The Melbourne Main Sewer Replacement (MMSR) is a major urban infrastructure project involving approximately 2.3 km of tunneling and construction works. The works extend from Docklands Precinct, on the west edge of the city, southwards to connect with the Hobsons Bay Main Sewer in Port Melbourne. A major component of the MMSR is the crossing of the Yarra River, upstream of the Charles Grimes Bridge. The 146m long river crossing works will be completed using a staged flooded cofferdam construction method within the weak alluvial sediments associated with the Yarra delta geology. The construction approach adopted has been designed to cause minimal environmental impact and river traffic disruption.

The crossing is to be constructed in three stages, beginning at the southern river bank. Each cofferdam stage is constructed as a twin-celled rectangular steel sheet piled box to enable sewer main construction at up to 10m below river bed level. The 40-50m long by 6m wide cofferdam stages consist of sheet piles and steel support framing system. The design was required to accommodate the construction staging, in particular, river bed backfilling and the delivery and installation of the steel sewer carrier pipes by flotation along the river. The existence of a soft highly compressible layer of marine sediment, Coode Island Silt, imposed geotechnical constraints on the design.

This paper reports on the various stages of construction and highlights the structural and geotechnical design aspects involved in the design of a staged flooded cofferdam construction in Coode Island and Fishermens Bend Silts.