AN ALTERNATIVE to traditional vertical sand drains has been used to accelerate consolidation of reclaimed land for the extension of Singapore's Changi international airport.
The vertical wick drains consist of a central core of highly permeable fluted polypropylene pipe, Mebradrain, manufactured by Geotechnics Holland. This 100mm wide and 3mm thick pipe is wrapped in Typar, a microporous spunbonded polypropylene geo- textile manufactured by DuPont.
Mebradrain is designed to keep water flow at a maximum even when distorted by ground movements, with Typar acting as a filter to prevent clogging by clay particles. This, says DuPont, ensures continuing operation of the drain, an advantage over conventional sand drains which often suffer from long term reliability problems. Typar is also claimed to be strong and durable enough to prevent it being pushed into the core in the event of any movement, further guaranteeing efficiency.
More than 45M.m2 of Mebradrain and 11M.m2 of Typar 5417 are being used on the 18 month contract. Drains are being installed down to 45m at a rate of one per minute using a proprietary installation system which uses a steel mandrel containing the drain to drive it into the ground.
A special anchor plate leaves the drain in place on withdrawal of the mandrel sleeve.
This is the third time Typar has been used at Changi. It was used in the early 1980s and in 1990 as a separation geotextile layer between the reclaimed landfill and the hardcore for the runways.