Test pile design, construction and testing – maximising the benefits of preliminary pile test results in pile design and construction at Euston Approaches

Abstract

A 450m long retained excavation to a maximum depth of 18m close to sensitive 3rd party assets will be constructed to house future HS2 tracks north of Euston station. Approximately 1700 bored piles between 1.2 to 1.8m in diameter, with over 700 located beneath the base slab and founded primarily in the Thanet Sand Formation, were designed to resist both tensile axial heave and compressive over-site development (OSD) loads. This paper presents the design thinking behind the testing of two preliminary test piles, as a part of the verification of the contract pile design with lower factors of safety as permitted by Eurocode 7. The tests addressed project geotechnical risks and allowed an efficient design. Collaborative discussions were carried out between design and construction teams, including the specialist piling contractor, to provide confidence from both design and construction viewpoints. The piling contractor’s preference for recent innovations such as polymer support fluid and distributed fibre optic sensing gave an opportunity to develop industry best practices. Clear communication channels between the parties responsible for analysis/design, management and testing and construction were maintained throughout the site works. Built-in instrumentation with automated live data transfer proved critical for rapid analyses during the first test. The construction programme was configured to allow adjustments to the second pile test to be made based on the preliminary results of the first test. Final load-settlement results were comparable between the two preliminary test piles. An allowance of 56 hours for the pile bore open time was established and the removal of base grouting for contract piles was achieved. Overall, the pile testing programme at Euston demonstrated its effectiveness on multiple fronts: sustainability, construction programming, geotechnical risk management and design validation.