Failure of the threaded region of rockbolts in underground coal mines

HSAC 840 steel rockbolts were examined, all of which had failed in service through the threaded region exterior to the rock face by brittle fracture transverse to the bolt axis. The bolts had been in service in an Australian coal mine in which the groundwater had a pH of 7.5-8.5 and a relatively high bicarbonate level of 515-2200 mg l(-1). All failures had occurred within the region comprising the domed washer and the end plate. The majority of the bolts showed a small thumbnail-shaped discoloured region 1.5-4.5 mm in length at the fracture origin, as is characteristic of stress corrosion cracking (SCC) in rockbolts. In half of these bolts, SCC had initiated immediately at the root of the thread, but in the other half of the bolts, SCC had initiated from a small pre-existing crack that had been produced in the thread as a result of a prior bending load.