SLADUS · SMALL TO LARGE DEFORMATIONS OF UNSATURATED SOILS: AN APPLICATION TO EARTH STRUCTURES

Small to large deformations of unsaturated soils: an application to earth structures

This Fellowship has been undertaken at the University of Glasgow in the United Kingdom and has involved a range of activities including field investigation, laboratory testing and constitutive modelling. The initial part of the project was undertaken at a site near Mantova in northern Italy, where an embankment has been erected for research purposes next to the flood defences of the Po river. The embankment, which is 4m high, 194m long with a crest width of 4m and maximum slope 2:3, is made of the same soil as the flood defences of the Po river (clayey silt). Construction and instrumentation of the embankment has been funded by a consortium of universities to enhance design of flood defences and to identify factors that cause slope instabilities, breaches and overtopping in such structures. During the initial stage of the project, one cubic meter of loose soil has been collected from the embankment for determination of basic soil properties as well as for preparation of samples in the laboratory by Proctor compaction. Undisturbed samples have also been retrieved from the embankment by core drilling inside uncased boreholes by using a Shelby sampler with a height of 50cm and a diameter of 10cm. Four different series of laboratory tests have been performed on undisturbed samples retrieved from the field. In particular, two series of Resonant Column Torsional Shear tests have been performed on unsaturated specimens subjected to isotropic loading at constant suction, in one case, and to wetting-drying at constant confining pressure, in the other case. Both these series of tests have investigated dependency of damping and soil stiffness at small strains (from 0.0001% to 1%) on confining pressure, suction and degree of saturation. Different degrees of saturation have been measured at the same suction level in different tests depending on whether the soil follows a drying or wetting path. This has enabled characterisation of hydraulic hysteresis and separation of the effects of suction and degree of saturation on soil behaviour. A third series of Triaxial Cell tests has been performed on unsaturated specimens under controlled suction to investigate soil behaviour during isotropic compression and shearing to critical state. A fourth and final series of Oedometric tests has been performed to measure water retention curves at different stress levels. Porosity and water content have also been measured on all samples prior to testing to evaluate material variability in the field. The loose soil collected from the embankment has been used to determine basic properties such as Atterberg limits, specific gravity, organic content and grain size distribution as well as for preparing laboratory compacted samples that have subsequently undergone similar tests as those performed on undisturbed samples retrieved from the field. In addition, results from previous experimental campaigns, performed at the University of Naples Federico II in Italy on laboratory compacted samples, have been analysed to gather further evidence of soil behaviour. Tests performed on field and laboratory compacted samples (at similar dry densities and water contents) have been compared in the light of the current state-of-the-art in constitutive modelling, which has led to the identification of interesting differences/similarities in terms of critical strength and small strain shear stiffness. An additional research activity, not envisaged in the original project plan, has been carried out in collaboration with the University of Cassino in Italy, aimed at the micro-structural characterisation of field and laboratory compacted soils via scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). This activity has provided further insight into the interpretation of laboratory tests and has enabled development of a constitutive framework where the phenomenological hydro-mechanical behaviour of the soil is related to the evolution of material fabric.

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