A Novel Method for Assessing the Reuse of Under Reamed Piles as Part of the Southbank Place Project-Paper

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The Southbank Place redevelopment in central London reused some of the under-reamed piles installed for the original Shell Centre in the 1950s. Reuse offered cost, construction and carbon benefits, but required confidence in piles that also help restrain movement of the Bakerloo Line tunnels beneath the site.<br /><br />The authors adapted CIRIA and BRE guidance, which primarily addresses straight-shafted piles. For under-reamed piles, they proposed checking that the new working load is supported by shaft capacity alone, with a minimum factor of safety of 1.1, to limit settlement. A separate check assessed ultimate capacity, including the pile base, with a minimum factor of safety of 2.0. Piles were assigned low, medium or high reuse risk according to existing and proposed loads and these capacity checks; the foundation layout was adjusted so no reused pile remained in the high-risk category.<br /><br />The assessment combined archival research, intrusive investigation and numerical modelling. Investigations confirmed pile geometry and examined concrete durability and nearby clay. The five-pile sample was selected using ISO 2859-1 sampling guidance, supplemented by engineering judgement. The tested concrete was generally dense and well hydrated, with no evidence of sulphate attack, although some local surface weakening was observed. Measured geometry agreed with archive drawings, and adjacent clay showed no discernible difference from clay elsewhere on site.<br /><br />A three-dimensional LS-DYNA finite-element model, using the BRICK soil model, represented the site’s construction and stress history to assess new foundation loads and effects on underground infrastructure. Residual uncertainty remained, as with any reuse of existing foundations. The project therefore included technical peer review and latent-defects insurance. The paper presents the approach as a means of managing reuse risk while retaining useful existing foundations on a constrained urban site.

Keywords

Southbank Place

pile reuse

under-reamed piles

foundation assessment

London redevelopment

Bakerloo Line tunnels

load capacity

geotechnical engineering

finite-element modelling

carbon benefits