3D FEM Back Analysis of the Observed Performance of a Very Deep Excavation in the Historical Center of Naples, Italy

Chiaia station is one of the art stations of Line 6 of the Naples underground network; it was constructed in a $50\text{-}\mathrm{m}\text{-}\mathrm{deep}$ excavation, a few meters from historic buildings and 4.5 m from the main façade of a sixteenth-century Basilica. The excavation, carried out partially in loose to medium dense sands overlying the soft rock formation of Neapolitan Yellow Tuff (NYT), was supported by a retaining wall made of contiguous bored piles braced with internal struts and prestressed ground anchors. The excavation sequence was quite complex due to archeological findings and to the presence of anthropic cavities used over the centuries to quarry NYT blocks. One of the key goals of the design was to limit movements around the shaft to prevent damage in the historical buildings. Long-term monitoring data obtained during nearly 9 years confirm the success of the overall construction process. A rather complex three-dimensional (3D) finite-element model with constitutive relationships for both the upper sandy layers and the soft rock is presented in the paper; this model was adopted to back-analyze the data from the monitoring and explore the influence of some of the key features of the case study. The role of the building bending and shear stiffness, of the soft rock stiffness, and of further apparently minor issues—such as the seepage and the ground anchors’ prestress—were investigated and discussed with the support of the model calculations. Observed settlements at the end of the excavation were in the range 10–15 mm, and in the long term they increased by 20%–50% to as much as 20 mm. The deflection ratios were very small, in the range $0.05‰–0.15‰$, and no visible damages to the buildings were recorded. These values were reproduced by the finite-element model only after the introduction of the relevant building stiffness.