GNGTS 2015 - Atti del 34° Convegno Nazionale

- soil anthropic fill / continental deposits; - silty clay (which outcrops along the SW border of the site); - Flysch (outcrop along the north edge of the area). The top of these three groups has been reconstructed on the basis of existing geophysical data and geological surveys. A 3D grid covers an area of approximately 700 m x 800 m, which has at its center the area of the planned excavation. The areal discretization is done with a cell area of 10 m x 10 m. The trend of contacts between the three groups is complex and highly variable in space, with the closure of the marine clay to the northwest on the top of the Flysch, and a direct contact of the fill in the north of this closure on the Flysch itself, which in turn emerges further north. Towards the SW the soil fill is absent, with an outcrop of the marine clay. This complex geometry has been reproduced in the 3Dgrid of the groundwater flow simulator. Contacts between the formations are represented by the finite difference discretization of the numerical model. The model has been stressed by the following boundary conditions: - a piezometric level fixed at the average sea level along the coast; - a height of the water table at the northern end in correspondence of the Flysch outcrop, set at values necessary to achieve the average piezometric level observed in the monitoring wells; - a conditions of no flow boundary on the east and west sides of the model. The model is also subjected to a recharge value estimated from average annual rainfall (975 mm/y) and reduced to obtain mean values of piezometric wells comparable with the measured data. The recharge is imposed zone by zone. Recharge is set to zero in areas where the hydraulic conductivity of the formation outcrops (marine clay) is too low to absorb it. It was also set to zero in correspondence of the planned excavation, for the same reason, in the simulation which represents the presence with a very low hydraulic conductivity. The main parameter of the hydraulic system is, given the stationary flow conditions, the hydraulic conductivity. Different hydraulic conductivity values are used in different zones. For the anthropogenic fill we used 3.46 m/d, the mean value derived from permeability tests. The area to the east has a larger hydraulic conductivity (15 m/d) that was introduced to adequately replicate the piezometric head distribution. In the area of the excavation, hydraulic conductivity was lowered to 1e-7 m/d. The marine clay, which outcrops in the south, has a hydraulic conductivity value of 5e-4 m/d. This value is greater than the mean of laboratory data in oedometer (1e-5 m/d) to take account of any inhomogeneity of the system and to reduce numerical problems of the model, but is much lower than the values measured by the permeability Lefranc tests (on average 0.15 m/d) that have been carried out, however, only in the most superficial, almost to the contact between the clay and the fill above.A homogeneus distribution of the hydraulic conductivity was given to the Flysch, estimated on the basis of the evidence given by Lefranc tests at 1.85 m/d (horizontal conductivity), and set anisotropic with a vertical permeability equal to 1/5 of the horizontal. The simulations predict a maximum rise of the piezometric head, as a result of the remediation action, equal to about 1 m within the very permeable area inside the planned excavation. The water table rise uphill of the excavation reaches about 40 cm. This is the reference value to be considered as a consequence of the excavation on the surrounding area. Note that this value does not pose a risk of raising the water up to ground level. However transient elevations can lead the water to reach the surface temporarily. This fact can happen even in the current situation without this fact represents an effective environmental risk. Acknowledgement. Trieste Port Authority for the support and the permission to present these data. GNGTS 2015 S essione 3.2 75