Pile tests to justify higher adhesion factors in London Clay

Abstract

The London District Surveyors Association guidance notes for the design of straight shafted bored piles in London Clay recommend an adhesion factor of 0·5 for shaft friction pile design. This value, based on the back-analysis of many maintained load pile tests to failure in London Clay, was selected to cover all design scenarios irrespective of site location, pile construction method, pile geometry and whether or not pile load testing is carried out. This ‘one size fits all’ approach to pile design is potentially over conservative. For sites where maintained load tests (MLTs) to failure on representative preliminary test piles are carried out, the results can be back-analysed to determine the appropriate adhesion factor for the site, which may then be used to improve the efficiency of the pile design. This paper discusses the pile design philosophy, pile load testing and construction sequence for a large development in Whitechapel, London, where MLT results were used to justify an increased adhesion factor of 0·6. This increased adhesion factor ensured that piles were ‘dry’ bored and founded above the water-bearing strata of the Lambeth Group, leading to lower project costs and risks.