Brisbane is a city of humid, sub-tropical summers and dry winters that rarely drop below 10°C. Lightning frequently shatters the skyline and rain falls for one day in three across the year. The city is divided by the river that bares its name and, as the capital of Queensland, it is home to approximately three million people.

The highly urbanised population structure of Australia makes the nation particularly dependent upon its cities. The upside of this is: if you or your business is in a city, you are looked after. The downside is that everything has to come through a densely populated and developed area. Conscious of this and an increasing population, Australia has been investing heavily in infrastructure development programs in recent years as a governmental priority.

Need and planning
The Merivale Bridge that spans the Brisbane River is the bottleneck of the entire South East Queensland rail network. Already at capacity, further train services passing north to south or vice versa must for the moment remain amongst the sweaty fantasies of transport officials. That is, unless another river crossing can be constructed.

By 2031 the problem will be worse. It is anticipated that the population of Brisbane will be in the region of 4.4 million and the number of workers moving to the region will also increase. On top of this, industrial freight looks to be ever increasing. The year 2031 also marks the end of the counter-measure, the South East Queensland Infrastructure Plan and Program 2010-2031 (SEQIPP). This AUD-134bn (USD-131bn) project, which was first released in 2005 and updated annually, will see a total of AUD 97.7bn (USD 95.5bn) spent on transport by 2031. It is the largest infrastructure plan in the country and is expected to finance almost a million jobs by the time it is concluded.

SEQIPP, along with infrastructure projects from each of the states stands alongside the Government of Australia’s Nation Building Program (NBP). The government is pumping AUD 37bn (USD 36.2bn) into infrastructure for the period 2008-2014 through the NBP alone.

“As a result of the record investment we’re making and the sweeping reforms we’re implementing, this century could well be another ‘golden age’ for rail,” announced Anthony Albanese, minister for Infrastructure, Transport, Regional Development and Local Government, and Lindsay Tanner, minister for Finance and Deregulation in a joint press statement.

From this background of heavy local and national government investment, the Cross River Rail project was born. Currently in the detailed feasibility phase, SEQIPP lists the funding granted for this stage as totalling AUD 25M (USD 24.5M), of which one-sixth is provided by the regional government and the remaining by the national government.

Civils consultancy for the project is being handled by an integrated joint venture team consisting of AECOM, Hassel and SKM Aurecon, which was appointed in December 2009.

With the reference design determined, the environmental impact statement, including the required duration for public comment or objection, and business case are the next steps. The entire detailed feasibility phase has an expected completion date somewhere in mid-2011. At this point the governments will make an investment decision. If the project goes ahead, the tender process could be underway as early as the second half of 2011. The procurement phase should take approximately 18 months.

Project vision
Under the plan, a tunnel will run for 9.8km from a southern portal at Yeerongpilly Station to a northern portal by Victoria Park, passing under the Brisbane River. The Queensland Government has endorsed these locations and they form part of the Cross River Rail reference design, which was released by the minister for Transport on 11 November.

It will take the form of a twin-tube, single-track tunnel. This format has been chosen because it poses the least risk from any potential changes in geology along the route. The tubes will be separated by a central rock pillar, which itself will be lanced at regular intervals by emergency evacuation passages. A permanent concrete lining connecting with the main tunnels will support the cross-passages.

The maximum grade of the tunnel will be 3 per cent, meeting rail tunnel specifications set by Queensland Rail. However, the majority of the tunnel will be less than this.

It is hoped that the rail tunnel will alleviate the stress on the existing network of an increasing population and increasing commuter workforce. The high capacity nature of the service should achieve this with little difficulty. Tunnelling as an option also results in minimal surface disruption to transportation, a necessity for this project that has increasing infrastructure efficiency as its sole objective.

The project is also intended to inject money into the economy and provide jobs during the economic downturn as part of a much larger Australian investment intended to offset the impact of global recession.

Tunnelling methods
Construction will primarily be undertaken by driven tunnelling, using two shield TBMs driving parallel from the southern portal to Woolloongabba Station in the middle of the alignment. A further two will run from Wolloongabba to the northernportal. Pre-cast concrete segmental linings will be used and the tunnel will be 6m internal diameter with a 7m bored diameter. When passing under the river, there will be approximately seven metres of rock cover to the water.

Where the sections are small and shallow, such as for cross passages, a roadheader or cut and cover will be used. ‘Dive’ structures connecting the tunnel to surface tracks will be excavated using open cut construction due. In areas with especially hard rock, it is foreseen that drill and blast may be necessary.

Underground stations are to be constructed using a combination of techniques, such as piled walls, rock cuttings, roadheaders and cut and cover. The south portal will be constructed with a reinforced concrete retaining wall with a soldier pile wall at the immediate portal and lattice arch girders to the floodgate shaft. The north portal will feature temporary soilretaining walls near the surface as well as cast-in-situ walls.

The reference design incorporates the existing Queensland Rail 25-kV traction and overhead electrification system though new power requirements have resulted in the addition to the design of two new traction feeder stations at each end of the tunnel.

Ventilation systems will be installed at the entrance and exit of each underground station. The longest section of the tunnel (between the southern portal and Boggo Road Station) necessitates an additional shaft providing ventilation and emergency access.

Stations are to be of a length to accommodate trains of nine cars (220m) though the additional equipment at each end of the stations pushes this to 250m.

Geology
A series of bores and geotechnical investigations have been undertaken to ascertain the nature of the geology along the study corridor. These included a seismic survey in the river itself as well as drilling and testing the ground at the planned location of each station and at relevant points along the riverbank. Drilling and testing was also done south of Boggo Road Station for the running tunnels. From this data and information from previous records, a 3-D geological model of the corridor has been constructed.

The tunnel has been designed to run along a course that allows the maximum length of the running tunnels to be through hard rock as it is the best way to minimize the effects of changing geology.

The proposed route is known to run through Neran-Fernvale Beds, Aspley Formation, mudstone, sandstone, Brisbane Tuff and alluvium.

One concern is the point at which the tunnel passes under the river, as the depth of the river crossing has been based on ‘desk-top analysis’ and seismic surveys.

Volumetric resumptions
A challenge in the reference design is that of present and future development requirements and stresses that will be placed on the tunnels and surrounding rock. The tunnel has been designed to cope with loads from existing buildings, proposed buildings and buildings that the land has potential to accommodate in the future. As an additional safety measure, volumetric resumptions of 7m on all sides (10m around the curve of the arch for station caverns) of the tunnel have been built into the design as an essential requirement. This distance of roughly one tunnel diameter ensures an adequate pillar of rock remains preserved against future development, in order to protect the integrity of the tunnel.

Alternatives to a tunnel
Cross River Rail has revealed that a number of other options had been considered to meet the objectives of the project during the Inner City Rail Capacity Study undertaken by the Queensland Government in 2008. These have included enhancements to heavy rail, expansion of the bus network, a light railway system, and a metro. None of the alternatives were deemed viable for the project.

Two statistics that Cross River Rail is particularly proud of are that, upon completion, the project will be able to move up to 120 000 commuters into the inner city from the north and south during the morning peak-travel and also that it would take a 30-lane motorway to provide the equivalent flow of passengers.

Constructing the periphery
The Cross River Rail project is not ‘just’ a tunnel under a river. As well as underground construction works on the main tunnel, the project also requires substantial peripheral construction, worthy of several separate projects in their own right.

Of the road and bridge requirements, these works include the reconfiguring of the Sherwood Road to Fairfield Road southbound slip lane to accommodate a new rail-over-road bridge next to the existing bridge, the reconstruction of the southbound ramp on Ipswich Road to pass under a new span beneath the road, reconfiguring the Station Street link to accommodate the new line, the creation of a new overpass, realigning a street to better favour the new line, creation of a cul de sac at Norbury and Dollis Street on both sides of the Ravena Road overpass to accommodate the adjacent rail, a rail bridge over Muriel Avenue as well as one over Moolabin Creek and a road-over-rail bridge at O’Connell Terrace.

Rail structures required include traction feeder stations near the two portals at Yeerongpilly (southern) and Victoria Park (northern) as well as one near Mayne Yard, an elevated structure for grade separation and stabling connection, an elevated structure near Mayne Yard for connection to the North Coast Line and tracks for the 9.8km main Cross River Rail tunnel from Yeerongpilly to Victoria Park.

Other tunnel works include both portals and the respective ‘dive’ structures, a southern floodgate shaft to the north of Yeerongpilly and a ventilation shaft between Yeerongpilly and Boggo Road.

Also to be included is provision for a ventilation shaft near the north portal and provision for a possible future connection to a North West rail corridor.


Plan of the proposed twin-bore tunnel with TBM drives Albert Street Station (artist’s impression) The Woolloongabba (Gabba) Station (artist’s impression) Brisbane River at night Longitudinal geological section on the tunnel route South East Queensland