2017 – Spillway Concrete Scour Detection and Repair at the Base of an 84m High Arch Dam

Oliver Giudici

A common concern for large spillways is erosion of the receiving plunge pool and potential impacts on the stability of the dam.Devils Gate Dam is an 84m high, double curvature arch concrete dam, located in northern Tasmania and constructed between 1968 and 1970.The full 134m long crest is designed as a free-overflow spillway and spill flows impact the downstream valley sides and plunge pool below, where energy is dissipated to reduce riverbank erosion downstream.To protect foundation rock,the plunge pool and large portions of the valley sides were concrete lined with 450mm thick reinforced and anchored concrete. During spill events the area is inundated by up to 12m of tail-water.In 2016 damage to the plunge pool concrete was discovered by divers during a special inspection of the impact areas, but poor visibility limited the understanding of the extent and severity. Subsequent investigations, including detailed sonar scanning, improved the understanding but it was not until the plunge pool was fully dewatered that the full extent of the damage was quantified.The damage commenced around 35m downstream of the dam arch and consisted of approximately 330 square metres of moderately to severely eroded concrete and exposed, deformed, and in some areas completely removed reinforcing bars. The most significant feature was a penetration through the concrete up to 2.5m into the foundation rock.A number of stressed anchor heads were also damaged or destroyed.A full appreciation of the damage necessitated the decision for immediate repairs given the impending power station refurbishment (commencing January 2018) which will subject the plunge pool to nine months of constant spill.This paper outlines the diving and sonar investigations undertaken in 2016, discusses the challenging tasks of dewatering the plunge pool and gaining access through the narrow canyon, and presents the physical works to strengthen the damaged areas.It discusses the difficulty of identifying and treating such damage, and serves as a cautionary tale for other owners who have fully submerged plunge pools downstream of spillways.

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