Processes in model slopes made of mixtures of wettable and water repellent sand: Implications for the initiation of debris flows in dry slopes

Abstract

a b s t r a c tDebris flows in slopes initially dry, such as post-wildfire debris flows, are initiated by surface runoff and sediment bulking due to reduced infiltration. Soil water repellency, extreme dry soils, and loose, cohesionless materials influence their initiation. The exact link between these features, the resulting infiltration processes and the initiation mechanism of a debris flow remains unclear. Here, we examine the relation between soil particle wettability and slope processes in physical models. Flume experiments were conducted in 10% increments of mass ratios of wettable to water repellent sand, subjected to artificial rainfall with monitoring of soil water content, pore water pressure, sediment and water discharge and failure mode. To date, wettability was considered only for the water repellent end, because it reduces infiltration, enhancing surface runoff. This study demonstrates that slight wettability changes, in the full wettable to water repellent range, impact a variety of slope processes. The two extremes, fully wettable and water repellent gave opposite responses, retrogressive slides for infiltrationinitiated in wettable sand and erosion by surface runoff in water repellent sand. The transition was dominated by surface runoff and preferential flow, yielding a combination of erosion and slides. From the tests, a continuous capping effect generated by water repellency was a necessary condition for erosion and sand bulking i.e., the generation of runoff-initiated debris flows. The sensitivity of the model slope response to artificial rainfall was particularly acute at high ratios of wettable to water repellent sand. For mixtures above a critical ratio of wettable to water repellent sand, the measurements with an index test revealed a fully wettable material despite differences in the infiltration, saturation and pore water pressure built-up trends. Implications for post-wildfire debris flows and debris flows in slopes initially dry in general are discussed.

The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the Associated Microscopic Mechanism

“…In addition, loess has very complex soil-water and engineering characteristics in an unsaturated state [10]. In general, most unsaturated slopes tend to lose stability during the process of seepage or when water pressure rises in soil pores [11,12]. Water infiltration causes a decrease in the suction of unsaturated soil, but the increase in pore water pressure when the groundwater level rises reduces the anti-slip force on the potential slip surface, which may unbalance the slope and cause slip damage [13].…”

The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the associated Microscopic Mechanism

“…At the same time, loess has very complex soil-water and engineering characteristics in an unsaturated state (Zhang et al 2020). In general, most unsaturated slopes tend to lose stability during the process of seepage or when water pressure rises in soil pores (Lourenco et al 2015;Schnellmann et al 2010). Water infiltration causes a decrease in the suction of unsaturated soil, but the increase in pore water pressure when the groundwater level rises reduces the anti-slip force on the potential slip surface, which may unbalance the slope and cause slip damage (Fredlund and Rahardjo 1993).…”

Water repellent soils: the case for unsaturated soil mechanics

“…It is important to note that, although tests in [25] used a permanently water repellent soil, water repellency in natural materials varies over time as a function of water content and water exposure (i.e., prolonged wet periods may break down water repellent surface coatings) [27]. An initially-water repellent soil may therefore become more hydrophilic as water contents increase, for example with changing seasons [4].…”

“…Effects of changes in water repellency on slope stability may also be of concern for deposition processes of treated material, for example mine tailings. Slope stability and debris flow mechanisms in slopes of varying degrees of water repellency were investigated by [25]. Slope models were prepared by mixing hydrophilic and water repellent sand ( =0.13 mm) in known ratios from 0 to 100%, in 10% increments, creating soils of heterogeneous wettability.…”

“…The WPDT is an index test that provides a rough assessment of water repellency for bulk samples [25,28]. A sample of roughly 100g (particle size <2 mm) is prepared at the desired water content and density.…”