Soft soil foundation improved by vacuum and surcharge loading

The Pacific Highway has been upgraded to support the high transportation demand between Sydney and Brisbane, along the north-east coast of Australia. To avoid the traffic through the busy town of Ballina, a bypass route was designed to traverse on a floodplain consisting of very soft, highly compressible, saturated marine clays up to 30 m deep in certain locations. A vacuum-assisted surcharge load scheme in conjunction with prefabricated vertical drains was selected to reduce the required time to consolidate the deep subsoil layers. The design of the combined vacuum and surcharge fill system and the construction of the embankment are described, and a comparison of the performance between the combined vacuum and surcharge loading system with the conventional surcharge only system is highlighted. Field data are presented and interpreted to demonstrate how the embankments performed during construction in both vacuum and non-vacuum areas. Suitable design charts for vertical drains are presented and discussed with a worked example, considering both vertical and radial drainage.