In the wild north-west of Australia’s island state Tasmania, unpredictable weather and rugged terrain presented a raft of logistical challenges when upgrading Murchison Dam. In order to ensure it is prepared to handle extreme rainfall events, upgrades to the dam will increase its flood capacity.
Constructed between 1978-1982, Murchison Dam is part of the 506MW Pieman scheme. The dam harnesses the power of the Murchison River, channelling its flow through the 6m Sophia Tunnel to Lake Mackintosh, the main storage for the Pieman Power Development. The tunnel discharge is uncontrolled and depending on the Murchison River inflow, it may lead to storage level fluctuations.
At 93m high, Murchison Dam comprises a 217m long concrete-faced rockfill embankment with a free-overflow frontal-approach and a side-entry concrete spillway on its left abutment. The spillway is 13m wide at the base, 10m high, 160m long, and is founded within a cut into a dolerite bedrock.
The spillway chute consists of reinforced concrete-lined slabs and chute walls that are both 300mm thick. The slabs and walls contain a single layer of reinforcement and are anchored into the surrounding rock foundation using 24mm-diameter rock anchors on a nominal 1.5m grid spacing, embedded 3m or 5m into the rock.
Two-stage upgrade
The dam is being upgraded in two stages. The first was completed in October 2020 and involved a 3m dam crest wall raise, shown in Figure 2, achieved using a combination of insitu and precast concrete construction.
