Articles, technical papers and presentations
Author(s): Charlie C. Li (Department of Geoscience and Petroleum, NTNU Norwegian University of Science and Technology, Trondheim, Norway),
Andreas Aure (Department of Geoscience and Petroleum, NTNU Norwegian University of Science and Technology, Trondheim, Norway) and
Greig Knox (Epiroc, South Africa).
Date: 20 August 2023
Overview: An inflatable rockbolt is a steel tube that has been rolled into an omega shape. During installation, water is injected into the tube, which expands the profile of the rockbolt, generating a radial force against the borehole wall. This reinforces the rock mass through the friction at the bolt–rock interface. This study aims to quantify the effects of borehole size, borehole roughness, and installation water pressure on the pull load capacity of the bolt. This was achieved through a series of pull tests under laboratory conditions. The test results showed that the load capacity was higher in boreholes that were either close to the initial profile diameter of the bolt or the fully unfolded diameter of the bolt, that were 26.8 mm and 38.7 mm, respectively, in the study. The load capacity and the radial stiffness of the bolt were lowest in the medium-sized boreholes (33 and 33.5 mm). In small boreholes, the shoulders of the bolt tongue are tightly compressed such that the outward elastic deformation of the bolt tube is locked in after installation. This deformation locking enhances the load capacity of the bolt. In addition, the load capacity was found to be higher in percussively drilled boreholes than in diamond-drilled boreholes. The additional friction angle of the percussive boreholes was back-calculated to be approximately 5.83º. The load capacity was also found to increase as the installation water pressure increases in the range of pressures tested. It was observed that the inflatable bolt was clamped against the borehole in three zones: on the two tongue shoulders and the side of the bolt opposite the tongue.
Download link: https://link.springer.com/article/10.1007/
Author(s): Greig Knox ( and John Hadjigeorgiou (University of Toronto, Canada)
Date: 15 March 2023
Overview: Rockbolts are an integral part of most ground support systems used to stabilise excavations in rock for both tunnelling and mining applications. Ground support design aims to match the selected rockbolts to anticipated rock mass behaviour and potential modes of instability. In seismically active underground mines, this has led to the development of chemically grouted energy-absorbing rockbolts capable of sustaining larger deformations than conventional rockbolts. In heavily fractured and/ or highly stressed rock, the installation of rockbolts can be problematic due to hole closures and blown-out holes. Even when rockbolts are installed successfully in the borehole, their effectiveness in fractured ground can be compromised by resin penetration into fractures in the surrounding rock mass. Self-drilling anchors are a potential solution to overcome the installation challenges observed in fractured, friable, and highly stressed conditions where large deformations are anticipated. Currently, there is a scarcity of data regarding the performance of self-drilling anchors subjected to shear and tensile loading. This paper presents the results of a comprehensive laboratory investigation conducted on both conventional and yielding self-drilling hollow core rockbolts subjected to shear and tensile loading. The results of this investigation provide quality quantitative data on the performance parameters of self-drilling anchors that can be used for the design of ground support in mining applications.
Download link: https://link.springer.com/article/10.1007/s00603-023-03288-1
Conference: CAVING 2022, Hilton Adelaide, South Australia
Author(s): Brendan Crompton and Greig Knox
Date: 30 August – 1 September 2022
Overview: The specification of a tendon is stated as the capacity of the tendon in new condition on the day it was produced. Rock support tendons are discretely tested quasi-statically (replicating closure) or dynamically (replicating seismicity) depending on the conditions expected with the mine environment. Although this testing is valuable, it is likely that a tendon subjected to rapid ground movement (seismicity) would first be subjected to some level of slow closure. Previous research investigated the possible correlation between the quasi-static elongation of a rockbolt prior to dynamic loading and residual dynamic capacity. The results from this research indicated that a correlation exists between the residual dynamic capacity and the energy absorbed quasi-statically when the tendon is elongated axially. It was proposed that a conservative approach would be to consider the total energy capacity of a tendon as the energy absorbed by a tendon in pristine condition during a single dynamic impulse resulting in the rupture of the tendon. The research sample set was limited to a single rockbolt configuration of fixed length and diameter, therefore it was noted that the results should not be extrapolated to other versions of this system or other ground support systems. This research will build on the previous work by repeating the testing regime on a larger diameter of the same configuration rockbolt. The results will be analysed to identify how the diameter of a rockbolt affects the energy and elongation capacity of the rockbolts under combined quasi-static and dynamic loading. This information will greatly assist geotechnical practitioners with support system design, product selection and decisions on when to rehabilitate an installed support system.
Download link: https://papers.acg.uwa.edu.au/p/2205_07_Crompton/
Conference: CAVING 2022, Hilton Adelaide, South Australia
Author(s): Rual Abreu and Greig Knox
Date: 30 August – 1 September 2022
Overview: In recent years self-drilling anchors (SDA) have received increased attention from the ground support industry. This includes studies and field trials at Oyu Tolgoi mine in Mongolia and Malmberget mine in Sweden which have highlighted the installation success rate of SDAs in fractured rock masses. Typical challenges associated with rockbolt installation in such conditions include hole closures and blown out holes, resulting in a reduction of bolt installation success rates and achieved support capacity. To improve the efficacy of installation in fractured rock masses, an SDA replaces the conventional drill steel required to bore the support hole, combining both the drill steel and rockbolt into a single component. This negates the need for equipment to alternate between a drilling operation and bolt insertion as an SDA combines these traditionally separate processes into one. For squeezing and seismically active ground conditions, yielding SDAs offer an additional performance benefit derived from the ductile mechanical properties of the bolt from which these anchors are produced. However, a consequence of this bolt design is that the SDA is subjected to the percussive loading normally applied to the drill steel during the drilling operation, which can affect the performance of the bolt. This investigation quantifies the influence of this percussive drilling on the performance of a yielding SDA when subjected to dynamic loading. An experimental group of bolts were drilled into quartzite and thereafter subjected to impact testing in a laboratory. The performance of these samples is compared to a control group of samples, not previously subjected to drilling forces. This investigation provides insight into the in situ performance of a yielding self-drilling anchor.
Download link: https://papers.acg.uwa.edu.au/p/2205_09_Abreu/
Conference: CAVING 2022, Hilton Adelaide, South Australia
Author(s): Greig Knox and John Hadjigeorgiou (University of Toronto, Canada)
Date: 30 August – 1 September 2022
Overview: The traditional installation of grouted rockbolts requires a support hole to be pre-drilled followed by the installation of a cementitious or resin grout. The rockbolt is then inserted into the grout. In highly stressed or poor ground, the presence of fractures, shear features and altered rock can lead to unravelling of the support hole and result in blockages and grout losses. Consequently, the support holes are often redrilled, resulting in oversized holes, increased installation times, and poor installation quality. Replacing the conventional drill string with a self-drilling anchor (SDA) improves both the quality and advancement rate of installation. These are critical considerations in poor ground. The SDA rockbolt is coupled to the rock drill, drilled to depth, then decoupled and post grouted. This one-step installation method overcomes the challenges of drilling into fractured rock where hole closures and grout losses are common. The operational advantages of the application of a one-step system using self-drilling anchors have recently been demonstrated in two caving operations, Oyu Tolgoi mine in Mongolia and Malmberget mine in Sweden. A yielding SDA offers additional performance benefits when in squeezing rock or seismic prone ground. There is limited data on the response of yielding self-drilling anchors to tensile and shear loading under controlled laboratory conditions. The undertaken experimental program has investigated the behaviour of a yielding SDA under tensile and shear loads. This contributes to an improved understanding of the response of selfdrilling anchors under a single loading mechanism and can provide an indication of performance in highly stressed and fractured rock observed in caving operations and deep underground mines.
Download link: https://papers.acg.uwa.edu.au/p/2205_12_Knox/
Published: Rock Mechanics and Rock Engineering (2022)
Author(s): Greig Knox and John Hadjigeorgiou
Date: 26 June 2022
Overview: Deep and high-stress mines are susceptible to mining induced seismicity that can challenge the installed ground support. Under these seismic conditions, conventional rockbolts such as mechanical, fully grouted rebar and frictional rockbolts are often inadequate. This has led to the development of several yielding, high energy-absorbing rockbolts that can carry large loads as well as accommodate large deformations. In this category, paddled energy-absorbing rockbolts are more widely used at seismically active mine sites. The performance of energy-absorbing rockbolts is generally determined by the impact testing method, which consists of dropping a known mass from a given height to transfer the kinetic energy of the falling mass to the rockbolt that is installed in a steel tube. All impact tests employ one of two configurations: continuous tube and split tube. The continuous tube configuration simulates an impact load directly applied onto the rockbolt plate while the split tube represents a loading condition on a rockbolt when a rock block is ejected by an impact thrust. However, the influence of the split location along the testing tube on the behaviour of energy-absorbing rockbolts had not been addressed in the past. This paper presents the results of a comprehensive testing programme whereby it was demonstrated that the location of the split within the host tube controlled both the maximum plate displacement and dissipated energy recorded prior to the rupture of the rockbolt. This has a significant influence on the performance of paddled energy-absorbing rockbolts under impact loading conditions.
Download link: https://link.springer.com/article/10.1007/s00603-022-02945-1
Conference: Breaking new ground: A Modern approach of destress strategies in deep (mass) mining, Johannesburg, South Africa
Author(s): Greig Knox
Date: 7 April 2022
Overview: Rock reinforcement products are typically laboratory tested under singular loading conditions. This presentation explores the benefits and limitations of conventional testing methods for ground support; and introduces new testing methodologies intended to improve industry understanding of ground support behaviour in application.
Conference: Ground Support 2019, Sudbury, Canada
Author(s): Greig Knox and Adrian Berghorst
Date: 23 – 25 October 2019
Overview: The specification of a tendon is stated as the capacity of the tendon in pristine condition on the day of delivery to the mine. A number of factors result in the degradation of the capacity of a tendon. Factors such as, but not limited to, installation quality, corrosion, and ground movement are all understood to reduce the residual capacity of a system. An understanding of the effects of the degradation of a tendon is important when determining the risk of an excavation. Rock support tendons are discretely tested quasi-statically (closure) or dynamically (seismicity) depending on the conditions expected with the mine environment. Although this testing is valuable, it is likely that a tendon subjected to rapid ground movement (seismicity) would first be subjected to some level of slow closure. This paper forms a basis for future work into determining the residual capacity of a tendon; the focus of this paper is to determine the residual dynamic capacity of an axially elongated tendon. Samples were axially elongated quasi-statically and fixed into place prior to being subjected to a single dynamic impulse, resulting in the rupture of the tendon.
Download link: https://doi.org/10.36487/ACG_rep/1925_14_Knox
Conference: Deep Mining 2019, Muldersdrift South Africa
Author(s): Brendan Crompton
Date: 24 – 25 June 2019
Overview: The use of resin-grouted tendons is a common ground support practice within the mining industry and various tendon designs are available. The support strength of a resin-grouted tendon is often constrained by the resin annulus between the tendon and the borehole. Effective mixing of the resin is typically achieved by ensuring the resin annulus does not exceed a specified maximum limit. Therefore, in some cases, the diameter of the tendon is dictated by the maximum allowable resin annulus and minimum diameter borehole that can be drilled and not by the support design requirements. The installation of tendons with mastic resin capsules is prone to gloving of the installed tendon by the capsule packaging, thereby debonding the tendon from the borehole, and compromising the mixing of the resin surrounding the tendon. This paper documents a practical investigation into the effectiveness of typical resin tendon designs in large annulus installations and the development of an improved tendon design for such cases.
Download link: https://doi.org/10.36487/ACG_rep/1952_13_Crompton
Conference: ARMA 2019, New York, USA
Author(s): Adrian Berghorst and Greig Knox
Date: 23 – 26 June 2019
Overview: New Concept Mining (NCM) has implemented the Dynamic Impact Tester (DIT) to conduct laboratory based dynamic testing on rock bolts. The DIT allows NCM to move rapidly through the R&D cycle for new rock bolts. This allows both a shorter time to market as well as comprehensive understanding of the performance of rock bolts. In addition to these benefits, the DIT is being used in several exciting ways to improve the understanding in the mining industry of the performance of dynamic ground support. An example is given where the dynamic testing database has been used to back analyze the quantitative performance of a Vulcan Bolt during an underground seismic event.
Conference: ARMA 2019, New York, USA
Author(s): Adrian Berghorst and Greig Knox
Date: 23 – 26 June 2019
Overview: New Concept Mining (NCM) has implemented the Dynamic Impact Tester (DIT) to conduct laboratory based dynamic testing on rock bolts. The DIT allows NCM to move rapidly through the R&D cycle for new rock bolts. This allows both a shorter time to market as well as comprehensive understanding of the performance of rock bolts. In addition to these benefits, the DIT is being used in several exciting ways to improve the understanding in the mining industry of the performance of dynamic ground support. An example is given where the dynamic testing database has been used to back analyze the quantitative performance of a Vulcan Bolt during an underground seismic event.
Conference: Caving 2018, Vancouver, Canada
Author(s): Adrian Berghorst
Date: 17 October 2018
Overview: A workshop presentation on current and future trends in qualifying dynamic ground support for underground mines. There are several facilities available to the mining industry when it comes to the qualification of ground support for dynamic environments in today’s underground environment. However are these sufficient, and what is the future of ground support qualification for this essential aspect of mining in the modern world.
Conference: Caving 2018, Vancouver, Canada
Author(s): Greig Know, Adrian Berghorst and Pieter de Bruin
Date: 17 October 2018
Overview: Laboratory-based dynamic testing allows rockbolt developers to apply impulses of energy to a rockbolt in order to approximate some loading aspects that the rockbolt would experience during a rockburst in an underground mine. This data can be used to compare a rockbolt’s dynamic performance providing geotechnical engineers with useful information for designing their required ground support system. There are two general sample configurations commonly used in this type of dynamic testing – split-tube test and continuous-tube test. This paper summarises a proposed a third configuration – a multi-split-tube test.
Download Link: https://doi.org/10.36487/ACG_rep/1815_58_Knox
Conference: Rock Mechanics – Experiments, Theories and Applications
Author(s): Koos Bosman (Open House Management Solutions), Martin Cawood and Adrian Berghorst
Date: 2018
Overview: The capacity of a rockbolt subjected to an impulse of energy varies as a function of the magnitude of the impulse of energy applied. This paper explores the relationship between the magnitude of the impulse of energy applied to a rockbolt and the resulting dynamic capacity. The result of this research shows that for a given velocity at impact, there is a linear relationship between the magnitude of the individual impulses of energy applied to a rockbolt and the resulting dynamic capacity of the rockbolt. The dynamic capacity of a rockbolt is not a constant value. During this research, the relationship between the magnitude of the impulse and the resulting displacement of the rockbolt is also examined.
Conference: RocDyn-3, Trondheim, Norway
Author(s): Greig Knox and Adrian Berghorst
Date: 26 June 2018
Overview: The limitations of the dynamic axial loading testing method are known, however, the DIT provides an efficient platform, on which a large number of tendon support systems can be compared under controlled conditions with increased agility. During the rapid development of new rock bolt products it is crucial to quantify the effects of high strain rates exerted by rock bursts on ground support systems. The DIT provides this capacity..
