Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Engineering geological investigation of the Tapora dunefield, Kaipara Harbour, New Zealand

Author
Bigley G
Year
2025
Journal / Source
Master of Engineering Geology
Publisher / Organisation
University of Auckland
Pages
124
Summary
The Tapora dunefield is a series of coastal sand dunes on the western coast of the Okahukura Peninsula, located within the Kaipara Harbour, North Island, New Zealand. There have been few previous engineering geological studies in the area. Today, significant residential land development at Tapora is now occurring and more is planned, driven by the expansion of Auckland commuter belt. Despite this, very little is known about the characteristic subsurface material properties. Tapora has historically been used for bovine (i.e., cattle) agriculture and plantation harvesting of pine (timber) and avocadoes. Farmers are now selling up land for the development of new residential sub-divisions. Particular issues that may arise, given the sand nature of the substrate, include (1) consolidation settlement when the sands are put under load (by a dwelling), as well as (2) slope instability, when a cut is made into a dune to accommodate a new road. The aim of this thesis is to develop a conceptual engineering geological understanding of the Tapora dunefield. This study uses several field, laboratory, and modelling techniques to develop an understanding of the engineering geology at Tapora. This investigates the relationships between soil composition, moisture and density in relation to slope stability, settlement, dilatancy, and liquefaction. Results show that Tapora consists of primarily pure (i.e., 100%) sand, with some silty sands located in the near surface of inter-dune flats. The load characteristics are governed by internal friction angle, which is a function of particle size distribution (PSD), angularity, and moisture content. With silt content altering the strength profile of the soil. Due to this, the dune slopes are very stable under all standard engineering cuts, only requiring reinforcement under a 90° cut. Settlement is very small, limited to the shallow silts and clay zones within the inter-dune flats. Footings in the near-surface sands (i.e., at 0.6 m depth) are expected to experience little to no consolidation or settlement issues. Sand extraction/dredging is a common practice in the nearshore Tapora area, and there are several ongoing resource consent applications to expand upon this onto onshore Tapora. Onshore extraction operations that create large excavations below the local water table will likely encounter issues stemming from shallow groundwater. Indeed, static-liquefaction issues may cause slope blowouts both during extraction operations and remediation/closure.