Numerical assessment of negative skin friction effects on diaphragm walls


Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Civil Engineering, Turkey

Approval Date: 2013

Student: CANSU GENÇOĞLU

Supervisor: KEMAL ÖNDER ÇETİN

Abstract:

Within the confines of this study, numerical simulations of time dependent variation of downdrag forces on the diaphragm walls are analyzed for a generic soil site, where consolidation is not completed. As part of the first generic scenario, consolidation of a clayey site due to the application of the embankment is assessed. Then two sets of diaphragm walls, with and without bitumen coating, are analyzed. For comparison purposes, conventional analytical calculation methods (i.e., rigid-plastic and elastic-plastic soil models) are also used, the results of which, establish a good basis of comparison with finite-element based simulation results. Additionaly, the same generic cases are also analyzed during the stages of excavation, when diaphragm walls are laterally loaded. As the concluding remark, on the basis of time dependent stress and displacement responses of bitumen coated and uncoated diaphragm walls, it was observed that negative skin friction is a rather complex time-dependent soil-structure and loading interaction problem. This problem needs to be assessed through methods capable of modeling the complex nature of the interaction. Current analytical methods may significantly over-estimate the amount of negative skin friction applied on the system, hence they are judged to be over-conservative. However, if negative skin friction is accompanied by partial unloading as expected in diaphragm walls or piles used for deep excavations, then they may be subject to adverse combinations of axial load and moment, which may produce critical combinations expressed in interaction diagrams. Neglecting the axial force and moment interaction may produce unconservative results.