Colne Valley Viaduct foundation design

The proposed 3.4km long Colne Valley Viaduct provides an excellent opportunity to review and optimise design methodologies for large diameter bored piles within Chalk. Whilst piles have been successfully designed and constructed in chalk for many decades, the HS2 Colne Valley Viaduct will be required to support the highest speeds and dynamic forces of any UK railway viaduct, in addition to maintaining the tightest of serviceability deflections and dynamic response. This paper illustrates how the project team have developed a co-ordinated design and construction approach to deliver value and certainty of performance for this major structure.

Current UK practice for design of piles in Chalk follows the guidance given in CIRIA C574 Engineering in Chalk. Whilst this gives guidance on determining ultimate shaft friction and end bearing resistance, it also suggests increased factors of safety on end bearing to allow for potential solution features within the chalk. This can lead to an overly conservative design. This paper describes the development of the methodology for the design of the piled foundations and to address the requirements of BS EN1997. The viaduct is supported on 56 piers and 2 abutments, each comprising groups of 4 – 6 piles, 1.5m and 1.8m in diameter. The piles are between 32m and 55m long and are founded in the underlying Seaford and Lewes Chalk (Upper Chalk). This paper sets out the assumptions made at early scheme design, the ground models used and preliminary pile load test results using bi-directional Osterberg cells. Additionally, this paper summarises how the results of the pile testing were incorporated to optimise and verify the detailed design of the piles.