Abstract
Rainfall is one of the most important triggers of slope failure. Weathered pyroclastic (tephra) deposits are especially vulnerable to slope failure because they commonly form slopes of high porosity and high clay content. Empirically derived thresholds for the triggering of landslides are commonly based on rainfall conditions and have been widely applied in volcanic soils. However, so far only few researchers utilized pore water pressure in the slope as additional variable for the threshold calibration. Here, we derived a new rainfall threshold for initiating the decrease in effective stress in the slope by analyzing a long-term record of rainfall and piezometer data from a slide-prone coastal area in northern New Zealand that consists of clayey, halloysitic tephra deposits. The level of effective stress decrease increased with rainfall intensity and duration. We observed highest effective stress decrease of up to 36% during rainfall events that triggered landslides in our study area. The effective stress threshold exhibits a satisfactory predictive capability. The probability of correctly predicting a decrease in effective stress is 53%. The effective stress threshold contributes towards the implementation of the decrease in effective stress into rainfall thresholds for the occurrence of landslides.
Highlights
Rainfall-induced landslides occur globally in all types of environments, posing a major natural hazard to people and infrastructure
We have studied a ~40-m-thick succession of weathered rhyolitic pyroclastic deposits dating to c. 0.9 Ma that are dominated by halloysite at Omokoroa Peninsula, Tauranga Harbour, the site of multiple landslide events
We showed that for moderate rainfall intensities (I = 4 mm/h) a rainfall duration of 25 h was a critical threshold for landsliding, and that the ensuing landslide dimensions, are both in keeping with parameters reported for landslides in halloysitic tephra deposits in Hong Kong and Japan
Summary
Rainfall-induced landslides occur globally in all types of environments, posing a major natural hazard to people and infrastructure. A large variety of rainfall thresholds have been derived at different scales (global, regional, local), by using different rainfall parameters (rainfall intensity, duration, antecedent precipitation), and for various types of landslides (Guzzetti et al, 2007, 2008; Segoni et al, 2018a). Segoni et al (2018a) recognized that most studies solely rely on rainfall data when calibrating rainfall thresholds They found only two case studies – Baum and Godt (2010) and Napolitano et al (2016) – in which pore water pressure data provided additional insight into the trigger mechanism of landslides. Understanding changes in pore water pressure due to rainfall is needed to calculate changes in effective stress in the soil slope (Duncan et al, 2014) and its evaluation would offer a new beneficial perspective in the determination of rainfall thresholds (Segoni et al, 2018a)
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