Foundation Optimisation and its Application To Pile Reuse

Andy Leung, Cambridge University
Geotechnical Engineering Group


Pile reuse has become an increasingly popular option for foundation designs, mainly due to its potential cost and environmental benefits and the problem of underground congestion in urban areas. However, key geotechnical concerns remain regarding the behaviour of reused piles, the modelling of foundations with old and new piles, and their interactions with the superstructures.

In this dissertation, a design and analysis tool for pile reuse projects is introduced. The tool allows direct coupling of superstructure stiffness with the foundation model through the matrix condensation method, by which a condensed structure matrix is extracted to represent flexural characteristics of the superstructure. The condensed matrix can be coupled with the foundation model for a holistic analysis of the superstructure-foundation system. Using this technique, superstructures with stiff structural components are shown to have significant impacts on the settlement response of foundations.

The proposed design tool also includes an optimisation algorithm to obtain the best configuration of new piles to work alongside reused piles. Foundation optimisation is first investigated using an exhaustive search technique, and then through a heuristic algorithm known as the Differential Evolution (DE). The DE is found to be an effective technique for foundation optimisation, which leads to both improvements of foundation performance and reductions in material usage. Under the concept of Pareto Optimality, the DE can be extended for multi-objective optimisation analyses, which reveal the relationships between optimised material usage and the corresponding foundation performance, providing a series of design options at various foundation costs.

Components of the analysis tool are validated against several case studies. To illustrate its application to pile reuse projects, a case history in London is presented, where 110 existing piles will be reused at the site to support the proposed new development. A series of optimised reuse options are obtained with various amounts of new pile materials, each leading to different foundation response. The various optimised solutions can help engineers and developers make informed decisions on the foundation design, depending on the finances of the project and the required level of foundation performance.