Wellington Town Hall Seismic Retrofit: A Case Study in Innovative Helical Pile Design and Installation-PPT Presentation

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The Wellington Town Hall, a Category 1 historic building opened in 1904, was assessed at just 20% of the New Zealand Building Standard and identified as earthquake-prone. Rather than replace it, Wellington City Council chose extensive seismic strengthening, including a base-isolation system designed to raise performance to 75% NBS.<br /><br />The site presented demanding geotechnical and construction conditions: reclamation fill and beach deposits susceptible to liquefaction, anticipated lateral spreading, and rock more than 50 metres deep. Work had to take place inside a vulnerable, occupied-by-structure historic building, with extremely restricted clearances and load-bearing walls that could not be removed.<br /><br />Helical (screw) piles were selected as the only feasible foundation solution. From 2019 to 2024, crews installed about 550 piles, 12–23 metres long, totalling 6.2 kilometres and 682 tonnes of steel. Piles were designed for six vertical load categories and significant lateral forces, with strict displacement limits and multiple liquefaction scenarios. Fixed-head and pinned-head connection details were developed to suit different structural demands, while the design was simplified to reduce installation complexity. Durability, corrosion loss, steel selection, welding compliance, and concrete infill were also carefully addressed.<br /><br />Installation required specialised, modified equipment and detailed sequencing. Piles had to be transported through narrow corridors and installed beneath stairwells, in basements, and in confined rooms; some areas were below groundwater or obstructed by existing foundations and services. Quality assurance included load testing up to 3,000 kN, torque-record verification, inspections, and video review of cleaned pile interiors.<br /><br />The project demonstrates how tailored helical-pile design, close coordination among structural, geotechnical, and construction teams, and rigorous installation controls can enable seismic strengthening of a major heritage building under unusually severe access and ground constraints.

Keywords

Wellington Town Hall

seismic strengthening

historic building

base isolation

earthquake prone

helical piles

geotechnical conditions

liquefaction

foundation solution

Wellington City Council