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Deep excavation: Legacy challenge

Construction of a hi-tech rail hub for the next generation of trains is taking shape near Bristol but dealing with historical site issues has called for some complex deep excavations. 

Delivery of Hitachi Rail Europe’s Intercity Express Programme (IEP) from 2017 will see an end to the use of 40-year-old trains on the Great Western Line. However, before construction of essential infrastructure in Bristol could get underway, Hitachi had to deal with the site’s legacy that dates back to the same era as the current rolling stock.

Nestled into a triangle of land defined by rail lines close to Bristol Parkway station is what will be the maintenance hub for Hitachi’s new rail fleet. Construction of the facilities on a former 1970s landfill seemed straightforward until a culvert running below the site collapsed and flooding threatened a nearby blood bank.

Deep excavation

The planned maintenance hub for the new fleet of Hitachi trains close to Bristol Parkway has hadto deal with a collapsed culvert

Construction of the £80M maintenance depot was planned to start on site but collapse of the culvert in April 2013 meant that further work was needed – and called for some very deep excavations and challenging propping conditions.

Volker Fitzpatrick managed to start work on the main contract to build the depot in August 2013 after a phased earthworks excavation plan was developed that would see the site turned into a giant game of solitaire.

“Replacement of the culvert was vital as at peak flow it can carry up to 5,000litres per second and the collapse resulted in flooding that threatened the blood bank that serves the whole of the south west of England,” says Volker Fitzpatrick culvert managing site agent Gary Kelly-Harris.

Work on the culvert started in November 2013 and was phased with the first £8M stage focusing on the 325m section under the new Hitachi depot and the second £2.2M phase of culvert replacement involved working on the 125m section that passes into Network Rail owned land.

“The landfill over the Stoke Brook culvert was up to 18m deep and gaining accessing for replacement called for around 180,000m3 of material to be moved within the site and then replaced,” says Kelly-Harris. The waste material was placed on the site in 1974 over a metal culvert that was never designed to take the loading from 18m of fill and had probably reached the end of its design life. “The landfill is highly variable – some of it contained asbestos – so it couldn’t be removed from site,” explains Kelly-Harris.

Despite the large scale movement of material around the site replacement of the culvert still called for some challenging deep excavations and propping solutions.

Volker brought Byrne and King to carry out the excavation and culvert construction, to a design developed by Arup, with ground support specialist Groundforce providing props and support advice.

The base of the culvert has been constructed at the base of the fill, resting on the natural geology that is primarily formed from clay and mudstones with limestone bands.

Sheet piling work for the culvert construction was installed by Volker Ground Engineering but with the landfill being so variable, the loads that could be placed on the piles by the props while the culvert was excavated were difficult to calculate.

“The result has been lots of design changes during the programme,” says Byrne and King contract manager Ben Thomas. “We have worked closely with Groundforce to make sure the props we were planning to use would be capable of meeting the loading requirements and layout of the chambers.

“The variable nature of the fill meant that not all of the sheet piles were in exactly the right place, so we had to design the propping system to cope with slight changes in dimension from the design.”

According to Groundforce’s Bryan Webb the actual loadings have varied between 60kN and 140kN. “Initially the loadings were expected to be much higher because of the steep batter of the temporary slopes created for access to the culvert excavation level and the close proximity of the work to the live rail lines,” says Webb.

Groundforce’s assistance with the culvert work also meant that its equipment was called in to assisting with temporary support of the wheel lathe and drop pits for the main depot construction.

Topping out of the main depot construction was completed at the end of January and Kelly-Harris expects work on the culvert replacement to be completed at the end of April.

Express delivery


Hitachi is supplying 92 of its new Super Express trains

The work at Stoke Gifford for Hitachi is part of a long-term plan – known as the Intercity Express Programme (IEP) – that the Department for

Transport (DfT) started work on in 2005 to look at how the UK’s fleet of ageing high speed dieselpowered trains should be replaced.

Following the government’s decision to electrify the Great Western Main Line to Swansea, IEP is integrated with the electrification programme in addition to Network Rail’s investment programmes, Crossrail, the Thameslink programme and HS2. As part of the IEP, Network Rail will deliver infrastructure changes and Agility Trains – a consortium of Hitachi Rail Europe and John Laing Investments – as the train service provider will deliver the Hitachi-made and maintained trains into passenger service.

DfT signed a contract with Agility Trains in July 2012 and for Hitachi Rail Europe, this meant the start of a programme that involves construction of a state-of-the-art train manufacturing plant in the UK, investment in new and refurbished depots – including the one at Stoke Gifford – across the countries and the design, build, and introduction into service of 92 highly reliable, fully serviceable, modern trains.

According to Hitachi, the family of new Super Express Trains will constitute a step change in capacity, improved reliability and reduced environmental impact on every train journey. Hitachi will carry out the maintenance for a period of 27.5 years from when the trains go into service in December 2017. 

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