Out with the old, in with the new at Wellington Town Hall

This marks the start of the transition from the building being supported on its old foundations to being supported by the new construction.

The original concrete foundations were identified as one of the main weaknesses in the 120-year-old building. To counteract this, the new construction involves installing a base isolated foundation beneath the entire structure – effectively building two layers of foundations under the existing 13-metre-high brick walls.

It has taken us nearly three and a half years and an immense amount of work to get to this point – careful construction, underpinning, countless hours in workshops, extensive temporary works, and lots of teamwork. Our team was heavily involved in the construction staging, as well as the design of the complex temporary works.

The loading of the first area is just the start – we need to jack 148 base isolators across the building footprint. This is done by inflating a flatjack under each one. Flatjacks are essentially steel balloons that are slid in under the base isolator, then filled with epoxy grout under pressure to pre-load the isolator/foundation assembly.

Each isolator has a different design load depending on where it sits in the building, so it is a slow and laborious process undertaken in confined conditions. In our case, the gap between beams and foundations is only about 475mm – it’s not a job for the claustrophobic. The team keeps a close eye on the dial gauges as the jacking is underway to make sure we know which parts of the building are moving as the load is transferred.

With the jacking of each individual base isolator, we bring the Town Hall closer to the 21st century. 30 down, 118 to go!