Country: New Zealand
Key Area: Buildings
Services: Architecture,Building Services,Geotechnical Engineering,Heritage,Seismic Engineering,Structural Engineering

When the New Zealand Police decided to build a new Police Station in the centre of Gisborne, they commissioned Opus to find an appropriate solution for their high profile site which was occupied by a notable historic building.

The heritage façades were retained as prominent features of the Gisborne streetscape. However, developing a foundation solution for the new police station presented a number of challenges for the geotechnical and structural engineers. The solution had to allow the retention of the façades, be suitable for the very soft ground conditions and address a variety of site-specific complexities, including much higher earthquake loading than that for conventional buildings. Importantly, the solution also had to be accommodated within the clients’ budget and time constraints. Settlement of a new building was likely to be significant, but due to the presence of the historic building, invasive site testing methodologies during the design stage were limited to the perimeter of the building. Conventional foundation options were cost prohibitive, typically too risky and would not allow swift construction.

The project team evaluated a number of complex solutions. The preferred option was highly innovative – modified steel screw piles with a reinforced concrete raft foundation. After conducting full scale load tests and Finite Element Analysis, Opus assessed the behaviour of the piles in soft material and eventually adopted this option which is considered a world first. For the first time steel screw piles were used as settlement reduction piles in soft soil. Due to the soft nature of soils, the piles were not capable of supporting the total building load, but took only a proportion of it, with the rest of the load being supported by the reinforced concrete raft. This innovative foundation design was tremendously successful. It eliminated most of the construction risks while also speeding up the construction and providing very substantial cost savings compared to other options.

The new police station building has performed well and was even subjected to a rigorous 6.8 Richter earthquake shortly after the construction of the building’s structure was completed in December 2007. The building survived it without any damage.