Engineering Geology of Coastal Slope Failures at Ōrere Point

Abstract

The focus of this study is the ongoing slope failures in soft sediment coastal cliffs in the Ōrere Point bay, east Auckland. Some of the weak materials exposed in the cliffs appear to be sensitive to changes in moisture content, and therefore, extreme weather events can pose a threat to slope stability in this area. For example, on 8 March 2017, a 1-in-100-yr rainfall event called the ‘Tasman Tempest’ brought ~210 mm of rain to the area in 24 hours, followed soon after by 157 mm of rain from ex-tropical cyclone Debbie. This led to significant slope failures at Ōrere Point in the months following the 2017 rainfall events, that were preceded, and followed, by periods of quiescence. In the present study, particular attention is paid to the morphology, failure history, triggering factors, and failure mechanisms of the Ōrere Point cliffs. The cliffs are formed of Plio-Pleistocene alluvial materials, deposited in a palaeovalley formed within Mesozoic greywacke. Field investigations included shear vane, penetrometer, and sedimentary logging, complemented by a ground penetrating radar (GPR) survey and unmanned aerial vehicle (UAV) surveys. Soils sampled in the field were also analysed using laboratory testing for soil index properties and microtexture, as well as a limited programme of shear strength testing. At least one weathered tephra layer of indeterminate age was identified during this study in the cliffs at Ōrere Point, which might be significant for stability. Indeed, elsewhere on the North Island (Bay of Plenty), such tephra layers exhibit a fundamental control on cliff instability. Characteristic failures at Ōrere Point include rotational slumps, which transition to earthflows. The conceptual model of cliff recession here is thus controlled by the rotational failures. These form a series of embayments, causing adjacent sections of exposed cliff to gradually lose lateral support and fail. In addition to the prevailing porewater conditions, the material properties probably part-control the exact volume and morphology of the cliff failures. Reducing porewater pressure in the cliffs through drainage at the Ōrere Point township would be advantageous for cliff stability.