Theory and modelling of negative skin friction on a pile in soil

It is well-accepted within the foundation engineering community that piles in settling soil can be subject to negative skin friction (NSF). Typical causes of soil settlement include surface loads (e.g. new fill, structures), lowering of the water table, and earthquake- or liquefaction-induced compression. From examination of the results of long-term monitoring of piles, Fellenius [1988. Unified design of piles and pile groups. Geotechnical Instrumentation –- Transportation Research Record, 1169, pp.75–82] concluded that essentially all piles in soil develop NSF. There are important fundamental differences in the behaviour of piles with NSF compared to piles without NSF that affect the structural and geotechnical design. The purpose of this study is to introduce a theory to explain the development of NSF for all piles in soil. It is hypothesised that, consistent with the results of long-term pile monitoring, very small soil settlements are sufficient to mobilise the side resistance. Mechanisms causing settlement (common to all piles in soil) include installation effects and consolidation under the permanent (head) load. A numerical model of a hypothetical pile and soil system confirms that settlement from these mechanisms result in NSF consistent with the results of long-term monitoring.