The city’s former Central Post Office building was built in 1912 and was subsequently transformed into Britomart Transport Centre, the terminal rail station. But now, in order to add a new loop to the rail network and enable its capacity to be doubled, a building reinforcement solution was needed to protect it from any damage or settlement while two new tunnels were excavated underneath it.

  • Name of the owner
    City Rail Link Ltd
  • Name of the client
    Downer Soletanche Bachy JV
  • Delivery date of the project
    October 2019
  • Partners of the project
    Engineering: Aurecon / Mott MacDonald
Central Post Office of Auckland - Building reinforcement by post-tensioning

Install post-tensioning to the underpinning beams

The tunnels were formed by installing diaphragm walls along each edge and excavating between them to the required level – a relatively standard procedure under normal conditions. In this case, however, the excavation was to take place directly below an historic building – one whose concrete pad foundations were right above the proposed tunnel routes.

Main contractor Soletanche Bachy carried out the diaphragm wall construction – a tricky operation within the limited physical space of the building.

Building reinforcement works

Freyssinet’s scope of works was to install post-tensioning to the underpinning beams, before gently jacking the building up by just a few millimetres to enable its load to be transferred onto the diaphragm walls, and allow excavation of the tunnels to begin.

In preparation for this building reinforcement operation, concrete underpinning beams were clamped inside and outside the main facades of the structure, secured by a series of post-tensioning cables of up to 13 strands.

 

  • 8,800
    Total load transfer (tons)
  • 150
    Number of flat jacks used
Central Post Office of Auckland - Building reinforcement

Column casing

Meanwhile longitudinal steel frames were used to enclose adjacent pairs of internal columns, with shorter steel elements creating ‘collars’ around each column, spanning between the longer beams, which were supported on the new diaphragm walls.

The steelwork was installed by a different subcontractor, but Freyssinet was responsible for grouting and post-tensioning each collar to ensure it had an immovable grip on the column in advance of the load transfer.

 

Flat jack technology was the obvious choice for the building reinforcement procedure

As the jacks can be used where there is very limited space – in this case just 50 mm gap. The transfer process was carried out in a series of moves, each within a zone incorporating one or two columns and based on analysis of the structural model.

Four flat jacks were used at each column and their operation was synchronised, to elimimate the possibility of any uneven loading or torsion being introduced. Movement and forces were monitored constantly during the operation, with a maximum elevation of just 2 mm permitted.

CPO Auckland - flat jack

Freyssinet was a major contributor to the success of this contract, controlling the jacking procedure to extremely tight movement limits with great success. This was a highly technical, high risk part of the project; Freyssinet worked to a very high standard in excellent engagement with the head contractor.

Scott Elwarth -  
City Rail Link Ltd, Head of delivery

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