Francesca Casini

Landslide triggering experiment in a steep forested slope in Switzerland

A landslide triggering experiment was carried out on a 37°- 40° steep forest slope in North-East Switzerland by sprinkling water artificially to represent an extreme rainfall event. This project is part of a multidisciplinary collaboration, which includes geotechnical engineering, hydrology, hydrogeology, forest engineering, geophysics and photogrammetry. A three dimensional model of the ground was developed from non-invasive geophysical surveys, insitu probing, combined sprinkling and dye tracer tests and shallow test pits. Laboratory tests were carried out on undisturbed samples under various degrees of saturation This paper mainly focuses on the characterisation of the site.

Unsaturated Hydraulic Conductivity of a Silty Sand with the Instantaneous Profile Method

The unsaturated hydraulic conductivity of a silty sand at different initial void ratios is measured using the instantaneous profile method. The variation of the suction and volumetric water content is recorded during the infiltration process as a function of time. Accordingly, an infiltration column was developed with a height of 600 mm and an inner diameter of 170 mm. The suction and volumetric water content were measured simultaneously every 100 mm along the column by means of small tensiometers and TDRs, respectively. Hydraulic conductivity is calculated by dividing the water flow velocity by the hydraulic gradient. The soil is reconstituted from Ruedlingen (Canton Schaffhausen, Switzerland), where landslide triggering experiments were carried out in October 2008 and March 2009. The hydraulic conductivity functions are determined and the laboratory values are compared to the in-situ measurements of hydraulic conductivity carried out in the course of the landslide triggering experiments.

The Effects of Hydraulic Properties of Bedrock on the Stability of Slopes

The transient process of rain infiltration in the soil and the effect of geometry and drainage properties of the bedrock on the pore pressure distribution and the stability of a slope are investigated. The simulated slope is a test field in northern Switzerland, where landslide triggering experiments were carried out. From geological point of view, the experimental site is located in the Swiss Molasse basin. The lithological units in the area are composed of horizontally layered and fractured sandstones intersected by marlstone. The stability of the slope is monitored at different stages of the infiltration using the limit equilibrium method of slices. Several cases were compared to study the effect of the fissures in the shallow bedrock on the stability of the slope. The approximate location and size of the fissures in the bedrock were determined by monitoring of spatial and temporal changes of electrical resistivity during rainfall and also geological investigations of the bedrock before and after the failure.

A microstructural model on the link between change in pore size distribution and wetting induced deformation in a compacted silt

The water retention curve expressed in terms of water content or degree of saturation and suction is hysteretic, stress path dependent (on void ratio), and depends on pore network. To properly describe its evolving nature, a multimodal retention model is proposed by considering a linear superposition of two subcurves of the Van Genuchten type. The model is constructed based on PSDs obtained with Mercury Intrusion Porosimetry (MIP) data at specified initial state, and thus, is representative of this initial pore network configuration. Based on the WRC model proposed above, the evolution of parameter ? will be evaluated based on the microstructural evolution of the pore network. The model is validated based on a series of oedometer samples performed at constant water content and then saturated at different initial void ratio and the same water content.

Deformation induced by wetting

This paper presents a simple model for predicting the deformation induced by wetting. The objective is to quantify the deformation induced by saturation of an unsaturated layer of homogeneous soil, causing variation of the initial void ratio and gravimetric water content. The soil is a low-plasticity silty sand. A simple expression for the normal compression line (NCL), which depends on the parameter c and one more parameter, will be proposed. The model may capture the progressive degradation induced by loading and wetting by linking the dependency of NCL by the parameter c and water retention curve by porosity. Cet article presente un modele simple pour la prediction de la deformation induite par le mouillage. L’objectif est de quantifier la deformation induite par la saturation d’une couche de sol homogene non saturee, causant ainsi des variations de l’indice des vides initial et de la teneur en eau gravimetrique initiale. Le sol est un sable silteux a faible plasticite. Une expression simple de la ligne de compression normale (LCN), qui depend du parametre x et d’un autre parametre, est proposee. Le modele permet d’integrer la degradation progressive causee par le chargement et le mouillage, en reliant la dependance de la LCN avec le parametre c et la courbe de retention d’eau avec la porosite.This paper presents a simple model for predicting the deformation induced by wetting. The objective is to quantify the deformation induced by saturation of an unsaturated layer of homogeneous soil, causing variation of the initial void ratio and gravimetric water content. The soil is a low-plasticity silty sand. A simple expression for the normal compression line (NCL), which depends on the parameter χ and one more parameter, will be proposed. The model may capture the progressive degradation induced by loading and wetting by linking the dependency of NCL by the parameter χ and water retention curve by porosity.

Grain size distribution and particle shape effects on shear strength of sand – gravel mixtures

This paper focuses on the grain size distribution and shape effects on shear strength of three sand - gravel mixtures from Switzerland. A total of 28 direct shear tests in a large direct shear box apparatus have been performed to investigate the strength and dilatancy of sand - gravel mixtures. A relation between the grain size characteristics, the shape and the shear resistance has been found. For each mixture, the void ratio extent (emax−emin), and angle of repose, ϕcv are determined following the standard ASTM procedure. The experimental results are analysed in terms of the frictional and dilatant contributions to the strength of mixtures as a function of the grain size distribution, shape effects and their relative density. The particle shape effects are evaluated using the criteria used by Cho et al. (2006) for natural and crushed sands. The results show the dependency of the dilation gradient on the grain size distribution and the shape of the particle. The minumun and maximum void ratios are also dependent on the grading and the shape of the particles. The shear tests are interpreted in terms of friction angle as function of the dilation angle both evaluated from the tests at peak value and at the end of the shear phase. A useful empirical equation has been developed to evaluate the friction angle at constant volume and the gradient of dilation.