Hydraulic and mechanical behaviour of volcanic soils and implications for evaluating slope stability

Landslides are more widespread than any other geological events on Earth. Due to their steep topography and contrasted weather conditions, volcanic regions are especially prone to water-triggered landslides. Indeed, examples of volcanic slope failures abound. Regions endowed with volcanic soils are often densely populated and landslides are causing devastating impacts including loss of human lives, damage to critical infrastructure and disruption to livelihoods. There is growing concern that the intensifying effects of climate change on the hydrological cycle – changing the amount and frequency of rainfall and meltwater input – will exacerbate shallow-seated landslide susceptibility. Soil is the weakest material involved in landslide-related disasters, and soil properties are pivotal in determining the susceptibility of a slope to mass movement. Volcanic soils have unique, but hitherto poorly constrained, hydraulic and mechanical properties. Depending on the volcanic parent materials and weathering conditions, these soils can display clay fractions of different mineralogical composition which likely influence their hydraulic and mechanical properties.

Our study aims to advance understanding of the relationships between the hydraulic and mechanical properties of volcanic soils. We sampled undisturbed volcanic soils in Tenerife (Spain) and Ecuador characterized by different mineralogies in order to measure their microstructural and hydraulic properties (particle size distribution ; pore size distribution ; hydraulic conductivity ; water retention curve). We also determined the mechanical properties (shear strength) of these soils and quantified the effect of soil water content on these properties conducting triaxial tests at different moisture level. The mineralogical analysis performed reveal clay fractions either enriched in allophanes or halloysites for the different sampled sites. The allophanic soils display large porosities and water retention values, whereas halloysites-rich soils are less efficient to retain water but seem to conduct it faster. Halloysites-rich soils also show higher, but more water content dependent shear strengths. Indeed, maximum shear stresses reached during triaxial tests are largely increased with drying while allophanic soils’ shear strength are less impacted by a decreased water content. This could be explained by the aggregation of allophane particles in clumps during drying, causing a reduction of shear strength offsetting the classic increasing shear strength due to the increased capillary effects.