The Connectus JV team is delivering Contract 2 for City Rail Link, which represents about 10 percent of the 3.45 kilometre length of the twin-tunnel underground rail link in central Auckland.
CONTRACT 2 INVOLVES the construction of twin tunnels within the Albert Street road corridor from Customs Street West to Wyndham Street, together with a new stormwater line from Swanson Street to Wellesley Street, and a large number of underground utility works, involving multiple network utility operators such as Vector, Vodafone, Chorus and Watercare.
From late 2015 to mid-July 2017, Connectus – the McConnell Dowell and Downer joint venture – has prepared for excavation of the City Rail Link’s trench and rail tunnels.
Working in the midst of New Zealand’s busiest city, the Connectus team commenced bulk excavation on Albert Street on July 17. They are digging to a depth of 18 metres at the deepest point (see photo), about the same height as a five-storey building.
The bulk excavation started from the southern end at Wyndham Street and will progress towards Customs Street. The excavation at the southern end will be completed late October while the northern end is expected to be completed mid-next year.
To build the underground rail tunnels up Albert Street from Customs Street to Wyndham Street, a 13-metre wide by 18-metre deep trench is excavated from the surface. The method used to excavate the trench is called ‘cut and cover’. The main reason for using the cut and cover method over a tunnel boring machine (TBM) is that the space along Albert Street is too shallow for a TBM and a number of underground utilities are running down the length.
Pre-bulk excavation works
From Wyndham to Customs streets, the top 1.5 metres of the road is removed and struts are installed on the eastern and western side. The struts prop the trench walls up as Connectus excavates down to the tunnel box level.
On the eastern side, concrete panels are laid to form a construction traffic deck for trucks and excavators to drive along the trench. The western half of the trench is left open in order to remove spoil from the trench via the eastern construction deck for the deeper excavation.
The section from Wyndham to Swanson Street was completed by June 26.
The section from Swanson to Customs Street started on July 17 and is expected to be completed in late October 2017.
There are about 40 staff working on site and 30 people working in the office. This will increase to around 60 people on site by October 2017, when Connectus starts preparing the construction of the tunnel boxes.
The deep excavation process
A 20-tonne excavator is located on the deck and excavates down to approximately three metres. A smaller 1.5 tonne digger is lowered into the trench to dig sections that the 20-tonne digger cannot reach from the deck. A bulldozer also works in the trench to move soil to areas where the bigger excavator, located on the construction deck, can remove it. When the excavation is down to approximately five metres, a long-reach excavator will replace the standard 20-tonne excavator and the 1.5-tonne digger will be replaced by a 5-10 tonne excavator. The long reach will be able to excavate down to 10 to 12 metres.
Around 250-400 cubic metres of soil is removed every day from the trench, and a total of 77,000 cubic metres will be removed by the end of this project. This volume is equivalent to almost 31 Olympic swimming pools. This soil goes to the old Three Kings quarry to help with construction of a large new housing subdivision (for more information go to: www.threekingsquarry.co.nz).
In summary, the bulk excavation works follow in behind the top 1.5-metre excavation. Part of the bulk excavation process is ensuring that the trench walls are stable and are not at risk of erosion or falling in. This is achieved by using a technique called “shotcreting”. Concrete is sprayed on the walls to a height of about 1.5 metres to reinforce and stabilise the trench walls and limit water infiltration.
The Swanson Street shaft was completed and reinstated on June 2. The TBM, affectionately named “Valerie” (after NZ’s world champion shotputter, Dame Valerie Adams), has completed a new two-metre diameter stormwater pipe between Wellesley and Swanson streets.