Abstract:
In the Auckland region, interest in the engineering geological properties of the underlying geology
has emerged due to large-scale infrastructure projects, but existing work has mainly been focused on
the alternating sandstone and mudstone units of the Miocene East Coast Bays Formation (ECBF).
The majority of the Auckland region is underlain by ECBF units. Hydrogeological issues during
tunnelling, tunnel boring machine performance, and slope instability associated with infrastructure
and transport corridors are common issues exhibited by the ECBF units. This research is the first to
characterise some engineering geological properties of a small glauconitic unit within the ECBF,
namely, the Orakei Greensand. The outcrop is exposed in a cliff and a shore platform north of Karaka
Bay beach at West Tamaki Point and is found intercalated within other lithological units of the ECBF.
Here, a combination of field and laboratory analyses were conducted to investigate the properties of
the greensand unit. Field techniques comprised characterisation and evaluation of in-situ strength
tests using the Schmidt hammer, Equotip Leeb hardness, and PUNDIT wave velocity apparatus.
Laboratory analysis included Particle Size Distribution (PSD), X-ray Diffraction (XRD), Scanning
Electron Microscopy – Energy Dispersive Spectroscopy (SEM-EDS) and direct shear tests using the
shear box technique. Properties of the Orakei Greensand were evaluated using a combination of
existing literature on glauconitic sands and published data relating to the engineering geological
properties of ECBF sandstone units. Preliminary results indicate that the engineering behaviour of
the Orakei Greensand raises several geotechnical implications. The findings of this study should form
the basis for more detailed work to be carried out on the greensand unit.