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Another milestone: Getting closer to Britomart tunnel excavation

Another Milestone: Getting closer to Britomart tunnel excavation

 

We’re a step closer to be able to start tunnel excavations in the former Chief Post Office (CPO) building.

Those excavations will turn Britomart into a through tunnel.

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Contract one of the CRL works involves Downer NZ and Soletanche Bachy (DSBJV) progressing the CRL work through and under Britomart Station and Queen Street to the former Downtown Shopping Centre site which is now Precinct Properties Commercial Bay development. 

They've achieved a new milestone.

This was the first area of the CPO building that has been load transferred onto one of the structural steel underpinning frames.

There are 12 frames.

This is significant because once all the load transfers are completed, we can then start tunnel excavations safely.

The weight of the building is carried by its columns down into the building foundations.

Before excavation of the rail tunnels can commence, a number of these foundations need to be removed – so the associated weight of the building firstly has to be transferred onto the underpinning frames.

If you're interested in the technical detail....

Each affected column is clamped by a steel collar which sits upon a steel beam located either side of the column. A hydraulic jack is located beneath each corner of the collar (four jacks in total per collar) and these jacks are used to exert a force on the underside of the collar equivalent to the building weight carried by the column.

  NORTHERN END:  The underpinning beams inside the former Chief Post Office at the northern end

NORTHERN END:  The underpinning beams inside the former Chief Post Office at the northern end


  SOUTHERN END:  The underpinning beams inside the former Chief Post office at the southern end

SOUTHERN END:  The underpinning beams inside the former Chief Post office at the southern end


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Each underpinning frame consists of two collars and the two beams. The beams span between two rows of diaphragm walls which were installed earlier during the project and are fixed to the capping beams which sit upon the diaphragm walls. Each frame supports two CPO building columns and the weight carried by both columns are transferred concurrently onto the frame during the load transfer for that frame.

During load transfer, the force exerted by each jack is slowly increased by increasing the hydraulic pressure within the jack and it is closely monitored along with any ‘lift off’ (displacement) experienced by the column. When the jacks exert a maximum specified force (predetermined by calculating the weight of the building carried by each of the two columns) or the column experiences ‘lift off’, the load transfer is complete.

Following construction of the rail tunnels and the subsequent re-build of the CPO building foundations, the weight carried by each column will be transferred back into the building foundations and the underpinning structures will be removed.

Tunnel construction beneath the CPO and across Lower Queen St is expected to begin late July and be completed in 2019.

All heritage values of the former CPO building have to be maintained - the project is working closely with the Heritage New Zealand. 

In January 2017, the Queen Street entrances to the Britomart Transport Centre closed for three years so that the CRL tunnelling work underneath could be done.

A new temporary building was built at the back of the centre resulting in the new main entrance for the Auckland transport hub being on Commerce Street. Commuters can also access the station from Tyler St, Galway St and Takutai Square.

CRL involves twin 3.4km long tunnels up to 42 metres below city centre streets.

CRL will extend the existing rail line underground through Britomart, to Albert, Vincent and Pitt Streets, then cross beneath Karangahape Road and the Central Motorway Junction to Symonds Street before rising to join the western line at Eden Terrace.

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The tunnels to be built from Britomart Transport Centre go underneath the new Precinct Properties downtown Commercial Bay development being built on the site of the previous Downtown Shopping Centre complex. The tunnels then go up Albert Street.

You can see the first signs of the CRL tunnels in this photo of the Fletchers Commercial Bay development.

  TUNNELS:  You can see the first sign of the CRL twin tunnels as they are being built in the Commercial Bay development area.

TUNNELS:  You can see the first sign of the CRL twin tunnels as they are being built in the Commercial Bay development area.