Effect of deep excavations on the construction site seismic demand during earthquake occurrence

Deep excavations in urban environment, mainly realized for the construction of metro stations, are often placed very close to existing above-ground buildings. The interaction in static conditions has been widely investigated, whereas in seismic conditions it is almost completely disregarded. This study starts from a case study, namely the San Pasquale station of Napoli Metro, that is excavated through different layers of sandy soils, with the water table being only 1 m below ground level and the Neapolitan yellow tuff constituting the bedrock of the site. To investigate the effects of the excavation and construction of this station on the nearby buildings, finite element models of the examined case have been developed, representing two scenarios. The first one refers to the excavation works phase, and it is calibrated to accurately reproduce the recorded response via monitoring instruments, installed on the buildings and inside the retaining structures of the station shaft, during the construction of the station. In this context the effects of stress-strain history experienced by the soil during the excavation process are accounted for in the subsequent seismic analysis phase. The second model represents a free field condition, since no excavation is simulated before the seismic analysis phase, and it is used as a benchmark for the first one. The results of the seismic analysis phase of the two models are compared and presented herein in terms of maximum acceleration at the ground level, and frequency content of the related time histories. Two main alignments are considered, parallel and orthogonal to the seismic input direction respectively, in order to highlight the modification of the seismic excitation at the ground level due to the metro station.