Some engineering geological properties of the Orakei Greensand, Auckland

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.