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TUNNEL VISION

TECHNICAL NOTE - Victorian brick tunnel joints provide clues to their construction that can help improve maintenance and reduce costs. Jack Knight and Scott Wilson report

Most rail passengers travel through tunnels every day and think nothing about them. Travelling from south Wales to London, for instance, trains pass through the longest tunnel in Britain: 7km of Victorian brickwork that is mainly unseen today, but is the result of the truest engineering skills beneath the River Severn.

Mainly built between 1830 and 1900, there are 628 operational tunnels in Britain covering 335km.

There are also, it is thought, more than 200 disused railway tunnels.

The engineering of these Victorian tunnels remains much of mystery even to the railway companies that own them. With no plans or paperwork, those responsible for safety and maintenance of the tunnels are, like their passengers, in the dark.

The only remaining details of early tunnel construction found to date have been from the construction of Colwell New Tunnel near Malvern, which was built around 1925.

Fortunately, the methods of construction used all those years ago have left a legacy, rather like a fingerprint, allowing engineers to chart the tunnels' construction and make strides in improving maintenance.

An early Victorian tunnelling technique was the use of 'breakups' - excavations upwards to the heading to form the final tunnel excavation. This method may have been a result of an accident at Watford Tunnel.

The line of the tunnel had been established and shafts were being sunk in brickwork to the top of the tunnel level. They were then carried on down in timber to the tunnel base and even a little bit further to form a sump to collect groundwater.

From the base of the shafts small timber headings 1.5m high by 1m wide were driven between the shafts to ensure that both line and level were correct. Candle-pots lit in each shaft bottom revealed any discrepancies in line and level.

Once the headings were through the tunnel could be started by excavating from the sides of each shaft in an excavated and supported advance of about 4.5m to form 'side lengths'. These were built in extra thickness brickwork, strong enough to support the weight of the shaft above.

In Watford Tunnel, however, the side lengths were under construction when a collapse occurred and both tunnel and shaft were lost, together with lives of several men.

It was decided that a more expedient way to proceed was to sink the shaft and the headings in the normal way but then to start the tunnel some distance away (up to 45m) from the shaft by forming a 'break-up' from the heading and in this excavated and timbered gap, build a section of full-sized tunnel.

This section of tunnel may have only been 3m wide, but had to be solid and very strong as the construction of the next sections of tunnel depended on it being able to support the roof, not only from one end but from the other, as it was realised that a second gang could now work towards the next shaft while the rst gang began to work back to the original shaft, knowing that there was now a section of tunnel completed behind for safety if anything happened at the shaft.

Break-ups soon became the norm because two or three breakups between shafts could increase production signi antly with new gangs introduced immediately to boost progress and improve the chances of the railway making money at an earlier stage than had been planned.

The tunnelling of Chipping Sodbury is known to have had 40 gangs working from only seven shafts. This could not have been achieved without the use of breakups between the shafts.

Tunnelling advances or lengths between break-ups were normally set at about 4.5m, governed by the length and thickness of the timbers put in to support the roof and the ability of the miners to carry this enormous weight of timber forward as they excavated the next length.

If bad ground was encountered or if water softened the roof, the main support timbers, which were spanning from the last length of completed tunnel brickwork on to supporting timbers in the excavated face, started to take weight from the ground above and might well have bowed or sagged under the load.

Sometimes, despite making an allowance for the sag in the timbers, there was too much bow or weight on the timbers and the brickwork could not be placed to the correct thickness in the roof. This caused problems as the timbers had to be reset, wasting time and effort.

Generally the foreman bricklayer or carpenter paid all the men on piecework (so much per yard) and it was in no one's interest to be delayed by having to reset timbers because of bad ground.

At the slightest hint of poor conditions the foreman bricklayer or carpenter would say how far the advance had to be, to be certain of getting the brickwork clearance in two weeks when it was time to complete the brickwork in the crown.

Advances in these circumstances would possibly be reduced to 2m or even 1m so that the timbers did not span so far and were therefore much stiffer, giving less sag under load.

But advance rates were paramount and, where possible, maximum lengths would be resumed as soon as was feasible, as the face support timbering was the same and took the same effort for a 1m advance as it did for a 4.5m one.

Junction lengths were sections of tunnels where two gangs met and it was sometimes a race to see who could claim the maximum yardage before the other gang. For example, if 7m was left then a gang may have chosen to excavate the standard 4.5m, leaving only 2.5m for the other gang to complete.

Shaft lengths were where a shaft used for construction was not required for ventilation and could be bricked across and sealed. The ground could then be backlled over the tunnel and the shaft backlled to the surface. These shafts have sometimes been found to have been left open with only a cover at the surface.

Tunnel lengths were always built with dog toothing at the end within the brick thickness to form a shear key between lengths.

This toothing was not always completely successful in forming a good joint and these joints are still visible today. Sometimes they are associated with attened crowns in the tunnels and sometimes with water ingress. They are very rarely tight and can almost always be seen at their set intervals along the tunnel. These are the only visible aspect of construction in Victorian tunnels today, apart from shafts.

As with ngerprints or DNA, these joints are different in each tunnel and can be mapped, revealing much about its history and construction.

Tunnel joint mapping can identify bad ground lengths, bricked up shafts, junction and even breakup lengths. Joint mapping takes up to an hour to complete over a 100m stretch and is therefore very time consuming on a long tunnel.

However, bulging or spalling in a tunnel can be joint mapped for a length of perhaps 50m either side of the problem within an hour and the mapping referenced against standard joint mapping profiles that might indicate bad ground, a buried shaft or, potentially, a junction length with failed supports.

Ground probing radar (GPR) investigations of tunnels has received a very bad reception from rail companies and maintenance contractors for its lack of definition in its results and for being inconclusive.

A number of common factors found in tunnels, such as too much water, soot and timber, have often been cited as reasons for poor results.

The results of the combination of joint mapping with a GPR survey is presented here and it is clear that there is an immediate division of the tunnel into discrete sections that have been identified by the GPR as separate sections of tunnel.

There are tunnel sections which are clear of faults and these possibly indicate break-up lengths, which as was stated above, were built very strongly, thicker than normal and wedged tight against the tunnel wall.

Other sections of tunnel between joints show signs of joint separation and possibly water ingress. Poor bricks or bad mortar may also be the cause of the sections revealed by the GPR survey.

Repairs can now be isolated to those lengths of tunnel bounded by the joints and not arbitrary fi gures of 50m or 100m of tunnel.

This should provide considerable savings in the general repairs to tunnel linings.

Joint mapping is relatively simple to undertake in a tunnel with the normal precautions for safety and protection. Interpretation can be difficult but is generally undertaken away from the tunnel environment where direct comparisons can be made to other investigations before a picture can be drawn of the possible tunnel construction in the area of the problem.

It is also possible to review older GPR surveys and to trace the joints on to the original survey in the particular area of concern to identify previous conditions of the tunnel.

Joint mapping might not have all the answers, but with the right interpretation can go along way to explaining how tunnels were built.

Jack Knight is associate director, tunnelling, at Scott Wilson

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