Prediction and Performance of Deep Excavations for Courthouse Station, Boston

Abstract

Construction of the Silverline Courthouse Station in South Boston involved 18-m-deep excavations at a site underlain by more than 24 m of normally and lightly overconsolidated Boston blue clay (BBC). The excavations were supported by 27-m-deep floating diaphragm wall panels and five levels of preloaded cross-lot bracing. This paper compares the measured performance of the excavation support system with the Class A finite-element (FE) predictions prepared during the original design phase and with the results of Class C analyses using information obtained during construction. The numerical analyses used data from a special test program of laboratory and in situ tests at a nearby site. The analyses represent coupled consolidation within the soil mass and the anisotropic stress-strain-strength properties of BBC using the MIT-E3 soil model. The Class A analyses generally overestimate the lateral wall deflections and underestimate the measured strut loads, as preloading was not included in the original FE model. However, they provide remarkably consistent predictions of the measured soil deformations, including settlements, lateral spreading, and subgrade basal heave. The Class C analyses refine the stratigraphic section, in situ pore pressures, construction time frame, and strut preloads using the data available at the time of construction but make nominal changes in soil properties. With these limited changes, the Class C model is able to achieve excellent agreement with the measured data.