This paper presents the results of on-going investigations in several underground hard rock mines in Canada. Since the summer of 2007 a series of regular site visits was undertaken in order to monitor the performance of support systems. Site visits originally focused on visual assessment of the performance of support systems and collecting failed support elements for subsequent laboratory investigations. Furthermore, water samples were recovered and analyzed. The influence of atmospheric corrosion on the loss of capacity of support systems was also recorded. Underground mine atmosphere is characterised by high humidity, gas from engines and blasting and fine dust particles of different mineralogical composition. The presence of these conditions has an adverse influence on the ability of steel based support systems to withstand corrosion. A field testing program was designed to evaluate the type and rate of corrosion in the encountered underground mining environments using corrosion coupons. Coupons were recovered at regular time intervals and were subsequently tested in the laboratory. Tensile strength tests were performed on corrosion coupons installed in different underground conditions to establish correlations between the observed corrosion rate (mass loss) and reduction of steel strength. This has significant implications on the long term performance of support systems in corrosive mining environments.
The design and installation of reinforcement and support systems in underground mines is well advanced. Nevertheless, despite great strides in design and quality control practices it has been difficult to eliminate ground falls. Although ground falls can be attributed to a range of causes such as inadequate design and poor installation, a common problem is the reduction in the capacity of support systems over time. In certain cases this reduction in capacity can be attributed to the corrosion of support systems. The impact of corrosion is illustrated in Figure 1 where a corroded metal strap has sheared and failed, i.e. it no longer provides the support for which it was designed. The loss of support capacity over time is not explicitly considered in the design of support systems. Quite often the emphasis would appear to be on fracture analysis to identify whether support corrosion was a contributing factor following ground falls [1]. Some operating mines have made efforts towards developing classification systems [2, 3, 4], pertaining to the susceptibility of an environment towards corrosion. For a variety of reasons these classification systems have not gained widespread acceptance. This paper summarizes on-going work to gain an improved insight on the factors that contribute to the corrosion of support systems in selected Canadian mines. The investigation program combines on site analysis with comprehensive laboratory experiments and testing. The long term objective is to develop sufficient data and understanding that can be used in the selection of appropriate support strategies, and eventually to a reliable assessment of the predicted useful life of a support system.
Previous work by the authors focused on monitoring environmental conditions at several underground mines displaying a range of mining and operational conditions, [5, 6].
