Clear the Decks

Hurray for search engines, especially the one that has street cameras! If you type in 'Caversham Road' you'll be presented with a limited range of options. Just two places seem to have a road of this name, London and Reading, and it's the latter we're looking at today. So what is revealed? Caversham Road is a fairly typical urban thoroughfare. Being a radial route from the centre of Reading it is also extremely busy. On its way out of town it encounters the railway which passes overhead with very little room to spare.

All changed

Back to the search engine with its grey images of a grey day in the grey end of town. The details of the railway structure are clear - but gloriously out of date. On the north side there's a wide wrought iron plate girder bridge. That's gone. Working your way south is a pair of (fairly) modern box girder spans. They've gone too. There's an approach ramp up to the tracks on the west of the structure. That's no longer there and the abutments too have been extended on both sides. On the east of the road is a Post Office depot with a whole load of shiny red vehicles. They've disappeared. In fact the only feature that's recognisable after the New Year's major possession is the fire station.

So, if not the search engine, where - or rather who - is the source of all this knowledge?

Mark Hepburn is BAM Nuttall's Design Manager for Reading CivIls Package 2 project. He is part of the Network Rail project team delivering the Reading Station Area Redevelopment Scheme and should know about the demise of the Post Office depot because that's where the project team is based. Taking over this old complex has proved invaluable  not only is there convenient office accommodation, the yard has been the site for prefabrication of the 1,000-tonne bridge deck and till beams that have replaced those described above.

Temporary works

The story started much earlier than the New Year though. The old northern span was taken out in September 2010. It carried the goods lines and a redundant siding on long timbers. It was in poor condition and sat in the way of the new track layout. Inevitably it also played host to signalling and other cables and these had to be accommodated on a temporary bridge, a rather elegant suspension structure as it turned out.

The other two spans were in good condition but, being single track through-spans fixed to an existing track alignment, they could not be adapted to carry the more complicated trackwork of the new scheme. In addition, the overall footprint of the new layout required the abutments to be extended towards the north. All this civils work would have disrupted the operation of the fire station which is close to the northern boundary. During the possession, this was temporarily relocated in the old Post Office complex.

Minimum construction depth

As Collin Carr mentioned in December's issue of the rail engineer, the new structure is an orthotropic deck which typically comprises a structural steel deck plate stiffened either longitudinally or transversely, or both. The stiffening elements can serve several functions simultaneously. They enhance the bending resistance of the plate, allowing it to carry local wheel loads, and also add to the total cross-sectional area of steel in the main bending elements, increasing the overall bending capacity of the deck. The deck plate acts as a top flange as in a box or l-beam girder. The design for this bridge avoided the need for transverse stiffeners thereby improving the fatigue performance of the deck and reducing the fabrication costs.

Designed by Gifford, its Project Manager Will Duckett says that the challenge was to build a deck that had the absolute minimum of construction depth so as not to worsen road headroom at the same time as giving track an acceptable depth of ballast. The designers have also undertaken a dynamic finite element analysis to prove passenger comfort for future high speed trains crossing the bridge. The structure, built up in the Post Office yard by Watson Steel Structures Ltd from three-metre modules, is large. It is capable of carrying seven tracks... 'and a football match' observes Project Manager Ges Roberts.

Careful manoeuvres

The design was influenced by the availability of technology to install it-the structure was going to be transported by road. Renowned specialists in this field are ALE (formerly Abnormal Load Engineering) who provided the Self Propelled Modular Transporter (SPMT) units and backup. This kit too was built up in the yard.

The route of the new bridge to its final position had to be carefully examined. There wasn't the luxury of a straight run in. The prefab yard was round the corner and the route took the cavalcade over a selection of delicate services including a large water main. All these had to be bridged or strengthened with the route being plated out on the night.

BAM's opportunity to start work came at the New Year with a possession of all affected lines taken at 20:00 on the 30th December. Caversham Road was closed and a long traffic diversion set up. Of course, in a town as complex as Reading there were many wily motorists who knew a 'better' way round the diversion. This alternative flow of traffic did not help the cause and it was inevitable that there were a few grumbles about the whole operation.

Attention was focused first on the two modern spans which were released from their bearings, lifted and carried away by ALE on transporter units. This gave a clear site, allowing breakers to cut down the abutments to the required level. While this was going on, the massive new bridge deck was being carefully manoeuvred around the yard so as to line it up for the final run in. Once set up at the northern end of the bridge, the new deck was slotted between the abutments complete with cill units and simply lowered into place.

Making tracks

Once down at 07:00 on New Year's Day, the waterproofing was completed around the edges and the tracks reinstated by Network Rail staff. These are in temporary positions, being tied into the existing alignments and levels. Later on in the scheme, additional tracks will be laid, along with switch and crossing units on the deck itself - something that would have been impossible with the old structures.

Hurray again for search engines! This time any old engine will do. Just type in 'Caversham Road', 'YouTube' and 'Network Rail' and you’ll be able to see a hypnotic timelapse video of the whole operation condensed into about three minutes. It probably felt slightly longer to all those involved over the New Year.

Article courtesy of The Rail Engineer- february 2011

Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
peter.bishop@bamnuttall.co.uk