Advanced geotechnical characterisation to support driven pile design at chalk sites

Abstract

Research is described that contributes to a major effort to improve current shortfalls in knowledge regarding pile driving, ageing, static and cyclic response under axial and lateral loading in chalk. More reliable design guidelines are needed urgently for offshore wind power, port, flood protection, high-speed rail and other applications. The ALPACA and ALPACA Plus joint industry projects aim to develop such new approaches through field testing at St Nicholas at Wade in Kent, UK. This Thesis describes the Author’s contributions. The main area considered is the projects’ intensive site characterisation through high quality sampling, in-situ and advanced laboratory testing. However, the Thesis also contributes a substantial analysis of pile tests conducted by other industrial consortia on steel driven piles in chalk at other sites in France, Germany and the UK, considering a wider range of pile geometries, chalk types, sites’ corrosion chemistry and geographical locations, while also allowing the evaluation of existing procedures for calculating driving resistances and axial capacities. After reviewing the outcomes from these parallel test programmes, the Thesis moves to describe the stratigraphy, structure and mechanical properties of the low-to-medium density chalk encountered at the ALPACA piling site, in research that underpins the field experiments’ interpretation and is central to their representative modelling. The experimental characterisation of the chalk identified aspects of behaviour that require careful attention when undertaking numerical analysis to model practical problems including the chalk’s marked sensitivity, brittleness, pressure dependency and anisotropy, as well as strain rate dependency. A clear hierarchy was found between profiles of peak strength with depth of Brazilian tension (BT), drained and undrained triaxial and direct simple shear (DSS) tests conducted from in-situ stress conditions. Highly instrumented triaxial tests revealed the chalk’s unusual effective stress paths, markedly brittle failure behaviour from small strains and the effects of consolidating to higher than in-situ stresses. The chalk’s structure, consisting of mainly sub-vertical joints, leads to varying properties dependent on specimen scale and in-situ stiffnesses falling significantly below equivalent laboratory measurements. The joints also promote stiffness anisotropy. Horizontal shear and Young’s modulus profiles fall well below the corresponding vertical trends. While vertical compressive strength and stiffness values are relatively insensitive to the applied effective stress levels, consolidation to higher pressures tends to close micro-fissures and reduces stiffness anisotropy. Additional laboratory research was incorporated that examined hypotheses regarding the role of corrosion in affecting the ALPACA pile field experiments, comprising mass loss and electrochemical experiments, as well as steel-chalk interface shear tests with different combinations of types of steel and chalk pore water chemistry. The corrosion reaction rates declined with time, were far faster with oxidizable steels when given access to air and faster still with saline water. The chalk’s dilation characteristics depended strongly on testing procedure, interface surface roughness, steel grade and the associated physiochemical interactions. The contributions described proved vital to the interpretation and analysis of the time-dependent, axial and lateral, static and cyclic behaviour observed in the 41 piles driven and tested as part of the ALPACA and ALPACA Plus projects.