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NZCN’s editor Lynne Richardson with City Rail Link head of delivery Scott Elwarth inside the tunnels

What lies beneath – By Lynne Richardson

At the bottom of Auckland’s Queen Street lies the Chief Post Office, and below this is Britomart Train Station and the new twin tunnels for the City Rail Link. But how do you hold up a 14,000 tonne heritage-listed building while two tunnels are constructed beneath it and the tolerance for damage is zero?

The Auckland Chief Post Office (CPO) was built in 1909–1912 at a time when the state-owned postal service was vital for public welfare and communication. The landmark building was located at the hub of the city’s transport system – next to the ferry building, tram terminus and original railway building, as well as the commercial wharves – befitting its importance, and was opened by Prime Minister William Massey in 1912.

According to Heritage New Zealand, the Category 1 listed building was designed in the Imperial Baroque style, which was used for many public buildings of the time, including the Auckland Town Hall. White Oamaru stone and Coromandel granite make up the main facades.

The Post Office provided a variety of services, such as postage, banking and the payment of pensions on its ground floor, while its first floor contained offices, including an electoral office for MPs – it was used by Prime Minister Robert Muldoon in the 1970s. The Post Office occupied the building until the early 1990s, and in the early 2000s it was converted into the new Britomart Station as the city terminal for the rail lines.

The CPO foundations encased in their steel collars with the weight of the CPO transferred onto the supporting beams

CRL is realised

In 2003, Auckland City Council’s transport committee received the business case for extending the rail lines from Britomart to run under the CBD to connect with the Western Line near Mt Eden Station, creating an underground inner-city link, but it would be a decade before a confirmed route would be identified and agreement would be reached with Precinct Properties, owners of the neighbouring Downtown Shopping Centre, under which the tunnels would run.

On 2 June 2016, Mayor Len Brown, Auckland Transport chairman Dr Lester Levy, Prime Minister John Key and Minister of Transport Simon Bridges officially marked the start of construction of the City Rail Link (CRL) with a ground-breaking ceremony outside the CPO in Lower Queen Street.

Behind the scenes though, planning and design work was already well underway. In the previous year in April, two joint venture (JV) design and construction ECI (early contractor involvement) contracts had been awarded to provide early input into the CRL design for the downtown area.

A JV consortium of Downer NZ and Soletanche Bachy (DSBJV) was chosen to progress the CRL work through Britomart Station, under the CPO to Queen Street and up to the site of the former Downtown Shopping Centre. Known as Contract 1 (C1), this included establishing temporary accommodation for Britomart Station’s ticketing and customer service operations, and, critically, the underpinning of the CPO building to allow the construction of the rail tunnels beneath.

A second JV, known as Connectus (comprising McConnell Dowell and Downer) was awarded Contract 2 (C2) to construct the cut-and-cover tunnels under and along Albert Street from Wyndham Street to Customs Street up to the former Downtown Shopping Centre.

C1 and C2 adjoin the site of the old Downtown Shopping Centre, being developed by Precinct and now known as Commercial Bay. The tunnels across this section were delivered by Precinct in a design-build model with FCC as contractor. These three sections have now been linked together and constitute 610 m of CRL tunnel from Britomart Station to Wyndham Street.

Supporting the CPO

Head of delivery for City Rail Link, Scott Elwarth, says buildings have been physically moved out of the way before to make way for tunnel construction – like the old Birdcage Tavern near Auckland’s Victoria Park road tunnel – but there was no room in central Auckland to do that for a building as large and heavy as the CPO.

“It’s one of the most historically important buildings in the country – a building with a top heritage rating. All our planning, design and construction of the tunnels has been dominated by the need to protect the CPO from any damage. Add in the tight working conditions for our teams under all that masonry and concrete and the ‘live’ Britomart Station on the other side of the wall, and you’re dealing with a challenging engineering operation,” Mr Elwarth says.

Instead, 64 diaphragm walls (or D-walls) were first built by the DSBJV (both outside the building in Lower Queen Street and then inside the building) using a 90 tonne piling rig affectionately named Sandrine by the JV team, after Sandrine Mussier, a long-time employee of Soletanche Bachy.

Inside one of the twin tunnels – the scaffolding and formwork used to create the concreate walls and roof are now being removed

D-walls are essentially panels of reinforced concrete poured directly into the ground and are the ideal solution in Lower Queen Street where the crews are working in reclaimed ground below sea level. The D-walls have been sunk 20 m below ground to the bedrock and have formed the supporting structures for the CPO during construction of the tunnels; they also prevent groundwater ingress into the tunnel box excavations.

Constructing the D-walls outside the building was relatively straightforward, with six walls poured straight from concrete supplier Allied Concrete’s trucks. However, inside the CPO, with a headroom of just 6.5 m, this wasn’t possible and the concrete had to be pumped in.

A challenge was encountered where the D-walls required significant steel reinforcing content, leading to congestion and constructability issues. This was overcome by introducing a smaller number of larger-diameter bars – the first time that 50 mm diameter high-strength seismic bar was commercially produced in New Zealand.

“Sourcing suitable steel from overseas, where steel of this size is usually manufactured, proved a problem because offshore manufacturers could provide it only in quantities much greater than what we required,” says Mr Elwarth, “so we liaised with New Zealand based suppliers over the possibility of manufacturing the bars locally, and Pacific Steel, based in Otahuhu, took up the challenge.”

Transferring the weight

Over an eight-month period, the ground below the CPO was removed in a ‘top down’ process following the construction of a series of post-tensioned concrete underpinning beams and installation of a number of steel underpinning frames.

Each of the CPO’s original columns was encased in a post-tensioned steel ‘collar’, which effectively ‘gripped’ the column. Load transfer from the existing foundations onto the underpinning frames was achieved by hydraulic flat jacks beneath the corners of the collars exerting a vertical load until either a maximum column movement (restricted to only a couple of millimetres) was met or until the calculated maximum load on the column was reached. This then meant that the column load was distributed across the underpinning structure and down the diaphragm walls into the bedrock. Each column foundation, now no longer bearing any load, could then be demolished.

A waterproof membrane is being applied to the tunnel box exterior before the 14 m deep excavation is backfilled to road level and the pavement reinstated

In this manner, the load of the CPO columns located within the tunnels’ alignment was transferred to the new foundations, allowing the JV team to excavate down to a depth of 14 m under the building for the tunnel boxes.

The east and west facade walls of the CPO (which lie above the tunnels) have also been supported through the use of post-tensioned concrete beams, tied together with cross-beams.

“This was a tremendous piece of engineering by the designers Aurecon and by the constructors DSBJV,” says Mr Elwarth. “The new foundations have completely protected the CPO from any possible movement, while providing a safe working environment for our crews.”

Indeed, the tolerances for movement are tiny. Right around the building, the JV team have installed reflective prisms, part of a network of monitoring total stations that ‘sweep’ the building every 20 minutes and feed data back to a central monitoring station. If any movement other than a predetermined amount is recorded, an automated alert is sent to the project team.

“With a building of this nature, we can expect movement of a few millimetres just with diurnal temperature changes, and we’ve used geotechnical modelling to determine what degree of movement may be created by say ground settlement, the foundations moving, or any other reasons,” explains Mr Elwarth.

The next stage

The two tunnels vary in width and height, depending on their location, but are typically 5.8 m wide and 6.3 m high. The extra height is to accommodate the overhead electrified lines above the trains and allowances to accommodate specific trackform below to minimise noise and vibration effects. Poured in situ, the base slabs for the tunnels were laid during February to June this year, with the tunnel walls commencing in April and construction of the roofs starting in June.

The tunnels line up with the existing lines on platforms one and five within Britomart Station, then curve gently under the CPO. Outside below Lower Queen Street, the twin tunnels run side by side and continue through a 25 m wide trench, curving under the new Commercial Bay tower (which replaces the old Downtown Shopping Centre) before linking seamlessly with the tunnels built under C2 and extending under Albert Street for 610 m as far as Wyndham Street. The tunnels have a gradient of 3–4% as they climb south.

Inside the CPO, work has now started on the intricate job of transferring the building back onto its new permanent foundations. The weight transfer will be gradual, over several weeks, and includes removing some 350 tonnes of steel used for the underpinning structures. “It’s a delicate, careful and well-planned operation,” Mr Elwarth says. “People will not notice any change to the CPO. It’s a very short journey – three millimetres at most – but it’s one of the most demanding engineering jobs undertaken in New Zealand and one rarely done overseas.”

Inside Britomart Station, work is underway to install new escalators and stairs

Inside Britomart Station, work is underway to convert it from a dead-end to a through-station. The northern curved blockwork wall has been removed and the stairs between the station’s platforms and intermediate levels have been demolished and narrower sets of stairs constructed to allow for the installation of an additional escalator. Four new metro-grade escalators will be installed, replacing the existing three commercial-grade escalators. Access and egress for station passengers is being maintained at all times.

The final stage of C1 will be to reinstate Lower Queen Street as a public plaza and return the CPO to its former grandeur, with the main entrance for Britomart Station on the ground level. It is expected that the CPO will reopen as a transport centre late next year. The temporary entrance and ticketing facilities, currently located on Commerce Street behind the CPO, will then be demolished.

It’s been over three-and-a-half years of construction to date for DSBJV on C1 and while they are now closing in on the final stages, the CRL’s next big contract of works is just about to start uptown.

The CRL, jointly funded by the NZ government and Auckland Council, is due for completion in 2024.

Watch a time-lapse video showing construction of the CRL tunnels under the CPO in Auckland’s Lower Queen Street here

Lynne Richardson is the editor of New Zealand Construction News and FTD - Supply Chain Management Magazine



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