Singapore Mass Rapid Transit SystemMott MacDonald was awarded the design contracts for the first two of the four sections of the Singapore Mass Rapid Transit System which required the railway to be carried on elevated structures. The length of viaducts on Mott MacDonald’s design contracts totalled 10.3km.

The elevated route passes through densely built-up areas and the viaduct is typically located either in the median of a dual carriageway or beside a road.

Precast post-tensioned concrete box girder beams were considered to be the most appropriate of a number of structural forms considered for the viaduct deck, in order to meet the very tight construction programme. This allowed the beams to be manufactured under factory conditions at a number of precasting yards along the route. They were then transported along the ready constructed viaduct and launched into position by specially designed gantries without disruption to the adjacent road traffic. The MRT standard viaduct carried two ballasted track a minimum of 4.3m apart, each track being carried on a separate beam. The required use of ballasted track permitted the use of simply supported spans without adversely affecting the track design. The precast beams, weighing up to 180 tonnes, are supported at their ends on in-situ concrete crossheads topping single piers on piled foundations. Two standard beam types were designed, approximately 17m and 23m in length. By adjusting the length of the crossheads it was thus possible to accommodate any span between 20m and 30m using the standard units.

Considerable attention was given both to the visual impact of the viaduct as a whole and to the detail of the surface finishes of the piers and the deck. A curved side profile to the box girder was adopted and the bottom corners of the girder were rounded to soften its appearance. For similar reasons the piers are circular in cross section. At a number of locations, where crossings of rivers or major road junctions were required, the standard viaduct design was inappropriate and a variety of long span structures were designed with spans of up to 55m. These were designed to be an extension of the viaduct form, so that the profile of the typical deck was retained, haunches being introduced where required to provide additional strength.

The design contracts required detailed design drawings and full contract documentation to be prepared in a very short time, and considerable use was made of computer aided design and detailing. A suite of programs was used to produce setting out data, the alignment design, contract drawings and bills of quantities.

Design Contract 102D comprised 3.3km of twin and triple track viaduct with elevated stations at Ang Mo Kio and Yi Chu Chang. Design contract 200D comprised 7km of twin track viaduct incorporating nine in-situ prestressed long span structures crossing road, railway and river, and an in-situ four span turnout to the depot. Elevated stations were at Redhill, Queenstown, Commonwealth, Buona Vista and Clementi.

These sections of the system were opened in 1987.