Queensland’s largest water utility, Seqwater, is upgrading its 26 water dams as part of the Dam Improvement Program, based on modern modelling of earthquake and flooding risk. The Leslie Harrison Dam supplies around 25 per cent of Redland City's drinking water and the $24 million upgrade will strengthen the dam wall and spillway to improve safety.
Seqwater’s plan was to fortify the 50-year old dam to a modern standard, using 16 anchors and 80 dowels to strengthen the dam’s concrete spillway by pinning it to bedrock. The project included a new weighted berm against the dam wall – which widened and strengthened it – and anchoring the spillway at the crest and the lower end, which would improve resilience to extreme weather events and earthquakes.
Constructing the new spillway and weighted berm required deep excavations across Tingalpa Creek and a substantial dewatering exercise was required to keep the work site dry.
The principal contractor for the dam upgrade is the engineering and construction company, Fulton Hogan. Alan Black, Fulton Hogan’s Engineering Manager - Northern Region, says the 50-year old dam required upgrading and remodelling to new design standards, to make it comply with ANCOLD (Australian National Committee on Large Dams) guidelines.
“The engineers at Seqwater wanted the spillway to stand up under larger floods than it had originally been built for,” says Black. “They wanted the dam to be weighted at its base, and the spillway had to be rebuilt to be proofed against extreme weather and liquefaction.”
Liquefaction occurs during earthquakes when underlying silt and sand layers become liquid-like under intense shaking.
“The spillway had been built on sandy silts, and Seqwater wanted the remodelled spillway built on, and anchored to, rock.”
The solution was to have the embankment excavated and rebuilt on installed layers of sand, gravel, rock and earth. The crest of the spillway would be anchored into the bedrock.
The works required excavating nine metres deep, to rebuild the base of the dam and the spillway, and those works necessitated a significant ground dewatering exercise.
“The works area was 150 metres wide and nine metres deep, and we had to control the water level during the works.”
Fulton Hogan contracted Coates Hire’s Engineering and Technical Services Team, to dewater the site.
Coates Hire’s State Manager – Services, for Queensland, Kevin Frankowski, says the engineers from Seqwater, Fulton Hogan and Coates Hire collaborated on the designs.
“The forces at work on a major dam are significant and everyone on that project understood the stakes were high.”
Coates Hire’s engineers developed a series of 200 dewatering wells at 1.5 metres spacing and 6.8 metres deep, at two levels. The wells were drilled around the embankment, and to them were attached seven high-capacity pumps. The pumps extracted the groundwater at such a rate that the construction teams were able to excavate without any groundwater infiltration, ensuring the stability of excavated slopes.
“Our design had to draw the groundwater at a sufficient rate to keep the work site safe and dry,” says Frankowski. “We were also required to filter, chemically treat and monitor the extracted water, so it met the relevant environmental standards before it was released into the creek.”
Frankowski says the project was a complete solution from design, to installation, commissioning, operation and support. Dewatering downstream from a dam – and treating the water to allow ‘safe’ discharge to the environment – were real challenges.
“We had the equipment, the experience and the engineers. We brought in our own chartered geotechnical and dewatering engineer, to certify the design and installation.”
Alan Black, from Fulton Hogan, says the dewatering operation was critical. “If the flows increased, and the water table rose, we could have had a washed-out excavation area and the dam could have been undermined.
“We contracted Coates Hire to get the water table down to 6 metres, and keep it there. They monitored the water and their pumps, and they ensured our pumped water was filtered. It all worked well – we’re very happy with the result.”