Dam safety | Murchison Dam Upgrading


Various challenges had to be overcome when Hydro Tasmania undertook recent upgrades at Murchison Dam to enable it to safely handle extreme flood events


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.


Murchison Dam Constructed between 1978-1982, Murchison Dam is


Above: Figure 1: Murchison Dam


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


Below: Figure 2: The completed crest wall raise at Murchison Dam


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


to access, at great heights and on a steep slope. 22 | August 2024 | www.waterpowermagazine.com 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


of concrete on an area that’s difficult


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.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45