54 / 2019-06-13 11:39:33

Solid Earth tides as contributors to landslide triggering

landslide,trigger,earth tide,effective stress

水库地质灾害防治与生态环境保护 > 地质灾害监测预警

Omokoroa Peninsula in the Bay of Plenty, New Zealand is bounded by ~ 30 m high coastal cliffs formed in Quaternary, halloysite-rich, sensitive tephra deposits. Flow-slides in these deposits are deep, extend considerable distances back into the slope, and generate long run-out debris flows. Two extratropical cyclones (Debbie (3/4 April 2017) and Cook (13 April 2017)) brought intense rainfall to the site. Each of these events was followed by renewed landslide activity, with some 26 new landslides observed.

Since 2013 borehole inclination, meteorological data, and pore water pressure have been measured. Kluger et al. (submitted) developed an effective stress threshold that indicated triggering of the landslides following both Debbie and Cook could be attributed to effective stress decreases. Elevated pore water pressures are thus considered the primary trigger for landslides in these materials. However, none of the landslides following Cyclone Debbie coincided with the peak of the pore water pressure, which occurred at 01.00 am on 5 April 2017. This peak decayed rapidly and when the first of the landslides was observed 10 hours later, at 11:00 am, 66% of the pore water pressure had dissipated. Many more landslides followed over the next 36 hours.

Borehole inclinometer data from a 42 m deep borehole shows no shear zone, but indicates that the borehole casing sways, with occasional large excursions away from equilibrium. Repeated measurements on a single day consistently show a pattern of movement in one direction reaching a peak and then trending back towards equilibrium. We interpret these fluctuations as reflecting the Earth tide, with motions enhanced by the sequence of weathered tephras at the site. Comparison of the predicted Earth tides for 5/6 (Debbie) and 13/14 April 2017 (Cook) shows that all of the landslides for which adequate timing data exist occurred at a similar point of the Earth tide cycle.

The static response of the tephra units exhibits considerable strain softening following contractive failure, from which we infer a progressive failure mechanism analogous to Northern Hemisphere sensitive soils. We infer that lowered effective stresses at peak pore water pressures initiate failure. However, the pore water pressures rapidly fall away, potentially halting the progressive failure mechanism. Earth tides provide the mechanism for driving continued progressive failure. Cycles of compaction and expansion associated with tidal displacements induce local stress concentrations, allowing fracture extension until the point is reached at which a full-slope failure develops. Global failure thus occurs some time post peak pore water pressure, with the delay duration depending on the magnitude of the Earth tides and the soil characteristics.