Analysis of existing levee systems: the Serchio river case



The floodplain embankments of the Serchio River (Tuscany, Italy) have experienced several failures during their life. After the event of December 2009 a detailed geotechnical investigation has been carried out. This paper discusses the necessary criteria for a cost-effective investigation taking into account the total length of the embankments (30 km), the requested level of detail and analyses and the capability of indirect methods (CPTu, 2D geo-electric tomography) to infer the soil stratigraphy. In this respect, it has been confirmed that CPTu test is an expeditious, economical and effective indirect method for soil profiling, if the results are correctly calibrated against borehole-logs. On the other hand, geo-electric investigations can be very useful to highlight anomalies and hetero- geneities in the embankment cross section. Following the geotechnical characterization of the Serchio River a number of analyses has been carried out for various purposes: to clarify the causes of the December 2009 failures, to design appropriate repair of the failures and retrofit of the embankments in proximity of the failures and to identify the risk areas considering the whole extension of the embankments. The following Ultimate Limit States have been considered:
– Slope failure and hydraulic heave (under steady state flow conditions) by the Limit Equilibrium Method. The stability analyses have been carried out using the Bishop simplified method with circular sliding surfaces and using three different commercial codes. Safety factor against hydraulic heave has been computed according to NTC 2008.
– Slope failure under transient flow conditions by means of FEM analysis (PLAXIS Flow), in order to assess the time to approach the steady state flow conditions.
– Susceptibility of the embankment and foundation soil to internal erosion phenomena. The paper also tries to clarify the redundant terminology that is used to describe the same phenomenon and, at the same time, points out the need to use appropriate terminology to describe different phenomena related to internal erosion.

The analysis results show that, for the selected cross sections, the safety factors are rather small and approaching to unit if the seepage forces are not considered. In the case of steady state flow, safety factor drastically reduces to values lower than one. The results of the FEM analyses suggest that the partial saturation of the embankment-material mainly contributes to its stability in the absence of filtration, leading to acceptable safety margins. A good characterization of the material under conditions of partial saturation would allow applying the most appropriate hydraulic conditions.


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AUTORI: Cosanti B., Squeglia N., Lo Presti D.C.F.
Numero di pagine: 49

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