Following detailed seismic assessments by WSP in 2013, Gisborne District Council decided to demolish their existing Fitzherbert Street office complex and redevelop the site to provide a modern office environment, with new Council Chambers and post disaster response facilities.
The new development consists of 3,100m² of new office space,configured in a single storey H-shaped arrangement. The building includes a central feature entrance,public contact space and general office space for the provision of local services plus a new Council Chambers complex designed to double as a post-disaster facility,from which the council can run their emergency response to a major earthquake or storm event.
CLIENT BRIEF/PROJECT CHALLENGES
The project had a several key challenges. Additional seismic joints were introduced into the design to split the overall building into smaller portions and allow fora phased construction. This initiative accommodated the required site pre-load over part of the building footprint and reduced the overall effect on the construction programme. The pre-loaded area was also continually monitored to measure the actual induced settlement compared to what was expected.
This meant that WSP’s computer modelling could be refined through a statistical process and confirm that enough settlement had occurred before the pre-load was removed,and construction of the building could continue. The Fitzherbert Street site is close to the intersection of the Taruheru and Waimata rivers and their outfall into Poverty Bay, which meant that poor ground conditions were prevalent.
WSP undertook a careful geotechnical assessment to ensure a cost-effective foundation design,while mitigating the risks of both excessive settlement of the final structure and the potential for earthquake-induced liquefaction.
To achieve the different seismic performance requirements of each portion of the building,WSP provided seismic joints to separate the emergency planning area from the remainder of the development. This enabled each area to behave as an independent structure, without damage caused by clashes or adverse interactions during an earthquake.
This separation enabled the emergency response area to be designed with additional roof and wall bracing to act as an independent rigid box and reduce seismically induced deflections during an earthquake to a minimum, while more flexible and cost-effective steel portal frames were used elsewhere.
Due to the presence of very weak soils underlying the building footprint and extending to over 30m below ground level, WSP judged that a piled foundation solution would be prohibitively expensive. A shallow foundation system designed to keep the building footings within the relatively strong near-surface soils level was required instead. The very weak underlying soils were found to be susceptible to excessive long-term settlements under the additional weight of the new building above. Over time this could have caused damage to the structure and the building finishes.
WSP undertook extensive computer modelling to determine the extent of expected settlements and develop a solution that limited the anticipated settlements to tolerable levels. This required a combination of a four-month site pre-load where an additional 4m height of fill material above ground level was used to “overload” the soil beneath and induce a high proportion of the calculated settlement to occur before construction of the building.
In other areas, light weight polystyrene blocks rather than traditional compacted granular fill were used to reduce the overall weight of the building.