Settlements of Embankments in Soft Soils

The major part of this paper is on the challenges in predicting settlements in highway embankments and reclamation works in marine, deltaic and estuarine type of deposits. The emphasis is on practical aspects and the difficulties experienced in confidently estimating settlements even after a century of developments and contributions. The influence of the general geology and soil conditions is discussed in relation to the site investigation works and the establishment of soil profile models. The fundamentals of preloading techniques with and without prefabricated drains (PVD) as ground improvement measures are also included. The observational approach in evaluating PVD performance and settlement estimations is then made with emphasis on curtailing residual settlements. Finally, the latter part of the paper is devoted to the analytical and numerical solutions of the behaviour of piled supported approach embankments with transfer layers. Geogrid reinforced pile supported (GRPS) embankment system is studied with the analytical approaches of Terzaghi, BS8006 method and Hewlett and Randolph method. Additionally, numerical analyses are also made with the Plaxis software. 1. INTRODUCTION The major part of the work presented in this paper relates to the challenges in predicting settlements in marine, deltaic and estuarine type of soft soils under road and highway embankments and during reclamation works. The early work on Soft Bangkok Clay is described by Muktabhant et al. (1967), Moh et al. (1969) and Eide (1968 and 1977). A comparison is then made with the practice in SE Asia and in Australia wherever possible. Concentration is made on the use of preloading with and without prefabricated vertical drains (PVD) and the restriction of post-construction settlements. Most of the material presented here is related to actual project works where the first author is involved over a period of forty years. In these activities classical and simple theories in Soil Mechanics are used and lesser emphasis is given to the recent excellent developments of more refined approaches. The primary and secondary settlements are calculated using the notable and well established works of Terzaghi, Mesri and Hansbo. The first comprehensive volume on soft clay engineering was by Brand and Brenner (1981) at AIT as based on the International Symposium on Soft clays held in Bangkok in 1977. The ground improvement conference held in 1983 at AIT also gave great impetus in the development of this field in SE

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