Laboratory Based Investigation of Stress Corrosion Cracking of Cable Bolts

Abstract

Premature failure of cable bolts due to stress corrosion cracking (SCC) in underground excavations is a worldwide problem with limited cost-effective solutions at present. To determine the cause and mechanism of SCC, identify potential technologies and eventually avoid catastrophic failure of cable bolts, a two-step methodology was implemented: (i) a long-term test using groundwater collected from underground mines, and (ii) an accelerated test using an acidified solution. Laboratory experimentation on both representative coupon and full-size cable bolt specimens was conducted. In the long-term tests, simulated underground environments were recreated in ‘corrosion cells’ which contained a newly designed cable bolt coupon together with a mixture of groundwater, coal and clay, to measure the potential for developing SCC. The incidence of SCC failures was not related to groundwater alone. Geomaterials in the corrosion cells accelerated the corrosion of cable bolts by increasing the concentrations of total dissolved solids and electrical conductivity of the water. Following this, an acidic solution containing sulphide, synthesised based on the chemical properties of groundwater from twelve Australian underground mines, was used as the testing solution for the accelerated tests. A coherent failure mechanism for SCC crack initiation, propagation and catastrophic failure of cable bolts was developed. Hydrogen embrittlement was determined to be the dominant mechanism for SCC. SCC only occurred when the environment could promote atomic hydrogen diffusion into the cable bolt, with the crack propagation rate being determined by the hydrogen diffusion rates. SCC resistance of full-size cable bolts was examined using designed tension loading apparatus and periodically increasing strain rate loading mechanism. The cable bolts with more wires took longer to fail. The cable bolts with plain wires provided greater resistance to SCC than those with indented wires. Galvanisation improved the resistance of cable bolts against SCC. The methodologies developed here can be applied to study SCC in other reinforcement materials and the results of this thesis can be used to develop guidelines to assess the environments causing susceptibility to SCC of high-carbon steel and set optimal support regimes in different geological and environmental conditions