Piled Raft Analysis and Viaduct Movement Assessment

The proposed development at Miles Street involved the construction of a 37 storey tower block with one double-storey basement and one single storey basement.

The site was underlain by 2m of Made Ground and 6m of Kempton Park Gravels over London Clay. A Network Rail viaduct carrying mainline rails to and from London Waterloo was present some 6m from the proposed basement wall.

CGL developed a piled raft design for the proposed structure with the geotechnical design carried out using PLAXIS 3D finite element software and a piled raft specific PLAXIS model. Using this software multiple loading conditions can be efficiently assessed and a detailed pile loading schedule provided. The CGL model was additionally able to show that there was redundancy within the proposed pile layout and potential for reductions in pile numbers.

Following the development of the piled raft design, a detailed PLAXIS model was constructed including the structure of the Network Rail viaduct, founded on concrete pier foundations with a formation level some 2m below ground level and with the viaduct track level supported on the piers. The full proposed loading of the structure was applied through a realistic construction sequence and ground movements from each stage assessed. The PLAXIS model was used to derive the secant pile wall deflection movements, with the 3D model able to capture the beneficial impact of the basement geometry of wall deflections. The PLAXIS ground movements through the construction sequence were combined with pile installation movements calculated based on CIRIA 760 to provide total movements at the viaduct and of the tracks on the viaduct.

This ground movement assessment method effectively showed that the proposed development at Miles Street, London, would not lead to movements of the network rail tracks would be above the relevant Network Rail assessment criteria and that the proposed development would not pose a threat to the structural integrity of the viaduct. The clear demonstration of this allowed the development to proceed swiftly, with construction currently underway.