Characterizing marshland compressibility by an in-situ loading test: design and set-up of an experiment in the Venice Lagoon

The fate of coastal marshlands in the near future will strongly depend on their capability to maintaintheir elevation above a rising mean sea level. Together with the deposition of inorganic sediments during hightides, organic soil production by halophytic vegetation, and organic matter decomposition, land subsidence dueto natural soil compression is a major factor controlling the actual elevation of salt-marsh platforms. Due totheir high porosity and compressibility, the marsh sedimentary body undergoes large compression because of theload of overlying more recent deposits. The characterization of the geotechnical properties of these deposits istherefore of paramount importance to quantify consolidation versus accretion and relative sea level rise. However,undisturbed sampling of this loose material is extremely challenging and lab tests on in-situ collected samples arenot properly representative of in-situ conditions due to the scale effects in highly heterogeneous silty soils such asthose of the Venice lagoon. To overcome this limitation, an in-situ loading test was carried out in the LazzarettoNuovo salt-marsh in the Venice Lagoon, Italy. The load is obtained by a number of plastic tanks that are filledwith seawater, reaching a cumulative load of 40 kN applied on a 2:51:8m2 surface. Specific instrumentationswere deployed before positioning the tanks to measure soil vertical displacement at various depths below theload (0, 10, and 50 cm) and distances (0, 40, and 80 cm) from the load centre. Moreover, six pressure transducerswere used to record overpressure dissipation over time. The collected datasets will be interpreted through a 3-Dflow-deformation model that, once calibrated, provides reliable estimates of the compressibility values for eachmonitored depth interval.