters is presented using the trial and error method (coded in Matlab). The proposed solution is used to determine two experimental pullout shear strength parameters. The results show good agreement between predicted and calculated load-displacement curves.

Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface

Abstract Shearing behavior and failure mechanism of bolt-grout interface are of great significance for load transfer capacity and design of rock bolting system. In this paper, direct shear tests on bolt-grout interfaces under constant normal load (CNL) conditions were conducted to investigate the effects of bolt profile (i.e. rib spacing and rib height) and grout mixture on the bolt-grout interface in terms of mechanical behaviors and failure modes. Test results showed that the peak shear strength and the deformation capacity of the bolt-grout interface are highly dependent on the bolt profile and grout mixture, suggesting that bolt performances can be optimized, which were unfortunately ignored in the previous studies. A new interface failure mode, i.e. ‘sheared-crush’ mode, was proposed, which was characterized by progressive crush failure of the grout asperities between steel ribs during shearing. It was shown that the interface failure mode mainly depends on the normal stress level and rib spacing, compared with the rib height and grout mixture for the range of tested parameters in this study.

Simulation Research on the Load Transfer Mechanism of Anchoring System in Soft and Hard Composite Rock Strata under Tensile Loading Conditions

Soft and hard composite rock strata are frequently encountered in transportation, geotechnical, and underground engineering. However, most of the current support is designed for homogeneous rock masses, which ignores the different anchoring effect in soft and hard composite rock strata. A numerical study is presented in this paper on the pull‐out behavior of fully grouted rock bolts in soft and hard composite rock strata. The nonlinear bond‐slip relationship of bolt‐grout interface that is anchored in soft rock and hard rock is obtained from laboratory test, respectively. Then, the nonlinear bond‐slip relationship is put into the numerical model. The numerical result shows a close match with the experiment tests and the proposed model. Lithological sequence, layer thickness ratio, and layer numbers are taken into consideration in numerical simulation models. Under the same layer number, the shallower‐soft and deeper‐hard composite rock strata (SHCRS) have a higher bearing capacity and deformation resistance than the shallower‐hard and deeper‐soft composite rock strata (HSCRS). As the soft‐to‐hard thickness ratio in SHCRS increases, the initial stiffness of the load‐displacement curve and peak load decreases continuously. The load‐displacement curve shows the same initial stiffness for different hard to soft thickness ratios in HSCRS. As the hard to soft thickness ratio increases, the load peak and the displacement at the peak load increase. Therefore, the closer the hard rock is to the loading end, the greater the initial stiffness of the load‐displacement curve is. The greater the hard rock thickness, the larger the peak load. Under the same anchor length, the peak load and the displacement at the peak load decrease with the increase of layer numbers, but the reduction magnitude decreases. This paper leads to a better understanding of the load transfer mechanism for the anchoring system in soft and hard composite strata and provides a reference for scientific support design and evaluation method.

Laboratory Investigation on Shear Behaviors of Bolt–Grout Interface Subjected to Constant Normal Stiffness

Shear behavior of the bolt–grout interface basically depends on site-specific boundary conditions, i.e., initial normal stress and normal stiffness of the borehole wall. Few studies have been conducted to investigate the shear behaviors between the bolt and grout material under different boundary conditions. To better understand the effect of boundary conditions on the shear behaviors of the bolt–grout interface, simplified two-dimensional (2D) bolt–grout interface specimens are prepared and tested using direct shear tests at various initial normal stresses and normal stiffnesses in this context. The testing results showed that both initial normal stress and normal stiffness significantly influence the bolt–grout interface shearing behaviors. Increasing the normal stiffness or initial normal stress would increase the peak and residual shear strength. However, the degree of the brittleness after the peak, normal displacement, and peak friction coefficient was reduced. It is noted that the peak and residual shear strength points under the constant normal stiffness conditions located near the peak and residual strength envelopes for the constant normal load tests, respectively. Besides, the shear failure process of the bolt–grout interface was captured by PAC acoustic emission (AE) monitoring and digital camera technology, a good correlation between the evolution of AE parameters and the shear stress curves was obtained.

An analytical model for estimating the force and displacement of fully grouted rock bolts

Previously proposed models for estimating the force and displacement of fully grouted rock bolts rarely describe the variations in shear stress and axial force in response to a changing load. In this work, an analytical model considering this variation pattern is developed. The model makes two assumptions: (I) failure occurs at the bolt-grout interface, and (II) the shear stiffness is constant along this interface. First, according to the variation in interfacial stress, the complete rock bolt deformation process is divided into three stages: (1) the elastic stage, (2) the softening stage, and (3) the debonding stage. Equations for axial force and displacement during each stage are derived under the two assumptions. Using these equations, the developed model accommodates the three stages. The model estimates of force and displacement show good agreement with pull-out test observations, indicating that the model effectively describes the complete rock bolt deformation process during a pull-out test. However, one limitation of the proposed model is that, for simplification, as in most related models, the radial stress along the bolt is not considered.