Stability of streambanks formed in partially saturated soils and effects of negative pore water pressures: the Sieve River (Italy)

Streambanks of alluvial channels are usually composed of loose materials, w hich are unsaturated in ambient conditions. Unsaturated soils are subject t o negative pore water pressures, which cause an apparent cohesion. The latt er is the main factor in allowing the stability of near-vertical banks. Eve n during moderate in-bank flow events, the apparent cohesion can be strongl y reduced as the material approaches full saturation; therefore, during the drawdown phase, as the confining pressure of the water in the channel disa ppears, a bank failure is likely to occur. Channel bed-level lowering along the Sieve River, Central Italy, has caused widespread bank instability. A geomorphological reconnaissance of forms and processes was followed by in s itu tests to determine the shear strength of the banks. Interpretation of t he tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils, in order to obtain relations between bank angle and height in limit equilibrium conditions. A stability chart was ob tained with curves for different apparent cohesion values, and a stability analysis was performed taking into account the effects of flow events. In o rder to investigate the pore pressure effects, a series of piezo-tensiomete rs were installed in a streambank of the Sieve River. Data from a 1 year mo nitoring period show variations in pore water pressure and matric suction a s a consequence of rainfall, evapotranspiration, and water stage variations . A planar failure with a tension crack occurred in the upper cohesive part of the bank during December 1996. The safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the mat erial. Safety factor variations through time are therefore shown as a funct ion of seasonal variations in matric suction. (C) 1999 Elsevier Science B.V . All rights reserved.