RAILWAY STRUCTURES MAINTENANCE

No.2 In Series of Articles - written by Dave Haynes

First published in FMRC Turntable Newsletter No 133 January/Febuary 1996

also in NWMRCA The Signalmen newsletter Vol 2 issue 10 Summer 1999

Bridge No 38, North Union Line

This bridge is located about 1/2 mile south of Wigan North Western station and was initially built in the middle of the last century to carry the West Coast main line over the River Douglas. The construction is of brick abutments and stone bedstones with a superstructure of wrought iron girders with dished steel plates spanning between, to carry the rail formation. ln connection with the expansion of Wigan's railway system in the 1890's, the LNWR widened the bridge by simply building a new one next to it, a century before the Department of Transport started playing around with motorway widening schemes. The construction was the same as the original bridge.

For various reasons, perhaps quality of workmanship, materials, ground conditions or increased train loadings, a bridge examination in early 1993 revealed various fractures in both the abutments and bedstones. The purpose of a bedstone is to spread the load from the beam to the masonry below, so as not to over stress any portion of the construction. Thus a fractured bedstone will quickly cause the underlying masonry to deteriorate and repairs required are greater. In Plate 1, a vertical fracture can clearly be seen running through the centre of the bedstone, as can the distortion of the underlying brickwork.

Plate 1 - Existing bedstone with vertical fracture

In addition to the normal 6 year bridge examination, detailed stress assessment of the main girders had revealed that a small joist half the size of the main girders between the old and 'new' decks, was severely overstressed. This joist had probably been inserted with the widening when no rails were directly overhead. Following 100 years of track reaIignments, this joist now carried the six foot rail of the 80mph Down main line!!

The first step in the remedial works was therefore to provide a scaffold platform which in itself was difficult, due to poor access, all scaffolding had to be fed in at the level of the deck, 5 metres above the river. No trouble for the experienced scaffolders who hung from the deck like spiderman. Next, the fractures in the abutments were pumped with a cement grout and substandard joist problem was partially cured by enlarging the bottom flange by bolting additional steel plates to it (the strengthening of this joist is still in the pending tray.

No a Grillage is NOT a way of cooking

The remaining work, the repair of the bedstones is where the difficulties arise. Consider the situation; when the bridge was built, a stone 1.5m long x 0.5m x 0.5m and weighing 0.5 tonne was placed on to a prepared brick wall. This was repeated for another 20 or so stones and then the steel deck, ballast and track placed on top. We now have a situation where the only access is from underneath and the deck must remain in place. The placing of a new bedstone is completely out of the question and so instead steel beams are built up in-situ and encased in concrete, this is a grillage.

Describing the work more fully and referring to the attached drawing, Figure 1. First, steel rails, are firmly concreted into the abutment beneath the damaged bedstone. After allowing the rail needles to reach full strength, the grillage installation can take place, during a Saturday night track possession. The girder over the damaged bedstone is jacked up using the needles and a 1.4 m long cross beam as a base for a 25 tonne hydraulic jack such as to take the load off the bedstone, a vertical distance of less than 10mm. Once the security of this arrangement is confirmed, the bed stone is broken up by the use of 200mm diameter core drills, followed by hand held pneumatic breakers; with the darkness and the headroom of 1.5m, there is some similarity to mining. Once the stone has been completely removed and the cavity adequately prepared, the grillage beams are individually placed and bolted together until finally the 3 layer grillage has been completed. The jack can then be released and the girder lowered onto the new steelwork. Once this stage has been reached, it is possible to return the line to traffic, after a nine hour possession and complete the work by encasing the grillage in concrete during the normal operation of trains.

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Copyright D Haynes
Last revised: July 20, 2000.

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