Reclamation and soil improvement on ultra-soft soil

In recent years more and more reclamations have been carried out on waste ponds or mine tailing ponds. The deposits in these ponds are ultra-soft and may still be undergoing self-weight consolidation. Reclamation and soil improvement works in these areas are very challenging. They require special attention. Conventional reclamation and soil improvement methods are no longer workable. This paper highlights the experience obtained from reclamation and soil improvement works on such soil in a project in Singapore. The fill was placed using a sand spreader, and the ultra-soft deposit was strengthened with a high-strength geotextile. Prefabricated vertical drains and staged construction were used to accelerate the consolidation process. The monitoring of settlement and pore pressure requires modification of conventional instruments to cater for the low strength of the ultra-soft soil and the large strain of deformation. The paper also outlines a new method to estimate the settlement based on large-strain theory. Finally a case study is presented to validate the proposed method in the determination of settlement magnitude and the time rate of settlement.