Scott Sutherland is a Chartered Geotechnical Engineer with Tonkin + Taylor, a New Zealand based Environmental and Engineering Consultancy. They have been brought on board the New Dunedin Hospital project to investigate the earth beneath the building site.
I came on board when T+T were engaged to provide geotechnical inputs during the Master Planning phase in 2018. At this stage we identified that the site straddles the original shoreline and is partly underlain by reclaimed land. In late 2018 T+T did some early investigation to understand the implications of this for the main ASB and ASC buildings. Involvement and continuity from the early stages of a project is a big advantage as it means we can help the team understand the geotechnical challenges and constraints throughout the process.
The geological history of the site is fascinating as it involves both natural marine/river processes and multiple stages of artificial reclamation from the mid to late 1800s. The original shoreline was only 1 – 2 blocks from the Octagon, so a lot has changed in only 160 years. The ground conditions can also vary a lot over a short distance. Our investigation strategy and foundation system both need to be designed to adapt to and tolerate these ground conditions while still meeting the demands imposed by the structures.
Some boreholes will extend to over 40m below ground level. The ground conditions up to this depth may have an influence on the buildings which then needs to be accounted for in the design, although this depends on the type of foundation system which is yet to be finalised (as of August 2019). We also need to understand the properties of the different layers in order to predict how the ground may behave during an earthquake.
The tight project programme means that we needed to start the geotechnical investigation while the site is still occupied by the existing buildings. This creates unique challenges around drill rig access, meaning that all drilling locations are carefully planned to provide sufficient coverage of information across the site. In some locations we have drilled from inside the existing buildings which requires specialist measures noise and around fume control and monitoring.
During the Master Planning stage investigations we encountered difficult drilling conditions; layers of hard basalt cobbles interbedded with clays and silts. This prior knowledge enabled us to plan for specific drilling techniques and ensure we reach target depths during the current investigation stage.