Large Diameter Pile Foundations in Heavily Over-Consolidated Strata: A14 Cambridge to Huntingdon Improvement Scheme Case History

Highways England’s A14 Cambridge to Huntingdon Improvement Scheme is approximately 37km in length and runs from the existing A14 trunk road at Ellington (near Huntingdon) to the Cambridge Northern Bypass at Milton. It includes 17km of improvement and upgrading of the existing road network in the west and east, and 20km of new highway in the central section to the south of Huntingdon. The scheme includes 34 new major structures including a 750m long viaduct. The route corridor is underlain predominantly by over-consolidated clays of Cretaceous and Jurassic age overlain by Glacial Till at the western end of the route and, locally by recent deposits associated with River Great Ouse. The nature of the underlying ground dictated that all major structures were supported on piled foundations. These comprised 900mm diameter CFA and 1800mm diameter bored cast in situ piles generally 15 to 30m in length, but up to some 40m in length for some structures. The founding strata for the piles included Glacial Till and heavily over-consolidated Oxford Clay, Ampthill Clay, Kimmeridge Clay and Gault Clay strata. Extensive ground investigations were carried out and interpreted for foundation design. Eight fibre-optic instrumented, preliminary pile load tests were also undertaken to determine performance in the various founding strata and were used to verify ground interpretation and pile design modelling. The paper outlines the scheme and structures and summarises the interpreted ground conditions along the route corridor. The interpreted pile load test results are also presented and compared with the interpreted ground investigation data. The paper also presents an overview of the pile design and construction.