Works are progressing well on all fronts with the surroundings of the main shaft changing dramatically from a wide open green space to a fully functioning construction site, complete with a 19 metre deep vertical access shaft and excavated launch chambers – despite the challenging hydro geological conditions experienced.

John Holland Tunnelling Melbourne Main Sewer Replacement (MMSR) project Manager Nick Lewis said several aspects of the project were notable for their technical complexity.

“One of the most technical challenges of the tunnelling project is the Coode Island Silt geology,” Mr Lewis said. “No one has tunnelled into the Coode Island silts since the 1960s and that was in a compressed air environment – so ours will be the first manned tunnel boring machine that has gone into the silts since probably the 1890s.”

“Significant geotechnical investigations have been carried out and the team has an expectation of the silts' behaviour when tunnelling through them,” he said. “We also move into some areas of Port Melbourne sand, Fisherman's Bend silt and Tertiary older volcanics which are known to have direct connection to charged aquifers. Strict contingency plans have been developed to deal with the potential high groundwater inflows, if encountered,” said Mr Lewis.

A TBM called Lucy Loo

Lovat have completed the manufacture of an Earth Pressure Balance machine of nominal cut diameter 2.952 metre, which will construct a 2.4 metre ID segmental tunnel lining. Measuring 92 metres in total length (including backup sledges), Lucy Loo, as she has been named by a local Port Melbourne Primary School student, has been lowered commissioned and has recently commenced tunnelling. The machine is expected to progress through approximately 490 metres of Coode Island Silt on its 1,100 metre first drive south.

Site establishment works at the Swallow Street site, the reception shaft for the southern tunnel drive have commenced with works to construct the diaphragm walls down to depths of 25 metre imminent.

The northern end of the works has progressed well with both shafts for the river crossing driven and commencement of the cofferdam in late May 2009. Steel sheet piles have been driven to depths of 21 metres to form the first of the three staged wet cofferdam construction. Excavation of the first cell has been completed with over 1,500 metres cubed of material being stored or transported off site.

Permanent tubular piles have been driven underwater into the riverbed, which will form the support for the 25 tonne precast concrete headstocks. The 2.4 metre internal diameter steel conductor pipe has been fabricated, with the first installation onto the headstocks due to occur in October 2009.

Additionally, the project team will also take delivery of an Akkerman ground boring machine (GBM) to construct 2.5 kilometres of branch and reticulation sewers and 28 manholes. These works will be undertaken in a densely populated residential area of Port Melbourne.

Due to the poor ground conditions and the ground water levels being approximately 2 to 3 metres below the surface the caisson method was chosen. The caissons will be fitted out as the final manhole. The wet caisson method reduces the requirement for dewatering around the manhole site and reduces the possibility of any ground settlement issues.

The 25 caissons have been standardised to an internal diameter of 2.44 metres with HDPE corrosive protection liner cast in. These will be built in 1 metre increments, with waterstop cast across the exterior joint. Cast in situ and 1 metre lifts have been necessary due to the number of overhead power lines in the area.

The GBM will be used to pipejack the vitrified clays pipes that range from 300 mm to 600 mm. The maximum drive length will be 123 metres.

John Holland is constructing the project on behalf of Melbourne Water. Other project team members include designers GHD, and Project Manager Aurecon.

The timely replacement of this major Melbourne asset is on target for completion in May 2012 and will deliver health and environmental benefits ensuring future growth of this great city.