Preload Design, Part 2 — An analytical method based on Bjerrum’s time line principle and comparison with other design methods

Following a review of time-dependent settlement behaviour and preload design methods presented in Part 1 of this paper (Wong, 2006a), Part 2 of this paper presents the development of an analytical approach for preload design based on Bjerrum’s (1967; 1972) time line model, or principle of “artificial aging”. This analytical approach can be readily coded using an Excel spreadsheet for assessing the required preload thickness to limit post-construction settlement to a specified value.

A worked example is presented and compared with other methods (Mesri, 1991; Poulos, 2004; PLAXIS Version 8, 2002) to illustrate the importance of geological and stress history on post-preload settlement behaviour.

The dependency of creep on stress level and stress history is used in the analytical approach introduced in this paper, and the results of the worked example clearly show the reasons for the possibility of significant post-preload creep settlement if the amount and/or the degree of consolidation during surcharging were insufficient.

The purpose of this paper is not to suggest any preference for a particular preload design method with respect to either correctness or accuracy in predicting post-construction settlement performance following preloading. Rather, through the worked example, it highlights the dependency of the results on certain assumptions used in the models, and perhaps also highlights the importance of being able to follow the stress path and applying fundamental principles to explain the predictions and their limitations.