Construction of Egg Shaped Digester Tanks with Post-Tensioning and SK4 Formwork System

In its effort to free up more habitable area in land scarce Singapore, the Public Utilities Board is currently embarking on a major change in the design of its sewage water treatment structures. Deep Tunnel Sewage System (DTSS) has been introduced island wide to channel all untreated wastes from the townships to two main sewage treatment works situated in the East (Changi) and in the West (Tuas) of Singapore.

However, the DTSS, especially the facilities in the western part of the island, would require many years of design and construction works to complete. In order to accommodate current needs, the Jurong Sewage Treatment Works, a major sewage treatment plant in western Singapore, has decided on a major extension project which called for the construction of 3 egg shaped digester tanks.

DSI’s licensee in Singapore, Utraco Structural Systems (USS), is engaged as the specialist sub contractor for both the design and construction of the digesters.

Each digester measures 33m high with its diameter at the equator measuring 20m. Each digester has a sludge storage capacity of 6,000m3, with wall thickness varying from 625mm at its base to 450mm at the top of the shell structure.

Through technical input and discussions with engineers from DSI Munich, USS developed the shell formwork system for the construction of the digester, a first from a local company. The jumping formwork system is capable of handling concrete pours of up to 1.45m per lift, and is equipped with internal and external suspended working decks.

Using DYWIDAG post-tensioning and the newly modified SK4 shell formwork system, the construction works of the egg shaped digesters were completed in November 2003 well within the stipulated construction schedule.

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Ministry of Environment, Singapore

ST Architects and Engineers Pte.Ltd., Singapore