Design of Quay Walls using the Finite Element Method

Abstract

This MSc thesis focuses on two types of quay walls, namely the anchored quay wall without relieving structure and the quay wall with relieving structure. In the case of high retaining heights the anchored quay wall mostly consists of a combined sheet pile wall, which means that the wall is composed by tubular steel members that are connected by means of sheet piles. In some cases quay walls are provided with a relieving structure. A relieving structure consists of a concrete structure founded on the combined sheet pile wall and bearing piles. The function of the relieving structure is to reduce the pressure on the retaining wall caused by the surface load and soil on the relieving structure. In this thesis the engineering aspects (moments and displacements of the wall and anchor forces) and the economic aspects (costs) of quay wall designs with and without relieving structure are analysed. Also two calculation methods are compared, the beam on elastic foundation and the finite element method. The results of the calculation showed that from an engineering point of view, the relieving structure is more favourable. The displacements, moments and anchor forces when a relieving structure is present are smaller than the ones calculated when the relieving structure is absent. Also in the case of the failure of one anchor a relieving structure gives a better redistribution of forces due to the high stiffness of the slab of the relieving structure. The cost estimation performed in this thesis lead to the conclusion that a quay wall with relieving structure is more expensive that a quay wall without it. The main contribution to the costs of the analysed quay walls come from the steel combined wall. The price of the steel primary members (steel tubes) depends on the way they are fabricated. When the tubes exceed certain limit in dimensions the fabrication costs increase with a factor 1.5 to 2. In this thesis, the calculated elements of all variants exceed this limit, which lead to high costs of the fabrication of steel tubes for both, the quay wall with and without relieving structure. During decision making the parties involved would have to consider whether the reduction of the costs by using a quay wall without relieving structure overrun the aforementioned engineering aspects. An important aspect is that when the relieving structure is absent the dimensions of the primary elements of the combined wall increase and are considered extraordinary in onshore projects. The companies that can install such elements are limited and even offshore equipment would be necessary, which could increase the total costs of the project. Regarding the calculation methods some remarkable differences where found, specially in the calculation of the anchor forces. The moments in the retaining structure for both cases with and without relieving structure calculated with D-sheet vary sligtly from the ones calculated with PLAXIS, a maximum difference of 20% is found. When the relieving structure is absent the anchor forces calculated with both methods differ in a range of 20%. When a relieving structure is present D-sheet estimates forces that are 30% to 35% lower than PLAXIS when the relieving structure has a length of 10 meters. When the relieving structure length increases to 20 meters, D-sheet calculates anchor forces that are 20% to 60% lower than PLAXIS. Literature review showed that the method to estimate horizontal pressures due to strip loads used by D-sheet underestimates the resultant force acting on the wall, which leads to an under estimation of anchor forces.