Chapter 8 Application of analytical method for preloading design of selected case studies

Abstract

Preloading is probably one of the oldest forms of ground improvement for soft grounds. Used with or without vertical drains to facilitate the consolidation process, preloading has consistently proven to be an effective means of reducing post-construction settlement for hundreds of years. Methods of designing the required height of preloads range from simple rule of thumb procedures such as factoring up the design loading based on engineering judgement, to graphical methods using the “artificial aging” procedure described by Bjerrum (1972 Proceedings of Speciality Conference, Performance of Earth and Earth-Supported Structures, American Society of Civil Engineers, New York, NY, Vol II, pp 1–54.) This paper describes an analytical method of preload design using the “artificial aging” concept that has been applied to a number of projects in recent times. It takes creep, target degree of primary consolidation for preloading and the design settlement criterion into account as well as the usual soil compressibility properties. The analytical method could be carried out by hand although for a layered soil profile, the use of a simple MathCad worksheet or an Excel spreadsheet would enable sensitivity analyses to be carried out, and to enable the relationship between post-construction settlement and preload fill thickness to be developed. The analytical preload design method is illustrated via two case studies.