A class C prediction of the settlements induced in a historical masonry structure by the excavation of shallow twin tunnels

In this paper a class C prediction of the possible damage induced in an existing old masonry structure by the construction of two parallel shallow tunnels is discussed and compared to the behaviour observed during the excavation stages. The structure under study is the ancient Felice Aqueduct in Rome, recently underpassed by the tunnels of the future underground line known as Metro C. The local geotechnical profile is mainly characterised by a sequence of relatively stiff soil and weak rock strata of volcanic origin, all subjected to an extensive investigation carried out by means of in-situ and laboratory tests. The structure itself has been properly characterised in terms of sub and above-surface geometry and mechanical properties of its block and mortar components. The back predictions were carried out by means of the FE code Abaqus, assuming plane strain conditions for the soil and plane stress ones for the structure. The masonry structure is schematised as an anisotropic elasto-plastic medium, whose elastic properties and yield criterion are derived via homogenization starting form a block structure with periodic texture. The macroscopic model is shown to retain memory of the mechanical characteristics of the joints and of the shape of the blocks. The soil strata are modelled by means of a linear elastic-perfectly plastic model, calibrated against the available experimental results. In the analyses a realistic construction sequence is simulated imposing volume losses consistent with those observed in situ: the resulting settlement profiles as predicted at different elevations of the Aqueduct are in good agreement with those monitored during the real excavations. The adopted approach proves to be valuable for the class of problems under study and, as such, is amenable to be used in future as a class A predictive tool for similar soil-structure interaction processes.