Experimental investigation of the erosion of unsaturated soil bed by debris flow

Abstract

Debris flows are one of the most destructive types of landslides, which travel downslope at high velocities and cause fatalities and damage to infrastructure. The destructive potential of debris flows increases with their volume, which is governed by the erosion of soil beds along their flow paths. Soil bed erosion occurs when stresses exerted on the soil bed exceed its strength. Thus, an accurate estimation of applied stresses and soil bed strength is essential for assessing soil bed erosion. Existing models usually assume a saturated soil bed, which contradicts the field measurements. Unsaturated soil exhibits different mechanical and hydraulic properties from saturated soil due to the influence of matric suction. Therefore, a realistic mechanistic soil bed erosion model necessitates unsaturated soil mechanics. Furthermore, existing mechanistic erosion models simplify the debris flow as an equivalent fluid and assume the frictional shear stress to be the sole driving mechanism of soil bed erosion. However, field observation shows that erosion mostly occurs at the boulder-enriched debris flow front, which is dominated by collisional stresses, and the liquefied body and tail of debris flow can also erode soil bed material. Despite the important role of collisional stresses and hydrodynamic shear stress, their effects on soil bed erosion are overlooked. In this study, a mechanistic erosion model is proposed to describe the erosion of unsaturated soil bed material by considering the effects of matric suction, collisional stresses and hydrodynamic shear stress on soil bed erosion. Then, unique flume experiments are conducted to validate the proposed mechanistic erosion model. Experimental results show that the proposed mechanistic model can well describe the erosion of unsaturated soil beds by different stresses. Contrary to the existing studies, the soil bed erosion rate does not increase linearly but demonstrates a parabola-like relationship with the soil water content because the shear strength of unsaturated soil varies nonlinearly with the water content under the influence of capillary stresses. Boundary collisional stresses drive soil bed erosion, while the boundary frictional shear stresses are counteracted by the soil strength mobilized by overburden induced by volume contraction. The erosion rate of unsaturated soil bed by hydrodynamic shear stress is significantly influenced by the infiltration of flow water. Findings from this study hint at the conjecture that existing erosion models may not be equipped with the necessary physics to provide realistic hazard assessments of debris flows eroding soil beds. The outcomes from this study present advancement towards improved delineation and assessment of debris flow hazards in mountainous regions.