Clayey Landslide Initiation and Acceleration Strongly Modulated by Soil Swelling

Abstract

Largely unknown mechanisms restrain motion of clay‐rich, slow‐moving landslides that are widespread worldwide and rarely accelerate catastrophically. We studied a clayey, slow‐moving landslide typical of thousands in Northern California, USA, to decipher hydrologic‐mechanical interactions that modulate landslide dynamics. Similar to some other studies, observed pore‐water pressures correlated poorly with landslide reactivation and speed. In situ and laboratory measurements strongly suggested that variable pressure along the landslide's lateral shear boundaries resulting from seasonal soil expansion and contraction modulated its reactivation and speed. Slope‐stability modeling suggested that the landslide's observed behavior could be predicted by including transient swell pressure as a resistance term, whereas modeling considering only transient hydrologic conditions predicted movement five to six months prior to when it was observed. All clayey soils swell to some degree; hence, our findings suggest that swell pressure likely modulates motion of many landslides and should be considered to improve forecasts of clayey landslide initiation and mobility.

Field and Remote‐Sensing Evidence for Hydro‐mechanical Isolation of a Long‐Lived Earthflow in Central California

“…Shear zone samples have relatively high liquid limits (LL = 44-46) and plasticity indices (PI = 23-24) relative to samples from the other sites and have cohesive indices much closer to those of clay than silt (cohesive index = 1.1). The residual friction angle of shear zone material determined by ring shear tests (12-14°; Figure S2) was low compared to values determined for other earthflows in California (Keefer & Johnson, 1983;Schulz et al, 2018). Several studies report decreasing peak and residual ϕ 0 with increasing LL and PI (e.g., Terzaghi et al, 1996, p. 152;Stark & Hussain, 2013;Sorensen & Okkels, 2013).…”

Widespread Initiation, Reactivation, and Acceleration of Landslides in the Northern California Coast Ranges due to Extreme Rainfall

“…Our study focuses on a ∼4,700‐km 2 area that contains hundreds of episodically to continuously active slow‐moving landslides located within the Eel River catchment, northern California Coast Ranges (Figure ). Due to high landslide activity, the northern California Coast Ranges have been a focus site for landslide investigations for over four decades (Bennett, Miller, et al, ; Bennett, Roering, et al, ; Booth & Roering, ; Booth et al, , Handwerger et al, , ; Iverson & Major, ; Kelsey, ; Mackey et al, ; Mackey & Roering, ; Mackey et al, ; Roering et al, , ; Schulz, Smith, Wang, Jiang, & Roering, ; Zhao et al, ). Nearly all of the slow‐moving landslides are underlain by the Jurassic‐Cretaceous Franciscan Complex mélange (Figure ), which comprises tectonically sheared sandstone, siltstone, shale, meta‐sandstone, greenstone, chert, blueschist, and serpentinite (Jayko et al, ; Jennings et al, ; McLaughlin et al, ; McLaughlin et al, ).…”

“…Slow‐moving landslides in the Eel River catchment are often large (>500 m long), deep‐seated (>3 m thick) masses that move downslope at rates up to several meters per year (Bennett, Miller, et al, ; Bennett, Roering, et al, ; Handwerger et al, , ; Mackey & Roering, ). Due to their flow‐like appearance, these landslides are often referred to as earthflows; however, most of their displacement occurs by sliding along narrow basal and lateral shear zones (Keefer & Johnson, ; Nereson & Finnegan, ; Schulz, Smith, Wang, Jiang, & Roering, ). Similar types of slow‐moving landslides occur in mountainous areas around the world (Cerovski‐Darriau & Roering, ; Malet et al, ; Miao et al, ; Rutter & Green, ; Simoni et al, ).…”

“…Similar types of slow‐moving landslides occur in mountainous areas around the world (Cerovski‐Darriau & Roering, ; Malet et al, ; Miao et al, ; Rutter & Green, ; Simoni et al, ). The slow‐moving landslides occur in a mechanically weak (friction angle ∼ 15°), clayey granular soil (chlorite, illite/mica, and smectite) with low hydraulic diffusivity (∼10 −6 m 2 /s; Iverson & Major, ; Keefer & Johnson, ; Nereson et al, ; Schulz, Smith, Wang, Jiang, & Roering, ; Schulz, Smith, Wang, Jiang, Deuell, et al, ).…”

“…These landslides display unsteady motion with velocities that are highly variable both within a single landslide and between neighboring landslides (Handwerger et al, , ; Mackey et al, ). Over seasonal timescales, the Eel River landslides exhibit velocity changes that generally correspond to precipitation‐induced changes in pore‐water pressure (Iverson & Major, ; Schulz, Smith, Wang, Jiang, & Roering, ; Schulz, Smith, Wang, Jiang, Deuell, et al, ). Typically, each landslide accelerates during the wet season and decelerates throughout the dry season.…”