The dynamics of reactivated landslides: Utiku and Taihape, North Island, New Zealand

Abstract

The primary aim of this research was to study the relationship between landslide motion and its causes, with reference to large, slow moving, reactivated translational rock slides. The movement of such slides has often been assumed to be uniform over time because poor temporal and spatial monitoring resolutions have not allowed the processes and mechanisms governing the velocity to be identified. The increased spatial and temporal resolution of the monitoring carried out for this research allows these processes to be better understood.

Two deep-seated, reactivated translational slides were selected to represent over 7,000 mapped landslides of this type in Tertiary-age sedimentary rocks of New Zealand. Each was closely monitored with an automated network of instruments to detect and measure the effects of rainfall, pore pressure, earthquakes and river stage on changing surface and subsurface movement patterns, with sufficient resolution to link periods of movement to their triggering factors. The dynamics and controls upon these landslides have been investigated by combining multiple interdisciplinary approaches including geology, geomorphology, geotechnics and geomatics. Without such an approach the mechanisms governing their motion could not have been adequately resolved.

The deformation behaviour at the two slides during the period of observation would best be described as episodic post-failure creep. The creep patterns observed typically comprised periods of accelerated-, slow- and vertical-creep, punctuated by intervals of rest, which recurred both seasonally and independent of season. Three systems were identified within the recorded unsteady, non-uniform motion: 1) basal sliding; 2) internal plastic deformation and basal sliding; and 3) seasonal surficial shrinkage and swelling unrelated to landsliding. Basal sliding by frictional slip along thin clay seams led to the largest horizontal displacements recorded at both landslides. However, once triggered by pore-pressure increase, accelerated-creep motion by basal sliding did not tend to arrest when basal pore pressure decreased. At both landslides slow horizontal- and vertical-creep occurred together over much of the monitoring period and was related to plastic deformation of the slide mass and basal sliding. This motion occurred at a constant velocity and did not vary with fluctuating pore pressure. Accelerated- and slow-creep motion was regulated by the geometrical complexity of the landslide mass rather than basal pore-pressure-induced increases in shear resistance, or rate-induced increases in material shear resistance.