ABSTRACT
The construction of the new North-East line of Singapore’s Mass Rail Transit system is arranged into twelve design and construct civil contracts. Contract 705 includes two underground two-level structures, which utilise a perimeter diaphragm wall to provide ground support. The method of construction chosen is a variant of the well-established top-down process for deep basements. Civil defence requirements mean that the roof is almost 2m thick, and is located 3m below ground, with much of the loading carried on internal walls. A method was required which would use the inherent strength of the roof whilst minimising the amount of support or temporary works. It was concluded that the access holes could be enlarged until a structural frame emerged that was simple to support but would still prop the excavation. This has been termed the Semi Top-Down Method.
