The world is experiencing unprecedented urbanisation. For the first time, more than half the globe’s population resides in cities or towns. This proportion is projected to reach 60 per cent by 2030.

This trend is not restricted to the developing world. In more developed counties, the current proportion of urban dwellers will increase from 75 per cent to over 80 per cent by 2030.

The extent of this migration is amplified when coupled with our expected overall population growth. In the next two decades, total urban population will reach almost five billion.

Cities are not only becoming more populated, they are also experiencing increased population density and crowding. Mega-cities – those containing more than 10 million people – will have grown from 19 in 2001 to an estimated 60 by 2015.

With available land at an increased premium, those charged with planning and designing essential urban infrastructure are increasingly relying on tunnels to deliver effective solutions.

Underground infrastructure already plays a significant role in housing the critical services required for effective functioning cities, such as water and sanitation utilities, energy supply, and increasingly, transportation and communication links.

For transport infrastructure in particular, tunnels provide a range of benefits:

• Tunnels provide new transport routes through built-up areas and allow multiple use of land parcels (below ground and at grade). 
• Undergrounding infrastructure allows designers and planners to retain and respect existing land uses, including the protection of sensitive environmental or cultural sites. 
• Depending on above ground topography, tunnels can allow for shorter distances and reduced gradients and level changes, which provides efficiencies in operational time, cost, energy use and emissions. 
• Tunnels can reduce air and noise pollution impacts and provide scope for transport operation without constraints of resident and business amenity. 
• The often negatively perceived visual impact of above-ground transportation is removed or reduced by placing infrastructure underground. 
• For many water crossings, tunnels often provide effective and cheaper alternatives to complex bridges that may need to be at significant height to allow shipping beneath them yet be substantial enough to carry large traffic loads.

However, tunnels do not provide a complete panacea to delivering urban infrastructure and addressing congestion in our bulging metropolises.

• Tunnels are often more expensive than surface options depending on surface land costs. Engineering and construction complexity, and the need to invest in capital-intensive equipment, such as tunnel boring machinery, often puts tunnel solutions at a disadvantage in traditional cost-benefit appraisals. 
• While tunnels themselves minimise the impact on surface features above, the tunnel approaches may still have significant surface impact. 
• While tunnel infrastructure typically has a long working life and requires less maintenance than above ground infrastructure, tunnel services, such as lighting, ventilation and safety systems, have high energy consumption and operational costs.

Overall, the increasing adoption of triple-bottom line appraisals of projects that balances economic (including whole-of-life costs), social and environmental elements highlights the benefits of tunnel solutions for urban transport infrastructure. 

Eastern Busway – applying a tunnel solution to a heritage precinct

As the capital of one of Australia’s fastest growing regions, Brisbane’s innovative Queensland Government owned busway network is helping improve public transport efficiency and manage congestion. It is unique in that it is leading the broad scale adoption of busway’s in Australia.

Brisbane’s recently completed A$366 million Eastern Busway – Eleanor Schonell Bridge to South East Busway, is a dedicated 2.1 kilometre public transport link that provides a critical connector in the city’s expanding busway network.

In order to achieve the necessary route alignment in an established urban environment, much of the Eastern Busway is underground and includes the longest dedicated busway tunnel in Australia. In particular the necessary route passes under a key heritage site – the Boggo Road Gaol - as well as existing infrastructure that includes nine electrified rail tracks and the Pacific Motorway.

Opened in early August 2009, the Eastern Busway is now operational and is predicted to take up to 450 buses, equating to up to 10,000 passengers, off the road every day,

The project was delivered by the Boggo Road Busway Alliance comprising the Queensland Government Department of Transport and Main Roads (owner), Thiess Pty Ltd (builder), and Sinclair Knight Merz (designer).

The tunnelling solution

The Eastern Busway alignment required the construction of a total of 910 metres of tunnelling including a 430 metre driven tunnel beneath the Boggo Road Gaol and Dutton Park.

Tunnel Types Explained A driven tunnel is constructed underground using a road header or tunnel boring machine. Soil and rock is removed from the end of the tunnel. A cut-and-cover tunnel is constructed from the ground surface down. Generally, walls and the roof are constructed first. Soil is then removed and the floor is constructed. This approach minimises the time the surface is unavailable for other uses. The alternative cut-and-cover approach is to remove the required soil, construct the tunnel, floor, walls and roof and then cover.

Like all urban tunnelling projects, designing and constructing infrastructure underground posed significant challenges. Critically in this case, the Boggo Road Busway Alliance team was required to build the driven tunnel that would pass very closely beneath the Boggo Road Gaol without damaging the heritage-listed complex above.

While driven tunnels avoid significant direct impacts on the surface in areas with suitable depth and ground conditions, buildings and infrastructure directly above the tunnel may typically experience some settlement, noise and vibration from construction.

The Boggo Road Goal’s brick buildings and brick perimeter walls were built in 1903. As the ground cover over the tunnel beneath the gaol site was relatively shallow (varying from 5.5 metres to 8 metres), careful consideration was required to avoid potential damage to buildings from construction works.

The gaol complex directly above the tunnel includes the six metre high perimeter wall, a single-storey building and two three-storey cell block buildings. Most of the brick walls of the buildings are founded on unreinforced concrete strip footings at shallow depth.

The design of the driven tunnel provided a horseshoe-shaped profile with a final excavated width of 15 metres and a full excavated height of 8 metres. The geology along the tunnel alignment was highly variable and complex.

To protect the buildings, careful consideration was given to how the tunnel was constructed.

Excavation and support measures

The excavation method under the gaol site involved heading and benching with a AM105 road header machine, which advanced at a rate of nine metres per week. The initial heading excavation (height) was around 6.5 metres.

A number of measures were undertaken to ensure the stability of the tunnel cut. Long steel ‘canopy’ tubes were installed forward over the tunnel arch ahead of the tunnel face being cut. These tubes acted compositely with sprayed concrete (shotcrete) up to 350 millimetres in thickness on the recently cut tunnel side and overhead surfaces close to the cutting face.

This initial ground support ensured very stiff tunnel lining could be installed as quickly as practical at the tunnel face. An admixture was also added to the shotcrete at the nozzle to attain high early strengths.

Settlement monitoring

Critical management to achieve minimal settlement under the heritage site included adoption of a rigorous Permit-to-Excavate procedure. This procedure required the construction team, the designer and the geotechnical engineer to meet each day to review the previous day’s construction, monitoring and geological data from the face mapping and agree on the tunnel support and construction methodology to be adopted for the next 24 hours of tunnel cutting.

This process ensured the settlement trends were reviewed daily and the adopted initial ground support and construction approach was adequate for the variable geology encountered.

The predicted settlement at the Boggo Road Gaol was 10 millimetres and the resulting observed settlement was 7 millimetres to 12 millimetres with no damage to the heritage structures recorded.

The alliance received a National Trust of Queensland Heritage Award in 2009 in recognition of this achievement.

Conclusion

The completed Eastern Busway Eleanor Schonell Bridge to South East Busway forms a vital link in the growing network of busways that play a key role in providing commuters with a congestion-free run on fast, frequent reliable services.

It is achieving this without impacting the heritage buildings above or nearby residents.


To gain a deeper understanding of the Australian experience and trends, logistics, challenges and benefits of busway design click here to view the related article: Busway Solutions to meet Rapid Urban Growth

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