Issue 100 of The Rail Engineer reported on the enabling works for Crossrail that were carried out at Christmas near Slough on the Great Western main line. On that occasion, principal contractor HOCHTIEF removed three brick arch bridges (two road bridges and one footbridge) and installed a new steel composite footbridge. At Easter the scores were reversed, with three bridges installed and one removed during a 104 hour, all lines rail possession.
These bridge works were carried out by Network Rail, on behalf of the Crossrail project, to enable installation of overhead line electrification on the Crossrail West Surface Works section between Paddington and Maidenhead. The four bridges had inadequate clearances for the new overhead lines.
HOCHTIEF and designers Hyder developed a repeated footbridge design which was used at three of the locations. One of these, at Horton, was the one installed at Christmas. This gave the team the chance to optimise the construction of the remaining two similar bridges, at Old Stockley Road and Trenches, which were installed in the Easter possession.
The footbridges cross all four running lines in single spans of 33 metres, which allows for future flexibility in the track layout. A clearance of 5.6 metres from rail level is provided, placing no restrictions on the overhead lines.
The decks are of steel-concrete composite construction. Weathering steel was used for the main girders to minimise future maintenance requirements. The twin girders and bracing were fabricated and assembled by Mabey Bridge in Chepstow and transported to site in single 33 metre lengths, one day ahead of the installation date. The beams were lifted as complete 29 tonne units by 500 tonne cranes at each location and landed on support trestles.
Once the beams were installed, eleven five-tonne precast deck units were landed using the same mobile cranes. The precast deck units were fitted with the parapet framework ahead of installation, providing immediate edge protection for operatives working on the bridge deck and also reducing both the work carried out at height and the total amount to complete within the rail possession.
There was sufficient time at Trenches to place the in-situ concrete element of the deck slab via a 50 metre mobile concrete pump. To do this over the Easter weekend, special arrangement had to be made for Hanson Concrete to open their Acton batching plant for a few hours on Easter Saturday.
Big lift at Middlegreen Road
The fourth bridge, at Middlegreen Road, is the only one to carry vehicles. Precast concrete portal units were installed on the existing piers and abutments to retain a three-span bridge. It was not viable to raise the approaches to the bridge to achieve the full 5.6 metre clearance, hence only 5.2 metres is provided at this bridge.
This part of the programme called for the largest crane of all.
A 1000 tonne mobile crane was used to lift the giant portal and parapet units into position. The Liebherr LTM 11000DS was supplied by Ainscough Crane Hires Heavy Lifting Division and is one of the largest mobile cranes available in the UK. Taking all three sites into consideration, there was a serious concentration of lifting power within a five-mile radius.
The Middlegreen Road bridge was the most complex and posed several logistical challenges. All lifting was carried out from Bloom Park, to the north-west of the site. Within the site compound, HOCHTIEF had to accommodate the 1000 tonne mobile crane complete with 56 metre long lattice boom, four precast cill beams (for the existing abutments and piers), ten precast portal units weighing up to 31 tonnes each, and eight precast parapet units. The precast concrete units were fabricated by Banagher Precast Concrete in Co. Offaly, Ireland, and delivered to site and unloaded throughout the week leading up to Easter.
The various elements had to be accurately installed to ensure fit and were connected together using temporary supports. As the items were being delivered from Ireland the week before installation, HOCHTIEF and Network Rail carried out a joint visit in early March to Banagher’s fabrication yard where they reviewed progress and quality and carried out key dimensional checks.
The programme was scheduled to take a total of 60 hours, with the lifting operation being completed in 36 hours and the steel fixing, shuttering and concreting of the in-situ deck pours completed in another 24 hours.
Down comes Horton
The demolition at Horton followed the completion of the access ramps to the new footbridge installed at Christmas, allowing the footpath to be switched to the new bridge a few weeks after the demolition of the old one. These works were carried out by Gilpin Demolition which was also responsible for the successful demolition of the three bridges at Christmas 2012. Horton bridge was demolished in a similar fashion with track protection consisting of Cordek polystyrene blocks covered by 150mm thick deep timber navvy mats used to form the crash deck beneath the bridge and the access track from the site compound.
The bridge was demolished from track level using two 35 tonne and two 21 tonne excavators fitted with grab and breaker attachments. The machines were located on both sites of the bridge with all three spans worked on simultaneously. The brick arisings dropped onto the timber crash deck where they were loaded into dumpers and stored in the site compound. The limited space in the compound meant it was a snug fit to store all of the demolition plant, track protection materials and spoil. The week after Easter was spent removing all the arisings, plant and equipment and, within a week, it didn’t look as though anyone had been there.
The demolition was a critical item in the Crossrail Outer Integrated programme. As the track protection blocked all lines, no trains could pass through the works whilst the demolition was underway. The works were therefore sandwiched between engineering train movements serving other projects within the scheme. Work started at 20:00 on Friday 29 March, following the movement of an engineering train through the site. It had to be completed, with a minimum of 50% of the track protection removed, 34 hours later, at 07:00 on Sunday 31 March, to allow the passage of a tamper and Kirow crane heading west. Everything went to plan, and HOCHTIEF were pleased to be able to hand back all lines ahead of programme after 33 hours.
There is still work to be done on Trenches, Middlegreen Road and Old Stockley, with construction of the access ramps, approaches, fencing, landscaping and street lighting to be completed prior to them reopening in Summer 2013.