Patterns of movement in reactivated landslides: Fid-Bau Portal

Patterns of movement in reactivated landslides

Massey, C.I. / Petley, D.N. / McSaveney, M.J.

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. Surface displacements of the 22×10⁶ m³ Utiku landslide, in central North Island, New Zealand were measured using continuous GPS (cGPS), for three years. The nature of the movement of such slides has often been difficult to determine because of poor temporal and spatial monitoring resolutions. After removal of tectonic plate motion, the temporal pattern of the landslide's surface motion could be understood to arise from irregular episodes of faster (up-to-21mm/day) and slower (up to 26mm/yr) post-failure landslide displacement, and seasonal cyclic displacements of about 20mm/yr–10mm per half year in alternating directions. Intervals of faster motion gave rise to displacements of between 10 and 120mm per event. Faster displacement was associated mostly with basal sliding (mechanism 1), involving deformation within a thin clay seam as recorded by borehole inclinometer surveys. Slower surface displacement involved permanent internal deformation of the larger landslide mass, consisting of plastic deformation within the landslide body and/or slip along existing internal planes of weakness, and slip on the slide base (mechanism 2); it accounted for up to 26mm/yr of displacement at a mean angle of about 49° from the horizontal, indicating that the slide mass was thinning as it moved down slope. Seasonal cyclic displacements were synchronous with changes in pore pressure, suggesting that it is a shrink/swell process (mechanism 3) associated with wetting and recharge of groundwater during the wetter winter months, leading to a downslope movement, and soil shrinkage leading to upslope rebounds during the dryer summer months. The brief periods of faster displacement were triggered by seasonal peaks in pore pressure, linked to long periods (12 to 20weeks) of increased precipitation and lowered evapotranspiration. Faster displacement, however, was not arrested by lowering pore pressure or by any other monitored factor. Similarly, periods of slower displacement did not correlate with pore pressure changes, or with any other monitored factor. This study has shown that the annual movement pattern of a reactivated landslide is a combination of these processes that generate a complex overall movement record. The field measurements showed real variability arising from variations in rainfall and pore pressure, which were overprinted with measurement noise that may mask some other processes.

Highlights • Relationships between landslide motion and its causes have been studied. • Displacements of the 22×10⁶ m³ Utiku landslide, New Zealand, were measured. • The landslide represents one of 7,000 mapped of this type in sedimentary rocks. • Displacement comprised patterns of episodic movement punctuated by intervals of rest. • A framework for the movement of reactivated landslides has been proposed.