Cracking at the unheated side of a tunnel during the heating and cooling phase of a fire

Abstract

This thesis focuses on assessing the crack width and durability at the outside of an immersed tunnel in case of fire inside the tunnel. Previous research (for instance Nieman [20]) addressed only the heating phase of a tunnel while also the inevitable cooling phase may have a large influence on the crack width. A user supplied code was written to calculate both the heating and cooling phase and handle the different reversibilities of the material properties. The material model is based on an explicit strain model where some simplifications had been made such as uniaxiality and the omission of the Poisson ratio. This material model is validated on some small models; the tunnel calculations are performed by making use of the geometry of the Wijkertunnel. The results of the tunnel calculations showed good agreement with the results of TNO 2007 [26] for the own weight and pressure loading. During the heating phase the results start to deviate from [26]; partly due to differences in the material model (load induced thermal strains reduces compressive stresses in the walls), partly due to instabilities in the calculation process. The load induced thermal strain decreases the thermal strain under compressive stresses and first time heating. One analysis could follow the complete fire, heating and cooling phase. During the heating and cooling phase the convergence behaviour was poor which is partly due to the complex material behaviour. The material model should be improved to obtain a more stable calculation process. During the cooling phase the tensile stresses increased in the roof and decreased in the walls of the heated tunnel tube as expected. The crack width at the outside of the tunnel is cumulative over a specific area. A lower limit is calculated and a crack width of more than 1 mm is found. This could influence the durability of the tunnel. To support this indication more calculations should be performed. The effect of the load induced thermal strain shows in a decrease of the tensile stresses in vertical direction in the walls and the compressive stresses in the side wall which are present for a longer time. In the mid wall an increase in compressive stress can be seen as a consequence of the net shrinkage which is a consequence of the load induced thermal strain during the cooling phase. It is possible to analyse the crack width during a fire. However more tests need to be performed to get a better understanding about the behaviour of the material properties during heating (high temperatures) and cooling. The code must be validated more thoroughly and only when the deformations of the walls can be explained for sure, a good assessment of the crack width after a heating and cooling phase can be made. In this thesis a foundation is laid for assessing the crack width in immersed tunnels during a heating and cooling phase. No such model existed until now. This model should be perfected, particular with respect to convergence, in order to obtain a more stable calculation and reliable results.