Monitoring and analysis of mechanisms of failure in a streambank stabilized by reinforced soil and vegetation

A reach of unstable riverbanks along the Sieve River (Tuscany, Central Italy) has recently been stabilised by a variety of different works, using bioengineering techniques, with the aim of identifying the most appropriate and the advantages of the different techniques employed. The works include gabions, fences, reinforced soil combined with vegetation, and reconstruction of an artificial levee. Reinforced soil, one of the methods employed, constitutes a promising technique for riverbank stabilization as it presents several potential advantages, such as a minimum ecological and environmental impacts when covered by vegetation. However, reinforced earths are still little employed for riverbank stabilization, because of the uncertainties related to the response to fluvial erosion and seepage phenomena that can induce possible failures. For these reasons, a research project has been undertaken with the aim to monitor the manufact and the possible causes of failure, with particular regard to the pore water pressures within the bank and their response to external hydrological factors (rainfall, river stage variations). The instrumentation installed on the reinforced soil and the reconstructed artificial levee consists of a battery of ten tensiometers, two piezometers, a river stage gauge, a rain gauge, and a TDR (time domain reflectometry probe), all connected to a data logger. Topographic measurements (cross-sections and terrestrial photogrammetry) are periodically conducted on the stabilised bank, on the channel bed and the opposite bank, to measure the overall responses of the river reach to the stabilization works. Before starting the monitoring activity, a first analysis was performed to verify the general stability of the stabilization work and its responses to flow events with high return times. The method of analysis includes the following steps: a) hydrologic analysis, to define the hydrographs of given return times; b) hydraulic analysis, to obtain the flood hydrographs in terms of river stages vs time; c) finite element seepage analysis in transient conditions, to model the saturated/unsaturated flow within the reinforced soil and the artificial levee and obtain the pore water pressure distribution during flow events; d) stability analysis, to calculate with limit equilibrium method the safety factor in different time steps of the event.