Assessing the Strength of East Coast Bays Formation Soils using in-situ Testing Data

AECOM undertook engineering geological mapping and geotechnical investigations to support resource consent for a proposed subdivision located west of Orewa. The development land consists of a central valley with gentle to moderate sloping upper sections, steep mid sections and moderate toe slopes. The site is underlain by a weathered East Coast Bays Formation (ECBF) sequence of the Waitemata Group, which comprises interbedded very weak sandstone and mudstone units that dip at a low angle northward. Past instability has affected the land and mapped features included head scarps, debris lobes and hummocky ground. Investigations included machine drillholes, machine excavated pits, hand augerholes, cone penetrometer (CPT) and dilatometer testing (DMT). Laboratory testing of select recovered materials was also undertaken, comprising Atterberg Limits, Standard Compaction, Moisture Content and Lime Reactivity tests. Ground conditions encountered in the geotechnical investigation comprise a mantle of weathered, reworked silt and clay to a depth of at least 3 m. Underlying this mantle is a weathered sequence of generally interbedded ECBF sandstone and siltstone. Alluvium/colluvium occurs in the valley floors, which in places are swampy. Geotechnical design parameters required for stability analysis were determined from both measured field vane shear strengths and those derived from CPT and DMT data. CPT empirically derived strengths were calibrated against in-situ vane shear strengths by adjusting the cone factor (Nkt) using Geologismiki software CPeTiT until a close fit between the field data was made. This paper presents a review of the investigation results, the geological model for the site and a comparison of measured and derived soil parameters.