FE Analysis of a Deep Excavation in Santos Clay

This paper presents a case history of an eight-metre-deep basement excavation in soft silty clay deposits in Santos, São Paulo, Brazil. The site consisted of a five-metre-thick sand layer overlying a twelve-metre-thick, soft clay layer, underlain by dense sands. A comprehensive geotechnical investigation, including CPTU, DMT, and laboratory testing, informed the initial design, which employed Plaxis for a five-metre-deep diaphragm wall-supported excavation. However, a subsequent unauthorised excavation of an additional three meters triggered significant wall displacements, necessitating emergency measures. The authors conducted additional Plaxis analyses to investigate this, modifying only the excavation depth while preserving the original soil models and parameters. The remarkably close agreement between predicted and observed wall displacements validated the accuracy of the numerical modelling approach. This case study underscores the critical importance of rigorous geotechnical design and the potential consequences of unanticipated excavations. The findings highlight the efficacy of Plaxis in predicting the geotechnical behaviour of deep excavations in soft clay conditions.