Preload Design, Part 1 — Review of soil compressibility behaviour in relation to the design of preloads

The method of treating soft soils by preloading has been used for over a century, and is still widely used today as one of most common form of ground improvement technique. Yet, every now and again, post-construction settlements have been observed to be more than those predicted after preloading. The author believes, in most cases, the poor preload performance is probably associated with lack of understanding of the time-dependent compressibility behaviour of the soft soils. And in particular, the behaviour of secondary consolidation (or creep) is still not well understood despite extensive research and numerous constitutive models that have been developed. The availability of powerful commercial computer programs does not help if they are used indiscriminately when the fundamental principles are not well understood.

Part 1 of this paper provides a review of the factors that influence creep. The dependency of creep on stress level and stress history expressed in terms of the over-consolidation ratio (OCR) is discussed, followed by a discussion on the commencement of creep. A brief overview of time-dependent consolidation and creep settlement analysis methods is provided, followed by a summary of the preload design approach given by Mesri (1991) that illustrates the possibility of the occurrence of higher creep rate some time following preloading.

In Part 2 of this paper, an analytical approach based on Bjerrum’s (1967) time line model, or principle of “artificial aging” will be presented for preload design to limit post construction settlement, and a preload design example is discussed to illustrate the importance of geological and stress history on post-preload settlement behaviour.