Many cities around the world have well developed sub-surface infrastructure comprising sewers, water pipes, underground railway lines and basements. In some cases, parts of this infrastructure are relatively old and fragile. With continued development and expansion of cities, these older structures need to be protected to ensure their continuing function.
Completed centrifuge model test investigating movements generated by simulated underground tunnel construction.
In the case of London, much of the underground transport network is relatively old and structures such as the running tunnels are sensitive to additional loads or movements being imposed upon them. New construction always causes movements within the ground, which can unfortunately be transferred onto existing structures, potentially causing damage. For example, a small movement of a running tunnel can reduce clearance between train and tunnel, meaning that remedial works must be carried out before the service can safely operate.
Because of its age, much of London's Underground is inaccessible to people with restricted mobility. The issue of accessibility is particularly acute in central London, where most stations are deep underground and existing access is by escalators and stairs only. One solution to this problem would be to install a lift down to the platform level. The installation of lifts would involve excavating a large vertical shaft, the construction of which would inevitably induce movements within the soil. These movements would then be transferred onto the existing structures (such as the platform tunnel, for example) potentially causing damage. The final effect is that the project would become uneconomical or impossible to safely construct.
The main objective of our research is to investigate what effect installing a lift shaft has upon existing infrastructure. For the purposes of this project, the existing infrastructure will be assumed to be an underground station tunnel and the soil within which this exists will be a clay of a similar type to that found in London. Therefore the entire scheme could be said to represent a project to upgrade the accessibility of an existing London Underground station.
The research will be carried out using City University London's geotechnical centrifuge, which allows researchers to model large scale problems in the laboratory. We will perform a series of experiments to investigate the geometry of deep shafts and the ground movements brought about during construction. By using a centrifuge to apply a high gravitational field to a small scale model, we can simulate the stresses that would be experienced within the soil at full scale. These small scale models then become representative of the full size problem.
The new understanding the research will provide will have an impact on designers and contractors by giving confidence in economic solutions to underground construction problems. Improved understanding of the ground behaviour will enable more complex designs to be completed efficiently without projects overrunning. The possibility of additional project costs in order to rectify damage to surrounding infrastructure will also be significantly reduced.
Dr Richard Goodey
City University London
Dr Goodey was awarded a Leverhulme Trust Research Project Grant in March 2013.