Stress corrosion cracking of rockbolts: An in-situ testing approach

Abstract

Premature failure of rockbolts due to stress corrosion cracking (SCC) is an unresolved global issue in underground structures, particularly underground mines. To date, SCC of rockbolts has been produced in laboratory-based studies under laboratory conditions, however no attempt to produce SCC in-situ in actual underground coal mine conditions has previously been made. In this study, an innovative testing methodology was used in which a rockbolt coupon was developed with multiple stressed sections. The test coupon could be placed in-situ within a borehole in an underground mine, exposing it to the borehole environment, the surrounding rock strata and the groundwater. In-situ coupons were made from both 1355 and HSAC840 grade rockbolt steels and installed within a typical rockbolted horizon where known SCC failure of rockbolts had occurred. SCC occurred in the 1355 grade in-situ coupons, but not in the HSAC840 grade ones despite service failures of HSAC840 rockbolts having occurred in the same underground mine. The difference in behaviour of the HSAC840 coupons and in-service rockbolts is most likely due to different stress regimes acting on the coupons and the rockbolts. Localised corrosion was observed on both steels although analysis of the groundwater indicated that it had low corrosivity. Microbiological analysis showed that a range of bacteria known to be involved in corrosion were present on the rockbolt coupons and in the underground environment. This suggested that the occurrence of localised corrosion and SCC in low corrosivity groundwater could be due to the presence of microbial organisms, in particular, sulphate reducing bacteria. The results of this study provide detailed insights into the SCC of rockbolts in the underground environment. The method developed here can be used to study other reinforcement elements and stressed settings for stress corrosion cracking.