Reclamation of a slurry pond in Singapore

A case study for the reclamation of a slurry pond as part of an offshore reclamation project in Singapore is presented in this paper. The slurry pond covered an area of 180 ha. The slurry in the pond was recently deposited ultra-soft high-plasticity clay. The water content of the slurry was more than 120% and the undrained shear strength was less than 8 kPa. The reclamation was first carried out by spreading sand fill in thin layers 20 cm thick using a specially designed sand spreader. The filling speed was carefully controlled to allow the slurry to be consolidated before more fill could be placed. Despite the precautions a failure occurred, in the form of mud bursting. As a remedial measure, geotextile sheets were used to cover a total area of 630 000 m2 before more sand fill was placed. After the completion of fill placement, fill surcharge and prefabricated vertical drains (PVDs) were used to improve and accelerate the consolidation of the slurry. As the performance of PVDs would deteriorate after they had undergone large deformation, they were installed in two passes. In the first pass PVDs were inserted with a square grid spacing of 2.0 m. After nearly 1.5 m of settlement had taken place, the second pass of PVDs with the same spacing was installed at the centre of the square grid of the PVDs installed in the first pass. After nearly 4 years of consolidation, the top of the slurry had settled more than 3 m. The undrained shear strength had also increased substantially. Therefore the use of PVDs for the improvement of the ultra-soft slurry was successful in this project.