Negative skin friction pile concepts with soil-structure interaction

Abstract

Code-based design of piles with negative skin friction (NSF) considers the NSF force (the drag force) as a load to be imposed on the pile as an unfavourable design action. These codes - for example Singapore Code of Practice CP4, UK Standard BS EN 8004:1986 and the recent Eurocode 7 (EC7) (BS EN 1997-1:2004) - would indirectly factor up the value of the drag force while at the same time disregard the shaft resistance above the neutral plane and factor down the positive shaft resistance below the neutral plane. Thus, the pile design in very deep soft clays typical of Singapore and Asian coastal plains will lead to very conservative pile lengths to meet the code requirements. The Fellenius unified pile design method recognised this deficiency, and it allows for better pile design with NSF taking into account the need for both force and settlement equilibrium between the pile and the soil. Fortunately, EC7 also allows for interactive pile-soil analysis using modern finite-element method tools that can optimise pile design for NSF, in particular when the remaining consolidation settlements around the piles are relatively small. This paper will compare these methods, provide insights into the proper understanding of NSF effects on pile behaviour and recommend the way forward for rational and economical pile design in settling soils.