RAILWAY STRUCTURES MAINTENANCE

No.1 In Series of Articles - written by Dave Haynes

First published in FMRC Turntable Newsletter No 128 May/June 1995

also in NWMRCA The Signalmen newsletter Vol 2 issue 9 Spring 1999

I've kept promising myself that I'd write something for the Turntable ever since Mike Garforth (Editor at that time) started begging for material, sorry Mike, guess I missed your deadline! What really tripped me into writing was Kev's (Pearson) March 1995 Railway Modeller article (first time I've bought the Modeller in 12 years!) especially his reference to a layout which was 'a total disaster.., curves were too tight.., too much complicated trackwork, poor electrics' not to mention the baseboards; "Guilty yer 'onor, but my field is bridge maintenance, I'm neither a p.way man, sparky or chippy".

Many enthusiasts only see the railway from the outside, apart from illegal sorties behind the lines bad pun aside, I'm sure some of you now respected and revered members will remember our various spotting outings such as Gateshead where some BR bloke chased us and we hid behind 6 foot high stacks of track panels!). It is with this in mind that I've wr itten this article to give a view from behind the scenes and hopefully thoughts on modelling ideas, after all, how often do models include evidence of 'model maintenance', certainly Bolden has a few.

I worked initially at the B.R. Area Civil Engineer s office at Crewe which was responsible for the West Coast Main Line from Nuneaton to Weaver Junction, Staffordshire and North/Mid Wales. I am now based at the Preston office which basically looks after the WCML from Weaver Junction to Dumfries and Quintinshill, site of the notorious Great War disaster. Although overall responsibility of bridges, structures, buildings and p.way now rests with Railtrack, B.R. civil engineers under the guise of British RaiI infrastructure Services still carry out the maintenance of these items, but with the 90's version of Beeching, the 'P' word, looming, this may change.

My work includes seeing maintenance schemes from initial surveys through scheme drawings and eventually to completion on site. The case I will describe is something that will become more of a problem in the future, that of Alkali Silica Reaction or ASR, which is basically where the components of concrete, i.e. aggregate and cement, react against each other causing expansion and ultimately self destruction; watch out for exploding bridges! After the mid 70's bridges are supposedly not affected due to better screening of materials. Time will tell!

A BRIDGE RECONSTRUCTION

Bridge number 91 on the Preston to Lancaster Line carries the 2 track overhead electrified WCML over the Lancaster Canal and adjacent road, approximately 1 mile south of Lancaster station. The road span was reconstructed in 1949 and the canal span in 1959. As with all B.R. operational bridges it is examined superficially annually and in detail every 6 years. Successive exams revealed worsening map cracking characteristic to ASR and following a visit by a concrete scientist from Derby Research, this was conf irmed. With the superstructure threatening to dismember itself, there was no option but to reconstruct. The options for a new deck were either steel, expensive but relatively light, or concrete, cheap in the short term but heavier and free (?) of any nasties. In the event concrete was chosen, due to the lower cost. Simple concrete decks tend to be deeper than the equivalent steel decks and thus the canal would have had to be lowered or the track lifted, both impractical, therefore to keep the same depth of deck, a transversely post-tensioned design was adopted. Put simply, this involves 15 no. 40mm x 7m bars passing through the bridge perpendicular to the span of 9m. These bars are then stressed, in this case to 70 tonnes and this causes the 6 deck beams to act as one slab, giving a similar strength to a deeper simple deck.

PREPARATORY WORKS

The B.R. bridge design office at York was engaged to produce a design and this started late in 1993. I became involved in this scheme in March 1994 and without going into to much detail, some of the processes involved are now described.

• Train operators needed to agree when we could close the line for 54 hours. This lead to their planning alternative timetables and the train diversions over the Long Drag so beloved by gricers. Some freight trains were diverted along the East Coast Main Line (a good excuse for running 'foreigner' trains during layout operation).
• As the bridge is in a conservation area, planning approval was required; the concrete had to be colour matched to existing stone.
• A survey for crane lifting was required, taking into account the various services such as gas and watern and a crane contract had to be let. The nature of the largest lift, a 32 tonne parapet beam at 27m radius, meant the use of a large road mobile crane, a 500 tonne Demag.
• The police, the canal and the road authorities had to be approached about the closures. The canal was closed for 1 week either side of the main weekend and the 2 day road closure meant some poor folk had a 4 mile detour.
• A contract for scaffolding was required and besides for access to the works, it also provided for an S&T cable route over the bridge and this was temporarily diverted onto the adjacent temporary cable bridge.
• A contract for some 40 diversion signs was required. Local residents were advised of the work as were local newspapers, television and radio.
• Temporary access steps were provided between road level and track level for staff. A village of temporary accommodation for staff, small tools and materials was up at the south end of the former Up side sidings.
• Military trestling was constructed on the canal towpath as a backup demolition system, described later.

RECONSTRUCTION WEEKEND - 12th/13th/14th November 1994

Before anything else the crane arrived to begin its lengthy rigging procedure, plate 1. Once possession of the line and isolation of the overhead wires had been obtained, the contact catenary wires were slewed over to the return conductors thus giving access for lifting. The track was then removed in panels on both lines, being stacked just off the bridge. This allowed excavators on the bridge to remove the ballast exposing the old bridge.

As you will see from the cross section, Figure 1, the existing bridge consisted of 8 no. 1m x 1m deck beams and 2 no. narrower parapet beams, all seated directly onto bedstones. Removal of the parapets was simply by coring through the beams and passing the lifting chains through, shown on Plate 2. Removal of the deck beams involved separating them with excavator mounted pneumatic chisels and thus passing chains around, Plate 3. This worked very well but in the event of this method not splitting the beams, horizontal and vertical jacks were located on the trestle beneath the bridge in order to 'lift and walk' these 2l tonne pieces of sick concrete apart enough to get the chains round. A deal was struck with the local council that if both of us covered our own costs, they could have the old beams as anti-caravan barriers at a development site in Morecambe next to Morrisons supermarket, shown in Plates 4 and 5. The new bridge design replaced the individual bedstones with two 11m long cill beams (imposts), one carrying each end of the deck. Therefore the existing bedstones had to be broken up and removed, thus allowing a dry concrete bed to be prepared in order to receive the new cill beams.

NOTE, Continuous Welded Rail would vary significantly in length due to thermal expansion and therefore to reduce this movement to something that expansion joints ('breathers') can handle, the rail is prestressed by an amount dependent upon the rail's temperature, thus at above average temperature, the rail is in compression whilst below it is in tension; cut a very cold rail and you hear a bang as the rail snaps back. This reconstruction may seem straightforward but I've not mentioned the various problems encountered in order to keep it simple!

Figure 3 - West Elevation of New Bridge

Plate 4 & 5 - Beams in use as parking control, Morecambe 6th August 1996

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