For years, Britain’s railways have lived with the restrictive loading gauge that was established 150 years ago by Victorian engineers. Bridges and tunnels which were perfectly adequate for small locomotive.
Today’s trains are built to run under those restrictions. They go through bridge arches and tunnel portals, often with only inches to spare.
However, not only is railway usage increasing these days, but the railway itself is getting bigger. As the electrification of more routes gets underway, clearance is needed for overhead wires and pantographs. In addition, freight containers are getting bigger, and these too demand larger clearances.
All this has resulted in a range of measures to make the envelope surrounding the railway bigger. Tunnel floors have been dropped. Special solid-busbar electrification systems have been developed which take up less room than conventional wiring. And some old and soon-to-be time expired bridges have been demolished and replaced by new, slimmer structures.
However, there are some perfectly sound bridges which are simply in the wrong place. Even with track lowering, which is sometimes not possible due to drainage problems or the fact that the bridge is over a station and track lowering would cause problems with platform height, the bridge is still too low.
Fortunately, technology exists to rectify this. For some time, many bridges have been raised by a few millimetres using hydraulic jacks to facilitate replacement of padstones or bearings. And if that is possible, it can be raised by a larger amount to give extra clearance beneath it. Such rehabilitation is significantly more sustainable than demolishing and rebuilding, not to mention the economic benefits.
Jacking up Butterley
A good example is Butterley Station overbridge, which has a 15-metre long deck with eight riveted steel girders spanning onto concrete abutments. Kevin Bennett, sales and technical director of specialist contractor Freyssinet, commented that the existing bearings were badly corroded and needed to be replaced with new ones made in the company’s own Telford factory.
“Fortunately, the bridge abutment was deep enough that we could get our 260-ton pot ram jacks in with only local modification of the abutment,” he explained. “We only lifted the bridge by 2mm, controlled by dial displacement gauges, with no more than 0.5mm difference between adjacent beams. Once the load has been relieved from the original bearings, the concrete around them can be removed by one of our Aquaforce specialist hydro-demolition teams. Fast-setting high strength grout and concrete is used to bed in the new bearings.”
Further work carried out at Butterley included the removal, by hand breaker, of the concrete casement to the main steel girders where this has cracked. The steel girder was cleaned back to metal and epoxy corrosion protection applied before finishing off with new hand-applied repair mortar. The work beneath the bridge required Freyssinet to remove the existing smoke deflection barriers and these were later replaced with new stainless steel baffles. Above the deck, brickwork repair to the parapets and pilasters, resurfacing works to carriageways and footways and replacement of the bridge deck joint completed the job.
In this instance, the requirement was only to raise the bridge by a small distance while these bearing replacements were carried out. However, to raise it by several inches, or even feet, would have been perfectly possible using the same techniques.
400mm at Valenton
Valenton rail bridge allows the Great Belt Line, which diverts TGVs and freight around Paris, to cross the tracks of Villeneuve yard, in the Val de Marne south-east of Paris. To allow track improvements in the area, and to permit the passage of a new type of larger rail carriage, the Valenton bridge had to be lifted by 400mm.
Preparatory works involved the construction of support corbels to carry the lifting jacks. Recesses were cut into the face of the abutment and new cantilevers cast. These were tied back to the abutment wall with horizontal, high tensile steel Freyssibars. Then, immediately before the lift, the jacks (blue cylinders in the photographs) and locking collars (silver cylinders) were installed, the hydraulics connected and the system tested.
The lifting operation took place over 30 hours during the Easter weekend. It began at midnight on Sunday with Freyssinet removing the track and ballast and terracing back the ballast at the rear of the abutments. Then the jacks were activated and the deck was raised by 100mm using computer-assisted lifting to ensure equal distribution of lift. 100mm shims were inserted below the locking collars which were then manually wound down to take the load. The jacks were retracted and a 100mm shim inserted beneath each of these. The operation was then repeated four more times to achieve a total lift of 500mm.
The next operation was to install precast concrete ballast walls of six tonnes apiece to prevent the fall of ballast into the space between the embankment and the deck. These were installed using two 100-tonne mobile cranes. Elastomeric bearing pads were installed and the deck lowered by 100mm to its final alignment. Replacing the ballast and track completed the 30 hour possession and trains were back running on Monday at 12:00 following the reinstatement of the catenary.
As well as its work with bridges, Freyssinet has developed a range of capabilities for tunnels. One of these also involves hydraulic jacking, in this case compensation jacking to counter the ground movement effects of tunnelling or sinking a shaft.
The latter is illustrated by a recent contract to preload the props of Farringdon Station shaft using 80 Freyssinet flat jacks. These steel capsules are inflated with oil for temporary use or grout for a permanent inflation and can achieve thrusts of up to 1000 tonnes for the larger sizes.
So, with expertise and a few jacks, it is quite feasible to stretch our Victorian loading gauge just enough for the next generation of railway vehicles.