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STRONG POINT

SLOPE ENGINEERING

Piling close to a railway in Loughborough allowed bridge strengthening to go ahead without disruptive long-term track possessions. Claire Symes reports.

 

Road bridges all over the UK have been undergoing strengthening work in recent years to allow them to carry the 40t lorries that are now permitted on the road network.

 

Many of these bridges carry roads over busy railway lines and strengthening work often requires lengthy track possessions.

 

Upgrading of a bridge over the Midland Main Line in Loughborough, however, was finished with minimal rail disruption thanks to some complex temporary works.

 

Foundation contractor Roger Bullivant installed piles to support two access platforms from which main contractor Galliford Try completed the strengthening scheme.

 

Bullivant also installed a set of piles to support new bridge approach safety barriers to prevent vehicles crashing on to the tracks.

 

Overbridge 78 (see box) is within sight of Loughborough Station and carries Meadow Lane over five railway lines - one siding, two ‘fast’ passenger lines and two ‘slow’ freight tracks. Meadow Lane and the railway are both at grade with the overbridge road approaches on two steep earth fill embankments, presenting a higher than usual risk for road/rail accidents.

 

‘In the case of bridges owned by Network Rail the work is undertaken in partnership with highway authorities, ’ says Network Rail engineer Andy Milne. ‘Improved bridge protection has been incorporated into this project and we are jointly funding the £1.5M scheme with Leicestershire County Council.’

 

Galliford Try’s work involved installing a new bridge deck and building cast insitu parapets and new safety barriers. ‘The reconstruction work itself was fairly straightforward but the temporary works for the scheme were more onerous, ’ Milne says.

 

‘But it meant only one major track possession was needed for the project - 80 hours over the Easter weekend - and that was for positioning part of the new deck.’

 

Temporary works for the upgrade involved installing access platforms on either side of the bridge, which doubled as a pedestrian link during the work. Before the platforms were built, Bullivant installed 14 bottom driven cast insitu piles, founded in the underlying mudstone, to support the structure.

 

‘The 220mm diameter piles varied in length between 5m and 9m and were cased through the made ground and terrace gravels to found in the mudstone, ’ says Bullivant area manager Keith Nation.

 

‘High strength tubular steel as well as traditional rebar was used for reinforcement to cater for the lateral forces placed on the temporary access platforms from wind loading.’

 

Bullivant had already installed a series of 450mm diameter hollow stem grout injected piles to depths between 6.5 and 11m below the embankments to support the new safety barrier.

 

Originally, the firm planned to install the 66 piles using a temporarily cased auger bored technique but switched to the grouted method when groundwater was encountered beneath the gravels.

 

Piles installed closest to the bridge abutments are the shortest but are heavily reinforced. The capping beams have also been tied together below the carriageway.

 

‘The safety barrier piles will hopefully never have any force put on them, but if the barrier was hit by a 40t lorry the overturning forces they would have to restrain would be huge, ’ Nation says.

 

‘We opted to use shorter tied piles close to the bridge to maximise the barrier’s load capacity and avoid the need for track possessions to carry out the piling.’

 

With Bullivant’s work complete and the access platforms in place, Galliford Try began permanent works in earnest.

 

‘Before piling started we stripped the carriageway off to expose the bridge structure, ’ says Galliford Try agent Emil Delport.

 

Fill was excavated to expose the brick arches and replaced with mesh reinforcement and lightweight concrete fill to form a ‘saddle slab’ The steel deck over the southern span was removed and replaced with new girders and precast concrete beams during a weekend possession.

 

‘During the 80-hour Easter possession we craned in the new precast concrete deck beams which span the slow lines and sidings, ’ Delport says.

 

Galliford then cast the new deck slab and parapets, installed street lighting and built the open box beam safety barriers before resurfacing Meadow Lane.

 

Overbridge 78, a 36m long, threespan bridge, is not a straightforward structure. It was built in three stages during the 19th century over a period of 50 years.

 

The first brick arch section, which now forms the central span, was built in the 1840s over what are now the fast lines.

 

The second section, over the slow lines, was added 30 years later to the south of the main span. It was formed using steel girders supported on a brick abutment.

 

During the 1890s the final part of the bridge was constructed in brick to span some sidings installed to serve a nearby industrial development.

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