A259 littlehampton bypass embankment construction with vertical drains : Anandan, J; Piercy, J Highways TranspV38, N2, Feb 1991, P10–15

Abstract

The bypass was constructed over soft alluvial soils next to the River Arun where undrained shear strengths were too low to permit construction of the six metre high embankment without consolidation. Comparative estimates of using methods of removing pore water showed that the least costly solution was to install Mebra wick drains at close centres of one metre. A total length of 150000m of wick drains were used and the embankment was built as an advanced earthworks contract, to half its full height at a rate of not more than 500mm per week. Pore pressure dissipation was closely monitored by nine piezometers and five magnetic settlement gauges; eight inclinometers were also used to monitor lateral movement of the embankment. The computer programs, SLOPE and ONEDIM were used to calculate slope stability and predict settlement. A granular drainage blanket was first laid on a layer of Terram 1000 geotextile taking care that no heavy loads travelled directly on the fabric. Wick drains were then installed with an average length of seven metres to penetrate the underlying soft chalk to about one metre. A smooth-wheeled vibratory roller was used to compact fill material with tipper lorries only being allowed to run on previously compacted material. It was necessary to raise instrumentation at about two week intervals. (TRRL)