Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy)

Mechanisms of failure occurring in two portions of a riverbank along the Ar no River (Central Italy), are investigated in detail starting by a series o f periodic field observations and bank profile measurements. Two dominant m echanisms involving the silty sand portion of the bank have been observed: (a) alcove-shaped failure in the middle portion of the bank; (b) slab failu re involving the middle-upper bank. A portion of the riverbank was subject to laboratory (grain size analysis; phase relationship analysis; triaxial tests) and in situ tests (borehole sh ear tests (BSTs)) to characterise the geotechnical properties of the overba nk deposits. Two different procedures of bank stability analysis have been performed: (1) a complete analysis, coupling seepage analysis with the limi t equilibrium method; (2) two simplified analyses, through the limit equili brium method with simple assumptions on pore water pressures distribution. For the complete analysis, saturated/unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions, us ing as boundary conditions eight hydrographs with increasing water stage. R iverbank stability analyses have been conducted by the Morgenstern-Price ri gorous method, dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step. The analysis revealed the occu rrence of two possible mechanisms of failure (slab-type and alcove-shaped s liding failures), according to the field observations, related to different river stages and pore water pressures within the riverbank: alcove failure s are likely to occur with moderate flow events, while slab failures are fa voured by flow events with higher peak river stage. A first type of simplified analysis, representing critical conditions reach ed during a rapid flow event, was based on the main hypothesis of the occur rence of a zero pore water pressures zone within the portion of the bank be tween the low-water stage and the peak stage reached. A second type of simp lified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event (worst case), with the main assumption of total saturation of the material up to the same elevation of the peak rive r stage. The first simplified analysis has given similar results to the com plete seepage/stability analysis, confirming slab-type and alcove-shaped fa ilure as the two main mechanisms of instability, while the second type of s implified analysis has conducted to too conservative results compared to th e other previous analyses. Field observations regarding different characteristic bank geometries in ad jacent sub-reaches have been summarised in a conceptual cyclic sketch, that include all the possible paths of bank evolution depending on the successi on of river stages reached during flow events and related pore pressure con ditions. (C) 2001 Elsevier Science B.V. All rights reserved.