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.
Highlights
The current state of practice for the design of piles is to place emphasis on the pile as a capacity determination problem
When the design codes treat pile design as a capacity problem, it often follows that the negative skin friction (NSF)-induced force in the pile is treated as an unfavourable load for the pile
Qb is the ultimate toe bearing resistance, Qsp is the ultimate positive shaft resistance below the neutral plane (NP), Fs is the geotechnical factor of safety, Pc is the dead load (DL) plus live load (LL) to be carried by each pile, Qsn is the accumulated NSF force, the drag force, and h is the degree of mobilisation, typically 0·67, 1·0 may be used in specific cases
Summary
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. EA pile axial rigidity Fs geotechnical factor of safety (usually taken as 2·5) Pc dead load (DL) plus live load (LL) to be carried by each pile Qal pile allowable load Qb pile ultimate toe bearing resistance Qsn accumulated negative skin friction force, the drag force at neutral point Qsp pile ultimate positive shaft resistance below the neutral plane h degree of mobilization typically 0·67, 1·0 may be used in specific cases
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