Waking up in Geelong

With all of the recent talk about the construction of the Melbourne Metro rail tunnels beneath Swanston Street and the Yarra River, it seems that many people have forgotten how our last major tunnel project was built – CityLink.

CityLink consists of two tunnels – the shorter Domain Tunnel that carries westbound traffic, and the longer Burnley Tunnel that carries eastbound traffic – with roadheaders being used to construct the majority of each tube. However at each end of the tunnel, the project faced the same problems as the Melbourne Metro tunnels.

At the western end of both tunnels is Grant Street and St Kilda Road, full of both road and tramway traffic.

Here a ‘top down’ methodology was adopted while digging the massive hole.

Excavations for cut & cover works at Grant Street May 1998 (Ian Hill, SLV image H99.20/24)

The 2002 book Journey and Arrival: The Story of the Melbourne CityLink published by the Institution of Engineers, Australia elaborates:

The conventional cut-and-cover method was not used at the Grant Street portal, in part to accommodate the impact on the adjacent Victorian College of the Arts (VCA). The size of the excavation necessary for cut-and-cover was huge: 35 metres wide and progressively increasing in depth to about 25 metres at St Kilda Road.

The top-down technique used along Grant Street involved the installation of columns along the alignment of the outer walls of the tunnels, and along the centre line which would ultimately form the barrier between the two tunnels. This was achieved by drilling holes about 900 mm in diameter to bedrock, and then backfilling the holes with concrete. A concrete beam was then constructed along the top of the columns to form the support for the tunnel roof. Once the spans were in place, Grant Street was reinstated and excavation took place unimpeded beneath the deck.

The top-down variation of the cut-and-cover method also reduced the disruption on St Kilda Road, a major thoroughfare with busy tram lines. In fact tram services were closed on only one weekend. The crossing of St Kilda Road was undertaken in stages. Initially the outer service roads were closed and traffic was diverted to the central roadway. This allowed the construction of the columns and tunnel roof in the same manner as along Grant Street. However, extensive propping was necessary under the surface to support the columns against the 25 metre depth of soil pressure. Once work on the outer service roads was completed, traffic was re-diverted to allow work on the central part of the road.

Meanwhile at the east end of the Domain Tunnel was the Yarra River, which passed only a few metres above the future tunnel roof.

It was decided to build the river crossing in two stages, with a coffer dam being used to keep the water out while allowing the river to keep flowing.

Coffer dam construction in Yarra River for Domain Tunnel 26/4/97 (Ian Hill, SLV image H99.20/10)

Again, Journey and Arrival: The Story of the Melbourne CityLink gives the technical details:

The construction of the cut-and-cover crossing of the Yarra River represented some of the cleverest engineering of the whole project. Essentially, a trench had to be cut in the riverbed, the tunnel built in it, and the riverbed placed back on top.

As these works could not proceed easily underwater, the river had to be diverted away from the riverbed worksite during construction. The most efficient way to do this, while ensuring minimum disruption to river flows and traffic, was to work on the crossing in two parts: initially shutting off about two thirds of the river from one bank, and, when completed, switching to the other bank, then closing the other third of the river to complete the process.

What does it mean for Melbourne Metro?

Back in April 2015 how to built the Melbourne Metro tunnels beneath the Yarra River was still unresolved – possible solutions being an immersed tube tunnel, or CityLink style cut and cover using a coffer dam.

This week the final solution was announced – a tunnel boring machine would do the work, with the tunnel running to the east of Princes Bridge, around seven metres below the riverbed, and 11 metres below the surface.

More photos of CityLink

The State Library of Victoria holds the ‘Melbourne City Link Project Series‘ – images taken by photographer Ian Hill during the construction of CityLink.

• Concrete pour for roof of Domain Tunnel inside coffer dam in Yarra River
• Grant Street entrance for Domain Tunnel showing cut & cover works
• Entrance to Domain Tunnel at Grant Street

There is a lot more technical details in Journey and Arrival: The Story of the Melbourne CityLink – I’ve just scraped the surface in this post.

VicRoads also have an interesting CityLink related document on their website – Exhibit I Project Scope and Technical Requirements.

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