Spillway chute wall raise – monolith concrete pour by crane kibble bucket

News from Tasmania – Murchison Dam Spillway Upgrade

Hydro Tasmania’s Murchison Dam on Tasmania’s West Coast has been undergoing a two staged risk-based spillway capacity upgrade since 2019. Stage 1 involved a 3m dam crest wall raise which was completed in October 2020. Stage 2, a partial raise of the right chute spillway wall commenced in early 2023 and is ongoing.

The Stage 1 parapet crest wall raise involved insitu and precast concrete construction as shown in the typical drawing section (Figure 1). The completed wall is shown in Figures 2 and 3.

The primary focus of the Stage 2 work is to partially raise the right chute wall by between 2.0-5.0m through construction of fourteen structurally connected concrete monoliths. Secondary works include the following activities:

  • a small raise of the transition wall between the dam crest and the chute
  • downstream dam toe erosion protection to resist damage from back water eddy action during spill events, and
  • a spillway reinforcement and anchor investigation program.

As Lake Murchison has very little flood storage buffer and typically spills on an annual basis, avoiding working within the chute for long durations is a key project and construction risk. This risk has been mitigated by eliminating work within the chute for the chute wall raise, active use of lake level forecasting models and a robust Dam Safety Emergency Plan and scheduling downstream toe and spillway anchor investigation works for lower flood risk months.

Stage 2 works are progressing well in large part due to the established collaborative approach being applied by all parties including Hydro Tasmania as the Principal, AECOM as provider of construction design support and site engineering, and Hazell Bros Group as the constructor. Challenging site conditions, particularly in the winter months, coupled with a dynamic construction program require constant communication between project team members to anticipate and remain on top of emerging issues.

Completion of Stage 2 works is currently likely to be towards the end of 2024.

Author: Oliver Giudici, Acting Project Manager

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