Fit for the future: Sheet piling at the Town Hall

This is the unique challenge that the Wellington City Council and Naylor Love teams have been tackling throughout their work on the Wellington Town Hall project. 

The Town Hall was originally built in 1902, meaning the building pre-dates Wellington’s main earthquake strengthening technology. The Council and Naylor Love have been hard at work retrofitting base isolators (invented in Wellington in the 1970s) and undertaking the sheet piling process (developed in 1906) to ensure the building is strong and safe for the next 100 years.

Sheet piling is a construction technique that creates a support structure of interlocking steel sheets driven vertically down into the ground to form a wall. These metal sheets fit tightly together, creating a barrier that can withstand the pressure of water and soil.

The sheets also provide the structure for the Town Hall’s new Auditorium basement. It will be a central city rehearsal, performance and teaching space for the New Zealand Symphony Orchestra (NZSO) and Te Herenga Waka—Victoria University of Wellington's New Zealand School of Music—Te Kōkī (NZSM) upon the building’s reopening. New walls and floors will be built within the sheet piles, after sucking the existing water out of the area, excavating the existing soil, and applying waterproofing membranes.

According to Craig Gibson, Naylor Love Site Manager for the project, driving the six-metre sheet piles into the ground was no easy feat.

“The Town Hall is a heritage building, which means its restoration must adhere to strict guidelines to preserve its historical and architectural integrity.

“Retrofitting an older building to withstand earthquakes is a technically demanding task, involving detailed engineering assessments and specialised construction techniques.” 

Anthony Pattison, the Council's Structural Project Manager for this aspect of the works, explains that the earthquake strengthening and base isolator design process took almost four years of design, and eight months of workshops leading up to the base isolator and sheet piling install process commencing onsite.

“These workshops determined how the works would be undertaken, judged how the building was likely to behave as a result of the sheet piling vibrations, and made a plan for how the building would be monitored throughout the installation - including what steps would be taken if the building was shaking too much.

“The main concern was that while some people on the team were familiar with sheet piling, no one was familiar with driving sheet piles immediately adjacent to the unreinforced brick masonry of the Town Hall. We weren’t sure how the building would behave once we started driving the sheet piles into the ground, as the vibrations would emanate through the ground and up through the structure.”

Visual monitoring of the site was undertaken by the team before the sheet piling construction started, so they could identify any new cracks or damage caused by the process.

“We were all ready to start the works, and then we had to check the building again because a magnitude six earthquake happened the night prior to the sheet piling commencing. The building remained safe and secure through this earthquake, and it was a testament to the structural design and construction," says Anthony.

“It was a really timely reminder about why we’re doing what we're doing, putting in the new foundations to strengthen the Town Hall and protect it against those sorts of events in the future.”

After around six weeks the sheet piling was completed. Since then, the team have dug out the basement for the NZSO rooms, toilets and stage lifts, and have begun the screw piling process for the auditorium foundations.