Effects of deep excavations in soft clay on the immediate sourroundings-Analysis of the possibility to predict deformations and reactions against the retaining system

Doctoral thesis, 2007

When excavating in an urban environment, the evaluation of the magnitude and distribution of ground movements is an important part of the design process, since excessive movements can damage adjacent buildings and utilities. In order to minimize movement of the surrounding soil, a retaining wall support system is used for deep excavations to provide lateral support. This dissertation describes different methods of evaluating ground movements adjacent to a deep excavation in soft clay and how to estimate the lateral earth pressure acting on the retaining wall system. A review is presented regarding: - Soil characteristics that are important for evaluation of deformations and earth pressure. - Current empirical methods of estimating ground surface settlements - Different classic methods of calculating lateral earth pressure - Various soil modelling methods, with focus on the theory of elasto-plasticity. This review is followed by an extensive case study performed at the Göta tunnel project, in the centre of Gothenburg, Sweden. Back analyses were performed in order to predict and interpret ground deformations and the development of stress changes against the retaining wall system. These analyses took the form of non-linear finite element analyses with three different constitutive models (an isotropic linear elastic Mohr-Coulomb model, e-ADP, a total stress based model capable of modelling anisotropic undrained shear strength as well as non-linearity in shear, and MIT-S1, a bounding surface model based on effective stresses. The different outcomes of these three models are compared and discussed. Special focus has been on evaluating the parameters to the MIT-S1 model and to evaluate the response of this model compared against advanced laboratory tests. The outcome of the analyses shows the advantage, compared to simpler models, of using finite element methods in combination with an advanced soil model, such as the MIT-S1, capable of simulating small strain stiffness, non-linear elasticity, non-linearity in shear and the development of shear induced excess pore water pressure. The analyses also show the importance of combining FE-analyses with empirical methods for estimating ground surface settlements. The field monitoring also demonstrated that deformations obtained by shear strength mobilization may be overshadowed by other engineering activities and poor workmanship. Close collaboration between the contractor and geotechnical consultant is of paramount importance.