Geotechnical and Microstructural Characterization of the Recent Delta Po River Flood Deposit, Adriatic Sea, Italy

During Fall 2000, a flood occurred in the northern Adriatic Sea, which introduced large quantities of sediments in the Po delta area in particular. Twelve samples were taken during June 2003 to investigate their geotechnical and microstructural properties in order to trace the signature of the flood layer in a downstream direction. Most of the analyses involved CAT-scan profiling, coupled to some X-ray radiographs and basic geotechnical testing including water content, Atterberg's limits, along with intact and remolded undrained shear strength. The sediment consists of a very soft light brownish silty-clay to dark gray silty-clay with some sandy layer and a high amount of organic matter. Some shells and warm tubes are observed. The flood deposit is found between 8 to 15 centimeters depth depending on the station and is about 10 centimeters thick. Typically, the flood layer is recognized on geotechnical profiles by a water content (105%); higher than for layers above and below; it less bioturbated and starts at the base with a sandy layer which has a much higher sensitivity (15). On the other end, the shear strength is in line with other layers. The pre-flood deposit is characterized by a water content of (60%) which is controlled by the consolidation process, with a normal higher shear strength but with a higher remolded shear strength in some case, around 1.5 kPa compare to less than 1 in the post-flood and flood deposits. The post-flood deposit is characterized by an hemipelagic deposit with an high percentage of bioturbation, by a water content of (75%) and a very low near surface shear strength increasing with depth. CAT-scan and X-ray radiograph profiles are also quite similar for all layers. In addition, the CAT-scan provides a 3D view of the biogenic structures. CAT-scan analyses coupled with geotechnical testing are being used to develop a measure of bio-porosity which could be used eventually as an index to correct the shear strength measured on the sediment matrix.