A variety of innovative methods have been used to build a major structure on Crossrail’s surface running section west of London, as Jon Masters discovers.
Earlier this summer a 100t Hiab truck-mounted crane was on hire to Carillion Civil Engineering working on Crossrail surface work at Stockley in west London. The machine was the only Hiab of its size in the UK last year when the contractor was working out how to lift and place some 6t precast concrete robust kerb units into place. As such, it is a good illustration of the difficulty of the contractor’s task here.
The kerb units completed a 300m long “up airport relief ramp”, which is one of six big structures on the Stockley main works contract of Crossrail. Carillion’s site is about 1.4km long and, for the most part, is less than 20m wide - as narrow as 14m at one point - with live railway lines on one side and the boundary of a row of business units on the other.
“Network Rail asked us what we would do if the Hiab broke down. The only answer I could give was: ‘If that happens then we’ll get it fixed’,” says Carillion project agent Ron Jolley. “The Hiab was the only thing narrow enough to sit on the up airport relief ramp with the capacity to lift the 6t units at an 11m radius. There was just no other way of doing it.”
The £36.4M Stockley main works is one of a number of contracts Network Rail and its suppliers are carrying out within the West Inner section of Crossrail’s surface work.
At Stockley, the Heathrow branch of the new railway joins the Great Western Main Line (GWML) from the south. Currently, up and down airport lines cross over the main lines of the GWML to connect with the down relief line and up main line via a reinforced concrete box bridge and a reinforced earth ramp built for the Heathrow Express in the 1990s.
“Network Rail asked us what we would do if the Hiab broke down. The only answer I could give was: ‘If that happens then we’ll get it fixed’. “
Ron Jolley, Carillion
The Crossrail works will give the junction more capacity. From the south, one more line will branch off from the Heathrow spur on a new embankment. This line will then cross over both the GWML main lines and relief lines on the new Stockley West Flyover - a 1,400t, two span steel warren truss bridge curved on a 300m radius, which was launched into place at the end of April (see box).
The new line will then run parallel to, and immediately to the north of, the existing relief lines on the North Viaduct - a 500m long structure made up of 19 spans of precast concrete beams.
Eventually, a set of points towards the eastern end of this viaduct will split the line as it crosses onto the next structure, the Stockley East Flyover: one line will continue onto the up airport relief ramp; the other will cross back over the GWML relief lines on the East Flyover, to connect with the down main line via a new up airport ramp.
For now, the East Flyover has been partially completed in the form of a 150m long reinforced concrete trough, formed of two 1m thick, 7m and 6m high cast in situ walls, set 6m apart on a 1m thick base slab.
Carillion Civil Engineering has begun a gradual handover of finished structures to sister company Carillion Rail, the holder of the Crossrail West Inner Track Infrastructure contract.
Carillion Rail has now started working towards a target of getting the up airport relief line operational in the New Year. The junction will then have additional capacity for when switches and crossings (S&C) and track alterations affect existing lines.
The track will be laid on lightweight fill placed within the concrete trough and the adjoining up airport relief ramp, while Carillion Civil Engineering starts the next stage of its works - construction of a single southern wall of the East Flyover between the GWML relief and main lines.
Once this is done, 133 precast concrete beams will be lifted in to place to span the 16m between the trough and the southern wall, prior to an in situ deck being cast on top to complete the East Flyover during 2015.
“We realised we could solve a number of problems with a double length system”
Ian Fryer,RMD Kwikform
In situ wall construction is a big part of this project. Both walls of the East Flyover trough (and the southern wall to come) have been built with a travelling gantry system designed and supplied by RMD Kwikform.
RMD has a lot of falsework equipment and materials on site. The company has supplied equipment for the in situ piers and diaphragm beams of the North Viaduct, plus it has a kit of parts at Stockley sufficient to build three travelling gantries.
Two assembled gantries were used to form the walls of the East Flyover trough, working in echelon at a rate of about one concrete pour every six days.
The system was developed for Carillion’s tender submission to solve problems of wall construction within 2m of live railway lines, as RMD’s Stockley gantries allow all steel and formwork fixing to be carried out from one side.
Each assembly includes - on its railway side - a full site hoarding and a shutter that can be moved on suspended rails from working platforms within the gantry. Each travelling gantry is 18m long - double the length of each pour - and weighs about 30t. They are moved on rails with a system of two hydraulic turfers.
“We realised we could solve a number of problems with a double length system,” explains RMD engineering director Ian Fryer.
“Formwork can be struck, moved for cleaning and fixed for the next pour from within the enclosed gantry.
Crossrail’s new Stockley West Flyover is the result of a multi-national effort. The steel of the 1,400t structure was supplied from Italy, prefabricated in Romania and then shipped to the UK in welded sections for bolting together on site by the Portuguese steelwork contractor Martifer.
The bridge is curved on a 300m radius and has two warren truss spans of 77m and 40m. The trusses have now been disconnected but were welded together with an additional 30m steel nose and 15m tail for launching the structure earlier this year.
Martifer carried out the launch with Swiss construction engineering specialist VSL. Two strand jacks mounted on the launch tail pulled the bridge into place by pulling on cables fixed to the bridge’s pier and abutments.
The first stage of the launch - 40m onto the central pier between the Great Western main lines and relief lines - was carried out over a single weekend in April, with all of the lines remaining open, as the majority of the 162m structure stayed on the launch rails south of the bridge’s southern abutment. The second stage made a much bigger overhang necessary, so was done during one weekend and one night possession of the relief lines during May.
“The bridge was designed to be launched. It made a lot of sense to build it to the side of the railway and then launch it into place,” says Carillion Civil Engineering project agent Ron Jolley.
Each truss is 7.6m wide and 8m tall, constructed of box sections of plate steel. Bottom chords are typically 1.2m by 600mm, with diagonal and top chord members of 600mm by 700mm in section.
Prior to the launch, construction of the flyover’s central pier presented Carillion with a problem, once it was discovered that there was insufficient room to place a crane for lifting the proposed sections of precast concrete crosshead into place.
“We had only about 150mm of space between the edges of the crosshead and the dynamic envelope of the railway at this point, so conventional in situ construction of the crosshead was impossible, and it became apparent that lifting in precast sections was not going to be possible either,” says Jolley.
“So [designer] Jacobs redesigned it. What they came up with they termed the ‘bathtub’, a 100mm thick precast concrete permanent formwork for the crosshead. This was light enough to be lifted into place with a smaller crane, and allowed us to build the rest of the crosshead inside of it.”
The system provides very good access and a lot of space for landing bundles of rebar. It’s very robust and saves a huge amount of time.”
The next - and last - stage of in situ wall construction at Stockley starts this month with installation of sheet piling in the 10m wide gap between the GWML main lines and relief lines. Carillion will then excavate and build the base slab for the southern wall of the East Viaduct.
A third travelling gantry, enclosed on both sides and 22m long, will be assembled for building the southern wall next year, concurrently with construction of another ramp of reinforced earth walls infilled with lightweight fill.
Carillion is aiming for a September 2015 finish.
“There is then some float in the programme before the Western Inner Track Infrastructure contract is due to lay tracks over the East Flyover in April 2016,” says Jolley.
For now, though, Carillion still has to finalise how it will get materials in and out of the southern wall work site.
It has the option of using temporary footbridges for access, but only nightly possessions of either main or relief lines are available for concrete pours and moving materials across the railway.
“We’re currently trying to find if there’s a way of doing the pours during the day, as night work brings issues of supply,” says Jolley. “We could do [something] similar to how the West Flyover pier was built, where we drove a narrow tunnel for pumping concrete under the railway and up into the pier site, but this carries greater risks for multiple larger pours, as pump lines can be vulnerable to failure.”
“It’s not like we don’t have a solution. We have options, but we’re trying to find better ones.”