The $3.4 billion City Rail Link has reached another milestone with the start of bulk excavation work for the cut-and-cover rail tunnels under Albert St in downtown Auckland.

Excavation will start at the Wyndham St end and move towards Customs St.

For those interested in watching the big dig, the CRL project team and the Connectus joint venture contractors have provided viewing windows at the Wyndham St pedestrian crossings.

The work will be undertaken progressively from Wyndham St at the southern end to Customs St at the northern end. Excavation at the southern end, to a depth of 18 metres, is expected to be complete by October this year and the northern end by the middle of next year.

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The tunnels will then be constructed with a cast-concrete floor, walls and roof before the trench is backfilled.

By spring 2019, this section of Albert St will be reinstated with a new road surface, bus lanes, widened footpaths and street furniture.

CRL project director Chris Meale says the start of bulk excavation is another milestone for the project.

Cut-and-cover construction is being used at each end of the CRL tunnels - between Britomart Station and the future Aotea Station on Albert St and, later, where it connects to the western line at Mt Eden. Between Aotea and Mt Eden stations, the tunnels will be between 13m and 42m below ground and bored using a 7m diameter tunnel-boring machine.

The first stage of the project, from Britomart to the Wyndham St intersection with Albert St, represents about 10 per cent of the 3.4km twin underground tunnels between Britomart and Mt Eden.

For more information, visit https://www.cityraillink.co.nz/crl-cut-cover-tunnel-excavation/