Geotechnical investigation and reuse of existing piled foundations in London

CGL was first asked to look at the feasibility of reusing existing foundations for the redevelopment of the Royal Pharmaceutical Society HQ at 1 Lambeth High Street at an early planning stage. In 2015-16 CGL worked with Iesis Special Structures to create a new piled raft for an 11 storey residential development from the original 1970’s piled substructure, following the principles of the BRE RuFUS framework (Reuse of Foundations for Urban Sites).

New core structures prevented direct reuse of the original piles, and increases in pile loads potentially rendered this route outside RuFUS. As originally conceived, the piled raft would require additional 20m piles of a similar capacity to the 1970 installations, and analysis was done to establish settlements once a new raft was overlaid on the original floor slab.

Detailed ground and geophysical investigations by CGL determined the condition of the existing concrete piles. Using the information gained along with very good construction records for the previous development, CGL was able to demonstrate the viability of piled raft designs for the new building.

CGL elected to use PLAXIS 3D finite element (FE) analysis software to reanalyse the composite soil structure and proposed designs. Routine 3D analysis of this form is relatively new to UK geotechnics on projects of this size. The result for the client was that the modelling showed no new piles would be required and that settlements predicted were well within the tolerance of the new structure. The conclusions of the analysis have been accepted by NHBC.

The construction works are due to commence shortly, but CGL’s geotechnical analysis has considerably reduced foundation and basement costs, shortened the construction programme and minimised use of new materials for the project.