A conceptual design of a quay wall on a steel slag subsoil

Abstract

This report addresses the design of a quay wall on a steel slag subsoil. The reason for this study is the realisation of a wind turbine assembly port. Worldwide hydraulic structures are seldom made in these artificial soils. This steel slag material present in the project area complicates the construction of a quay. Is it possible to install foundation piles in this material? What environmental aspects need to be taken into consideration? Which quay wall type is most suitable for realising the port area?
The study starts by explaining the motivation behind the quay wall structure on this unconventional soil. The main reason for the need for wind assembly ports is to increase the wind turbine installation and maintenance capacity. The problem analysis explores the challenges associated with the steel slag materials, this leads to the problem statement and design objective. The problem can be summarised as follows: Despite the large experience in port developments and quay wall constructions, the ability to efficiently design a quay wall on a varying soil system like steel slag, is still considered complex. This leads to the goal of this thesis, which is to create a conceptual design of a quay wall on a steel slag subsoil at the location.
The design analysis aims to find the most efficient quay wall design, which is possible to construct and even take advantage of the presence of steel slag material to increase structural performance and stability. In the thesis approach, the steps taken to achieve the goals of the study are shown. The report proceeds with the development of a method in which the different characteristics of the steel slag material are examined.
The steel slag materials have some positive and negative effects compared to regular soils. The relatively high friction angle and high density can benefit the structure when applied at the right location.
One of the issues with using steel slag is the risk of environmental implications. When steel slags come in contact with air and water, heavy metals can leak out of the slag causing damage to the ecosystems and humans. The design solution aims to mitigate the environmental risks without exponentially increasing the costs.
A system analysis follows including an area, stakeholder and function analysis. This helped to illustrate the broader environment and requirements for the quay wall construction. The basis of design section outlines the starting points and boundary conditions whereafter the programme of requirements and evaluation criteria are defined. The report includes a functional and structural design.
After analysing potential alternatives for constructing a quay wall, the cofferdam variant was the most promising given the required bearing capacity, height and subsoil. The cofferdam design consists of two combi walls connected with tie rods at two levels. For stability, a grout anchor is connected to the backside of the combi wall. A low permeable environment was created because a clay layer is present between the two walls. The cofferdam dimensions were chosen so that most of the steel slag material would be enclosed between the combined walls. The residual volume of steel slag material is used as a fill material for the piles.
In the structural design, a detailed construction sequence and the design model were provided.
A PLAXIS 2D model based on Finite Element Method (FEM), was made for two cross-sections of the quay wall. Based on the outcome, the elements of the quay wall were verified and optimised. The installation method of the combi wall has a large impact on the cost. Results were analysed and risk-mitigation measures have been advised to provide a controlled construction. Various checks on stability, strength, stiffness and deformation were conducted to ensure this design meets the technical standards.
The validation of the design was then performed to check whether the design objective was adequately formulated and correctly translated into the requirements. As the client was Port of Amsterdam, the design was validated in correspondence with this company.
The report concludes with a discussion of design considerations and the implications of the design choices. Finally, the conclusions and recommendations section summarises the key outcomes of this report.
Based on the outcome of this report, it can be concluded that the construction of a safe and stable quay wall is possible with the right construction measures. It was recommended that further analysis of the environmental impact of the re-usage of steel slag material be conducted.

For further research, it was recommended to perform detailed calculations on the connection between the elements. In addition, a hydrological test could be performed to understand the flows of the rain and groundwater in this design. For the execution of the structure, it was recommended to perform an additional pile driving test with the driving shoes to prevent failure. The test results will show it this setup is suitable for the realisation of the quay wall. Additionally, it can provide extra certainty on the construction time, cost and knowledge.