455 Proceedings of the XVI ECSMGE Geotechnical Engineering for Infrastructure and Development ISBN 978-0-7277-6067-8 © The authors and ICE Publishing: All rights reserved, 2015 doi:10.1680/ecsmge.60678 Influence of pile raft stiffness on building behaviour in a tunnel-pile clash scenario Influence entre raideur d’un massif sur pieux et comportement d’un bâtiment dans un scenario de collision tunnel-pieu V. Dubasaru 1 , L. Zdravkovic *2 , D.M.G. Taborda 2 and S. Hardy 3 1 ARUP, London, UK (formerly Imperial College London, UK) 2 Imperial College London, Department of Civil and Environmental Engineering, London, UK 3 ARUP, London, UK * Corresponding Author ABSTRACT In a modern urban environment, the underground space becomes increasingly congested due to the high value of the land that forces the new infrastructure projects to be constructed deeper into the ground. For each new project, the potential of both expected and un- expected clashes between new tunnel alignments and the foundations of the existing structures becomes more probable. However, to date, the research on tunnel-pile clashes has been scarce. In the current study, the effects of such a situation are studied by carrying out finite el- ement analyses for a scenario that is typical in the London ground profile. A parametric study was conducted to investigate the influence of the pile raft bending stiffness on the building settlement and the change in piles’ axial forces. It is shown that an increased raft bending stiffness helps to transfer the load from the trimmed pile to the adjacent piles, thus reducing the settlement of the trimmed pile. In the pro- cess of tunnel excavation, the pile settles due to the soil-induced downdrag and the loss of both its base and part of its shaft capacity. It is concluded that the tunnel-pile clash has a large impact on the surface structure, piles and tunnel itself. RÉSUMÉ Dans un environnement urbain moderne, l’espace souterrain est devenu très prisé face à la montée des prix des terrains. De ce fait, les nouveaux projets d’infrastructure sont dorénavant forcés de s’installer plus en profondeur. Pour chaque nouveau projet, la probabil- ité de collision, attendue ou non, entre de nouveaux tunnels et les fondations de structur es existantes a augmenté. Cependant, à ce jour, les études sur les collisions tunnel-pieux sont peu nombreuses. Dans cette étude, les conséquences de cette situation sont examinées au travers d’analyses par éléments finis pour un scénario qui est typique d’un profil de sol Londonien. Une étude paramétrique a été conduite pour étudier l’influence de la rigidité flexionnelle d’un massif sur pieux sur le tassement d’un bâtiment ainsi que sur le changement de forces ax- iales des pieux. Cette étude montre que l’augmentation de la rigidité flexionnelle du radier contribue à transférer les charges du pieu coupé vers les pieux adjacents, réduisant ainsi le tassement du pieu coupé. Lors de l’excavation du tunnel, les pieux se tassent du fait des forces d’enfoncement induites et de la perte en résistance provenant de la base du pieu coupé et d’une partie de sa surface de frottement. Nous en concluons que la collision tunnel-pieux a d’importantes conséquences sur les structures à la surface, sur les pieux et sur le tunnel lui-même. 1 INTRODUCTION In large urban environments with congested subter- ranean spaces, many large infrastructure projects are currently being proposed. Due to the high value of land, the new projects are constructed deeper into the ground. This increases the potential of both expected and unexpected clashes between new and existing structures. Such clashes can have a major impact both on the new tunnel lining and on the existing structure. Several types of mitigation measures are known to have been adopted on various projects to reduce and control the effect of clashes. These can be grouped in in-tunnel and out-of-tunnel mitigation measures. The major disadvantage of the former is that their implementation causes delay in the tunnel construction, while the latter can be prepared before the tunnelling commences and can be applied inde- pendently of the underground works. The in-tunnel mitigation measures are highly de- pendent on the tunnelling method. The Sprayed Con- crete Lining method (SCL), being a more flexible technique than TBM or EPBM, gives the opportunity for a larger variety of mitigation measures to be im-