In the early days of the construction industry in Singapore, sand quarries operating in the Old Alluvium formation (cemented sandy deposit) were left behind as ponds filled with silt and clay in suspension. Among them was a 5.6 ha pond, about 12 – 15m deep, which was required to be restored to its original condition for sand and granite stockpiling. The relatively clear water at the top of the pond was pumped out, resulting in a smaller pond filled with concentrated silt and clay in slurry form. The shear strength of the slurry, which reached a depth of 5 to 6m, was about 1 to 2 kPa. This paper reports an innovative solution adopted in restoring the pond by capping it with geotextiles. Long-term stability analysis of the sand and gravel stockpile on the capped silt pond showed that a number of smaller ponds capped with geotextiles between interlaced ramps would be more favourable than a large pond capped with a single piece of geotextile. Hence, a restoration strategy was established which consisted of, firstly, constructing a main bund across the pond along the region with shallow depth of siltyclay slurry, and subsequently, constructing transverse bunds branching out of the main bund. This action called "geotextile capping" was followed by construction of a series of "finger berms" across the pond on top the geotextiles. This paper reports the salient soil properties and the construction method used, together with the field observations after capping. It also highlights the importance of understanding the soil behaviour and ground conditions, as well as the need to innovate in difficult geotechnical engineering projects.
The project site was a defunct sand quarry which was situated off Tampines Avenue 10 in the eastern part of Singapore (Figure 1).