THE Australian city of Melbourne touts itself as one of the world's most liveable cities. Yet with a growing population placing ever greater strain on its public transport infrastructure, particularly in the city centre, further investment is needed to provide the capacity required to meet current and future demand.

The chosen solution is the Melbourne Metro Rail project, a 9km twin-bore tunnel from Sunbury in the west under the central business district (CBD) to Cranbourne-Pakenham in the southeast. The line will add five new underground stations to the network at Arden, Parkville, CBD North, CBD South and Domain allowing an extra 20,000 people to use rail services during the peak.

MMRAIn addition, and most importantly, the new tunnel will allow Sunbury and Cranbourne-Pakenham trains, two of the city's busiest services, to avoid the already congested City Loop, increasing frequencies on the Werribee, Craigieburn, Upfield, Sandringham and Frankston services. This is projected to take 18,000 cars per hour off the streets of central Melbourne during the peak.

"The Melbourne Metro Rail project will start the transformation of Melbourne's rail network into an international-style metro system," says Mr Evan Tattersall, CEO of Melbourne Metro Rail Authority. "It will increase the capacity, reliability and efficiency of lines serving Melbourne's growth areas in the north, west and southeast and is the largest overhaul of Melbourne's metropolitan rail network since the completion of the City Loop in the early 1980s."

The project is currently in the planning and development phase, with an updated business case currently being finalised ahead of public release in the coming months. This includes traffic modelling and forecasts as well as the rationale behind recent decisions regarding the alignment and technical design. An expression of interest for major construction work will also be announced this year with this work due to get underway in 2018 following the completion of the preliminary works package in 2016-17. This encompasses relocating utilities and preparing construction sites. Expressions of interest in this work closed on December 23 and the contract is set to be awarded in mid-2016. The complete project is due to be completed by 2026.

Swanston Street was selected as the preferred alignment for the tunnel in mid-2015 and Tattersall says as one of the largest infrastructure projects in Australia, the work poses a series of unique challenges, not least due to the complex urban environment in which construction is taking place. As a result he says the project requires a robust planning, design and procurement process.

"Significant supporting works are required before construction can commence on Melbourne Metro including planning approval, enabling works relocating tram and utilities along the project corridor, and procurement," Tattersall says. "Site investigations are currently underway to gather further information about ground and environmental conditions to inform the preferred alignment."

Specifically on Swanston Street investigations have already provided sufficient information for Melbourne Metro Rail Authority to reveal that the CBD North and CBD South stations will be constructed entirely underground, which will minimise disruption to light rail services, pedestrians and cyclists at street level. For this work, shafts will be excavated next to Swanston Street, and the CBD North and CBD South station boxes will be dug out from underneath.

"This construction approach will significantly reduce surface disruption and enable the world's busiest tram corridor to keep operating during construction," Tattersall says.

A variety of sources of finance are in use for the project. After fast tracking a $A 40m ($US 28m) allocation to allow work to continue, in its 2015-16 budget, the state government of Victoria committed to fund $A 9-11bn of the project's cost, with $A 1.5bn allocated to complete the planning and design of the project and to start construction in 2018 over the next four years. An additional $A 3bn is coming from a metro bond agreement while discussions are ongoing with the federal government over securing financial support.

Construction of the twin-bore tunnels and five stations is structured as a multi-billion dollar "availability-based public-private partnership (PPP)" under which the private sector will be responsible for designing and building the tunnels and stations, as well as financing, operating and maintaining the infrastructure. Prospective private sector participants have been invited to formally register their interest and Melbourne Metro Rail Authority is set to conduct an extensive industry briefing and market-engagement process in the next few weeks.

The 2015-16 state budget also included $A 1.3bn for 37 new high-capacity trains to serve the line. The EMUs will accommodate up to 1100 passengers and could be lengthened to carry 1600. The tender will require that 50% of the vehicles' content is produced locally.

Before taking up the role at Melbourne Metro Rail, Tattersall worked on the city's 47.5km Regional Rail Link project, which opened in June 2015. He says that in addition to others which have transferred from rail projects in Melbourne, Metro Rail is benefiting from people who have worked on other major tunnelling projects, including London's Crossrail. "They are sharing crucial lessons about construction in complex urban environments," he says.

The new line will provide a direct rail connection to some of Victoria's key medical, educational and commercial locations for the first time and Tattersall says the project is for "all Victorians." In addition with the line designed and constructed to allow for future enhancements to Melbourne's public transport network, it will play a key role in dictating future rail and public transport policy in the Victorian state capital.

"Melbourne Metro Rail is a building block enabling the future construction, extension and electrification of rail lines across Melbourne," Tattersall says.