Bolt Grout Research Articles

A pioneer in-situ investigation on the bearing capacity and failure causes of real scale fully grouted rockbolts

The actual study presents a thorough investigation on the bearing capacity and failure causes of fully grouted rockbolts. For this purpose, 144 real scale in-situ rockbolt pull-out tests were conducted on Fan River hydropower plant’s power tunnels in Rreshen region, Albania. The pull-out tests were performed on full grouted rockbolts of two types: SN-Stor Norfors and PG-Post Grouted, two different bolt diameters: 22 and 25 mm, three bolt lengths: 3, 4 and 6 m and three inclination groups such as: horizontal, 0–30° and 30–90°. The results showed that, the SN bolts have lower bearing capacity and smaller displacement compared to PG bolts. On the other hand, the rockbolts’ bond strength and displacement decrease as the installation angle to the horizontal increases. This is due to the difficulties faced in filling the drilled hole with grout as the inclination opposes the gravitational direction. In their entirety the results indicate that grouting quality is a crucial factor on the bearing capacity of fully grouted rockbolts and the system failure is mainly related to the shear failure along the bolt–grout or grout–rock interfaces.

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.

Safety assessment of cable bolts subjected to tensile loads

Cable bolts are one of the most common tension anchorage systems for reinforcing the stability of rock/soil masses; however, the quantitative assessment of cable bolt safety via numerical simulations has not been deeply explored until now. In this technical study, a bolt tri-linear tensile model and a bolt–grout (B–G) interface tri-linear shear model were integrated into a cable bolt tri-linear model to reflect the large tensile deformation-broken failure of the bolt and the B–G interface slip-softening failure after cable bolt yielding. A dimensionless index was developed by defining the maximum of the failure approach index (FAI) for bolt tensile failure and the FAI for B–G interface shear failure based of this cable bolt model to quantitatively assess the cable bolt safety. The comparison between the laboratory and numerical test results and engineering case analyses showed that the cable bolt FAI combined with the cable bolt tri-linear model, which could quantify the cable bolt safety and determine the cable bolt failure position and sequence as an improved substitute for cable bolt yield state indicators in numerical simulations. The present study is expected to offer a more accurate numerical simulation technique than existing simulation techniques in the quantitative assessment of cable bolt safety, finally contributing to the quantitative evaluation-based support design.

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.

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.

Effect of Contributing Parameters on the Behaviour of a Bolted Rock Joint Subjected to Combined Pull-and-Shear Loading: A DEM Approach

The mechanical performance of fully grouted rock bolts is essential in the stability of underground excavations in jointed rock masses. This research implements a new cohesive contact model in distinct element codes (PFC2D) to investigate the fracturing response of rock-like and grout material, as well as the bolt–grout interface. The results are compared in detail with experimental observations. The proposed modelling approach, used in conjunction with the distinct element method (DEM), successfully predicted the behaviour of grout failure and the bolt–grout interface’s shear response. We then developed a novel numerical, stepwise pull-and-shear test (SPST) scheme to further analyse the mechanical behaviour of bolted rock joints subjected to simultaneous pull–shear loading. The cohesive DEM framework proposed in this paper was used to carry out the SPST scheme numerically. The mechanism involved in enhancing the shear strength of bolted rock joint was determined by monitoring the $$ \sigma_{\text{n}}^{\text{i}} $$ and its corresponding contact chain force network during the pull-out test. The influence of pretension stress, the rig angle of the bolt profile, and the constant normal stiffness (CNS) condition are assessed systematically. In particular, the pretension stress magnitude at which the synthetic rock bolting system exhibits the highest shear resistance is identified. The findings from this research highlight the sensitivity of bolted rock joints to the simultaneous pull-and-shear loading, boundary conditions, and bolt–grout interface configurations.

Determination of the bond–slip relationship of fully grouted rockbolts

The bond–slip relationship of fully grouted rockbolts with long encapsulation lengths is critical to the bolt axial performances. However, how to properly determine its profile still remains a challenge. It is proposed that the pullout tests of short encapsulated rockbolts could be used to estimate the bond–slip relationships of long rockbolts under the same conditions. This method is based on two assumptions: (1) The bond–slip relationship simply calculated from the load–displacement curve of a short rockbolt could represent its interfacial shear stress characteristics and (2) bolts with different embedment lengths have the same bond–slip relationship when subjected to the same conditions. Pullout tests were carried out on instrumented rockbolts with short embedment lengths to verify the first assumption, and pullout tests on rockbolts with various embedment lengths were numerically modeled to validate the second assumption. It was found that the shear stresses were not uniformly distributed along the bolt–grout interface; the load–displacement curve of a short rockbolt could still be used to derive the bond–slip relationship of a bolt. The bond–slip relationships computed from short grouted rockbolts tend to underestimate the interfacial shear bond stresses of longer bolts.

Pull-out behaviour of wood bolt fully grouted by PS-F slurry in rammed earth heritages

ABSTRACTThis article presents a laboratory and field investigation of pull-out resistance of wood bolts in rammed earth heritages. The laboratory testing involved axial tensile tests of four grout specimens prepared in specially designed moulds using PS-F (Potassium Silicate solution-Fly ash) slurry to investigate the performance of anchors. The field testing involved pull-out tests of three wood anchors installed in rammed earth heritages by gravity grouting. Experiments reveal that the failure mode is the pull-out of bolt from grout and average skin friction resistance of bolt–grout interface is approximately 0.34MPa. Load–displacement curves and elastic–residual displacement derived from cyclic loading indicate such anchor system has strong ductility with small elastic deformation and large residual deformation. Bond collapse firstly occurs at the loaded end and then propagates towards the full bonded length. Bond stress distribution is not uniform along the bonded length. Maximum bond stress distributes at the range from 0.3 m to 0.4 m. Emergence of compressive strain in the interface reveals such anchor system enjoys the advantages of both tensile and compressive anchor types, which significantly differs from results from other regular bolts. The research conclusion makes scientific senses to traditional material and craft in rammed earth heritages.

Improvement of rock bolt profiles using numerical method

Current developments in numerical modelling techniques are presented to investigate the load transfer between the steel bolt surface and grout. Underground measurements of steel bolt performance indicate that amongst many parameters bolt profile configuration play an important role in load transfer capacity between the bolt and grout. This study proposes a new numerical solution to predict displacement, shear stress and shear strain of a fully grouted rock bolt intersected by single rock joint. The main characteristics of the numerical solution, consider the bolt profile and joint movement under pull test condition. In this paper, five types of rock bolts are used with different profiles. The rock bolts are modelled by ANSYS software. Profile rock bolt T3 and T4 with load capacity 180 kN and 195 kN in the jointed rocks, were choices as the optimum profiles. The results showed that there is a promising agreement between numerical and experimental methods.

Numerical modelling of failure propagation in fully grouted rock bolts subjected to tensile load

Numerical modelling of fully grouted rock bolts loaded in tension is presented by implementing a non-linear bond–slip relationship of bolt–grout interface into a commercial finite difference rock mechanics code. The proposed model shows a close match with the experimental results and analytical predictions in terms of load–displacement relationship, rock bolt axial force distribution and interfacial shear stress distribution. A detailed debonding failure process is also presented, which includes five successive states: elastic state, elastic-softening state, elastic-softening-debonding state, softening-debonding state and debonding state.

Time-dependent system reliability of anchored rock slopes considering rock bolt corrosion effect

This paper aims to investigate the effect of rock bolt corrosion on time-dependent system reliability of anchored rock slopes. First, a corrosion degradation model for reinforcing steel bars in concrete is selected to model the uniform corrosion of rock bolts. Second, two typical failure modes of rock bolts due to corrosion and the resultant slope failure modes are identified. Subsequently, a Monte Carlo simulation-based reliability approach is proposed to perform system reliability analysis of anchored rock slopes. Finally, an example of an anchored rock slope is worked out to investigate the effect of rock bolt corrosion on the time-dependent reliability of the anchored rock slope. The results indicate that the probability of slope failure upon yield failure of rock bolts at the free length only increases slightly with time. On the contrary, the probability of slope failure upon bond failure at the bolt–grout interface increases dramatically with time. The system probability of failure of the anchored rock slope decreases with increasing thickness of bolt cover and increases with increasing water–cement ratio of the grout. During the design and construction of pre-stressed rock bolts, a certain thickness of bolt cover should be guaranteed and the water–cement ratio of the grout should be strictly controlled to enhance the long-term stabilization effect of rock bolts.

Analytical model for rock bolts reaching free end slip

This paper presents the difference of behaviour between fully grouted bolts with and without free end slip when loaded in tension. An analytical approach is proposed for fully encapsulated bolts when the free end of the bolt slips. This model is based on the existing bond–slip relationship of bolt–grout interface with no free end slip. The derived analytical solutions of load–slip relationship, slip distribution, the shear stress and strain distributions presented in this paper are all connected with free end slip. The analytical approach is validated by experimental results. Free end slip has a significant influence on rock bolt behaviour and should not be ignored.

Probabilistic modelling of the bond deterioration of fully-grouted rock bolts subject to spatiotemporally stochastic corrosion

Developing predictive models for quantitative assessment of the deterioration of rock bolts exposed to corrosive environments is essential for rational planning of maintenance activities for anchorage structures. This article presents a probability-based computational model for predicting the time-dependent deterioration of bond capacity of corroding rock bolts due to the attack of chlorides. The inherent stochastic nature involved in the corrosion and degradation process is identified and simulated. A method is developed based on fundamental Mohr–Coulomb theory to evaluate the effect of corrosion on the bond strength due to a failure mode involving splitting of grout cover. Parameters affecting both strength and stress state at the bolt–grout interface are quantitatively related to the degree of corrosion. Integrating the realised corrosion stochastic process and the developed bond loss evaluation method, a computational algorithm based on Monte Carlo simulation is presented for evaluating the bond deterioration in a probabilistic framework. The presented model is used in an illustrative example to show its capability to trace the evolution, throughout the entire design life, of several representative performance indicators of rock bolts as a structure unit or of the interested location within the anchor length.

An analytical model of fully grouted rock bolts subjected to tensile load

An analytical model for fully encapsulated rock bolts subjected to tensile load in pull-out tests is presented. This model is based on the bond–slip relationship describing the mechanical interaction at the bolt–grout interface. The model takes into account the residual shear stress in addition to the complete decoupling mechanisms. Formulations are also derived for the load–displacement curve, shear stress distribution at the bolt–joint interface and axial load distribution in the bolt. The model was validated with experimental results from both the laboratory experiments and in situ studies.

Experimental and numerical investigations of stresses in a fully grouted rock bolts

Rock bolts are widely used for rock reinforcement in hard-rock mining and civil engineering since a long time. However the use of fully grouted rock bolts and cable bolts is limited in coal mines. In order to improve performance of the rock bolts as a supplementary roof support system for any type of roof condition in coal measured formations, it is necessary to have a good understanding of the behavior of the bolt–grout and grout–rock interactions as well as the mechanism of load transfer in rock bolts. As the performance of grouted bolts depends on bond strength, extensive laboratory pullout as well as pushout tests were conducted in the present investigations with the variations in the bolt diameters, length and cement–water mixing ratios of grout. The load–displacement curves were developed and were verified with the numerical results obtained from finite element analysis using ALGOR software. Numerical models were validated for pushout tests and a detailed analysis was carried out to know the displacement, stress, strain distribution along the bolt.

Effect of bar shape on the pull-out capacity of fully-grouted rockbolts

This paper presents experimental results obtained from the direct pull-out test using different types of rockbolts having different shape of lugs. These are smooth surface bars, ribbed bars, single conical lugged bars, double conical lugged bars and triple conical lugged bars. It is found that the failure mechanism of the conical lugged rock bolt is different from that of conventional rockbolts, in which the adhesion or the shear strength at the bolt–grout interface provides the load bearing capacity. From the results of experimental study, the use of the conical lugged rock bolts is suggested, because it provides a greater anchorage strength due to wedging effect that is a combination of the shear and compressive strength of the grouting material.

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.