Performance of floating piled raft with varying lengths in soft compressible subsoil

Abstract

Conventional piled foundation usually provides adequate load carrying capacity to limit the settlement within allowable limits. However, in deep layer of soft compressible subsoil with settling platform, this foundation system faces numerous problems namely requiring very long piles, lower pile capacity due to downdrag forces, and hollow gap formed beneath the slab of piled structures when the earth platform settled causing services to break and poses health hazard. This research proposed an analysis and design methodology for an alternative foundation system of ‘floating’ piled raft (FPR) with same or varying pile lengths to resolve the problems stated above. The design objectives are to control differential settlement, angular distortion and bending moment rather than only limiting total settlement. The proposed analysis and design methodology bridges the research gaps of using piled raft in soft compressible subsoil. This incorporate long term settlement in the analysis to cater for piles of varying lengths and can be used by practicing engineers for design works. Parametric studies were carried out to verify the proposed analysis and design methodology through modelling of ‘floating’ piled raft with different numbers of piles, lengths configurations, spacing of piles and also different raft thickness. The vertically loaded pile rafts analysed are 3x3, 6x6 and 9x9 number of piles respectively with total combination of 108 cases that cover different pile lengths of same and varying lengths, different pile spacing and different raft thickness. The research findings showed that piled raft with combination of varying pile lengths is generally more effective in reducing differential settlement, ratio of (Δr/rmax), bending moment of the raft and angular distortion (b) compared to pile raft with similar pile length (even with longest piles). The findings from the parametric studies contributed to a better understanding on the performance and behaviour of ‘floating’ piled raft in soft compressible subsoil especially on the piled raft of varying piled lengths. The proposed analysis and design methodology in this research has also been successfully used to design ‘floating’ piled raft foundation system in deep and soft compressible subsoil to support low rise buildings of 2- storey to 5-storey that have been constructed and occupied for more than 10 years. This confirmed the benefits obtained from this research to have a realiable and efficient analysis and design methodology through better understanding of the performance and behaviour of ‘floating’ piled raft foundation with same or varying pile lengths.