The contract was awarded in 2022 to upgrade Murchison Dam, a 95 m Concrete Faced Rockfill Dam, with a free overflow frontal approach and side entry concrete spillway on the left abutment.

The aim of the project was to reduce the risk position of the dam for Hydro Tasmania, by providing additional flood protection, so the dam can safely pass floods of rarer frequency.

Risk mitigation was provided by the following project scope:

• Raising the existing RHS chute wall by up to 5m vertically, with a gravity block section, founded on a rhyolite rock outcrop,
• Raising the training wall,
• Spraying 100 m3 of shotcrete at the downstream toe, to provide scour protection during a spill, and
• Anchor investigations to confirm the integrity of passive anchors in the existing spillway wall and slabs.

The main component of the project was the chute wall raise, involving 1,700 m3 of concrete, and was constructed as follows:

• Install a 120 m long pedestrian stairway to provide access to the work front.
• Build two crane pads, consisting of heavily reinforced concrete and post-tensioned anchors to secure the pads back to rock.
• Installation of over 100 passive anchors using an A-framed drill rig.
• Development of an S15 mix design with a fly ash and cement blend that minimised heat of hydration, through several mix trials and a full-scale placement trial.
• Utilisation of timber formwork to form up concrete that interfaced with undulating rock.
• Design, fabrication and implementation of a steel formwork and access system for the upper pours.

The project was challenging, with extensive temporary works requirements and the need to work around a live spillway. The reservoir level in Lake Murchison can rise rapidly due to the catchment area being large relative to the volume of the storage, combined with outlet works providing minimal ability to control lake level.