Shear strength and retention models of a partially saturated riverbank silty soil

Soil mechanical behaviour is strictly related to its natural water content, which is primarily dependent on hydraulic boundary conditions. When soils are partially saturated, as frequently occur in river embankments, soil suction also turns into a fundamental variable worth of investigation and monitoring, both in laboratory and in the field, for a reliable interpretation of the related soil response to external actions. The case of earthen water retaining structures and their safety condition assessment towards local or global collapse mechanisms requires special attention for soil characterization and site measurement of unsaturated soil states. Moreover, strength and hydraulic models used for relevant stability analyses should be adequate for representing the site-specific behaviour of soil in terms of water content, pore water pressure and suction values, relying on the effective possibility to properly calibrate all required parameters. Nevertheless, these aspects typically receive only limited attention, especially in standard practice, leading to inaccurate estimates of flood hazard and related risk management. In this context, with the aim of comprehensively studying the strength and retention properties of a riverbank silty soil, a series of laboratory investigations - including oedometric and direct shear tests under suction-controlled conditions - have been performed. Laboratory tests are here used as the main direct source of information to model unsaturated soil behaviour, monitoring different state variables under suction-controlled conditions. The results have been then interpreted using both literature and soil-specific mechanical and hydraulic models, to be possibly implemented in integrated methodologies for the stability assessment of river embankments.