Development of negative skin friction on pilesinstalled through soft compressible soils in Sri Lanka

Use of grounds with soft compressible layers at shallow depths for development projects requires the use of pile foundations to support the structural loads. Piles installed through soft compressible soil layers are subjected to Negative Skin Friction (NSF) as a result of consolidation of the soft compressible layers due to placing fills, lowering of the ground water table, pile driving etc. The NSF, or commonly referred to as the drag force, is generally estimated from the skin frictional resistance due to the consolidating layer and layers above it. However, it is shown by the researchers [Bjerrum et al. (1969) and Endo et al. (1969)] that the NSF is not developed up to the bottom of the consolidating layer but only upto a point, referred as the neutral point, above the bottom of the consolidating layer. Moreover, the results of full scale field tests conducted by researchers clearly indicates that the development ofNSF is not due to the total stress but due to the effective stress acting on the pile shaft. Moreover, it is experimentally shown that the development of the ultimate NSF requires very small relative displacement between the pile and the soil. Furthermore, it is experimentally shown that the action of short-term live loads reduces the developed NSF. Therefore, the NSF and short-term live loads are not simultaneously acting on the pile