Compressibility behaviour of preloaded highly plastic clays

Preloading is a proven technique for improving the shear strength and reducing the compressibility of clays. This paper presents the results of laboratory oedometer tests, undertaken to explore in detail the changes in compressibility characteristics of a remoulded, highly plastic organic marine clay upon preloading. The test results indicate that the application of preload pressure as small as one-sixteenth of the footing pressure would cause a reduction in post-construction settlement by about 40%. Using the findings of the present study, a mathematical model was developed to predict the magnitude of preload pressure to be applied to achieve a specific reduction in consolidation settlement such that the residual settlement would be within acceptable performance limits. Experimental results also suggest that the relationship between the deformation (expressed as a percentage of soil thickness) and the applied pressure for preloaded highly plastic clays could be treated as a rectangular hyperbola and hence the behaviour could be characterised by a two-parameter model that was dependent on the magnitude of the preload pressure. Expressions were also developed for the satisfactory prediction of these parameters to calculate the post-construction settlements.