Control of suffosion along riverbanks: the case of the Olona River (Italy)

Piezometric fluctuations may affect the stability of the riverbanks by determining suffusion, i.e. the removal of fine particles from the soil when the filtration rate is high. Suffusion, or underground erosion, can reduce the mechanical properties of the soil and promote its landslide. A huge number of Lombardy Valley (Northern Italy) is affected by problems related to the progressive degradation of the mechanical strength of the alluvial deposits due to underground erosion or piping In some cases it results in the formation of large cavities whereas in other ones the phenomenon is limited to a removal of fine particles (overall silty sands and silt). The critical velocity able to trigger suffusion is exceeded when the hydraulic gradient is particularly high, i.e. due to piezometric fluctuations during flood. Therefore, horizontal drainage systems can be installed nearby riverbanks in order to reduce these fluctuations in piezometric levels during the floods of watercourses. The goal of this research project was to investigate the effectiveness of horizontal drainage arranged parallel to the river to reduce piezometric oscillations in the riverbanks in transient state and considering the position of drainage system respect to the bedrock. The new analytical formulation proposed in this paper allowed to design a drainage system that can increase the slopes stability, reducing suffusion occurring in the riverbanks. Estimates of piezometric levels and their changes with time were made on the basis of hydraulic boundary conditions, soil properties and river flood. Results showed that the effectiveness of horizontal drains depends on the diameter of the drains and their location relative to the aquifer bedrock. Finally a comparison of the results obtained with new equation and a numerical groundwater flow model showed a very good agreement, indicating that the analytical relation is sufficiently accurate to be used in a preliminary design of the drainage system, even if. additional work is required to develop lower-cost method of installing the drains.