Management of acid sulfate rock in a Sydney road tunnel

This paper describes the investigation and management of acid sulfate rock (ASR) conditions encountered in a Sydney road tunnel constructed in the Hawkesbury Sandstone. ASR conditions in Hawkesbury Sandstone have not been previously reported in published literature. The main focus of this paper is the investigation of ASR and its derivative products in an emergency egress long passage directly adjacent to a mainline carriageway. Sulfate induced heave has locally caused severe cracking, debonding and failure of shotcrete from the walls of the long passage. Rock degradation has also occurred on exposed rock walls in other parts of the tunnel as a result of sulfate efflorescence. Testing has revealed the sulfate is comprised almost entirely of one mineral; ferricopiapite, an iron sulfate member of the Copiapite Group of minerals. Given the unusual nature of the sulfate efflorescence and the proximity to the mainline, an intrusive investigation was carried out into the tunnel walls to assess the source and mechanisms responsible for the ASR conditions. The results of the study provide an important insight into potential hazards for current and future Sydney tunnelling projects and the need for ASR testing as part of geotechnical investigations for tunnels.