CRL UPDATE excavation completed

The City Rail Link is the largest infrastructure project ever to be undertaken here, creating a 3.45km twin-tunnel rail link up to 42 metres below Auckland’s city centre. It recently reached a milestone.

The project includes a redeveloped Britomart Station, two new city centre stations and a redeveloped Mt Eden Station, where the City Rail Link (CRL) connects with the existing Western Line.

Key dates:

• November 2018 – excavation completed

• Late 2018 – start of the construction of the tunnel box under Customs Street intersection

• Mid 2019 – tunnel box completed

• Late 2019 – trench backfilling completed

• Late 2020 – road re-instatement and streetscape works complete.

How much material is the Connectus JV using for the job?

• 4,060m3 of structural concrete poured – a total of 9,800m3 is needed to form the full Albert Street tunnel box

• 1,180 tonnes of steel reinforcing installed so far out of a total 2,500 tonnes

• 6,200m2 of waterproofing membrane so far out of a total 14,300m2

• A total of 50,000m3 of backfilling material into the Albert Street trench.

Completed works:

• Approximately 250 services relocated.

• 201 concrete slabs installed over the trench, providing a suspended working and public traffic deck along Albert Street

• A total of 152 struts, 142 walers and 76 rock dowels installed as ground support for the trench walls

• 516m of stormwater pipe realigned using micro tunnelling techniques and 40m of a 100-year-old sewer main strengthened.

Connectus (the McConnell Dowell and Downer joint venture) is delivering Contract 2 for the CRL.

This involves the construction of rail 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 the diversion and protection of a large number of underground utilities.

Connectus has now started backfilling the trench above the tunnel box. Behind the hoardings along Albert Street, the Connectus team reached another big milestone on October 8 commencing the backfilling of the trench. With half of the Albert Street tunnel box construction completed, it is now time to start working back up to road level using a mixture of materials.

The first 70 metres are getting backfilled with 10,000 cubic metres of materials. In total, 50,000 m3 of material will be used to backfill the Albert Street trench – the equivalent of 20 Olympic swimming pools.

“The start of the backfilling is a big milestone for the team,” says Aaron Ghazal, Connectus Site Engineer in charge of the backfilling.

“The team has been working very hard on Albert Street for the last three years and reached quite a few milestones, such as services relocation, piling, supporting services, bulk excavation and tunnel box construction.

“This milestone marks the start of the last stage of the Albert Street works before we reinstate the road.”

The excavation programme is now 91 percent complete. The last 5000 m3 are being dug and removed from the trench. In total, 77,000 m3 of spoil will be removed – the equivalent of almost 31 Olympic swimming pools.

Meantime, City Rail Link’s Albert Street tunnel box is well underway, with more than 50 percent completed. The total length of the tunnel box to construct is about 350 metres and 13 metres wide. So far, 240 metres of floor have been constructed, 192 metres of walls poured and 108 metres of roof completed. The tunnel box is being constructed in 29 sections, from the southern (Wyndham Street) end downhill to the Albert/Customs Street intersection.

The first 60 metres of the trench have had their lower struts removed. The team is now waterproofing and protecting the tunnel rooftop at this location, ready for backfilling.

“The project has come a long way since construction started in December 2015,” says Mark Anderson, MCD area manager.

“A considerable amount of temporary works has been completed over the past couple of years in order to enable the construction of the permanent concrete tunnel box at the base of the cut and cover trench.

“This is an enormous achievement for everyone involved, and it is exciting to now be able to walk through sections of completed tunnel box, 18 metres below the road surface.”

Grant Maclean, MCD site senior executive and project engineer, is in charge of making sure the team complies with the Underground Health and Safety Act and mining regulations. Although there are only 30 metres of tunnelling under Customs Street, the stringent controls still need to be in place and monitored.

Completing this section of work will connect the City Rail Link tunnel on Albert Street to the tunnels built through the adjacent Commercial Bay site, while keeping traffic and pedestrians moving above ground on Customs Street at all times.

Last month, the team installed the temporary struts and walers to support the walls of the cut and cover trench.

Currently, the team has passed the halfway mark through the excavation under the Customs Street traffic deck.

“It was a great achievement to complete the installation of 80 tonnes of walers and props to support the excavation within the tight confines of the underground environment and with limited machinery assistance at times,” says Maclean.

“We now move on to complete excavation of the remaining section over the next eight weeks, working two 10 hour shifts per day in an urban environment to achieve our targets. This requires careful consideration of planned night works in order to minimise disturbance to our neighbours.”

Grant Myocevich, MCD superintendent.

The sequencing of the tunnel box construction

To build the tunnel box, there are phases the team goes through for each section. The construction starts with pouring the floor and shotcreting the walls, then the team install a waterproofing membrane on floor, walls and roof, before tying the steel bars together ready for the concrete pour.

Once three sections of the tunnel box have been built, the team can remove the struts. They then waterproof the top of the roof and pour the final protective concrete layer. The remaining 14 metres of trench is then progressively filled up with backfill materials.

The final stage is the reinstatement of the road. This sequence will continue all the way down to the Customs Street intersection.

“This project has been very challenging, working on one of the busiest main streets in central Auckland,” says Grant Myocevich, MCD superintendent.

“The site is extremely confined. Good logistical coordination is critical to ensure we get concrete, pumps and steel deliveries daily. Good communication between engineers, supervisors, the workforce and the deliveries is a must on this project.”

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