All go at Googong Dam21 March 2011
The timely spillway upgrade at Googong dam in Australia enabled the structure to handle the largest rainfall event since original construction was completed in 1978
The Googong dam is a 66m high zoned earth and rockfill embankment on the Queanbeyan river in New South Wales, Australia and is one of the main water supply reservoirs for the Australian Capital Territory (ACT) and surrounding region. The dam has an ANCOLD hazard category of ‘extreme’ because of its location upstream of Queanbeyan and Canberra.
The Googong dam spillway (GDS) upgrade project became necessary for two reasons. Firstly, after completion of the original dam and spillway in 1978, a series of relatively small flood events up to 400m3/sec caused dramatic erosion of the rock in the steep unlined portion of the spillway chute. Progressive remedial measures were undertaken to mitigate further erosion. The reliability of these measures was in question as they had not been tested in flows near the original design probable maximum flood (PMF) peak discharge of 4500m3/sec. Secondly, reviews of flood hydrology in recent years have increased the PMF peak outflow to 10,500m3/sec.
For these reasons, ACTEW Corporation engaged GHD Pty to develop and compare options and subsequently prepare a detailed design to upgrade the spillway and bring it in line with contemporary design standards. A review of delivery mechanisms was also undertaken, and an alliance structure selected because of the: uncertainty of approvals; potential for change as the projects evolved; and desire to be part of the delivery process.
The Bulk Water Alliance (BWA) came into being in May 2008. The BWA comprises ACTEW Corporation Ltd, (in cooperation with the dam owner ActewAGL), GHD Pty (the designer) and Abigroup Contractors in joint venture with John Holland (the constructor).
Construction commenced in February 2009 and was completed in December 2010. The project cost was A$56M (US$55.5M).
Existing spillway details
The existing spillway through the right abutment consisted of:
• A quarry forming the curved approach channel.
• An uncontrolled ogee crest with a crest length of 124m.
• A 64m long concrete-lined chute which converges to 64m at the downstream end.
• An 80m long unlined chute in rock with a width of approximately 64m.
• A stepped drop of approximately 24m to the downstream quarry which forms the plunge pool.
The upgrade project scope included the upstream training wall. This, on the left side of the spillway approach channel, was showing signs of outward movement and settlement of backfill behind the wall. Excavation, anchoring and concrete backfill works were undertaken to arrest this movement and restore structural integrity and functionality.
The existing spillway chute is a concrete lined portion of the spillway that was designed and constructed to accommodate the original 4500m3/sec PMF. Work in this area was to allow increased discharge capacity. Hence both spillway training walls were raised by between 2m and 6m. Over 1200 passive rock anchors were installed to increase the resistance of the spillway slabs to uplift pressures.
The chute extension was previously an unlined section of the spillway which had experienced significant erosion in flood events. As such, a large cavity in the rock was filled with mass concrete. The area, on a 23% grade, required installation of passive rock anchors and lining with a 400mm thick 32MPa concrete slab. The chute terminates in a small radius flip bucket. Large concrete gravity walls were constructed on both sides of the chute. These walls ranged in height from 9.5-17m, with a base width up to 9m. They were cast from two types of concrete simultaneously with the skin being 32MPa for durability and the core being 15MPa to assist with minimising peak temperatures to reduce the likelihood of cracking.
Work below the flip bucket was known as the downstream apron. It consisted of a series of steep, near vertical batters and horizontal benches. The profile of the apron dictated that mass concrete was needed to treat erosion on the left hand side and blasting of approximately 25,000m3 of rock on the right hand side. Staged excavation of blasted rock and both temporary and permanent passive rock anchors were required to address rock stability while a 300mm thick 32MPa structural concrete facing was required to prevent further rock erosion.
At the base of the downstream apron, a plunge pool for energy dissipation with dimensions of approximately 40m square by 10m deep was required. Another 20,000m3 of rock required blasting and excavation followed by passive rock anchors and structural concrete lining on the upstream face of the plunge pool as per the downstream apron.
In total, the project involved the excavation of 43,000m3 of rock, the placement of 23,000m3 of mass and reinforced concrete, plus the installation of 16,000m of dowels for anchorage of the reinforced concrete structures and as rock support.
Construction challenges and changes
The alliance framework allowed for a collaborative approach to change as the project developed and issues were identified. Key issues reviewed as construction progressed included:
• Increasing the width of the benches on the downstream apron below the flip bucket to improve construction and maintenance access.
• Revising the design of the approach channel training wall remedial works to account for the worse than expected structural cracking encountered once the backfill was excavated.
• Reviewing the construction methodology and sequencing for the concrete gravity spillway walls to minimise heat generation and thermal cracking.
• Reviewing the construction methodology and sequencing for the new chute slabs to increase production whilst maintaining the design intent of minimising joint opening and cracking.
• Replacing the plunge pool upstream face concrete lining with shotcrete to increase production and reduce construction risk.
The project benefited from ongoing drought conditions throughout much of the project, with reservoir levels generally hovering around 40-50% of capacity. The completion of the construction works was timely as heavy rainfall in the region during October to December 2010 filled the reservoir and it spilled for the first time in 12 years on 3 December 2010.
The last of the concrete pours in the spillway was completed in November 2010. A storm event on the night of 8 December 2010 resulted in a peak discharge of 550m3/sec, which was the biggest event to pass through the site since construction of the dam was completed in 1978. It was estimated that this event had an annual exceedance probability of about 1:20.
Over the first two weeks of December, the total rainfall was 3.3 times the monthly average, while the October and November totals were 1.5 to 2 times the monthly averages. The spillway was found to operate as anticipated during the design phase.
Through this basic alliance principle, the Googong dam spillway project developed a culture of planning all works or resolving any problem through innovative thinking.
Some examples where innovation improved the performance of the works were:
• The use of 1m3 precast concrete blocks for formwork in the 6,500m3 mass concrete backfilling of a void in the eroded spillway. This improved the programme and reduced any risk associated with installing conventional formwork.
• Design and certification of a concrete vibrator system that can be hung from the concrete pump when placing deep mass concrete pours negating the need for personnel to enter the pours.
• Design of an edge protection barrier system utilising surplus concrete piles from a previous project.
• Design and development of a water monitoring system in association with the discharge pump to shut the pump down above specified discharge limits.
Many challenges have been faced by the BWA team in delivering the project. Decision making has been a collaborative effort involving all parties based on transparency, trust, professional respect, integrity and abilities. Through this, a difficult project with many changes was delivered with the projected cost within budget, to the owner’s satisfaction and in line with the functional requirements.
Ben Greentree is the project manager for the Googong dam spillway upgrade. Email: [email protected]