Landslides in sensitive soils, Tauranga, New Zealand

V. G. Moon, M. J. Cunningham, J. Wyatt, D. J. Lowe, T. Mörz, M. E. Jorat

Research output: Chapter in Book/Report/Conference proceedingConference contribution


In the Tauranga region sensitive soil failures commonly occur after heavy rainfall events, causing considerable infrastructure damage. Several notable landslides include a large failure at Bramley Drive, Omokoroa in 1979, the Ruahihi Canal collapse in 1981, and numerous landslides in May 2005; recently the Bramley Drive scarp was reactivated in 2011. These failures are associated with materials loosely classified as the Pahoia Tephras - a mixture of rhyolitic pyroclastic deposits of approximately 1 Ma.

The common link with extreme rainfall events suggests a pore water pressure control on the initiation of these failures. Recent research on the structure of the soils shows a dominance of halloysite clay minerals packed loosely in arrangements with high porosity (51 – 77 %), but with almost entirely micropores. This leads us to conclude that the permeability is very low, and the materials remain continuously wet. The formation of halloysite is encouraged by a wet environment with no episodes of drying, supporting this assumption.

A high-resolution CPT trace at Bramley Drive indicates induced pore water pressures rising steadily to a peak at approximately 25 m depth; this depth coincides with the base of the landslide scarp. We infer that elevated pore water pressures develop within this single, thick aquifer, triggering failure through reduced effective stresses. The inactive halloysite clay mineral results in low plasticity indices (13 – 44 %) and hence high liquidity indices (1.2 – 2.4) due to the saturated pore space; remoulding following failure is sudden and dramatic and results in large debris runout distances.
Original languageEnglish
Title of host publicationProceedings of the 19th NZGS Geotechnical Symposium
Subtitle of host publicationHanging by a thread? Lifelines, infrastructure and natural disasters
EditorsC. Y. Chin
Place of PublicationWellington
PublisherInstitution of Professional Engineers New Zealand
Number of pages8
Publication statusPublished - 2013
Externally publishedYes
Event19th New Zealand Geotechnical Society Symposium: Hanging by a thread? Lifelines, infrastructure and natural disasters - Millennium Hotel, Queenstown, New Zealand
Duration: 21 Nov 201322 Nov 2013
Conference number: 19

Publication series

PublisherInstitution of Professional Engineers New Zealand
ISSN (Print)0111-9532


Other19th New Zealand Geotechnical Society Symposium
Abbreviated title19th NZGS Symposium
Country/TerritoryNew Zealand


  • Sensitive soil
  • Tauranga
  • Halloysite
  • Pore water pressure


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