Numerical development and implementation of a constitutive model for clays with application to deformations around a deep excavation

In heavily congested urban environments the development and use of underground space often offers the only viable alternative for the improvement of transportation infrastructure (e.g., MUNI Turnback, San Francisco). Traditionally, the design of the shoring system for excavations on soft soils was the responsibility of the Contractor. The Engineer's responsibility was limited to specifying a system that would result in a safe excavation. However, the practice is changing rapidly because of the very significant costs associated with litigation related to properties that have been damaged as a result of relatively small ground movements. Thus, accurate prediction of ground movement is not only an important part in the design of lateral earth support systems, but is essential for estimating the risk of damage to surrounding facilities. The usefulness of numerical methods for the analysis of excavations is determined, in general by the following parameters: (a) "accurate" modeling of the generally complex construction sequence; (b) accurate characterization of the mechanical behavior of the soil material, and (c) modeling other construction activities.; This dissertation addresses the modeling of lightly overconsolidated clays and its application to the prediction of deformations around a deep excavation using a well-instrumented case history. The numerical modeling of clay behavior is subdivided into two areas: (a) the formulation of inelastic laws describing the material response and (b) the robust numerical implementation of these laws for the solution of boundary value problems. The formulation of inelastic constitutive laws for clays is investigated for both finite strain and small strain kinematics. A new constitutive rate independent model, referred to as Bear-Clay, is developed to describe the anisotropy stress-strain-strength relationship of lightly overconsolidated clays. The new model requires nine parameters, obtainable from standard laboratory tests. New fully implicit and explicit algorithms with error control are proposed to integrate the constitutive laws in both non-linear finite element and finite difference computer codes. The accuracy, stability, and efficiency of these algorithms are investigated through theoretical considerations and numerical simulations. Finally instrumentation data collected during the construction of the MUNI Turnback, are used to illustrate the capabilities of the proposed numerical tool.