Tunnels are generally more expensive than bridges and at-grade structures, so they may not be selected for some projects even though they could result in a more sustainable project. In this article we explore different forms of cost-effective tunnel ventilation designs with reference to several tunnel projects. Such designs would make tunnels more viable options, particularly when there is a call for sustainable development. The resulting environmental and societal benefits could outweigh the initial higher costs when looked at over the life-cycle of the project.

Roadway Tunnels: Minimizing Costs and Environmental Impacts

The type of tunnel ventilation system designed and the features of the tunnel that affect ventilation can have a major impact on the sustainability of a roadway tunnel.

Tunnel Ventilation Systems. In general, tunnel ventilation systems can be divided into three major types, longitudinal, semi-transverse, and full transverse.

Reference is made to the Singapore Underground Road System (SURS), a 15 km (9-mile) –long, two-lane (in each direction) ring tunnel around Singapore’s central business district with 17 underground entry ramps and 16 underground exit ramps. PB is the concept design consultant for the entire project. For a sustainable design, the ventilation system was recommended to be a composite of longitudinal system with point exhaust. Axial fans are housed in a number of strategically located ventilation buildings with easy access for inspection and maintenance. Supply air is injected into each tunnel section through a Saccardo nozzle at constant velocity in the direction of traffic flow, resulting in a longitudinal airflow that is then exhausted just upstream of the next ventilation zone. Apart from significant saving in initial cost (US $200 million), this concept uses the piston effect of the vehicles to self-ventilate the tunnel, which lowers energy consumption and operating costs.

For immersed tube tunnels, proper design can significantly reduce the services space and, hence, the submerged volumes of the immersed tube tunnel. This was the case with the Western Harbour Crossing (WHC) in Hong Kong, which comprises a 1.36-km (0.8-mile) long immersed tube tunnel and 0.64-km (0.4-mile) -long cut-and-cover approach tunnels. PB is the project designer responsible for the concept design for the mechanical & electrical (M&E) system and immersed tube tunnel. The tunnel ventilation system and the tunnel services were optimized to reduce the air duct and the services ducts inside the immersed tube tunnel. The tunnel was then re-configured from a five-box tube to a four-box tube with integrated services box and ballast concrete. This arrangement not only reduced the overall dimension of the immerse tube but also reduced the seabed preparation.

Figure 1: Stack Tunnel Arrangement-Eastern Distributor Tunnel

Site Constraints. Sometimes it is not easy to construct tunnels for a densely populated urban area, particularly if they are in a soft ground and located very close to the ground level. Under such circumstances, a stacked tunnel may resolve the land issues. This was the case with the Eastern Distributor in Sydney, Australia, a 6-km (3.6-mile) -long route with a new 1.7-km (1-mile) –long main tunnel and the 600 m (2,000-foot) -long Dacey Todman Underpass (Figure 1). PB is the tunnel ventilation designer for the project.

The main tunnel is a double-deck, dual three-lane tunnel with entry and exit ramps. Due to a very stringent environmental requirement at the tunnel portals (zero portal emission), a ventilation building is located near each of the main exit portals to extract the polluted air from the tunnel. This air is then exhausted vertically to the atmosphere at a high velocity. This project is one of the major sustainable developments in Sydney as it links northern and southern Sydney via a narrow corridor in a densely populated urban area with minimum portal emission to maintain air quality in the neighborhoods.

Use of Construction Adits. The selection of a suitable ventilation system and the use of construction adits can optimize the tunnel cross-section area and shorten the construction period with multi-face working, as was the case with Tate’s Cairn Tunnel in Hong Kong, a 4 km (2.4-mile) long, twin bore, two-lane structure. PB is the project designer for the M&E system. The structural and M&E design for the winning build/operate/transfer (BOT) tender saved 14 months in programmed construction time and an estimated 15 percent in the construction cost. A major contribution to these savings was the deletion of ventilation shafts and the use of two 500-m (1,650-foot) -long construction/ventilation adits that allowed up to twelve faces to be available at any one time for excavation. The semi-transverse ventilation system was designed to take advantage of the two construction adits that intersect the main road tunnels at their quarter points.

Access/Location of Ventilation Buildings. PB is the M&E project designer for the 3.8-km (2.3-mile) -long Tai Lam Tunnel in Hong Kong, the longest dual three-lane tunnel in the world. There are 24 supply axial fans and 15 exhaust axial fans housed in two ventilation buildings, one at each portal. Semi-transverse ventilation system is adopted with point exhaust at the first and the third quarter points. The construction program was shortened by 12 months and a cost-effective design was achieved by the following features:

• Space at the “crown” above the traffic gauge created by constructing a concrete slab was used as an air duct.
• Point exhaust at the first and the third quarter points minimized portal emissions.
• Ventilation tunnels constructed to one-quarter of the tunnel length from both portals eliminated the need for vertical vent shafts, which were not permitted because the tunnel runs under a country park, and because of the associated site formations and access roads.

Railway Tunnels: Cost-effective Designs Generate Energy Savings

Several factors need to be considered to make a railway tunnel a viable option for sustainable development, including those discussed below.

Environmental Control System. Mainly two environmental control system concepts are used for mass transit systems in tropical countries: closed system (CS) and platform screen door (PSD). The PSD concept, which has been used for several projects in South East Asia, can generate substantial energy savings because it isolates the air-conditioned platform environment from the hot, humid air in the tunnel, improves the train-induced ventilation for tunnel ventilation, and reduces the air leakage into the station when the tunnel ventilation fan is operated. The overall effects are to achieve a sustainable design, which can reduce the costs for plant space and M&E equipment, reduce the operating cost on the environmental control system and tunnel ventilation system, and enhance the station environment and safety.

Site Constraints and Use of Temporary Launching Shaft. Similar to road tunnel projects, it is difficult to construct tunnels and stations in a densely populated urban area. The use of temporary construction launching shafts as permanent building can eliminate the need of additional site formation, access roads and building structures. As part of Quarry Bay Extension for an existing railway line in Hong Kong, the design made use of the launching shaft, having it become a permanent ventilation building that incorporates vent shafts, plant rooms and evacuation staircases. Extra space and an assess route were not required for the permanent building. As a sub-contractor, PB served as the mechanical and electrical designer.

Rolling Stock. Rolling stock has significant impacts on the design of a tunnel and the associated M&E systems. In particular, the impacts due to:

Braking Methods. Of the two types of braking methods, rheostatic and regenerative, regenerative results in sustainable design in that it recovers part of the generated current to power on-board auxiliary equipment, and the remainder is available to the current distribution for other trains operating in the vicinity of the regenerating train or back to the station power supply system. If the line is not receptive, the excess energy is dissipated by on-board resister grids to the tunnel. Rheostatic braking consists of bank of resistors to dissipate the generated electricity in the form of heat, which then heats up the tunnel air.

Ceiling mounted A/C units aggravate the problem because the heat discharged by the upstream A/C units cascades along the train in the upper regions of the annulus and can make the condenser intake temperature of the downstream A/C units about 7° C (12° F) higher than the average tunnel air temperature at the same location, even with the operation of the tunnel ventilation system. In comparison, underneath train A/C units can reduce the airflow requirement to ventilate the tunnel during congested operation due to the stratification effect and, in turn, reduce the associated civil provision.

Pressure Transient. Subject to the permeability of the trains, pressure waves that can affect passengers inside a train or on a station platform are generated when a train enters or exits a tunnel portal, so need to be controlled to ensure a safe and comfortable environment. Cost effective designs to mitigate the pressure transient includes the control of train speed at portals, the use of pressure relief dampers or pressure relief shafts, the use of isolation walls and the use of special portal design.

Conclusion

Tunnels can be an important component of sustainable transportation infrastructure design. Often they can readily meet two components of the triple bottom line—environmental and social outcomes. It is incumbent upon engineers to develop cost-effective designs that satisfy the third component—economic outcomes. As illustrated by projects discussed in this article, multi-discipline knowledge/input is essential to arrive at a cost-effect design. We need knowledgeable and innovative engineers to come up with ideas. We also need experienced contractors who can work with designers to help come out with a tailor-made cost-effective design.