Risk Based Approach In A Spillway Upgrade

In line with worldwide best practice, major dam upgrades are currently being undertaken in New South Wales (NSW) to ensure the continued safe operation of these assets in line with the Dam Safety Management Guidelines set out by Australian National Committee on Large Dams (ANCOLD) and NSW Dams Safety Committee requirements. Burrendong Dam, which is one of the major dams owned by State Water, is located in the greater Murray Darling River catchment on the upper reaches of the Macquarie River. The dam is currently being upgraded to increase the flood handling capacity of the spillway to cater for extreme flood events.

The conventional spillway design was found to be expensive therefore State Water embarked on a risk based approach to reduce the risk below As Low as Reasonably Practicable (ALARP). Saddle Dam A, which is located approximately 1.5km east of the existing spillway, was considered to be a viable option for an auxiliary spillway. The main issue was, however, the presence of a shear zone containing brecciated rock and clay gouge, which consisted of broadly graded silty sandy gravels and sandy clays. Hence, the potential for piping and foundation erosions were identified as the major risks for the project. The conventional design approach of a traditional concrete apron slab with ground anchors was found to be very expensive. Therefore, the erosion risks for not providing an apron slab to protect the shear zone were thoroughly investigated. The geotechnical conditions were analysed in order to satisfy the nominal criteria for energy dissipation of spillway discharge flows into the natural environment with an acceptable degree of erosion risk. The outcome of this analysis led to a significant reduction in cost as well as risk.

This paper focuses on the analysis and design of the geotechnical aspects adopted in the risk based approach, which led to significant cost savings to the spillway upgrade project. This paper is intended to provide valuable information for geotechnical professionals handling similar types of projects – especially where there is a potential to reduce cost by accepting a risk based approach.