Rock Mass Characterisation of existing concrete dams – Making the most of limited historical data – Tinaroo Falls Dam

Understanding a dam’s foundation, including its rock mass properties and the shear strength of its inherent rock defects, is an essential component in ensuring its safety. This paper discusses the use of limited historical data to assess rock mass classification and defect shear strength, which were required as inputs to assess the stability of the concrete monoliths of one of Sunwater’s dams, built in the 1950s. The dam’s foundation comprises mostly massive granite, but no laboratory strength testing of the foundation rock mass, or its defects has been performed. A review of the dam’s foundation including a limited geotechnical drilling program was performed in 2009 to inform dam upgrade works at that time. The current review considered all available geological/geotechnical data, including original investigation borehole and limited geological mapping data, the 2009 data and its assessment, and ultimately contributes to understanding the current overall risk position of the dam. The review aimed to improve the estimates of foundation rock mass classification and rock defect shear strength by statistically reviewing the available historical data and applying the methodologies of Barton and Bandis (1990) and Hoek and Brown (1997). The analysis involved assigning three foundation types: sound foundation, hydrothermally altered foundation, and downstream hydrothermally altered. Sensitivity analysis was then performed to account for uncertainties in assigning the various parameters of the foundation rock mass.