Managing the risks associated with acid sulfate rock in NSW road projects

Nicholas Bridgement

Acid sulfate rock (ASR) is unweathered rock that contains metal sulfide minerals (commonly iron sulfides). When ASR is exposed to both oxygen and water, oxidation of sulfides leads to the formation of sulfuric acid, sulfates and salts. The probability of ASR being present, can to some extent, be predicted from the geological origins of the rock or later hydrothermal depositions of sulfides. An ASR risk map has been prepared to assist in the pre-design phase of road construction projects.

ASR has the potential to be problematic (depending on concentrations) with respect to environmental, structural and durability risks. It is becoming increasingly common for ASR to be encountered by roadworks in New South Wales where designs include deeper cuttings into unweathered rock that has generally not been the case historically. Examples are given of New South Wales where ASR has been encountered, together with an American example where significant environmental penalties and remedial costs occurred.

Other than low risk geological formations, site investigation for roadworks must include identification of ASR and, where present, screening, detailed testing and interpretation of the distribution of sulfide contents. The details of each aspect of this assessment need to be fully understood. Where ASR is present, the design, specification and construction must include control measures to reduce environmental risks associated with exposing ASR and potentially releasing leachate into the environment. Control measures include dilution, encapsulation and treatment with crushed limestone. Control measures must also be developed to protect structures such as bridges, culverts and retaining walls, stormwater drainage pipes and pavements. The locations of where ASR is placed within the earthworks formation must be limited with respect to environmental, structural and durability constraints. For successful management of ASR in construction projects, careful planning and staging of the earthworks is critical.