Finite element modelling of a deep excavation in the overconsolidated miocene soil of Vienna

Abstract

This contribution deals with the numerical modelling of a 33-meter-deep excavation in the overconsolidated soil of Vienna. The retaining walls of the pit were realized by diaphragm walls which will be integrated into the newly constructed shaft of the metro station Matzleinsdorfer Platz. A comprehensive deformation monitoring system had to be installed on the site due to deformation-sensitive buildings in the vicinity of the shaft. Numerical 2D- and 3D-models, based on the finite element method, are created and compared with each other. Elastoplastic material models with isotropic hardening (Hardening Soil Model with and without small strain stiffness) are used for the calculations. The soil stiffness parameters of the Hardening Soil Model are determined through laboratory as well as field tests. Due to the low hydraulic permeability of the soil, the study also explores the impact of various drainage conditions (drained, undrained, consolidation) on the deformation behavior of the diaphragm shaft, resulting in a set of 24 calculation models. The calculated deformations are compared with the in-situ measurements of the actual shaft to validate the used soil stiffness parameters and material models. After a thorough analysis of the results, an appropriate calculation model is chosen for soil parameter and sensitivity analyses. In the soil parameter study, the effect of the pre-consolidation pressure of the overconsolidated miocene soil layers on the deformation of the diaphragm walls and excavation base is analyzed by varying the pre-overburden pressure (POP) in the calculation model. Furthermore, assuming that all applied soil parameters are statistically descriptable by a normal distribution, shear and stiffness parameters are varied based on standard deviations from literature. The sensitivity study further explores the influence of the diaphragm wall thickness and concrete beam prop width on the deformation behavior of the shaft.