Client: Downer Soletanche Bachy JV

Location: Auckland

The City Rail Link (CRL) is the largest transport infrastructure project ever to be undertaken in New Zealand. It is a 3.45-kilometre twin-tunnel underground rail link, up to 42 metres below Auckland’s city centre, that will transform the downtown Britomart Transport Centre and provide a better connection to the Auckland rail network.

With its technical expertise and ability to deliver a high-quality product consistently, Allied Concrete was proud to be the preferred supplier of concrete to this large project. A project of such scale, the construction of CRL has been divided into several different contracts, with Allied Concrete chosen as the main concrete supplier to Downer NZ and Soletanche Bachy (DSBJV) who were awarded Contract 1 (C1). The project team consist of Auckland Transport, Soletanche Bachy, Aurecon, Freyssinet, Allied Concrete, Acrow and Downer NZ.

C1 took place from June 2016 to September 2021 and included construction of tunnels from Britomart Station, westwards underneath historic former Chief Post Office (CPO) building (which was converted into a central transport hub and termination point for Auckland’s train lines in 2001). Allied Concrete’s work included D-walls, underpinning beams, tunnel box, special low shrinkage concrete to replace grout in steel underpinning collars, flowable fill and no fines concrete.

A significant aspect of C1 was working with the CPO, a Category A-listed heritage building. To facilitate the tunnels built beneath, the CPO building had to be lifted just 2mm and placed on 340T of steel beams spanning between rows of new diaphragm walls built on either side of the tunnels. At 4000T, the stone building is the heaviest in New Zealand to be moved. Following construction beneath the CPO, the weight of the building was then transferred onto permanent foundations. The permanent foundations involved concrete underpinning beams consisting of approximately 830T of concrete structures, made up from 38 reinforced concrete beams – 32 of these post-tensioned using a series of stress bars and post tensioning multi-strand tendons.

One of the most technically demanding aspects of the concrete supply was the design of concrete mix to grout the collars used to transfer the load of the building onto the new diaphragm and tunnel walls. This work was carried out by sub-contractor, Freyssinet NZ. In this type of operation, a proprietary high flow shrinkage compensated grout mixed on-site is usually used. Due to the complexity, location, and size of the pours a ready-mixed solution was critical to the success of this part of the construction. The key construction objective was to avoid/minimise loss of bond between steel collars and concrete, and concrete with existing column in the middle. There were 10 collars in total requiring a volume between 0.39m³ and 11.3 m³. After extensive laboratory trials along with Freyssinet NZ, Allied Concrete developed a special high performance low shrinkage 50 MPa self-compacting concrete mix which could flow easily into the tight and constricted collars without segregating, and without requiring vibration to meet project requirements.

Performance of this mix throughout the project was excellent and it fully met the specified requirements of the project, ensuring this heritage building remained intact and able to continue to be an integral part of Auckland’s train lines.

Over the course of the project, more than 20,000 truckloads of concrete have been delivered to site, nearly 100,000m³ of concrete has been poured and over a dozen concrete mixes have been used – including some unique mix designs. On average, more than 880m³ of concrete is delivered to site every week. Fly ash has been used to replace virgin cement lowering the carbon footprint of the concrete and saving the fly ash from otherwise going to landfill.

When fully operational, 54,000 passengers an hour will use CRL stations at peak times. This is the rail equivalent of an additional 16 lanes of road or three Auckland Harbour Bridges.

CRL construction is planned to be completed in late 2024.