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Balfour plays Reading festival viaduct just right

Network Rail’s near-£1bn station upgrade hinges on a de-congesting flyover that has seen its contractor overcome flooded plants and shocking ground conditions.

Project: Reading Viaduct
Client: Network Rail
Contract value: £45m
Region: South-east
Main contractor: Balfour Beatty
Start date: June 2013
Completion date: February 2015

Network Rail has certainly put itself in the limelight over the past few years.

Huge planned infrastructure spending on projects around the country has seen some rail schemes help prop local construction sectors, providing vital work for subcontractors, specialists and suppliers during the downturn.

At the fore of that spending has been the redevelopment of the facilities, station and track around Reading station in Berkshire.

This one scheme alone has swallowed £895m of Network Rail’s cash as it bids to improve the train running times along the busy route from the west into London Paddington.

One vital project within the delivery of the whole Reading scheme has been undertaken by contractor Balfour Beatty.

Its £45m Reading Viaduct project is critical to the success of the overall scheme and pivotal in helping increase capacity along the congested line.

In essence, the project will see the fast main line services operated by First Great Western that run in and out of Reading station from Bristol and south Wales transferred onto a new precast concrete-decked viaduct.

These will fly over the ‘festival’ lines used by rail operator CrossCountry for services to Newbury, Basingstoke and other destinations in the South-west.

Freight transport will travel beneath the fast lines, avoiding clashes and helping increase both passenger and freight capacity through the area.

Vital upgrade work

“The Reading station area has become utterly inadequate for the amount of traffic it now needs to accommodate,” Network Rail senior programme manager Kevin Brown says.

“It gets upwards of 80,000 people a day travelling through the station and all the lines going into it are at the same grade.

“Any train failure means there is a huge knock-on effect on all services running through the area – both freight and passenger. This work will be of massive benefit.”

With contractor Balfour Beatty starting on site early in the summer of 2013, the schedule has focused the minds of the project managers across the delivery team (see box).

“As far as the construction work itself goes, we have carried out around 95 per cent of it in under a year”

William Smith, Balfour Beatty

Despite the complexity of the project, most of it has been carried out in less than a year, according to Balfour Beatty Major Projects project director William Smith.

“The railway needed to be moved for us to be able to build the viaduct,” he says.

“Really we have been going since Easter 2013, with the first four weeks of that taken up by piling and ground engineering work. As far as the construction work itself goes, we have carried out around 95 per cent of it in under a year.”

The Balfour Beatty team called on the expertise of its specialist ground engineering arm BBGE to carry out the installation of piles and vibro concrete columns beneath the viaduct piers and the track ramps up to the mainline and viaduct.

Miles of piles

Ground conditions in the area are difficult, with made ground and alluvial deposits from the nearby River Thames making up the first few metres before sand and gravels are reached at 5 m to 6 m below ground level.

At 10 m there is a thick layer of weathered chalk, but the BBGE specialists had to bore as deep as 24 m before hitting reliable, solid chalk strata.

In total, some 32 km of 900 mm and 1,050 mm-diameter CFA piles have been installed, typically at 2 m centres, with 24 of the 1,050 mm-diameter piles clustered beneath the viaduct piers.

“This is a heavy structure subject to heavy imposed loads which is being built on a site where the first 10 m of ground is poor”

William Smith, Balfour Beatty

Most were augured without the necessity of gaining a track possession, despite its close proximity.

“There were only 25 or so that we needed to install under a possession,” Mr Smith says.

“This is a heavy structure subject to heavy imposed loads which is being built on a site where the first 10 m of ground is poor. Those piles need to be big enough to carry all that.”

Beneath the east and west track ramps the BBGE team elected to install an array of 3,000 vibro concrete columns to help improve the ground’s load-bearing capacity.

These 300 mm-diameter concrete columns are set at 1.5 m centres and are formed using a vibroflot attached to a concrete pump, which forms the concrete column as the vibroflot penetrates the ground. A steel reinforcement cage can then be placed within the concrete column.

“It was BBGE’s preferred method around the ramps,” Mr Smith explains. “There are a few still to go in, but it has gone very smoothly.”

Festival viaduct construction

The viaduct itself is being constructed using a total of 325 precast concrete beams bearing across 25 spans along the main-line viaduct, with a further six across the festival line viaduct.

These 1,400 mm-deep beams weigh in at 50 tonnes each and have been manufactured in Ireland by specialist producer Shay Murtagh (Precast) before being shipped over to Liverpool, transported to site by road and lifted into position on top of the piers.

With the huge loading on the structure, the design on the viaduct calls for every other pier to be locked into the deck, effectively creating a single T-shaped structure with arms that span 25 m each way from the central pier.

“The lap-length between bars is very different to British Standards and getting the continuity of steel from the abutments into the deck has been difficult”

William Smith, Balfour Beatty

Not only does this mean the viaduct is very stiff, it also saves on the amount of bearings required and the amount of piling beneath the ‘free’ pier. It does make for formidable reinforcement detailing, though.

“The project has been completely designed under the new Eurocodes and it has been very challenging,” Mr Smith says.

“The lap-length between bars is very different to British Standards and getting the continuity of steel from the abutments into the deck has been difficult.”

Difficult or not, it’s been achieved with the main line now ready for track laying and the festival viaduct ready for final handover in the new year.

With just a few months to go before the whole Reading station area project is set for completion in the spring, it’s not long before rail passengers and freight operators start to enjoy the full benefits of Network Rail’s investment.

Flooded batching plant setback

With most of the construction work on the scheme concentrated into such a short space of time, there has been no room for slippage in the programme delivery schedule.

The appallingly wet weather of last winter arrived as work was really beginning to kick on and saw the River Thames burst its banks, flooding the concrete batching plant earmarked for the scheme.

This potential setback was dealt with in part thanks to the strong bond developed between all members of the project team.

It is a theme that continues throughout the scheme, with Mr Smith claiming the ‘can-do’ attitude of everyone involved has pushed the project through the difficult times.

“We haven’t allowed the programme to drift at all – even during the winter last year when we thought at times we would be better off building an ark,” he says.

“We helped pump out the Tarmac batching plant and any site queries raised by us were dealt with by Network Rail and the designers Atkins within 24 hours.

“All parties are represented in the same office so we have been able to engender a real common purpose.

“If an issue was encountered on site we could have an Atkins designer working on a solution within an hour, then it’s just a question of getting the paperwork to follow that trail.”

 

Rebar modelling helps aid clash detection

With precast concrete beams weighing as much as 50 tonnes being shipped across to the site from the Republic of Ireland, the project team has been keen to ensure all potential installation difficulties have been ironed out by the time they are lowered into position.

This has included the continued modelling, remodelling and clash detection between the precast concrete beams and the steel reinforcement that runs through into the deck.

The decks are heavily congested with rebar and the computer modelling has proven invaluable, not only in sorting out clash detection details but also ironing out any other potential difficulties – a move which has proved a fillip to site safety, according to Mr Smith.

“The logistics of the build has been the great focus,” he says.

“We always want to limit the amount of work we do at height as far as possible and use the computer model to go over and over the installation sequence, to work out where there might be a potential issue and to minimise disruption and hazards.”

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