Non-linear finite element prediction of the performance of a deep excavation in Boston Blue Clay

The work investigates the behavior of a deep excavation which forms part of a 100 m wide basement excavation located in Boston, Massachusetts. Two different types of tied-back retaining walls were used, i.e., a soldier pile tremie concrete wall and a traditional reinforced concrete diaphragm wall. The glacial marine clay (Boston Blue Clay) deposit was modeled with the Kinematic Hardening Model for Structured soils (KHSM), its reduced bubble model version (KHM) and the well-known Modified Cam Clay (MCC) model. The difference between the models is the prediction of softening with loss of structure as plastic strains occur. The values of the optimised soil parameters used in the simulations were obtained by a careful calibration of the models against a range of advanced laboratory and field tests performed at the site. Comparison of the available horizontal wall movements monitoring data with the undrained finite-element predictions revealed a very satisfactory agreement when the KHM was used in conjunction with a small-strain elastic formulation. The relatively small increase in lateral wall deflection in the presence of initial structure accounted for in the KHSM confirms that the small-strain properties of the soil control the magnitude of excavation deformations. Finally, using a coupled-consolidation analysis and the KHSM, an excellent agreement between the observed and measured pore water pressures and ground movements of the excavation base was achieved.