Search

Recent studies into rock bolting parameters have suggested that increasing nut length has a positive influence on the pretension achieved during rockbolt installation. A literature survey on bolts and fasteners was undertaken, as well as a laboratory testing program to examine this suggestion. The testing program examined M24 nuts ranging from 1D to 1.5D (24 mm – 36 mm). These nuts were used to tension a rockbolt using an instrumented drill rig to measure torque, and a through-hole load cell to measure pretension. It was found that nut length has no impact on the pretension values achieved during rockbolt installation. The largest factor affecting pretension values was found to be the levels of friction acting between bearing surfaces. Overcoming this friction is estimated to account for 85-90% of the applied torque from the drill motor.

Resin bolt parameters, such as back pressure and gloving, and their effect on ground support system performance, remains one of the fundamental areas of rockbolt research. The majority of previous studies into resin bolting parameters have utilised various methodologies to investigate the effect of a singular parameter. Unfortunately, due to the variability in methodologies and the relatively narrow field of study of each research project, a holistic conclusion into the exact science behind various results is unable to be drawn. It is the focus of this research project to conduct a detailed and consistent testing program, which attempts to simulate real world conditions as closely as possible, in order to provide the industry with engineered roof bolting solutions to specific underground roof properties. Recently published studies have implemented steel piping as a simulated borehole and have reported relatively high back-pressure measurements. As part of this project, testing conducted both underground and in a cement block, have recorded back-pressures in the range of 4-10 MPa, which are substantially lower than previous tests conducted in steel piping. Rockbolts installed in an underground coal mine using a continuous miner have been over-cored, the core has been cut into 100 mm lengths and each sample has been push tested. After push testing, the samples from the top 300 mm of each bolt were inspected for gloving. Almost all of the recovered rockbolts experienced some degree of gloving within the top 300 mm of its length. The average severity of gloving within these specimens was found to be relatively minor. It was found that gloving can reduce load transfer by 4-6 kN per 10% of gloved surface area.

Stress Corrosion Cracking (SCC) is a failure mechanism which has been observed to affect rockbolts installed in a number of Australian and overseas underground mines. SCC requires the application of a tensile stress on a material placed in an appropriately corrosive environment. SCC is characterised by the growth of fine fractures, originating from the surface of a material, which will continue to grow until they reach a critical length at which thematerial fails by mechanical overload.An extensive literature survey into rockbolt corrosion and SCC identified the need for a dedicated testing system for investigating SCC in full-scale rockbolt specimens. This thesis has focused on the design and development such a testing system. This system included a novel testing methodology, the Bending and Tension Loading Apparatus (BaTLA), which was housed within a Controlled Mine Environment (CME) laboratory, capable of recreating theatmospheric conditions present in an underground mine, as well as conduncting ‘accelerated’ tests using synthetic testing solutions. The development of this laboratory was based on the findings of a detailed field study conducted to characterise the geological, geotechnical, hydrological and atmospheric environment of two underground coal mines which have presented cases of rockbolt SCC. This newly developed laboratory was used to carry out an extensive testing program consisting of over 40, 000 hours worth of experimentation.This extensive testing program assessed the performance of both bending and tension loading in the BaTLA for investigating SCC in rockbolts. Both static and Slow Strain Rate (SSR) loading conditions were examined and each were found to be useful for investigating specific aspects of SCC. It was found that static testing provided the most appropriate means for examining mechanistic aspects of rockbolt SCC such as critical stress threshold, while SSR testing was found to be better suited to comparing material and environmental f

A new laboratory facility designed and constructed at the University of New South Wales, aims to continue and offer a new approach to researching the phenomenon of the stress corrosion cracking. This new approach includes the use of full sized specimens, a specially designed frame, as well as a new loading regime, known as the Periodically Increasing Stress Test, to closely simulate the loading encountered by bolts in service. Coupled with a detailed water testing program to be undertaken at a number of partner sites, this new approach hopes to further increase understanding of stress corrosion cracking and its causes.