Steel screw settlement reduction piles for a raft foundation on soft soil

The new Gisborne Police Station building was built on a soft site occupied by a historical retail building. The façade of the historical building had to be retained, and the building demolished. Based on complex geotechnical investigations and consideration of various foundation options, Opus recommended that a piled raft foundation be adopted. 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. A full scale pile load test was undertaken on 12 m, 18 m and 24 m long screw piles. Finite element (FE) analysis was undertaken to assess the performance of a single screw pile and of the raft-pile-soil system. The FE analysis also allowed to assess settlement and the proportion of load taken by the piles and by the raft. Post construction settlement monitoring indicated that the actual settlement of the building was close to the predicted settlement. The innovative foundation design eliminated most of the construction risks, speeded up the construction and resulted in substantial ($1M) cost saving compared to other foundation options. The new police station building has been constructed and performed adequately under static load; it also survived the December 2007 Gisborne earthquake, which measured 6.8 on the Richter Scale, without any damage.