Soft soil foundation improved by vacuum and surcharge preloading at Ballina bypass, Australia
The Pacific Highway is being upgraded to support the transportation demand between Sydney and Brisbane, Australia. To avoid the town of Ballina, the bypass route has to traverse on a floodplain consisting of very soft, highly compressible, saturated marine clays up to 40m deep. A vacuum assisted surcharge load scheme in conjunction with prefabricated vertical drains was selected to reduce the required time and consolidate the deeper subsoil layers. The design of the combined vacuum and surcharge fill system and the construction of the embankment is described. A comparison of the performance of the vacuum combined surcharge loading system with a standard surcharge fill highlights the benefits of vacuum consolidation. Field monitoring data is presented to demonstrate how the embankment performed during construction. Suitable design charts for vertical drains were developed considering both vertical and horizontal drainage. As a result, the conventional and often cumbersome trial and error methods to estimate the appropriate parameters could be avoided.