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Soft Ground Tunnelling in Melbourne – Investigation and Design of the Melbourne Main Reliever


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Author P Clark, M Dixon and T O’Shannessy


Melbourne Water (MW) is responsible for a section of
existing gravity trunk sewer servicing the Melbourne central business district,
Docklands, Port Melbourne and other adjacent areas, known as the Melbourne Main
Sewer (MMS). MW has determined that the 2.2 km long 110 year old sewer will not
adequately service these areas into the future or provide the level of
reliability needed. The Melbourne Main Sewer Replacement project (MMSR) involves
the replacement of the MMS with a new gravity sewer main and six key manhole
shafts, known as the Melbourne Main Reliever (MMR).
The majority of the MMR
sewer tunnel is situated south of the Yarra River and will be constructed using
an earth pressure balance (EPB) tunnel boring machine (TBM) and will be lined
with precast concrete segments. The tunnel will be 2.4 m internal diameter and
will have a 1.8 m diameter glass fibre reinforced plastic (GRP) sewer pipe
positioned within it.

This project represents the first large-scale, modern, mechanised tunnel
project in Melbourne’s Yarra Delta Quaternary sediments. There is little
precedence known to the authors of similar scale tunnelling projects in these
particular geological units. During the recent detailed investigation and design
stages of the project, significant effort has been spent on characterising the
geotechnical conditions along the alignment and predicting the effects of the
new tunnel on the existing environment.

Some of the key
design issues that have been identified and managed through the design process
include short-term surface settlement due to tunnel excavation and the predicted
effects on surface infrastructure, potential spalling of the steel fibre
reinforced concrete segments under full TBM ram loads and tunnel lining design
in geological units with highly variable stiffness.
Construction of the new MMR sewer tunnel is
expected to commence in mid-2008 with a capital cost of approximately $160
million. Sewer system commissioning is scheduled for completion by early