Bored pile design in stiff clay II: mechanisms and uncertainty

Abstract

The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state.Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay.Estimates of settlement using a non-linear soil stress-strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays.

Geodatabases to improve geotechnical design and modelling

“…The model reported in Vardanega et al (2012) was found to match the bounds of Patel's (1992) database of test results well, however, it was not applied to individual pile tests in London Clay. In this paper a preliminary investigation of the model's applicability to the full-scale pile load tests reported in Whitaker and Cooke (1966) is presented.…”

“…(2) is valid when M is greater than 1/α, at which point the ultimate shaft resistance is reached. Vardanega et al (2012) related M to overall factor of safety, F total , using Eq. (3), assuming the factor of safety on the shaft resistance, F shaft , is sufficiently similar to F for straight-shafted piles.…”

Discussion: How Eurocode 7 has affected geotechnical design: a review

“…For piled foundations in stiff clays, Vardanega et al (2012c) advocate a 5th percentile design line (c u with depth) for base resistance, and a 50th percentile design line for shaft friction. Future developments in Eurocode 7 concerning the determination of characteristic values should require that the design calculation in question (e.g.…”

“…Equation (5) is used in Vardanega et al (2012c) along with a 'Randolph-style' calculation method (e.g. Fleming et al, 2009) to estimate settlements of bored piles in stiff clays.…”

Editorial

“…The second paper also by Vardanega et al (2012b) forms the companion paper to the first paper and deals with the mechanisms and uncertainty related to bored pile design in stiff clay. Using the same base data for London Clay, this paper explores the challenges involved in design with regard to total stress and effective stress conditions.…”