AbstractsEngineering

3D FEM analysis for sequential excavation:

by T. Zhou




Institution: Delft University of Technology
Department:
Year: 2015
Keywords: 3D FEM model
Record ID: 1272116
Full text PDF: http://resolver.tudelft.nl/uuid:13cbbb3d-d1be-4cc7-9c02-448d463347c4


Abstract

This thesis discusses the use of 3D finite element method (FEM) in deep excavations, especially the simulation of sequential excavation method in 3D FEM model. It is an independent research project based on the Spaarndammertunnel project in Amsterdam. This thesis only focused on the excavation part of the tunnel project. Soil parameters used in models were derived from in situ soil investigation. Cone penetration tests and boreholes were carried out along the tunnel location. The soil data was compared with Amsterdam North-South metro line database as well. The excavation plan, retaining structure and building material were developed from the preliminary design of this project. The data for soil investigation and preliminary design were provided by CRUX Engineering BV. The research focus was to investigate the advantages and disadvantages of applying 3D FEM modelling for deep excavations. Since the construction of Spaarndammertunnel starts after the completion of this graduation project, no field data was available for verification. Instead, 2D models were made for comparison and verification purpose. The soil and building material parameters were first tested in 2D spring model and 2D FEM model. The deflections of sheet pile walls in both 2D models were at similar level. The 3D FEM model adopted the model set-up tested and adjusted in 2D FEM model. Parametric study was done in order to investigate how the domain and mesh set-up influence the results in 3D FEM model. Certain geometry (50m * 18m) of excavation section was determined so that the results from 3D FEM model and 2D FEM model were comparable. The excavation method applied in 2D and 3D FEM model was normal staged excavation. The excavation was designed to be dry excavation. After the comparison between 2D and 3D FEM model, the sequential excavation method was implemented into 3D FEM model. Different excavation rates, excavation directions and lateral support design were tested in order to optimize the sequential excavation model. The responses of sheet pile walls as well as surrounding soil were recorded and compared with results from normal 3D excavation model. The parametric study of 3D FEM model showed that the geometry of the excavation zone and the mesh set-up has great influence over the response of sheet pile wall. If the excavation section was longer than 50m, the whole domain can be considered as plain strain, so that the results in 2D and 3D FEM models were almost the same. If the section length was lowered from 50m to 30m, the bending of sheet pile wall could be reduced by 20%. This is because the sheet pile wall can benefit from the corner effect in 3D scenario. It is advised to apply 3D FEM model when dimensions and shape of the excavation zone allows the model to maximize the corner effect. The mesh set-up is also crucial to 3D FEM model. Coarse mesh leads to low accuracy results that certain stress points receiving much high stress than it should. Refine mesh requires a lot more time for calculation. For future research, it is still…