Sensitivity analysis of numerical models for diaphragm walls with cross-walls in soft soils

Abstract

This master thesis presents a sensitivity analysis of parameters used in modelling a deep excavation in soft soil. The main purpose is to understand the impact of different parameters while modeling for the deep excavation of a new railway station in the West Link project, Gothenburg, Sweden. One of the challenges in this project is to predict how the usage of an international proven construction method will preform in the specific soft soil properties of the Gothenburg clay, and how to best model it. The thesis presents the experience of similar projects conducted with similar conditions and how the support structure of diaphragm wall and cross-walls is used and installed. In order to create a model representing these particular circumstances, the Lilla Bommen section of the Göta Tunnel project is modelled as a reference model using the PLAXIS 2D program with two different material models representing the clay behavior. The chosen material models are the Hardening soil model using small strain and the NGI ADP-model. One of the difficulties in modelling the excavation, supported by the diaphragm wall and cross-wall, was to simulate the 3D effects of the perpendicular cross-wall in a 2D model, where the out-of-plane spacing and the interface between the structure and soil of cross-wall is unable to recreated. However, this was solved by converting the lateral support of the cross-wall into imaginary steels truss and having the cross-wall parallel to the diaphragm wall instead. The comparison between the material models showed that the NGI-ADP-model had greater displacements than the Hardening Soil small strain model. The sensitivity analysis of the parameter study also showed some interesting results of what parameters have the larger impact. The interface strength Rinter is hard to estimate due to several reasons but had an affect on the bending moment and lateral deformations of the diaphragm wall. The stiffness parameters of the Hardening Soil model were hard to study separately since they are depended on each other. In the NGI-ADP-model the ratio of un-/reloading shear stress modulus and active plane strain shear strength Gur/SA u showed to have notable impact on the heave at the excavation bottom.