When the first Blackwall Tunnel was built under the River Thames in 1897, it was designed for horse-drawn vehicles. At the time, there were probably fewer than 10,000 cars worldwide. Now that same tunnel carries 40,000 vehicles each day. Likewise with railway tunnels; the high speed trains which travel through some of the oldest existing infrastructure in the UK run on highly sophisticated fully-welded track and trackbed, not the wooden sleepers and ballast which was laid down at the time of construction.

The original illumination for the Blackwall Tunnel included electric lighting, probably highly innovative at the time. The lighting specification, described in the ICE Proceedings of April 1897, was "three rows of incandescent lamps of 32 candle-power, fixed 10 feet apart, on alternate lines, with additional lamps and a few arc lights in the shafts" – a far cry from the lighting requirements of a modern day highway tunnel, which needs to provide for the safe transition from full daylight to tunnel environment for a vehicle travelling at up to 70mph.

Similarly, many of the shallower tunnels that lie beneath busy modern urban streets were designed and built when the streets carried only horse-drawn vehicles. Many of these tunnels, rail, metro or sewer, were brick-lined structures, but the loadings from the surface were not significant at that time. Apart from the increased loadings from heavier vehicles, over the past 100 years the ground all around the tunnels is likely to have been subjected to significant additional stresses from deep foundations and piles to support high-rise buildings, or deep basement structures.

With the increase in population and the development of the metro system, the ground underneath the original tunnels may also have been removed for the construction of new tunnels and sewers. The new construction will have caused deformations and movement of the original works and, as all tunnellers know, disturbing a dry and stable tunnel has every likelihood of inducing cracking and water leaks.

Modern regulations on fire, health and life safety in tunnels are becoming more and more stringent. As an example, recent UK Highway Agency recommendations state that highway tunnels should be equipped with emergency exits, or cross passages at a maximum spacing of 100 metres. Further, the Permanent International Association for Road Congress (PIARC) recently commissioned a survey of the major road tunnels in western Europe. The survey gives serious consideration to the fire and life safety features of the tunnels, and also their internal appearance. Quality of finishes to provide clear vision for drivers, and appropriate walkways beside the carriageways, are key features of a safe tunnel environment. They will need to be addressed for the major highway tunnels not only in the UK, but for all over Europe.

The risk of fire is not restricted to older tunnels. The blaze in the Channel Tunnel happened within four years of opening to rail traffic, and the fire in the Storebaelt Tunnels was during construction. The repair works in the Channel Tunnel were carried out by the installation of sprayed concrete within the damaged lining. At Storebaelt, the ground conditions were extremely poor, and it was decided that the damaged linings would stay in place. The reconstruction used spare cast iron rings from the Channel Tunnel, machined to the correct dimensions. These were erected within the damaged rings, which maintained the necessary clearance envelope for the railway.

In California, existing tunnels are being re-appraised to incorporate the modern rigorous seismic design codes now in place in that State. Many older structures are undergoing retro-fitting to ensure their safety in the event of a major earthquake.

Ultimately of course, tunnels just get old. The pointing of brickwork deteriorates; slow but gradual seepage takes its toll; trackbed or carriageway decking runs its lifetime; long-term chemical or physical attack cause structural distress; stray currents may induce corrosion; new fixtures and fittings are required to improve the overall quality of the tunnel.

Consideration must be given to renovating or to abandoning the facility and constructing a new tunnel. This seems drastic for large transportation tunnels, and it would be difficult to envisage an economic evaluation finding in favour of abandonment.

New materials and techniques for extending the design life of existing structures are being developed continuously. The economic arguments would also be expected to prevail for refurbishment of the smaller tunnels for sewers or cable and communication tunnels, rather than completely new construction. Even in a state of total disrepair, one would anticipate the provision of a waterproof membrane and sprayed concrete lining to extend the life for a further twenty-or-so years, albeit with reduced internal space. However, the choice remains: to build a new tunnel or to refurbish the existing facilities.

Difficulties and solutions

A major difficulty in refurbishing an older tunnel is often the lack of drawings and records of the original construction. Masonry tunnels may involve a lining of many concentric layers of brickwork. The most effective method to determine the construction is to core through the lining, but such destructive testing and the means to repair the coring, may prove detrimental to the refurbishment. The less accurate, but non-destructive, ultrasonic measurement has often been used as a substitute. There have been attempts to develop automatic techniques to appraise tunnel linings, such as ground radar to assess the thicknesses of masonry layers. However, the success of these innovations is limited and it seems that there is no magic fix. Tried and tested, but labour intensive, manual inspections remain the only reliable means of condition assessment.

Even when full details are known, access to the refurbishment area may prove problematic, particularly for transportation tunnels where flow of traffic or the rail service needs to be maintained. Detailed inspection of tunnels is an extremely time-consuming procedure because of these difficulties: panels and cladding often have to be removed to expose the tunnel segments, to establish the root cause of the problems. Often, tunnel levels and wriggle are not known, again requiring new surveys to be conducted.

The repair works are equally laborious: the tunnel linings need to be cleaned and structurally assessed; segments may need to be re-caulked, or masonry re-pointed, to seal leaks; new fixtures require installation; drainage replaced or improved; and the new final linings erected. These works are generally carried out in off-peak hours for road tunnels, or in very limited possession times for rail or metro tunnels, adding greatly to the overall programme for the works.

Like for like replacement of older materials may not be an option and substitute materials need to be used. Refurbishment of some of the older cable tunnels in the UK has revealed that asbestos was used in the original construction.

A key tool in refurbishment and repair is sprayed concrete. This can be used to fill voids, replace deteriorated sections, or to provide a completely new lining. Many new tunnels are formed using sprayed concrete as a final lining, with or without a waterproof membrane, so its use in refurbishment is fully justified. However, the application of an additional internal lining impinges on clearances. For major transportation tunnels in particular, where space is at a premium, the use of sprayed concrete may not be acceptable. Indeed, it is understood that for the forthcoming refurbishment of the River Severn Tunnel, Railtrack has ruled out the use of sprayed concrete.

Clearance envelope was the key issue during the refurbishment of the Waterloo and City Line in London. The rolling stock was to be upgraded, which required enlargement of the tunnel. Following a sophisticated ‘wriggle’ survey of the tunnel, it was realised that the lengths of tunnel requiring enlargement could be significantly reduced, and modifications were made to the track alignment and track support system to accommodate the new rolling stock. In other areas within the tunnel, back-analysis of the cast iron linings revealed that the flanges could be cut back to improve clearances.

Lowering of trackbed has been a successful way to extend the life of rail tunnels in the UK and in Europe. Upgrading railway lines often includes the introduction of an overhead catenary line. Developments in trackslab designs have allowed the vertical clearances to be achieved. One of the first successes with precast concrete trackslabs was at Penshurst Railway tunnel, on the Redhill – Tonbridge Line, which required upgrading to carry international freight rolling stock from the Channel Tunnel. The precast slabs were prestressed longitudinally and transversely, resulting in a slab thickness of only 200mm.

Further research and development continues in this specialist area, and this can only be of benefit to overall infrastructure improvements.

Future works, clients and projects

The tunnels of London Underground have already been mentioned. If and when the PPP franchises are awarded, then significant refurbishment works will have to be carried out. Each of the Infracos is conducting maintenance and repair work, but it would be expected that the bulk of the refurbishment and modernisation will start only when the private companies are in place. The amount of upgrading necessary for the older lines, and hence the costs associated with this work, will only be fully appreciated once work commences. Many of the underground lines still run on wooden sleepers, with the associated rocking of trains which eventually necessitates speed restrictions. These sleepers will eventually require replacing with modern slabtrack. The logistics of this undertaking is significant, not least because of the need to maintain services, and because the new rail operators will be anxious to maintain their target service levels.

Knowledge of the condition of tunnels, and the amount of upgrading required to ensure their suitability and safety for high speed rail traffic, must be a cause of concern to Railtrack. Incidents do occur, such as the collapse of the tunnel roof in Strood in Kent, requiring immediate response to re-open the line. However, in light of the extensive modernisation currently under way, including the East Coast and West Coast Main Lines, the amount of repair and refurbishment contracts let to date on these high-profile projects does seem minor. One would hope that this demonstrates the quality of workmanship of past railway engineers, rather than that the logistics of undertaking the necessary inspections have been labelled ‘too difficult’. The recent plans published for the ECML indicates remedial works to a number of tunnels. The feasibility studies for these works are currently under way, but these contracts for the construction are identified for the later stages of the upgrade programme.

The Mersey Kingsway tunnels were completed in 1974. Designs are being prepared for upgrading these tunnels to provide additional lay-bys and cross passages for emergencies. As a result of the Mt Blanc Tunnel disaster, many highway tunnels, in the UK and Europe, will be modernised and improved in the areas of fire and life safety. Such schemes will include improving ventilation and lighting, enhancement of the controls and communication systems, provision of additional emergency exits or compartments, and the general improvement of the condition and appearance of the tunnels.

When the public becomes familiar with how a new or refurbished road tunnel should appear, the case for improving other tunnels gains momentum. The start of repair work to the Mt Blanc Tunnel is imminent. The consensus is that the tunnel should be twinned, but the costs would be significant.

Some tunnels have been abandoned, only to be repaired and re-opened at a later time, either for changed usage or because of change in public opinion. The Woodhead Tunnels linking Yorkshire and Derbyshire were part of a disused railway line. This had suffered collapses and the masonry lining was in a poor state over considerable lengths. After remedial measures, these tunnels now house the 400kV trans-Pennine transmission cables of the National Grid.

British Waterways values its heritage and has been seen to be improving many of the facilities on the network of navigable waterways throughout the country, as well as re-opening many disused canals. The Blisworth Canal Tunnel, on the Grand Union Canal, was reconstructed almost 20 years ago.

In a 200 year old tunnel with an invert full of water, some deterioration of the structure may be expected However, the general deterioration has been found to be within the sidewalls and crown of the tunnels. A number of canal tunnels have been refurbished recently. These include the Standedge Tunnel in Huddersfield, and the Islington Tunnel in London. Future works include the Dudley Tunnel in the Midlands.

Overseas spending

The major spending on infrastructure in the USA has also extended to improvements to the existing facilities. The studies of the PATH Tunnel in New York City were completed in 1995. This tunnel was built between 1874 and 1890, under the Hudson River, and consisted of a 30-inch brick ring with 1/4 inch boiler plates at the extrados. Slow, long-term settlement measurements of up to 20 inches had been recorded in the tunnel. Various techniques were used to determine the nature of the progressive deterioration mechanisms, including coring the brickwork and impact echo methods to determine the limits of the void between the iron liner plates and the brick. The recommended remedial works accommodated a phased programme of refurbishment but, as with most transportation tunnels, this would need to be undertaken within four-hour nightly track closures.

The inspection of a number of Amtrak tunnels in New York City is scheduled to start in the near future. Six railway tunnels under the East and North Rivers, of between 4 and 4.5km in length require upgrading or rehabilitation. Seismic retrofitting of two of the oldest immersed tunnels in the world, the Posey and the Webster tubes which run parallel into Oakland, California has already been done. These immersed tunnels are unusual in North America, in that they are of reinforced concrete construction. The studies for the seismic retrofitting of the BART tunnel in San Francisco, which remains the deepest and longest immersed tunnel and is of the more conventional American steel design, are imminent.

Back in Europe, work is now complete on the modernisation of the 200km-long Beira Alta Railway in Portugal. This project involved the refurbishment and enlargement of 12 brick/ masonry lined tunnels to accommodate overhead electrification equipment. In Germany there are plans to renovate 18 railway tunnels, totalling 15km, between 2000 and 2003. These works will consist mainly of installing a new inner shotcrete lining, draining, and in some cases renovating the tunnels to accommodate twin tracks.

In the Far-East, the original Cross Harbour Tunnel in Hong Kong, opened in 1972, is the subject of ongoing detailed inspection. This immersed tunnel is a single-shell steel binocular construction, and it is understood that the integrity of the steel shell may have deteriorated significantly. Refurbishment may be expected in due course.

Cost benefits

The repair and rehabilitation works outlined above have one main objective: to prolong the active life of the tunnel. After renovation, the tunnel may continue with its original usage, may provide an enhanced facility, or may be used for a completely new purpose.

The generic costs for new tunnels can be assessed fairly readily and accurately. This would not be the case for renovation of existing tunnels, primarily because of the different circumstances for each scheme. In all cases discussed, the costs of the refurbishment, and the lengthy programmes necessary for inspection and construction will have been studied and assessed in detail. The market in asset management and inspection work, in the development of new assessment techniques and time-saving methods of testing, and of the actual repair and refurbishment work seems set to continue or even accelerate.

With many of the world’s transportation tunnels now dating back well over 100 years, refurbishment and repair must be seen as a cost-effective alternative to expensive replacement.
Work on the Beira Alta Railway in Portugal involved the refurbishment and enlargement of 12 brick/ masonry lined tunnels to accommodate overhead electrification equipment. Work on the Beira Alta Railway in Portugal involved the refurbishment and enlargement of 12 brick/ masonry lined tunnels to accommodate overhead electrification equipment. Work on the Beira Alta Railway in Portugal involved the … The logistics of reconstructing canal tunnels, such as at Blisworth, can be impeded by the water-filled invert. The logistics of reconstructing canal tunnels, such as at Blisworth, … In spite of recent, innovative technology, manual inspections remain the only reliable means of condition assessment. In spite of recent, innovative technology, manual inspections remain the …