Abstract

Chalk is present in a large area of the East and South coast of the UK where renewables are now being installed. In most cases they are founded on displacement piles, whose design uses empirical methods or partial (wished-in-place) simulations. Installation effects such as grain crushing and pore pressure generation are conservatively estimated (if considered at all), as the change of chalk properties around the pile due to pile installation cannot be easily quantified. In this work it is shown how advanced numerical modelling can be used to guide the design of the foundations in these complex soils.

Abstract

Chalk is a silt-sized soft biomicrite rock often encountered as a low to medium density, high porosity, structured material within a fractured mass. In recent years, there has been increased interest in the behaviour of chalk and the development of new design procedures for pile foundation installation design, motivated by several large-scale onshore and offshore infrastructure projects. Recent modelling has demonstrated the importance of accurately characterising the operational stiffness of the chalk mass. While several methods exist to measure the chalk’s stiffness in situ, they are often subject to significant scatter, with no guidance available to the end user on interpretation or on which method should be used as a baseline. A new programme of multi-method in situ geophysical testing in chalk at a well-characterised onshore test site in Southern England is described that forms part of a wider research project. The chalk deposit is shown to be relatively uniform with depth which provides a unique opportunity to apply multiple methods and interpretations without the influence of significant layering. The experimental programme is described and the interpretation and selected results of downhole geophysical tests at depths up to 40m are presented. The chalk’s remarkably high shear stiffnesses are shown to be highly repeatable and consistent when rigorous test execution and analysis is applied