Soil water content and suction monitoring in model slopes for shallow flowslides early warning applications

The results of laboratory infiltration experiments on instrumented model slopes are presented. Loose granular volcanic ashes from the mountainous area north-eastern of Naples, responsible for several large flowslides during the last decades, have been tested. The experiments were aimed at a better understanding of the hydraulic processes leading to slope failure, in order to identify the most useful variables to be monitored for building up effective early warning systems. With respect to this, useful information was provided by coupled measurements of soil suction and volumetric water content, respectively carried out by minitensiometers and TDR, and by slope surface settlement measurements, made with optical laser sensors. In particular, monitoring of soil volumetric water content seemed more useful than soil suction monitoring for early warning purposes, since water content grew smoothly during the entire infiltration processes, while soil suction showed abrupt steep fronts. Furthermore, the obtained results showed how rainfall infiltration was significantly affected by soil volumetric collapse, in turn related to its initial porosity. In addition, during infiltration flume tests with soils showing volumetric collapse, suction resulted nearly in all cases much smaller than expected from laboratory water retention curves. Since steep slopes equilibrium is often guaranteed by cohesion increment due to suction under unsaturated conditions, the obtained results indicate that soil volumetric collapse may be responsible for flowslide triggering when soil is still unsaturated.