Cable Bolts Research Articles

Behavior of Fully Encapsulated Cable Bolt in Various Double-Shear Methods and the Sequence of Wire Failure in the Newly Developed Shear Apparatus

The use of long-tendon ground support elements (cable bolts) is now a common practice in modern underground coal mines, hard rock mines, tunnels, and other underground structures. The paper briefly introduces the development of test methods for rock and cable bolts, including single- and double-shear methods, in the last 30 years and focuses on the novel development of the fourth generation of cylindrically shaped shear test apparatus (MK-IV Double Shear Box) for assessing tendon performance in shear based on the experience gained from the development of previous versions of shear experiment apparatus. The results were compared with similar test findings using a rectangular-shaped double-shear apparatus with and without friction across joint faces. The sequence of wire failure was studied by using the newly developed double-shear test apparatus, and the performance of abnormal phenomena in the double-shear tests was analyzed. The sequence of wire failure can be inferred based on the positive and negative values of strain gauge readings, and the wire of cable bolt under shearing may be broken in the joint area. The failure possibly occurred from one side to the other instead of from top to bottom. The modified double-shear test apparatus with a long section of concrete cylinder was proposed to conduct further research on cable bolt debonding.

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.

Field evaluation of flexible support system with radial gap (FSRG) under a squeezing rock condition in a coal mine development

A wide range of alternatives is available to design a support system to stabilize an underground opening under squeezing conditions. An underground mine development was selected for the research where the underground openings reach to a depth of 1000 m and weak geological formations are present. The operational concerns in the mine development led to a shaft support design having a thick reinforced concrete with very high early strength while the decline was supported by a traditional support system in the squeezing ground. The rigid support in the shaft was typically capable of sustaining high ground pressure and stabilized the opening within negligible radial displacements. It should be noted that the design of the rigid system was straightforward and high early strength concrete had key importance in this case, however, it could only be applied in the shaft. The traditional support system for the decline was a light and conventional system, consisting of thin shotcrete layer, cable bolts and steel arches suffered excessive deformation as high as 60 cm or more. A light-flexible support system with a radial gap was developed for the decline in the same geological unit, allowed substantial ground deformation to occur in a controlled manner, eventually, the opening was stabilized. The newly adopted radial gap allowed improved ground relaxation and reduction of the ground pressure acting on the secondary support layer. The design and application of the yielding support system with a radial deformation gap was effortful, hence the system can be used where available equipment and operational factors force practitioner to use a light and flexible system. The significance of the new system arises from keeping the ground support elements and the operation cycle the same while modifying the support design and installation procedure.

Analytical Modelling of Load–Displacement Performance of Cable Bolts Incorporating Cracking Propagation

The application of cable bolts tends to increase in both underground mining operations and civil tunnel excavations have given its outstanding performance to support the excavation surface and hence minimise the hazards and risk of failure in the field. Understanding the mechanical behaviour of cable bolts under axial loading subjected to different field environment from analytical perspective is essential and economic for selecting the most appropriate cable bolt for a specific scenario and also the ground support system design. This study develops a novel analytical model coupling the crack propagation in the grout annulus to capture the load–displacement performance of cable bolts under axial loading. The majority of the input parameters associated with mechanic properties of the cable bolt and grout are readily determinable by laboratory tests. Additionally, the model simulates the ongoing varying conditions of the confinement to the grouted cable bolt during the pull-out test based on the proposed crack propagation equation, leading to a more realistic reflection of the field conditions. Finally, the capability of the proposed model to simulate the performance of cable bolts under axial loading is demonstrated by validating against experimental results in the literature.

Numerical Study on the Effectiveness of the Grouting Cable Bolt in Deep Retained Goaf Side Gateroad

Grouting cable bolt was employed to control the stability of the retaind goaf side gateraod (RGSG) in the Panyidong coalmine by considering severe stress disturbances and soft rock mechanics associated in deep mining environment. To have a deep insight of the effectiveness of the grouting cable bolt supporting on the stability control of the deep RGSG, a FLAC3D numerical model was built according to the geotechnical condition of the Panyidong coalmine, in which strain softening constitutive material was introduced to model the softening behavior of the surrounding rock supported by end-anchored cable bolt or grouting cable bolt. Simulated results demonstrated that: Confining stress release during roadway driving and stress concentration during longwall face retreating are the main driving force to the large deformation of the RGSG supported by end-anchored cable bolt; Grouting cable bolt supporting can improve the stability of the deep RGSG considerably thought providing high bolting strength and grouts injection, and ideally, the RGSG’s roof sag and right coal rib convergence making main contributions to the RGSG section shrinkage could be reduced by 22.0% and 23.0%, 26.6% and 29.4%, 30.2% and 34.9% when the RGSG is supported by grouting cable bolt I, II and III, respectively. Field application also verified that the integrity of the surrounding rock of RGSG was improved and the residual section of the RGSG was large enough to reserve for the second longwall panel mining when the grouting cable bolt technology was applied in the Panyidong coalmine with 850 m depth.

Assessing the mechanical performance of different cable bolts based on design of experiments techniques and analysis of variance

Many experimental studies have been carried out to characterise the mechanical performance of cable bolts under axial or shear loading. The common limitation in all these studies is that only single-variate descriptive analysis was included leading to an insufficient understanding of interactive influences of a range of parameters on the mechanical performance of cable bolt. Also, defining a reasonable number of experiments for the testing program that includes wide range of parameters has been a challenge for the scholars and practitioners. Thus, to address these limitations in a sound and logical manner, two statistical techniques being Taguchi and Response Surface Methodology (RSM) were introduced to demonstrate their effectiveness for a proper design of experiments leading to a minimum number of tests without compromising the output quality. Such a task was performed on the data reported in the literature. In addition, the quantitative post-data processing was conducted using Analysis of Variance (ANOVA). To be specific, ANOVA was mainly performed to quantify the contribution of different independent parameters (e.g. confining medium and grout strengths as well as borehole dimeter) on the dependent ones (e.g. peak and residual loads as well as the initial stiffness) individually and interactively. RSM was also used for the post-data multivariate statistical analysis. Finally, it was concluded that with the aid of Design of Experiments techniques such as Taguchi it is feasible to develop a minimum number of experiments to obtain the most representative tests results without compromising the quality of research outcome along with some quantitative post-data processing using ANOVA. Also, it was demonstrated that one of the most important specifications of RSM is to provide the quantitative and visual interactive effects of independent parameters on the mechanical performance of different cable bolts using multiple regression modelling.

Pre-Tension Losses in Cable Bolts and their Progress in Time

Bolting and anchoring products are mostly pre-tensioned after their putting into the construction. Pre-tensioning force is a force with clearly defined size and direction and helps the construction to transfer loads. After pre-tensioning a cable bolt some pre-tension losses occur immediately after releasing the jack and other losses progress during their working time. These losses may occur due to the mechanical properties of the cable bolt and its structure itself, but also due to the properties of the surrounding rock mass.The article deals with the enumeration and description of individual losses in accordance with applicable legislation and their measurements both in the laboratory and in actual mining conditions. The comparison of calculated and measured losses and a brief overview of pre-tensionable bolting technology within OKD coal deep mines is a part of the content as well. The result of the tests is quantification of these types of pre-tension losses including proposals for their reduction.

Analysis of Support Design in Weak Rock Drift Using a Systematic Approach

The aim of this study is to develop a systematic approach for support design of weak rock drift based on empirical, analytical, and numerical method, which is employed to estimate weak rock support demand and design support system. Detailed engineering geological investigations and rock mechanics test have been carried out in weak rock drift. The Q‐system and GSI‐system were used to determine the primary support design and rock mass properties, respectively. The numerical model of RS2 finite element program has been calibrated by analyzing the relation of falling height observed in the field to the frictional angles obtained from empirical method, rock mechanics test, and calculated rock mass parameters, respectively. In an attempt to check the validity of sophisticated support, support suggested by Q‐system, and the combination support system proposed by analytical approach, the RS2 program was employed to analyze the depth of plastic zone and total displacement surrounding the weak rock drift. Numerical results show that the depths of plastic zone and total deformation surrounding the weak rock drift supported by the combination support system significantly descended 87% and 90% of those of sophisticated support. In particular, the rock bolt and cable bolt provide enough frictional and interlocked forces to resist weak rock falling which change the weak rock mechanicals properties and the surface holding function reinforced by the shotcrete, wire mesh, and steel strap. The factor of safety (FOS) of 8.28 of the combination support system is much more than the FOS of 1.5 for permanent drift. The combination support system with rock bolts, cable bolt, shotcrete, wire mesh, and steel straps has been applied to stabilize the weak rock drift and found to be successful to prevent further deformations surrounding the drift.

Stability Analyses and Cable Bolt Support Design for A Deep Large-Span Stope at the Hongtoushan Mine, China

This study presents stability analyses and a cable bolt support design for a typical deep large-span stope (1-1# stope) at the Hongtoushan mine in China, using an integrated empirical and numerical method. Detailed field work including quantification of joint distribution and surface quality, along with laboratory test on intact rock samples, were performed to obtain the geotechnical properties of rock masses. The rock mass of the 1-1# stope was characterized by rock mass rating (RMR), rock mass quality (Q), and geological strength index (GSI), and then the modulus, peak strength, cohesive strength, and internal friction angle of the rock mass were estimated. The stability of the stope was then evaluated by empirical (RMR, stability graph) and numerical approaches (limit equilibrium analyses with UNWEDGE and stress-strain analyses with FLAC3D), considering of the effects of rock mass quality, induced stress, and large-span. A cable bolt support system obtained from the empirical method, was then further analyzed using the FLAC3D and UNWEDGE codes. The results show that the maximum plastic zone thickness and vertical displacement at the stope roof decrease significantly and the safety factor of the unstable wedge block increases significantly after installing the cable bolt support systems recommended by the empirical method. Therefore, it is suggested that an integrated empirical and numerical method is used to obtain quantitative stability assessment and optimum cable bolt support design for deep large-span stope roofs.

Stress corrosion cracking fracture mechanism of cold-drawn high-carbon cable bolts

Premature failures of cable bolts due to stress corrosion cracking (SCC) in underground reinforcement systems are universal issues with limited practical countermeasures at present. To determine the SCC fracture mechanism which is an essential step to develop the potential solutions, the environment conditions leading to SCC failures were examined using four-point bend tests. Fractographic features of both serviced-failed and laboratory-failed cable bolts were investigated and compared. It was identified that the fracture surfaces of cable wires were characterised by step-shape fracture mode. The highly-oriented wavy lamellae microstructures of cable wires, produced during cold-drawn manufacturing process, have significant effects on the direction and path of crack propagation. Hydrogen-assisted SCC was suggested as the dominant fracture mechanism in cable bolt failures. SCC only occurred in the environmental conditions that promoted the diffusion of hydrogen diffusion into the cable wires, and crack growth rates were determined by the hydrogen diffusion rates. This study provided fundamental knowledge on fracture mechanism of cable bolts which can be applied to further study in identifying potential countermeasures against the SCC of cold-drawn high-carbon cable bolts.

Parametric study of fully grouted cable bolts subjected to axial loading

The laboratory short encapsulation pull-out test (LSEPT) has been widely accepted as the most efficient method to characterize the mechanical behaviour of cable bolts under axial loading. In this study, a number of LSEPTs was performed on conventional (Plain SuperStrand) and modified (MW9S) cable bolts using the improved pull-out test design. The effects of several parameters including the uniaxial compressive strength (UCS) of the confining medium and grout and the borehole diameter on the mechanical behaviour of MW9S and Plain SuperStrand cable bolts were investigated. Response surface methodology (RSM) was employed to quantify the contribution of these parameters on the responses including peak and residual loads and initial stiffness. RSM revealed that the UCS of the confining medium is a key contributing factor to the mechanical behaviour of both cable bolts. Also, it was demonstrated that the borehole diameter had a negligible impact on the overall behaviour of the MW9S cable bolt while the peak load of the SuperStrand cable bolt was increased due to an increase in the diameter of the borehole. Finally, from a comparative analysis, it was confirmed that the improved design can better represent the field mechanical behaviour of cable bolts through maintenance of the embedment length during large deformation.

Supporting problems at an island mining face in shanxi, China. Case study based on field observation

Abstract In this paper, field observation is conducted in Fujiaao Coal Mine, Linfen city in Shanxi province, China, which is unique because of its island-typed mining layout. Firstly, geology, layout of the coal cutting face, supporting pattern are presented and details are also provided. Then the problems are discussed and analyzed based on existing bolting theories. It finds out the length of the cable bolt is too long, which requires long time to drill boreholes, whilst the effectiveness is limited due to the illogical combination between cable bolt function zone and rock bolt function zone. Also, the face place allocated to rock bolt is very likely to cause line contact between its edge and steel belt, thus the bearing force is tremendously weakened and solely determined by strength of belt. Furthermore, field observation proves the low active supporting effects to roof, thus leading obvious roof sink even without the disturbance of mining activity. Eventually, some solutions are proposed with the intention to improve the situation based on the economic and technical considerations, including bolting in humidity environment, rational cable bolt length, wise choice of face plate, and ways to increase pretension force. The viewpoints and comments of this study can be referred by practitioners in coal mines sharing same difficulties.

Practices, experience, and lessons learned based on field observations of support failures in some Chinese coal mines

Modelling, experimental study, and field observation are prevailing research methods in coal mine support application. Many criteria and rationales have been established to evaluate the stability of coal mine excavations from both qualitative and quantitative perspectives. Nevertheless, most coal mine support designs are still highly dependent on empiricism or semi-empiricism, even with the existence of the aforementioned theories. Oftentimes, most lessons are learned via the trial and error approach. A more critical fact is that further in-depth improvement of or research into coal mine support appears to be largely bottlenecked. Under such circumstances, support failures in four representative coal mines in China are selected as research objects, through which dominating implied principles are investigated and re-clarified by field observations. In the Datong coal mine, an extremely fractured roadway is successfully maintained by simply changing the end-encapsulated bolting system to a fully bonded bolting system, with the results indicating the formation of an artificial compressive stress arch. In the Kongzhuang coal mine, a deep-buried shaft with a super-large section is effectively stabilized by tieback cable bolts, and a 561-day-long load-monitoring procedure verifies the effectiveness. In the Donghuantuo coal mine, the support ideology of retaining, reinforcing, and holding functions is presented to interpret bolt support failure under an aqueous environment. In the Hulusu coal mine, 5400-mm-span entries are stabilized by simply installing four tendons on the roof; the effectiveness is further proven, and an enhanced beam building mechanism is interpreted. At the end of this study, the effectiveness of a pretension force, a fully bonded bolting system, longer tendon support, and mesh/shotcrete is discussed. All of the examples given in this study are proposed to meet the principles of high efficiency, low cost, and operational friendliness. Additionally, the authors hope that through this paper, more comprehensive research/analytical methodologies will be able to bridge support theories and applications, which is the main purpose of this work.

Laboratory-Based Investigation into Stress Corrosion Cracking of Cable Bolts.

Cable-bolt failures due to stress corrosion cracking (SCC) could significantly compromise the sustainability and long-term stability of underground constructions. To fully understand the SCC of cable bolts, a two-step methodology was implemented: (i) long-term cable-bolt coupon tests using mineralogical materials collected from underground mines; and (ii) accelerated full-scale cable-bolt tests using an acidified solution. In the long-term tests, a novel three-point bending coupon was designed. The effects of mineralogical materials on SCC were evaluated under the simulated underground bolting conditions through the application of “corrosion cells”. For accelerated tests, SCC resistance of different type of cable bolts was examined using the new designed tensile-loading apparatus under the periodically increasing strain-rate loading mechanism. It was identified that mineralogical materials and applied stress intensity accelerated the corrosion process of the cable bolts. The number of wires and wire surface conditions in different types of cable bolt directly affected SCC susceptibility. The cable bolts with a greater number of wires provided higher resistance to SCC. The developed experimental methodologies can be applied to study SCC in other reinforcement materials and the results can be used to design optimal support systems in different environmental and geotechnical conditions.

Innovative approach to strata reinforcement in coal mines with reference to evaluation cable bolts shear strength

There is an increasing need to determine accurately the strength properties of tendons for an effective ground control on mines and underground structures as well as on modelling simulations. The strength properties of cables, used as cable bolts, have been evaluated mainly by their ultimate tensile strength, as this kind of test can be carried out in the field as well as in the laboratory. Only recently, there has been a growing interest in cable bolt failures in shear because of the documented field failure evidence. Accordingly, this paper reports various methods of shear testing of rock bolts and cables using different shear testing rigs, some have been developed by the rock bolting research team at the University of Wollongong. A programme of shear testing of a variety of cable bolts marketed in Australia was undertaken, the results of which were reported and conclusions were drawn. It was concluded that plain cable bolts were de-bonded during shearing when compared to spiral cables under the same testing conditions. In addition, both the single shear and double shear testing methodologies will result in the same outcome if there is no de-bonding, and a proper confinement is applied.

Design principles and key technologies of gob side entry retaining by roof pre-fracturing

This study investigates gob side entry retaining by means of roof pre-fracturing. Based on short cantilever beam theory, the entry roof was pre-fractured from gob roof via orientated blasting. The gob gangue was then blocked and used as one side wall of the retained entry. The blasting hole length relied on site geological conditions, and a design method was formulated. Yieldable cable bolt anchored into stable overlaying strata is one key technique to stabilise the entry roof under double dynamic pressure. For the gangue management I-steel and retractable U-steel formed frameworks were investigated with regards to varying seam thicknesses. A real-time remote monitoring system was developed and used to investigate surrounding rock mass deformation of the proposed retaining method and to provide early warning for mining safety. We present two working cases to show the applicability of the retaining method under different geological conditions. Field monitoring reveals that the periodic weighting loads decreased at the working face after roof blasting. The entry roof and gangue wall became stable after the working face passed a certain distance. Our proposed mining method provides an innovative approach for mining safety and economy.

Mineralogically influenced stress corrosion cracking of rockbolts and cable bolts in underground mines

Stress corrosion cracking (SCC) of rockbolts and cable bolts in underground mines has been identified as a serious international problem over the past two decades. The frequent occurrence of rockbolt and cable bolt SCC in strata containing clay and groundwater suggests the presence of certain mineral types may be an influencing factor. To identify the environmental factors leading to SCC and evaluate the effects of mineralogically influenced corrosion, a laboratory testing program capable of recreating the underground bolting environments was designed and conducted. A static load was applied to specimens of rockbolts and cable bolts placed in “corrosion cells” containing groundwater, clay, coal or a combination of those materials for about 300 days. Results from these tests revealed that the mineralogical materials indirectly affected the rate of corrosion by significantly altering the local water chemistry. The mechanical load applied to the specimens was found to accelerate the rate of corrosion, and corrosion pits were observed to have formed on the steel surfaces. The generation of corrosion pits would likely result in local stress concentration and contribute to SCC. The results of this study provide further insights into the environmental factors leading to SCC and the proposed methodologies can be used for investigating this type of failure through in-situ experiments.

Laboratory Testing and Modeling of a High-Displacement Cable Bolt

Laboratory Testing and Modeling of a High-Displacement Cable Bolt

Analysis of the influence of groundwater and the stress regime on bolt behaviour in underground coal mines

The service failure of rock bolts and cable bolts are frequently reported issues in underground coal mines. Whilst numerous experimental investigations concerned with the service failure of bolts have been conducted, numerical modelling offers an alternative approach in evaluating the factors contributing to service failures of bolts in underground mines. In this study, analysis of the influence of groundwater and tensile stress on bolts in underground coal mines was studied through the numerical modelling of a grouted bolt in the immediate roadway roof. Bolt tensile stress and groundwater dripping rates in the immediate roadway roof were analysed using a package based on finite element method to assess the effect of coal roof thickness and claystone bands, as main contributors of known service failures of bolts in roadways of underground coal mines. Increasing coal roof thickness was found to increase bolt dripping rates. Probable location of stress corrosion cracking (SCC) occurrence was established through examining the shift and increase in maximum bolt tensile stress that was exhibited along the bolt length with increasing coal roof thickness. Claystone bands situated at the top and centre horizon of a grouted bolt produced lower bolt dripping rates compared with scenarios with no claystone bands. Intersecting claystone bands at the centre horizon of a bolt for a fully grouted bolt could increase the likelihood of SCC corrosion and bolt failure by contributing to microbial corrosion processes and grout fracturing by tensile stress. This study improves the understanding the bolt failure associated with the presence of groundwater and changing stress environments, which in turn is imperative in formulating strategies to mitigate support element failures and improve the ground support viability.

Neural Network Assisted Analysis for Longwall Gate Road Stability Using Measured Roof Convergence Data

Immediate roof of a longwall panel is composed of layered coal, shaly coal, shale and clay strata overlaying by a thick sandstone main roof. The average depth of the coal seam is about 420 m and it has dip of 10°. The gate roads of width 5.2 m and height 3.5 m are developed in either sides of the panel and the roof is supported by 2.4 m resin grouted rock bolts. A special resin grouted 60 tonne capacity, 6.1 m long bulb type cable bolt is also installed for effectively supporting 8 m width shield installation roadway and gate roads during retreat of the longwall panel. The paper emphasizes on the monitoring and analysis of roof convergence in gate roads by convergence recorder as well as double telescopic telltale devices. Based on the analysis results, design is modified to improve the stability of gate roads. Cumulative roof convergence (CRC) data during development of gate roads are found to vary as tangent hyperbolic function with passage of time and are analyzed using artificial neural network to estimate the key parameters of the function. Convergence increase rate along with the maximum CRC is then used to prepare a Trigger Action Response Plan for immediate notification of the ground control problems. Data analysis also suggests that during retreat of longwall panel, CRC increases exponentially if the longwall face approaches within 10 to 25 m from a measuring station installed along main and tail gate roads. In this paper, methodologies are described for the analysis of CRC data for better understanding of roof behavior and for taking timely decision for any unusual event.