The effects of roots on the hydro-mecanical behavior of unsaturated pyroclastic soils

Abstract

Pyroclastic soils are widely diffused all over the world and they are characterized by high porosity and an open metastable internal structure. In situ they usually cover the shallowest layers of slopes in unsaturated conditions. As consequence, they are often involved in rainfall induced flow-like landslides triggered, during the rainy season, by water infiltration in unsaturated pyroclastic soils on steep slopes. The rain water infiltration leads to the volumetric collapse of the metastable structure in unsaturated conditions, and to liquefaction in fully saturated conditions. Once triggered, the propagating mass can reach great distances and cause many damages when it impacts with structures or infrastructures. These damages can be count as loss of life and economic damages. As risk mitigation measures for these rainfall induced flow-like landslides, structural and passive control works such as dissipative basins and/or brindles have been usually adopted over the centuries. An alternative sustainable risk mitigation measure can be represented by bio-engineering techniques, since they use natural elements such as woods or vegetation for stabilizing slopes prone to failure. The effectiveness of bio-engineering practices depends firstly on the soil properties. This aspect was investigated by carring out an experimental study on the effect of soil nutrients on the plant growth and how this is reflected on the soil hydraulic response. It was found that nutrient availability in soil enhance the plant growth, particularly the root number, and this increases the effectiveness of the vegetation on induced soil suction during evapotranspiration. After this preliminary study, the hydro-mechanical behavior of pyroclastic soils (widely known as rich in nutrients) permeated by roots of perennial graminae, typically used for controlling surface erosion, was investigated. From drying (Evapotranspiration) and wetting (Infiltration) test results it can be claimed that the presence of roots influences mostly the shallowest layers of the soil (up to 1.2 m). In particular, during drying the effect of roots on induced soil suction is highlighted in dry season, when air temperatures are high and the vegetation is florid. On the other hand, during wetting, the presence of roots tends to delay the water infiltration, even if the magnitude of suction reduction depends on the initial condition. Oedometer tests provided original insigths on the role of roots on the internal structure of these collapsible soils. In particular, it was found that during root growth, the soil structure tends to reduce its porosity and this is reflected into a reduction of the collapsibility of the root permeated soil during wetting in unsaturated condition. Shear strength of rooted soil, performed trough consolidated drained and undrained triaxial tests, show that the presence of roots increases both total cohesion and the internal friction angle, proportionally with the root biomass in the soil. Moreover, consolidate triaxial test results in undrained conditions showed that during post-failure stage the presence of roots reduces drastically the increment of pore water pressures avoiding the probability of static liquefaction of the material. All those insights allow having a basic framework to design further experimental investigations in order to consider this technique a sustainable risk mitigation measure in unsaturated pyroclastic soils of the Campania region. [edited by author]