Tunnelling is by necessity a more demanding activity –from the technical viewpoint at least – than most other aspects of civil engineering. Few jobs are so confined, so fraught with risk, both financial and to health and safety, and so unpredictable for the designers and contractors.
But while the lay person focuses on the technical challenge of boring the tunnel itself, few notice the equally challenging task facing the team who install the finishes and the lighting.
In fact, in many cases, it is the mechanical and electrical installations rather than the excavation that create a real headache for a tunnelling project – witness the Channel Tunnel and the more recent Jubilee Line Extension on London’s underground system. After all the heavy civils work was complete, these projects were plagued by programme and cost over-runs on the M&E packages.
Lighting is often taken for granted in tunnels, at least by those who use them. Motorists expect to plunge into a motorway tunnel out of bright sunlight at 100km/h and still be able to see well enough to drive safely. If the tunnel lighting is done well, the driver will be barely conscious of a change in luminance; but if done badly, the driver will be disorientated and safety adversely affected. The very lack of sunlight means that lighting is of the utmost importance in a tunnel.
Even less appreciated are the tunnel finishes, the secondary linings which are exposed to the view of tunnel users, and so to dirt and damage as well.
Tunnel linings often have an aesthetic role to play, especially in underground railway stations, but also within the running tunnels themselves. Most important, though, is their physical performance, and in particular, their interaction with the tunnel lighting.
"The comfort and light intensity inside a tunnel is probably affected more by the choice of tunnel lining than by the choice of lighting", says Rene van den Bosch, managing director of Euro Panels Overseas, maker of Glasal lining panels and a subsidiary of Belgian building materials giant Eternit.
Van den Bosch explains that the light intensity inside a tunnel is dependent on the combination of lighting system and internal lining: "The lining needs to be clean at all times, because if it gets dirty it will absorb more light and the efficiency of the lighting system will be reduced," he says.
Reflected and diffused
Glasal, a product developed about 40 years ago for general cladding and building applications, has become a preferred choice of many tunnel designers for the past 30 years, van den Bosch adds.
Glasal is a cement-based fibre-reinforced sheet material with a hard, semi-matt enamelled finish that reflects light, but also diffuses it to prevent glare. "Tunnel curves tend to create reflections off the lining material, so the light reflected from the surface of the lining should be diffused," explains van den Bosch.
Glasal is also chosen by designers for a number of other reasons. Fire resistance is a major consideration, as recent events in the Mont Blanc and Channel tunnels have demonstrated.
"Tunnel linings have to be incombustible and, if there is a fire, it’s important that the lining does not give off toxic gases," says van den Bosch. Glasal, being made of fibre-reinforced cement, contains no solvents or other harmful substances to be released in a fire. When exposed to very high temperatures, the material crumbles into a granular mass, says van den Bosch. He points out that Glasal, plus Eternit’s Promat fire insulation product, were both used in the Mont Blanc Tunnel and, unlike metallic products, did not twist or distort to prevent access to emergency services.
Fibre cement linings like Glasal also have maintenance benefits for tunnel operators. Van den Bosch explains that Glasal’s paint-enamel surface is washable, so the material’s reflectivity can be easily maintained, but he also says that slight accidental damage will not interfere with the material’s performance.
Ceramic tiles can chip and crack, necessitating replacement, while the joints between them can harbour dirt and in time reduce reflectivity by an appreciable degree. Enamelled steel also chips, and the problem here is the subsequent corrosion of the underlying steel.
A fibre-cement material is stable, moisture resistant and resilient. Scratches and chips in the "glazed" surface might expose the cement base, but no further deterioration occurs at the site of the scratch, says van den Bosch.
Glasal might be a prime choice for tunnel designers, but ceramic tiles, vitreous-enamelled steel and plain fair-faced concrete are still very widely used. The choice depends on the tunnel type, the required balance between performance, initial cost and maintenance requirement, and of course aesthetics.
Concrete system
Whereas Glasal is a flexible sheet material with no inherent strength of its own, Tunneline is an entirely different approach to tunnel lining. Developed by Kinloch Tunneline of Sandbach, England, this is an in situ concrete system that can be used in conjunction with mass concrete, conventionally reinforced concrete or fibre-reinforced concrete.
Tunneline, which was developed in the early 1980s and first used in 1984, uses a patented formwork system to construct a concrete lining inside the tunnel. This produces a structural lining with a fair-faced internal diameter and is suitable for sewers, culverts and road or rail running tunnels. Originally developed for live sewer and culvert renovation, the Tunneline system has subsequently been used on many kilometres of rock, railway and segmental tunnels, including London’s Ring Main.
"We have used the Tunneline system on just about everything from small diameter sewers to 8m-diameter road tunnels," says Kinlock Tunneline project manager Dave Shaw.
Current projects include a contract with Chinese contractor Paul Y Tunnelling for constructing 4.2m, 1.35m and 1.2m diameter sewage tunnels under Hong Kong harbour, and future projects include work on the London Underground at Brixton to create new lift shafts and concourse tunnels.
Modular lighting system
Product developments in the lighting sector tend to occur more frequently than in tunnel lining, largely due to on going innovation in the field of electronics. One recent development that has brought major benefits in terms of speed and efficiency is the Flexo cabling system, developed by BICC General, now owned by Italian industrial giant Pirelli.
Flexo is a modular cabling system comprising a high integrity Class 2 stranded copper cable encased in galvanised steel wire armour and either XLPE or EPR insulation with an outer sheath of LSF. The cable has integral sockets spaced at intervals (the exact spacing determined by the customer’s requirements) enabling lights to be installed at precisely the right location, with or without having to install sockets after installation of the cable itself.
This means that the Flexo system can be used during construction for temporary lighting or for permanent and emergency lighting circuits. According to Flexo, installation of the system is very simple, fast and cost effective. The plug leads can be connected to the lights prior to them being secured. The main cable system, with integrated sockets, is supplied on standard drums which can then be installed as simply as a normal cable. When installed, the sockets align with the light fittings and connection is made by simply plugging the light fitting into the socket.
According to Ian Wilkie, international sales manager for Flexo, the use of a modular system is not only quicker and less labour intensive than traditional systems, but quality and environmental integrity are more easily safeguarded too.
"Cable systems are vulnerable to vibration, moisture and dust. Flexo conforms to all relevant standards and is protected to IP67," he says. Being factory-assembled, the risk of on-site damage or faulty installation is reduced as well.
When it comes to the lights themselves, the global market for lamps is controlled by only a handful of major players, led by the likes of General Electric, Philips and Osram. A far more competitive market is that for luminaires and control systems. Hugh King, spokesman for Thorn Lighting, says that such fittings are frequently designed and made to customer specification: "Fixing arrangements, such as mountings and brackets, are often bespoke because of customer preference. Most tunnels are one-offs, so the lighting system has to fit the tunnel design."
"The most unusual thing about tunnel lighting, though, is that it is designed to work the opposite way to what you’d expect", adds King. By this he means that the most light is needed at the entrance to the tunnel, where there is plenty of natural light; the luminance is lowest in the middle of the tunnel which is naturally the darkest part of the tunnel.
This apparent reversal of logic is of course designed to prevent the sudden change in ambient light levels disorientating drivers as they enter the tunnel. As you penetrate deeper into the tunnel, you are gradually acclimatised to lower and lower light levels, and then exposed to increasingly brighter lighting as you approach daylight again.
Energy consumption
Since tunnel lighting is usually in constant operation, energy consumption is an important factor. During the world oil crisis of the mid-1970s, low pressure sodium lighting, with its characteristic orange-yellow glow, became the preferred form of street lighting. It was cheap and energy efficient, but so poor in colour rendition that objects lit by it appeared in monochrome.
High pressure sodium, with its whiter light and higher energy consumption, did not become very widely used until the 1980s. It is now a primary choice for tunnel lighting, and is frequently used in conjunction with fluorescent lighting. "High pressure sodium is the first choice in tunnels now because it is more energy efficient than previously", says King.
Les Brown, principle lighting engineer for British consulting engineer Babtie Group, points out that high pressure sodium was available for over 20 years before it really took off. "Tunnels usually use fluorescent lamps for the base lighting because it gives good visual guidance. It can be arranged in a continuous strip which does not create a "flicker effect" as you pass through the tunnel", says Brown.
"High pressure sodium is used largely as a booster, say, at the entrance to the tunnel," he adds.
A relatively new development in the UK is the adoption of a method of lighting known as "counter-beam". This is not a new kind of technology, or even a new product, but a new method of designing tunnel lighting to reduce visual disturbance and improve lighting efficiency.
Counter-beam lighting places luminaires laterally, across the axis of the tunnel, and oriented to shine towards on-coming traffic. To the uninitiated, this might suggest an attempt to dazzle tunnel users but, according to Brown, it does the very opposite:
"Counter-beam lighting is very cost effective, although it’s not suitable for all tunnels," he says. "Traditional lighting, using symmetrical fittings, lights the whole tunnel, and that’s a waste when traffic is moving all in one direction".
Counter-beam lighting makes clever use of less light (and hence less energy) to give tunnel users a clear view of the road ahead and all objects within the tunnel. "Instead of trying to illuminate objects, counter-beam lighting illuminates the road and the tunnel walls and bounces light off these surfaces toward the on-coming traffic so objects are seen in silhouette."
The system does this by orienting the light in one direction: down and back towards the driver. "If you were to look out of the rear window of a car travelling along a tunnel lit with counter-beam lighting, the tunnel behind you would look darker than in front," says Brown.
This has major benefits, he explains. Not only does the lighting system consume less energy, but fewer light fittings are required. This means less cabling, and fewer controllers and, because lower intensity lamps can often be employed, those light fittings, cables and controllers can be rated lower, and hence cheaper.
This all adds up to considerable cost savings. As Brown says: "installation, maintenance and energy costs of tunnel lighting can be horrendous."
After many years of accepting the orange glow of low-pressure sodium lights in urban streets, motorways and some tunnels, lighting engineers have begun to turn their attention to white light. Not only is this more attractive to the eye than the sickly, yellow hue of low pressure sodium, it also helps with colour rendition and so improves visibility. High pressure sodium lighting is less orange than the low pressure version, but whiter still is CDMT (or metal halide) and compact fluorescent lighting.
"These types of lighting are gaining ground in street lighting," says Brown, "and I think that before long they will take over completely from high pressure sodium. They’re not used very often in tunnels at present, but I think that is about to change".
