Moon, V. G., de Lange, W. P., Garae, C. P., Morz, T., Jorat, M. E., & Kreiter, S. (2015). Monitoring the landslide at Bramley Drive, Tauranga, NZ. Presented at the 12th Australia New Zealand Conference on Geomechanics: ANZ 2015 Changing the Face of the Earth - Geomechanics and Human Influence, Wellington, New Zealand.
Abstract
Omokoroa Peninsula, Tauranga Harbour, is prone to landslides in sensitive pyroclastic soils, especially in coastal bluffs. The largest is the landslide at Bramley Drive that first occurred in 1979, and was reactivated in 2011 and 2012. Since 2012 the landslide has been monitored with laser scans, vibro-and static-CPT, pore water logging at 3 depths, and a borehole inclinometer. Laser scan results track degradation of the scarp and allow development of a preliminary magnitude-frequency curve for failure events on the scarp. To date the borehole inclinometer has shown no obvious shear surface development. However, deformations in phase with the solid earth tides are evident in the cumulative displacement plots. Layers of weak soils separated by sharp boundaries are believed to exaggerate the deformations of the solid earth tides to the extent that they are measurable with a simple inclinometer. Residual deformation after subtraction of the earth tide effects indicate some movement over winter of 2014 associated with sensitive soils at or near the failure surface. The depth of this movement corresponds with a zone of high induced pore water pressures under vibratory CPTu. Pore water pressures indicate two discrete aquifers: an upper aquifer in tephra layers high in the upper part of the sequence that responds to atmospheric pressures; and a second aquifer in the underlying ignimbrites. Pressures in the bottom aquifer are lower than in the overlying aquifer in summer and higher in winter. Large spikes in pore water pressure have been observed during winter of 2014; these coincide with the time of deformation noted in the inclinometer traces.
