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WALL STORY
If the only light you see is at the end of the tunnel – it needs washing says Andrew Back.

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02.jpg Road tunnels and vehicle underpasses play a major role in the highways network and must be maintained with special care – and that includes cleaning – to ensure that road safety is not compromised and lighting levels remain efficient.

Few, if any motorists take notice of tunnels: their prime objective is to get to the other end as quickly and safely as possible; and fewer still reflect on the construction and maintenance that is required to keep them at the right standard.

A high standard of lighting is a priority: too bright, and the lamps or lights may dazzle or distract drivers; too dim, and the tunnel can seem claustrophobic and daunting, as well as increase dangers from obstructions and other hazards. The solution, which is designed-in by the architect and civil engineer, is to use the walls of the tunnel to reflect artificial light and create a ‘false’ ambient lighting effect.

The level of lighting must encourage drivers to use their dipped beam, or, in many modern vehicles, trigger the automatic headlight sensor, and also provide sufficient light for any pedestrians or emergency incidents, such as breakdowns.

Reflectivity uses all surfaces of the tunnel, including the roadway, but by far the most important are the walls, which is a reason for painting them in light colours and keeping them clean. The Highways Agency calls for a minimum LUX lighting level of reflectivity of 0.6 on the walls, which must be at least four metres high. Reflectivity on the roof can be 0.3.

Detergents are frequently used to wash off heavy grime and fuel emissions, but if possible should be avoided to prevent the wash from polluting drainage systems. However, to be more environmentally friendly the tunnel surfaces require washing more frequently to make it easier to keep clean and maintain the levels of reflectivity.

On open roads, street lamps are at their brightest when it is darkest: in tunnels the opposite is true. Motorists’ eyes must adjust to the dark and therefore at the entrance and exit to the tunnel lighting is at its brightest, diminishing in intensity as the tunnel progresses. But cleanliness of the tunnel walls is a constant factor in road safety. Where tunnels incline, taking drivers under rivers, for example, drivers entering decelerate and generate little pollution or emissions, but on the upward incline, as they leave the tunnel, the acceleration creates a sharp increase in emissions. These fuel particulates, especially diesel, are not only dirty and greasy, but they can act aggressively on the structure of the tunnels.

In winter there is also the problem of high acceleration to combat cold weather, plus the road salt that can be picked up by all vehicles before they reach the tunnels. Some tunnels burrowing under tidal rivers may also be affected by airborne and residual marine salt. It is this combination of diesel particulates plus a small amount of condensation that allows the aggressive and acidic pollutants to get to work on the walls of the tunnel.

Tunnel walls under rivers and estuaries often have facings of light-painted ductile cast iron plates for security against tidal flows and the depth of the structure. At their entrance and exits – and with most horizontal road tunnels – the facing may be lightly curved or straight and made of smooth white or light enamel to aid reflectivity. In either case aggressive and acidic emissions and deposits can adversely affect the appearance and longevity of the wall covering.

So what are the benefits of tunnel cleaning? Road tunnels are constantly bombarded by various aggressive pollutants from vehicle emissions and road salts that can, if not controlled, corrode and deteriorate the tunnel structure. The most cost-effective way of reducing these effects and the structure’s life is to keep the tunnel clean.

Prolonging structural benefits is only part of the big picture. Millions of pounds are spent creating road tunnels and inner-city vehicle underpasses of stature and character featuring decorative murals and friezes, which make a tunnel individual and attractive. However the desired effect is very quickly hidden by a film of dirt and grease from a constant flow of heavy traffic building up deposits. If the tunnel is not cleaned frequently then the wall staining will not be suppressed, allowing it to get worse and worse, reducing the reflectivity of all the tunnel surfaces, and the appearance of any art works.

Government Guidelines
Environmentally, a combination of energy saving lighting and clean reflective materials reduce the lighting requirements resulting in reduced energy in compliance with Governments guidelines and the topical carbon unit debate.

What is required is an efficient frequent cleaning service using high technology to create a purpose-designed procedure that can provide a fast and productive cleaning operation reducing tunnel closure downtime and the inconvenience to motorists.

Outsourcing this maintenance is a popular option, with companies such as Avondale Environmental Services offering tunnel washing as part of their services. In fact, Avondale has a large fleet of specialised vehicles for this purpose, fitted with an automatic washing system provided by Avondale’s systems partner Schmidt Mulag.

The procedure is a fast, effective and a reasonably simple procedure: based on a brush assembly fitted to a manoeuvrable hydraulic arm, mounted on the front of a specialised vehicle. Water is sprayed onto the wall followed by a large rotating brush that agitates and removes the dirt and grime followed again by a water spray that rinses the surface. The whole operation is combined within the brush assembly head and to prevent the potential risk of the operation damaging the tunnel wall panels, automated pressure sensors are attached to the brush mechanism. These automatically control the height, pressure and position of the brush, following the tunnel’s wall curvature and varying height.

As a high profile environmental company, Avondale is committed to sourcing environmentally friendly solutions, which, in this case, avoid the use of detergents or bleach where possible. The solution it has developed with its systems partner is so effective that detergents are not required and the procedure uses as little water as possible, removing the film of dirt and grease without damaging the walls. The use of specialist, low-emission vehicles completes this highly productive and efficient, environmentally friendly ‘package’.

Highway road tunnels and inter-city vehicle underpasses are significantly more than uninviting holes in the ground: they are vital components of an essential busy highways network. If that light at the end of the tunnel is brighter then the designer’s walls, then light reflections are simply not working safely and efficiently.  CT-E

Andrew Back is managing director of Avondale Environmental Services, a professional asset management company involved in the maintenance of road and rail environments. For further information tel: 01634 823200, fax: 01634 844485, or email: info@avondaleuk.com.

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