Localised Failure of Cliff Top Waitemata Group Residual Soils, an Approach to Remediation

Cliff top soil failures are a common occurrence along the coastal margin of the Auckland Urban Area. With the continued development of this space for residential infill housing the impact of localised slope failures have increased. As the majority of development along the coastal margins are privately owned, any remediation measures proposed must be cost effective for individual landowners, constructible and provide a robust engineering solution to meet local authority requirements. WSP Opus and Earth Stability Limited have remediated numerous localised shallow slope failures along Auckland’s coastal margin through the use of soil-nailing and the installation of drainage within the overlying residual soil mantle.

Due to access, budgetary and health and safety constraints, geotechnical investigations undertaken for cliff top properties are typically limited to the areas adjacent to the failure and terrain evaluations. The prominence of terrain evaluations in the development of geotechnical models has facilitated the use of WSP Opus’s UAV capability to determine any underlying factors driving slope failure. Shallow slope failures are often the initial driver for the landowner to engage geotechnical specialists, however as geotechnical practitioners and stability contractors, the underlying cause of slope stability issues along the cliff line cannot be ignored.

The use of soil-nails to enhance the stability of a slope has long been established, but is typically constrained by site geometry to primarily provide support perpendicular to the slope. Therefore, it must be communicated to the client that soil nailed remediation of slope failures are not intended to prevent global cliff failure or lateral regression adjacent to the soil nailed section.