Abstract
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.
Highlights
Grouted bolts are the primary form of roof support in underground coal mines in Australia
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
Highest maximum bolt tensile stress was observed in claystone band configurations that had (1) thin claystone bands, (2) no claystone bands in the centre horizon of bolt length and (3) claystone bands situated at the immediate roof of the roadway
Summary
Grouted bolts are the primary form of roof support in underground coal mines in Australia. Gray (1998) determined that the primary failure mechanism for rock bolts in service in Australian underground coal mines was due to Stress Corrosion Cracking (SCC), causing rock bolts to fail in a brittle manner at less than their recorded ultimate tensile strength. The most common methods employed in the numerical modelling of ground support and groundwater interactions in underground coal mines are the FDM and FEM (Anderson et al 2015), in cases where the rock mass behaviour in the area of interest is not significantly controlled by high-density discontinuities and the interaction with support elements. Groundwater discharge flow rates and bolt tensile stresses in a roadway of an underground coal mine was analysed using a FEM code, RS2 (Phase2 9.02), in this study. Each model had a run time of around 8 min
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