Comparison of the measured and finite element-predicted ground deformations of a stiff lodgement till

The use of the finite element method to model excavations and tunnels in Dublin black boulder clay has, in the past, had limited success owing to the failure of the available constitutive models in commercially available software programs to adequately represent the essential features of a stiff soil. A stiff soil - lodgement till - underlies much of the city of Dublin, Ireland; consequently, the successful prediction of deformations in this soil arising from structural and infrastructural projects is of considerable importance. Research has shown that the stress-strain response of a stiff soil is complex and depends on many factors including stress history, stress level, and strain direction. These important features are included in the hardening plasticity small strain stiffness (HSS) soil model that is incorporated in the Plaxis V8.4 finite element code. This paper describes the field and laboratory methods that were used to determine the parameters for incorporation in this soil model and the validation of these parameters. These parameters are used to model the deformations around two excavations: a 4.5 m deep excavation with a vertical face, and a 10.7 m deep excavation with a face slope of 70 degrees-75 degrees. Good agreement was found between the predicted and observed deformations.