An uplifting experience – new bridge helps Chiltern achieve its Evergreen aspirations

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Civic Way. Now that’s an impressive name for a road. Somewhere for good citizens to perambulate, to take part in a parade or embark on some other worthy venture. Well, perhaps a parade at South Ruislip would be slightly more on the prosaic side. It’s the road to the tip – or, in modern-speak, the Civic Amenity Site and Waste Transfer Station. It’s the inevitable sort of use for land that’s rail-locked on all sides. Rubbish trundles down Civic Way, it’s loaded into rail wagons and taken away to…another tip. Sorry, landfill site.

Getting your fridge to the dump has to involve crossing a railway. At South Ruislip, this takes you under Bridge No.1 on a short chord between Northolt Junction and Northolt Park Station on the Marylebone to Birmingham main line. Originally, this was built for a single-track and Bridge No.1 – although fairly modern – was a single-track, three-span structure.

“Was”…because it is no longer. It is now a double-track, single-span bridge – a structure lifted into position on the weekend of 29/30th January by BAM Nuttall under the civils project management of Giles Brookes. Giles looks after the whole of the civils works for Project ‘Evergreen 3’ – the £250 million upgrade of the Chiltern main line.

Linespeed improvements

As Giles explains, “At the moment, trains travelling from London to Banbury and beyond have to negotiate the tight radius of a curve that passes beneath the line from Greenford, with a permanent speed restriction of 50mph. This has a significant impact on running times and timetabling. So this scheme is all about linespeed improvements.”

Up trains, via the previously single-line chord do not have such a restriction. The Northolt scheme, designed by Atkins, involves the widening of the chord line’s embankment to accommodate two tracks on an optimum alignment. BAM Nuttall is busy at the moment constructing a variety of widening structures in addition to plain fill placed alongside. These range from simple gabion baskets to more complex king post retaining structures, a contiguous bored pile retaining wall and soil nailing.

In all, the widening on both sides involves about 1100m of embankment. Work on it started in October last year and will continue through to the end of this month.

But, smack in the middle of the chord is Bridge No.1 which had a single-track superstructure and single-track bank seats and piers. With the widened embankment designed to optimise linespeed, it was inevitable that the bridge deck would be in the wrong place. The option of just placing an additional span to one side or the other would not achieve anything useful. So the design used the space between the bank seats and the piers for the construction of new, wider abutments, thus reducing the structure to a single span – albeit longer that the existing road span.

The new reinforced concrete wing-walls and abutments are founded on augered piles that were installed by Van Elle using low-headroom mini piling rigs. Most of this work was carried out without possessions. Once completed, the new substructure was backfilled to the soffit level in preparation for the main erection possession.

Pre-stressed concrete demolition

As the possession for the removal of the existing structure approached, there was a need to consider carefully how the bridge deck was to be lifted out. Probably built in the 1970s, it was a pre-stressed concrete deck, post-tensioned transversely. Single pre-stressed beams can be lifted out fairly easily so long as they’re handled carefully. There are considerable forces retained in the pre-stressing cables mainly in the lower parts of the beams. If they are lifted at the ends then the forces will be restrained and the beam will remain stable. If however the beam is lifted away from the ends – or if it ‘falls over’ – then it can destroy itself violently. The beams at Bridge No.1 were held together with post-tensioned cables and this gave an added level of complication. Designer Tony Gee and Partners was called in to analyse the structure and design special rigs to ensure stability throughout the demolition process.

The deck slabs were lifted out intact, span by span. A 1,000-tonne Ainscough crane was assembled across Civic Way immediately prior to the possession. The parade of old fridges had to stop. With the track and ballast out of the way and with cabling diverted, special lifting beams were threaded under the deck. These were fabricated with a hogged profile. As the lift took place, the portion of lifting beam at the centre of the deck contacted the underside of the slab first. As load was taken by the crane, the beam deflected slightly so that the slab was progressively supported uniformly along the length of the beam. Thus the slab was not subject to transverse bending and the post-tensioning was not disturbed. Once on the ground, the post-tensioning cables were located and severed through the existing joints between the beams. The whole process therefore contained any potential risk from unstable tensioning forces.

New steel bridge

The outer spans were removed first so that the backfilling above the abutments could be completed, with the existing bank seats remaining inside the new embankment. The replacement superstructure is a steel bridge fabricated by Briton Fabricators of Sutton-in-Ashfield, Nottinghamshire.

“The track was ready for Monday morning with a temporary speed restriction”, concluded Giles. “It was welded and stressed the following weekend, re-tamped and brought back to full linespeed. It has been reinstated in its original position, in the centre of the new bridge deck. In future, when the embankments are finished, the track will be slewed to the north, so giving room to install the new Down Main adjacent.”All this is a great achievement and comfort for Giles, his civils and track team and the Evergreen 3 project. It must also be a comfort to all those with scrap fridges weighing down the backs of their cars. They may now continue their parades down Civic Way – which really is a much grander name than Tip Road.

Article courtesy of the rail engineer magazine.

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