Laboratory Methods for Investigation on Stress Corrosion Cracking of Rockbolts and Cable Bolts

Saydam, Serkan (University of New South Wales) | Wu, Saisai (University of New South Wales) | Ramandi, Hamed Lamei (University of New South Wales) | Crosky, Alan (University of New South Wales) | Timms, Wendy (Deakin University) | Hagan, Paul (University of New South Wales) | Hebblewhite, Bruce (University of New South Wales) | Vandermaat, Damon (University of New South Wales) | Craig, Peter (University of New South Wales) | Chen, Honghao (University of New South Wales) | Elias, Elias (University of New South Wales)



Catastrophic failure of rockbolts and cable bolts, due to stress corrosion cracking (SCC), is a major problem in many underground excavations that can compromise both safety of the workers and the economic viability of the operations. This paper reports on development of laboratory instruments and methodologies at UNSW Sydney for simulating SCC in laboratory environments. Both representative coupon testing, and full-scale rockbolt and cable bolt testing methodologies are presented. Coupled with a detailed environment characterisation and field tests, the laboratory methodologies will aid in further understanding of SCC and identifying the potential countermeasures to prevent SCC occurrence in underground excavations.

1. Introduction

In underground structures, excavation of rocks reduces the confining pressure on the surrounding rocks, allowing the strata to separate, fold and buckle into the void created (Aydan, 2018). Because rock is weak in tension, this buckling action can lead to fracturing of the strata and a roof failure. To prevent the relative movement and fracturing of the strata, rockbolts and cable bolts are often used to stabilise an excavation (Chen et al., 2016; Hadjigeorgiou and Potvin, 2011; Kilic et al., 2002; Oliveira and Diederichs, 2017; Windsor and Thompson, 1994). Rockbolts used in underground coal mines are usually manufactured from steel rods, typically 22 mm in diameter and 1200-2200 mm long, which are installed by drilling a hole into the rib or roof strata. Cable bolts are an evolution of rockbolting technology which are usually comprised of a number of wires wound together around a central king wire. Cable bolts usually offer a higher flexibility and load capacity than regular rockbolts (Chen et al., 2015; Galvin, 2016; Windsor, 2004). These, together with cable bolts greater length, allow for anchoring to a greater depth where the potential of presence of stable rockmass is high.

With the decline in the global coal reserves accessible for open-cut mining, underground mining at greater mine depths has increased the reliance of coal industry on rock reinforcing techniques. As the mining operations continue in greater depth, rockbolts and cable bolts encounter more challenging geological conditions. In the past few decades, a particular attention has been paid to failure of rock bolts and cable bolts in underground mines. One of the main causes of such failures has been identified to be stress corrosion cracking (SCC), which had been simply overlooked in the past. SCC requires synergistic occurrence of three key elements: stress, an appropriately corrosive medium and a material susceptible to SCC (Gamboa and Atrens, 2003; Jones, 1998). This synergy is described in the schematic shown in Fig. 1. The conditions required to induce SCC vary depending on each of the key element. The stress required to induce SCC is usually below the yield stress of the material. Stress corrosion cracks generally grow at a slow rate until the stress in the remaining section exceeds the fracture strength of the material, at which point the material will fail (Enos and Scully, 2002; Scully, 1975; Wu et al., 2018b). SCC results in a dramatic reduction in mechanical strength with only a very minor removal of material. In most cases, SCC is not noticeable by a casual inspection. Structures affected by SCC generally fail in a fast, sudden, brittle and catastrophic manner (Schweitzer, 2010).