1. Lorong Halus Interchange Introduction: Project to build a new $26 million interchange for the Tampines Expressway starting 1994. The site was situated on a disused landfill site which had been used for approximately 10 years. Site deserted in the mid 1980’s. 20 hectare site.

2. Problems (1) Expressway had to be built on site due to scarcity of land. The unknown effect of leachate, landfill gas and waste material on the road pavement and structures. Leachate was perched in the landfill. The leachate itself had too high a composition for direct discharge into the sewer. Soil was in the phase where landfill gas was being formed by the decomposing products. There were some pockets of gas at relatively high pressures. This could have potentially posed a fire hazard during construction.

3. Perched leachate as a result of a low permeability soil layer below the waste layer. (3.0 x 10 -9 m/s).

4. Problems (2) Potential problem of settlement of fill beneath the overpass. Potential problem with piles encountering obstacles in the landfill. Need to prevent more water percolating in landfill embankment and creating further spread of leachate and waste substances. Problem of erosion of surface from heavy tropical rainfall. Risk of biofueling – (Where encrustation blocks the stones in the gas collection layer.)

5. Adopted Solutions (1) A 1500mm sand cut-off wall to prevent underground fire between disused and active dumping sites. Bored piles not driven piles were used for the carriageway to avoid obstacles in landfill. Concrete bored piles and substructures protected from aggressive leachate by High Density Polyethylene (HDPE) sleeves. Leachate drains were installed at the side of the carriageways to prevent pavement surface damage.

6. Protective Sleeve for Concrete Structures

7. Cross-Section of Expressway

8. Adopted Solutions (2) Gas in landfill was intercepted at the surface by a low permeability layer and vented under its own natural pressure to safe discharge points. (Passive Venting) Gas under carriageway was contained in a permeable venting layer of granite stones and carried to vent stacks along perforated plastic tubes. (Active Venting) Mound created from excavated landfill needed some form of capping layer. Capping layer prevents surface runoff water getting into landfill. Designed for 1 in 100 year storm event.

9. Cross-section of Adopted Capping Layer

10. Alternative Solutions Move excavated landfill to another landfill site. (Prevent the creation of mound). If contamination of leachate was higher then the use of a sodium bentonite cut-off wall would need to be considered. Use of alternative capping layers? Lack of alternatives, since contamination isn’t a direct risk to human health and the project is ‘Unique’.