Cable Bolts Research Articles

Mining-Induced Deformation in the Malmberget Mine

The Norra Alliansen orebody of the Malmberget sublevel caving mine consists of iron ore interspersed with biotite schist and granitic inclusions. The schist is squeezed between the ore and the host rock and in direct contact with the ore along the majority of the length of the footwall. The schist exhibits high deformation when exposed to stress. SMART cable bolt roof deformation measurements are re-analyzed to draw conclusions regarding the patterns of deformation in the mine. Each bolt’s head is placed at the origin of a spherical coordinate system and the radius and inclination angle between the bolt and every production blast occurring during the bolt’s recorded lifetime are calculated. The deformation experienced by each instrument is investigated by comparing the long-term recorded movements with the developed geometric variables. Patterns of deformation magnitude and rate are found with respect to production-blast distance and inclination angle, instrument location, rock quality designation, and likely mining-induced stresses. Results show that deformation magnitude tends to be higher when driven by production blasting occurring on the production level above the instrumentation when accounting for the effects of distance, but average deformation magnitude for very-near production blasts tends to be higher than that for production blasts occurring directly above the instrument. Correlations also exist between the measured RQD, estimated rockmass parameters, and the measured deformation. Empirical evidence allows the identification of six scenarios which account for 91% of the recorded high-deformation-rate events. These scenarios help determine which production activities are most likely to cause high deformation rates.

Load Transfer Characteristics of a Cable Bolt in DEPT

Cable bolts help surrounding rock masses of underground excavations to form internal rock structure and thus self‐stabilize through effective load transfer between rock masses and cable bolts. The load transfer characteristics of cable bolt play the most significant role in cable selection for support design. There are many factors potentially affecting the load transfer characteristics of cable bolt via strengthening or weakening load contributors such as cable bonding, mechanical interlocking, frictional resistance at cable‐grout interface and grout‐rock interface, and shearing of grout anchorage. Double embedment pull test (DEPT) is an effective, reliable, and repeatable test method of investigating the load transfer characteristics of cable bolt and thus was selected in this study. With DEPT, the influence of factors such as cable rotation, grout age, grout anchorage annulus thickness, cable wire profile, and failure mode on the load transfer characteristics of cable bolt were investigated in terms of tensile strength and stiffness in different loading stages. Conclusions drawn from the study can be used to improve cable installation quality and to select appropriate cable bolting technique.

Ground response to high horizontal stresses during longwall retreat and its implications for longwall headgate support

Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site. The roof behavior in the headgate near the face was carefully observed during longwall retreat. Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.

Experimental and analytical study on the mechanical behaviour of cable bolts subjected to axial loading and constant normal stiffness

This study aims to experimentally and analytically investigate the behaviour of two different cable bolts, including Sumo and TG using an advanced pull-out testing facility capable of testing the cable bolts under axial loading and constant normal stiffness. The quantitative contribution of a range of parameters (e.g. uniaxial compressive strengths of grout and confining medium and the borehole diameter) on the overall behaviour of Sumo and TG cable bolts was examined based on Analysis of Variance (ANOVA). It was found that the confining medium strength was the most effective factor to the peak load and the initial stiffness of Sumo cable bolt. For the TG cable bolt, the grout strength was determined as the most significant parameter. Overall, a rise in the strength of grout and/or confining medium resulted in an increase in the peak load of both Sumo and TG cable bolts.In addition, a new analytical model was developed to simulate the load-displacement behaviour of modified cable bolts. The model can predict the linear elastic behaviour of cable bolt as well as the non-linear response. The non-linear behaviour was captured through an algorithm in which, an increase in the axial displacement led to an incremental increase in the axial load where the cable bolt was subjected to a constant normal stiffness. The model was then validated using the experimental data resulted from the pull-out tests on the Sumo cable bolt. An acceptable agreement between the experimental data and the model simulations confirmed the capability of the developed analytical model for the behaviour prediction of modified cable bolts under constant normal stiffness.

Gateway stability analysis by global-local modeling approach

Longwall mining is the most widely used mining method in underground coal mining. The stability of gateways is crucial for longwall mining as they provide many essential services such as transportation of excavated coal, ventilation of longwall face. High in situ stresses, mining induced stresses due to advancing longwall face, and poor rock mass quality leads to large deformation jeopardizing the stability of the gateway in Guobei coal mine. In this paper, the global-local modeling approach is used for analysing the stability of the gateway in the mine. In this approach, a mine scale 3D global model was constructed to extract the stresses acting on the gateway. The extracted stresses from global model were implemented into 2D local model of the gateway for detailed analysis. The used approach was verified by the field measurements taken from the modeled gateway. In an attempt to deal with large mine induced displacements, a support system composed of rock bolt, cable bolt and shotcrete was proposed. The verified model was used to evaluate the performance of proposed support system. It was concluded that the proposed support system can significantly reduce the displacements. The verified model facilitates the performance evolution of other alternative support systems considered by the mine management. The presented approach can be applicable for other deep underground coal mines, experiencing large gateway convergence.

Numerical investigation of the shear behaviour of a cable bolt in single shear test

Cable shear behavior in reinforcing rock discontinuities involves both the axial shear behaviour along the cable-grout interface and the local lateral shear behaviour of the cable deflecting section. This is a complex condition which is difficult to be solved in a theoretical manner. Pile structural elements in Flac3D were used here to investigate this shear behavior with special attention to the influence of cable anchorage length. The employed numerical cable model was firstly calibrated using the laboratory pullout test and a single shear test on a plain Superstrand cable bolt. Then the calibrated model was used to investigate the anchorage length effect on the cable lateral shear behavior. As expected, numerical results show that two demarcation anchorage lengths existed in cable shear tests. In the case of plain Superstrand cable bolt subjected to lateral shear loading at an angle of 90°, the two demarcation anchorage lengths were approximately 2.09 m and 0.61 m.

EFFECTS ON THE FINAL INTENSITY OF INPUT FORCES IN LONGBOLTS INSTALLED AT THE MINING OPERATION 2 AREA, OKD, INC.

In the deep coal mines of OKD, Inc., both bolts and long bolts of different designs are used for the rock massif and steel arch support reinforcement. Continuous measurement of forces in 6 strand bolts and 1 cable bolt (long bolts, generally) was carried out during the trial operation of the modified Room and pillar mining method at Mining operation 2, site North, OKD, Inc. Hydraulic dynamometers were installed on these long bolts and a monitoring of forces took place throughout the life-time period of the mining panel No. V. From this measurement, a knowledge of their different load behavior with respect to the input stress parameters was obtained. The input intensity of the force applied to the bolting elements is burdened by losses of various kinds. The subject of the article is a description and analysis of the intensity of the initial stressing force applied to individual long bolts (with a threaded clamping bush or wedge barrel) and quantization of short-term stress losses with a description and analysis of these.

Numerical simulation of the pull-out behaviour of fully grouted cable bolts

Fully grouted cable bolts have been used in the mining industry for many decades. To understand their mechanical performance, numerous laboratory pull-out tests have been conducted on cable bolts. The most commonly used method is to install a cable bolt in a passive confining media, such as a metal tube and pull the grouted cable bolt from that confining media. However, a common problem occurring in those tests is that the confinement generated by the confining medium is unknown. This paper provides a numerical approach to estimate the maximum confinement in pull-out tests of cable bolts. The commercial numerical code of FLAC2D was used. The structural elements of a rockbolt were used. The pull-out behaviour of a plain cable bolt and a modified cable bolt were simulated. Results of the numerical approach were compared and validated against laboratory pull-out test results. The model showed that compared with plain cable bolts, modified cable bolts could generate more confinement within the rock sample or confinement medium. The effect was more apparent in a weak strength confining medium. Based on the findings of this study, it is recommended to use modified cable bolts in high in situ stress conditions, especially when the surrounding rock mass are weak.

The application of distributed optical strain sensing to measure the strain distribution of ground support members

A distributed optical strain-sensing technique is presented as a solution for measuring the strain distribution along ground support members used in tunnelling and mining works. The technique employs a Rayleigh optical frequency domain reflectometry technology, which measures strain at a spatial resolution of 0.65 mm along the length of a standard optical fiber. A rationale for selecting this technology as a potential monitoring technique for ground support elements over alternative commercially available technologies is discussed. The development of a technique to couple optical fiber sensors with rock bolt, umbrella arch, and cable bolt support members is also demonstrated. A robust laboratory investigation of such optically instrumented support members demonstrated the capability of the technique to capture the expected in situ support behaviour in the form of coaxial, lateral, and shear loading arrangements as would be anticipated in the field. Moreover, the micro-scale data obtained by this optical sensing technique are shown to provide unprecedented insight into the local/micro-scale geomechanistic complexities associated with the bearing capacity of ground support members, especially when compared with data obtained by discrete strain-sensing technologies.

Interactions of stress corrosion cracks in cold drawn pearlitic steel wires: An X-ray micro-computed tomography study

X-ray micro-computed tomography (micro-CT) was used for analysis of stress corrosion cracking (SCC) of cold drawn pearlitic steel wires from a cable bolt from an underground coal mine. The results were compared to those obtained from magnetic particle inspection (MPI) of the wire surface and optical microscopy (OM) and scanning electron microscopy (SEM) examination of sections through the wires. The high-resolution two-dimensional (2D) OM and SEM images were registered into the micro-CT image to allow direct comparison with the corresponding slice from the three-dimensional (3D) micro-CT image. Subsequently, the micro-CT image was processed to segment the cracks in the steel body and the attributes of each crack, i.e. the surface area, surface exposure, and maximum radial and longitudinal penetration depths, were calculated and plotted to determine their intercorrelation. It was found that MPI and micro-CT had a similar ability in revealing surface cracks. The 2D techniques provided high-resolution information but unlike micro-CT they were unable to map the complex 3D structures of the cracks and their interactions.

Numerical and analytical simulation of the structural behaviour of fully grouted cable bolts under impulsive loading

Designing reliable yielding support system to mitigate the effect of the kinetic energy in burst-prone conditions in mining and tunneling excavations is one of the challenges for geotechnical engineers. A combination of the support elements can be used to increase rock strength and minimise the displacement of unstable rock mass. It is important to understand how the support system works to ensure the stability of underground excavations. Cable bolts have been commonly used as an effective underground support system and an element of reinforcement to improve rock stability. Cable bolts are usually considered to be subjected to static loads under relatively low stress environments, however, in burst-prone conditions, they might be subjected to dynamic loads. Cable bolts as well as other support elements are used in burst-prone conditions to absorb the kinetic energy of the removed rock to avoid sudden and violent failures. This paper develops numerical and a novel analytical simulation technique for cable bolts to assess their structural behaviour under static and dynamic loading conditions. The numerical and analytical models are then validated against experimental observations reported in the literature, which demonstrates the reliability of the proposed models.

Investigation of cable bolts for stress corrosion cracking failure

Premature failure of cable bolts due to stress corrosion cracking (SCC) in mining and civil industries is a worldwide problem with limited cost-effective solutions. A load frame capable of applying tensile loads was designed and constructed to simulate and examine SCC of full-size cable bolts (up to 2.4 m in length) under simulated in-situ conditions. The tension loading apparatus was used to conduct periodically increasing strain rate (PISR) tests on cable bolts in simulated underground environments. The PISR test was appropriate and reliable for examining SCC susceptibility of TG and Superstrand cable bolts including the effects of wire arrangement, galvanisation and indentation. Overall, TG cable bolts were more susceptible to SCC than Superstrand cable bolts. In each type of cable bolt, the one with plain wires was approximately 30% more resilient to SCC in terms of time to failure than the one with indented wires. Galvanising was not able to prevent SCC, however it considerably delayed SCC occurrence. The results of this study provided detailed insight into SCC of different types of cable bolts and can be further used for setting up optimal support regimes in different geological and environmental conditions.

Numerical modelling of staged stope extraction in a tabular steeply dipping deposit

ABSTRACTStope stability is a key factor for the success of a mining operation. To optimise ore productivity while maintaining stope stability, the mining block/stope must be extracted in stages. Ore dilution will occur if the stope is not properly excavated/blasted. This study examines stope stability during mining in three stages, where the height of each stage stope is 10 m. The paper also presents simulation analysis of a typical steeply dipping tabular orebody at 1200 m depth below the surface, which is common in many Canadian underground hard rock mines. Numerical modelling analysis was conducted using the finite element program, RS2D, where the non-linear elasto-plastic Mohr-Coulomb failure criterion was adopted. The rock reinforcement system (i.e. cable bolts) was modelled/installed in the stope footwall after each mining stage to strengthen access drifts and stabilise the rock mass around the stope that was disturbed by mining activity. Results are discussed in terms of depth of failure zones, total deformation and axial forces in cable bolts with respect to mining stage.

Mechanical Characterisation of Modified Cable Bolts Under Axial Loading: An Extensive Parametric Study

A wide range of modified cable bolts are currently used for ground support in different conditions in Australian underground coal mining operations. This is mainly based on the belief that the performance of modified cable bolts is better than conventional cable bolts. Despite this positive view, very few studies have characterised the performance of modified cable bolts in service. A new laboratory-based pull-out testing facility was employed in this study to investigate the behaviour of two different types of modified cable bolts under axial loading including Garford twin strand bulbed and MW9P (smooth wire). The impact of different parameters including the compressive strength of the grout and the confining medium as well as borehole diameter on the performance of Garford twin strand bulbed cable bolt was investigated. In the case of the MW9P cable bolt, only the effects of borehole diameter and grout compressive strength were examined. A full factorial experimental programme was designed based on Taguchi method to explore the performance of these two cable bolts under different testing conditions. Consequently, an analysis of variance (ANOVA) was conducted to assess the weighting contribution of each parameter (e.g. borehole diameter, the compressive strength of grout and confining medium) to the load–displacement performance of Garford twin strand bulbed and MW9P cable bolts. For the Garford twin strand bulbed cable bolt, it was concluded that the compressive strength of the confining medium was the most influential parameter in the determination of the peak and residual loads as well as initial stiffness. It was also observed that an increase in the compressive strength of either the confining medium or the grout led to an increase in peak and residual loads in the Garford twin strand bulbed cable bolt, while borehole diameter had no sensible effect on peak and residual loads as well as initial stiffness. In the case of the MW9P cable bolt, the compressive strength of grout had the greatest impact on the peak and residual loads whereby, the peak and residual loads varied directly with grout compressive strength. A 10-mm increase in borehole diameter of MW9P cable bolt had negligible effect on the peak and residual loads as well as initial stiffness.

An experimental framework for simulating stress corrosion cracking in cable bolts

Stress corrosion cracking (SCC) of cable bolts in underground mines is a universal issue with limited cost effective solutions at present. Understanding the SCC mechanism of cable bolt failure is crucial in maintaining effective ground support and hence increasing the safety and productivity of underground mines. However, to date, no practical laboratory system has been developed to replicate the SCC mechanism which occurs in underground mines. In this study, based on the chemical properties of mine water, an acidified solution containing sulphide is synthesized. The solution is exposed to the wire strands from cable bolt in the laboratory to model service failure. It is shown that the applied stress intensity and time to failure have an inverse power law relationship in wires of cable bolts. Similar features are observed in both laboratory-failed specimens and the service-failed cable bolts under macroscopic and microscopic examinations, demonstrating the ability of the proposed framework to simulate failure of cable bolt subjected to SCC in the laboratory. Furthermore, the impact of stress intensity and hydrogen concentration on SCC are examined and a cohesive failure mechanism on the cable bolt SCC crack initiation, growth and catastrophic failure are presented. The proposed framework can be applied to the reinforcement materials for improved understanding of the SCC mechanism in underground mines and tunnels.

An Experimental Study on the Shear Performance of Fully Encapsulated Cable Bolts in Single Shear Test

A set of single shear tests on fully encapsulated cable bolts was carried out using a newly developed and integrated Megabolt single shear apparatus. The instrument is designed to determine the pure shear strength of cable bolts where there is no contact between the host body faces during the shearing process. Eight different types of cable bolt were encapsulated in 40 MPa concrete cylinders, using Stratabinder HS grout. Prior to encapsulation, cable bolts were pretensioned at the desired value using a manual pretensioner. Effects of surface profile, pretension value and debonding on shear strength of cable bolts were investigated. It was found that the shear strength of spiral/indented cable bolts was lower than that of plain cable bolts. Increasing the pretension load decreased the peak shear load of cable bolts. In general, no debonding was observed for spiral/indented cable bolts during shear testing; however, all tested plain cable bolts were debonded.

A New Laboratory Short Encapsulation Pull Test for Investigating Load Transfer Behavior of Fully Grouted Cable Bolts

A new laboratory short-encapsulation pull test was developed to better understand load transfer behavior of fully grouted cable bolts that are commonly used in underground mines. Within the embedment section, a series of tests was performed to examine the effect of confining medium diameter on the maximum bearing capacity of cable bolts. A bearing plate appropriate for the investigation of bond failure at both cable/grout and grout/rock interfaces was designed. To prevent a cable bolt from rotating during the pull-out process, a system of locking keys and nuts was integrated into the design to couple together the holder tube, anchor tube, and bearing plate while still allowing axial movement of the cable bolt. Three experiments were conducted. First, different termination methods were used and compared to evaluate the effect of termination instruments on the performance of cable bolts. Then, the confining medium was subjected to a range of preconfinement pressures to determine its effect on cable bolt performance. And finally, the effect of borehole roughness on the pull-out behavior of plain and modified cable bolts was investigated. Once the testing environment was determined, pull-out tests were undertaken on plain and modified cable bolts in a medium-strength material.

Shear behaviour of a cement grout tested in the direct shear test

Portland cement grouts are widely used in the mining industry to bond cable bolts with the surrounding rockmass. Numerous laboratory and field tests showed that bond failure of the cable/grout interface is the dominant failure mode. Previous research has found that shear behaviour of the grout along a pre-defined plane plays a significant role in determining the nature of the bond failure in a cable bolt reinforcement system. In this study, the shear behaviour of a Portland cement grout was investigated based on a direct shear test. Two different boundary conditions were considered being a constant normal load (CNL) and constant normal stiffness (CNS). Under CNL condition, five different normal pressures between 0.1 MPa and 6.0 MPa were examined. While under CNS condition, the initial normal pressure was set to value within the same range. Also, a CNS of 10kN/m was added. The cohesion, internal friction angle and shear strength of the grout were acquired. The results showed that there is a linear relationship between the shear strength of the grout and resultant normal pressure. However, under the CNS condition, the shear strength of the grout was found to be generally higher comparing to the CNL condition, most likely because sample dilation resulted in an increase in the normal pressure. Consequently, shear strength of the grout also increased.

Results of pull tests of rock bolts and cable bolts in Sydney sandstone and shale

This paper discusses the result of 181 pull tests of rock bolts and cable bolts in the sandstones and shales of the Sydney region. The pull tests were carried out as part of the Sydney Metro Northwest tunnelling project. The tested bolts include: end-anchored corrosion-protected steel bolts, resin-grouted fibre reinforced plastic bolts, cement-grouted multi-strand cable bolts and friction bolts. Recorded and corrected bolt stiffness characteristics are presented by bolt type, grouting agent, lithology, bolt bar material and borehole diameter. Rock-grout and grout-bar interface bond strengths are assessed for different rock mass units and bolt parameters. Common problems with typical pull test arrangements and their impact on the resulting data are discussed. Examples of issues encountered during bolt installation, such as partial grout encapsulation and insufficient mixing of resin, are presented. Recommendations are made on how some of these problems can be detected and prevented.

The importance of pre-tensioning cable bolts used to reinforce of arch yielding support

Widespread in Polish hard coal mining, cable bolts are used to reinforce the arch yielding support of roadways. This article attempts to prove that change in approach to long bolting technology can bring significant benefits in terms of improving the stability of roadways, including limiting the development of the fracturing zone in the roof, and ultimately also increasing the frame spacing of arch yielding support. The article contains the results of numerical calculations using the finite element method showing the effect of pre-tensioning on the work of cable bolts installed in a situation when a zone of a fractured rock mass appeared in a roof excavation. As a result of the calculations, the dependence of the final vertical displacements of the roadways roof on different pre-tensioning variants of the cable bolts was presented. The second part of the article presents the practical aspects of the use of cable bolts with pre-tensioning in reinforcing the rock mass and arch yielding support of roadways. In addition, the article identifies the developments in bolting technology which could lead to an increase in work safety and reduce the costs of arch yielding support.