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.
The second stage began in early 2023 and focussed on the raise of the right chute wall to improve the chute’s containment of the spillway flows. The wall raise structure varies in height from 2-5m above the existing right wall and is being constructed on the rock foundation behind it.
The new concrete structure will extend 110m and comprises 14 mass concrete monoliths with a total concrete volume of 1600m3.
Logistical challenges
The team faced a range of logistical challenges when carrying out the upgrades. Not only is the site in a remote location and subject to the wild weather of Tasmania’s North-West Coast, but the dam is prone to spill on an annual basis and there is a high risk of rockfall and treefall. Then there’s the challenge of placing 1600m3 of concrete on an area that’s difficult to access, at great heights and on a steep slope.
The contractor, designer and Hydro Tasmania representatives acknowledged these challenges from the outset and had to work within them.
One of the first logistical challenges was the remoteness of the site. To produce the large amount of concrete needed for the chute raise structure, the contractor had to establish a project-dedicated mobile concrete batch plant.
The Lake Murchison catchment is also known for rises and falls – historical data indicates the lake can rise up to 10m in 24 hours. The lake has a limited flood storage buffer and the Murchison Dam, located within a gorge with steep rocky slopes, typically spills annually.
The high likelihood of dam spill events during the construction phase was identified as a key project risk. Steps to mitigate this included the contractor minimising work within the chute, actively using lake-level forecasting models, ongoing daily lake level checks and the implementation of a Dam Safety Emergency Plan.
The project team witnessed several dam spill events during construction, with some lasting a number of days. With careful planning, works were able to progress during such spill events, as shown in Figure 3.
Access challenges
Gaining access to the structure’s rock foundation area was one of the biggest challenges. The complexity and magnitude of this challenge can be fully appreciated when standing on the dam crest and looking down onto the rock foundation footprint area.
The rock foundation area is only accessible by rope and starts at a height of 93m above the dam foundation before sloping steeply to its lowest point of 43m.
To gain access to the area, the project team upgraded the existing upper and lower crane pads from the 1980s when the dam was originally constructed. These are located on the opposite side of the spillway to the works. The upgraded crane pads provided sound and level surfaces to operate two 250t hydraulic cranes, as shown in Figure 4.
The onsite cranes performed the lifting operations required to construct the wall raise, but the first task was to install a stairway alongside the wall raise rock foundation.
A roped access team was required to construct the stairway footings and the stairway installation was a celebrated milestone on site. It enabled safe and easy access to the rock foundation area and marked the commencement of permanent works on the chute wall raise structure.
Constructing the chute raise
Preparation of the rock foundation area was undertaken. Some sections were too fractured and needed to be removed or treated with rock bolts. The next stage was the installation of 95 passive vertical rock anchors spaced at a nominal spacing of 1.5m along the wall raise footprint, as well as the installation of 15 sub-horizontal anchors.
The passive vertical rock anchors were designed to provide additional shear stability for the wall raise. They comprise 32mm diameter reinforcing bars with a double corrosion protection made of an inner grouting, corrugated polyethylene sheathing extending across the concrete-to-rock interface, and an external grouting.
Sub-horizontal rock anchors connected the chute lining and wall raise to the rock face. They were installed in areas where the rock foundation level extended above the top of the existing chute wall and were constructed on a 1.5m horizontal and 1m vertical grid. Their location was determined on site to avoid interference with the vertical anchors. The anchor installation work was sub-contracted to an independent engineering company, which demonstrated the effectiveness of an A-frame drill rig and the skill of working at heights, as shown in Figure 5.
As of November 2023, all anchors were successfully installed on site. Since then, the contractor has been connecting the monolith reinforcement to sacrificial frames and lowering them to the final positions. They shall then be fixed to the rock foundation with sacrificial footings, as shown in Figure 6.
The sacrificial frames were also used to internally support the formwork, as access and the foundation profile made it difficult to use traditional externally supported formwork.
Once installed, the contractor assembles the formwork around the frames and pours the monoliths in stages. The biggest risk is the potential for tensile stresses and cracking to develop due to the hydration of the mass concrete and the foundation’s restraint. The design allows for additional reinforcement to counter tensile stresses from shrinkage and temperature effects. The monoliths’ concrete (S15) also has a minimised cement content to reduce hydration heat and must be placed via kibble, which the cranes lower into its concrete placement position, as shown in Figure 7.
Collaboration key to success
A key takeaway from the Murchison Dam upgrade project has been the importance of establishing a collaboration framework and has led to Hydro Tasmania implementing this process in other projects.
The Murchison Dam upgrades are progressing well and the team acknowledges the efforts of many to get the project to this stage. Once completed, the upgrades will significantly increase the flood handling capacity of the dam, mitigating the risks of extreme weather events. The project is scheduled for completion in December 2024.
