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True resilience

007 93Q8089

The phrase “resilient infrastructure” gets thrown around a lot.

But for real “resilience” built into a project, look no further than Auckland’s NZ$230M (£130M) State Highway 16 Causeway Upgrade.

It improves, widens and raises a 4.8km long section of motorway to provide additional capacity and to safeguard against coastal erosion and flooding.

Ninety-thousand motorists per day use the route in the city’s west as a key route for the city and its airport.

007 92Q0748

007 92Q0748

The route heading south east crosses the Whau River Bridge, then Causeway Bridge.

The entire causeway will be raised 1.5m, with work completed in stages to minimise disruption to the community and environment.

To begin the phases, an eastbound embankment will be created on the motorway’s southern flank. With traffic then transferred on to the embankment, the raising work will expand northwards into the westbound lanes.

phases to raise causeway

phases to raise causeway

The causeway has been raised in phases to avoid disruption.

As the road space grows, the environment is still a key consideration. The route passes directly though the stunning Motu Manawa – Pollen Island Marine Reserve, 500ha of the inner reaches of Auckland’s Waitemata Harbour. In this national park, disturbance of marine life is strictly prohibited.

Aerial jpg

Aerial jpg

The 4.8km State Highway 16 Causeway Project contract, shown in green at centre.

Its intertidal mudflats, tidal channels, mangrove swamps, saltmarshes and shellbanks are unaffected by harsh weather most of the year. But flooding is a regular issue when high tides and storm surges combine. This is before even mentioning the future rise in sea levels from climate change.

Two ageing bridges lie within the 4.8km causeway section. They are the Whau River Bridge at the western end and Causeway Bridge, further east. As the most valuable assets in this stretch of work, they have pulled the majority of focus from the design team.

The team, or Causeway Alliance, is made up of client New Zealand Transport Agency, with international expertise from designers Aecom, Jacobs, Coffey, and Australasian contractors Fulton Hogan and CPB.

past conditions

past conditions

The causeway floods in poor weather conditions.

Aecom regional director Christian Christodoulou says the alliance model enabled innovative thinking.

“The client is there, day-to-day, so we can discuss with them what we want to do, and any efficiencies or value-engineered alternatives.”

Most of these efficiencies are to be found around the Causeway and Whau River bridges. Each bridge consisted of two separate structures: a northern structure carrying three-lane eastbound traffic, and a southern structure carrying three-lane westbound traffic and a cycleway.

007 93Q8089

007 93Q8089

The completed Whau River Bridge

The original structures dated back to the 1950s and 1960s with the northern structures initially carrying single lane traffic in two directions, prior to duplication by the southern structures. Further widening was then carried out in the 1990s.

At the western end of the project is Whau River Bridge, a 182m long, eight-span crossing comprising reinforced concrete rectangular deck beams and a deck slab cast monolithically on reinforced concrete supports. Causeway Bridge is shorter but of similar construction – a 75m long, five-span bridge crossing an inlet.

The client’s requirements were for Whau River Bridge to add two lanes to become a dual four-lane carriageway. Causeway Bridge is to get an extra three lanes so that it has four lanes eastbound and five lanes westbound. It will also get a minimum 3m wide pedestrian/cycle path.

052 93Q8182

052 93Q8182

An aerial view of the causeway widening and Causeway Bridge at top.

Widening the Whau River Bridge was a fairly straightforward task: additional deck width on both outer edges was provided through new precast prestressed concrete “super-T” beams and a reinforced concrete deck slab. The additional structure was cast monolithically onto reinforced concrete headstocks sitting on top of new 1.5m diameter, permanently cased, bored, cast insitu piles.

Meanwhile, on the Causeway Bridge, the client again called for a symmetrical widening. But a better, asymmetrical, solution was found.

This involved building a new, independent bridge structure to the north of the existing northern structure to carry four lanes eastbound. While the existing northern and southern structures would be combined into a single, wide deck to accommodate the five westbound lanes, along with the pedestrian/cycleway.

Whau cross

Whau cross

Whau River Bridge cross section.

Adopting this design was recognised as offering benefits in traffic phasing – the new structure was programmed for completion before modification works to the existing structures, allowing traffic to run on the southern structure, then the northern structure, before it was spread across both. Utitilising as much of the existing structures as possible also reduced the overall footprint made in the sensitive marine environment.

The new structure for eastbound traffic appearing north of Causeway Bridge comprises three spans formed of precast, prestressed concrete “super-T” beams and reinforced concrete deck slab, supported monolithically on reinforced concrete headstocks and permanently cased, bored, cast insitu piles.

The southern structure for westbound traffic at Causeway Bridge connecting the two existing structures, is achieved with transverse reinforced concrete headstock stitches, which also support new precast pretensioned concrete beams and reinforced concrete deck slab forming the deck infill at the median.

Causeway cross

Causeway cross

Causeway Bridge cross section.

These monolithic designs replicate the structural performance of the existing structures, to minimise any differential movement.

To make sure the existing structures were indeed up to scratch, corrosion investigations were undertaken. Many of the piers found to have visible signs of corroding reinforcement and chloride levels  measured in concrete were well above accepted industry thresholds.

Christodoulou says the client was well informed about the challenge ahead, but did not budge on any prior commitments.

052 93Q8182

052 93Q8182

An aerial view of the causeway widening and Causeway Bridge.

“They contractually required us that over the next 25 years the maintenance plan would be cut to no more than 20% of the capital for bridge replacement, about £4.5M. The client had a very clear idea of what they wanted.”

Christodoulou and the team then introduced the idea of a hybrid corrosion protection system. First used in the 90s on the Kyle of Tongue Bridge in Scotland, it was proven technology but a relatively new technique in New Zealand.

It involves a series of interconnected anodes installed into a grid of drilled holes over the faces of the substructure piles and portal frame legs. A current is applied to the anodes for an initial period of several weeks. Thereafter the current is switched off and the anodes act passively in galvanic mode for the remaining service life.

typical installation hybrid anodes

typical installation hybrid anodes

A typical installation of the hybrid anodes.

“It’s similar in principle to a zinc battery. It is a self-powered solution, you don’t need any expensive power supplies,” says Christodolou.

“It involves low capital and ongoing costs. The last thing we wanted to give them was something expensive and requiring experts to maintain and operate over the next 50 years.”

And when thinking about infrastructure threats in New Zealand, inevitably the focus turns to earthquakes. Auckland is not at high seismic risk like Christchurch or Wellington, but seismic activity is still a relevant concern for the government.

A structural assessment, along with digital modelling and spatial investigations ascertained the bridges’ capacity under dynamic loading. Under both 1 in 1,000 year and 1 in 2,500 year seismic event loading, only “minor” or “moderate” damage is predicted, and emergency traffic would be able to use the structures. Still, just for a little extra resilience, 20 piles were strengthened by a concrete jacket.

Overall, the alliance claims the causeway tender came in about NZ$50M (£28.3M) under the client’s initial expectations due to a range of measures. These included: reducing the alignment width; using alternative sub-water treatments; using alternative ground improvement methods; and cost savings built into the sub-structure refurbishment.

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