Artist’s impression of how Auckland’s Quay Street will look like when the Downtown project is completed next year
Strengthening the Quay Street seawall for another 100 years
Running the length of Quay Street is an historic seawall that has retained and protected land reclaimed over 100 years ago to form Downtown Auckland.
Over the last 18 months, strengthening of a 600 m section of the seawall has been underway and lays the essential foundations for the Downtown transformation. The project has also been hitting some critical deadlines.
Eric van Essen, programme director of the Downtown Programme, is happy with how the work is progressing, with full completion expected in early 2021. “We reached a major milestone early this year with the completion of the section between Queens and Marsden wharves and have just completed the Princes Wharf section.”
The strengthening has been approached in four sections, with the design team from Tonkin + Taylor developing different methodologies with the Downtown Joint Venture through early contractor involvement (ECI) to match the ground conditions, location and design requirements, and to minimise street-level disruption.
“Between Queens and Marsden wharves, a palisade wall approach has been taken which involves constructing 96 reinforced concrete piles along the length of the seawall. Each pile has a diameter of 1200 mm with pile lengths varying from 10 m to 25 m,” Mr van Essen says.
Seawall strengthening between Queens and Marsden wharves has involved constructing a palisade wall
For Princes Wharf, a different method has been required to avoid relocating the high concentration of underground utility services in this area. Jet grouting, a relatively new technique, involves drilling approximately 200 mm diameter holes into the East Coast Bays rock layer some 10 to 15 m below ground. A rotating nozzle then injects a water cement mixture into the soil, forming a column as it moves back from the rock up to ground level.
There are now 153 jet grout columns arranged in groups of four positioned approximately 3 m apart, extending 110 m along Quay Street in front of Princes Wharf.
Jet grouting has been used to strengthen the seawall beside Princes Wharf
With these seawall works completed, the enhancement of Quay Street with wider footpaths, an integrated bi-directional cycleway, raingardens and new street furniture can get underway.
Ferry Basin seawall strengthening
Another approach altogether has been adopted for the Ferry Basin and the area in front of the Ferry Building. Mr van Essen says in these two sections, the seawall is being strengthened by drilling a series of inclined rock anchors inserted through the seawall. These comprise high-strength steel tendons that anchor the seawall to the bedrock and protect it from sideways ground movement in an earthquake.
Inclined anchor drilling has been used for the Ferry Basin section of the seawall
The Downtown Programme is one of the largest urban transformation projects to be undertaken in Auckland, requiring the coordination of multiple related projects and deadlines. Mr van Essen says a strengthened seawall is the foundation upon which all the other projects depend.
“While strengthening works have been highly visible, requiring some very impressive construction and drilling equipment, the seawall itself is largely unseen beneath Quay Street. It is reassuring to know that this critical strengthening work will secure the resilience of the city against seismic events and climate change for another 100 years and beyond.”
Watch a video that shows the construction of the palisade wall between Queens Wharf and Marsden Wharf that was completed earlier this year: https://youtu.be/eDD1nGKxRU4