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The railway challenge of the 21st century

OVERVIEW

The Channel Tunnel Rail Link will provide the UK's first highspeed rail link from the Channel Tunnel to London.Rail Link Engineering ground engineering manager Nick O'Riordan gives an overview of the complex geotechnics involved.

The 109km long Channel Tunnel Rail Link (CTRL) is being built in two phases and, when complete, will halve the 70 minute journey time from London to the Channel Tunnel.

The route has been designed to follow existing transport corridors - alongside major roads or existing railways - to limit environmental effects. New stations will be built at Stratford in east London and Ebbsfleet in north Kent, and London's St Pancras will be substantially expanded to become the high-speed rail link's international terminus. CTRL will also connect into the existing Ashford International station.

Construction of Section 1 began in October 1998 to link the Channel Tunnel with the existing line to Waterloo station at Fawkham junction in north-west Kent. This section will be commissioned for use by autumn 2003.

Section 2, which will bring the new line into St Pancras, will be completed in 2006 and will involve about 21km of tunnelling beneath London and the river Thames and a similar length of viaducts, atgrade slabs and earthworks for the permanent way.

The CTRL project touches people at many levels. At its most superficial level, it will reduce rail travel times to continental Europe through the transfer of Eurostar trains from the UK's congested railway network on to a dedicated high speed line. However, this very act will free up rail pathways to improve mobility - potentially removing cars and freight from the road network.

Also, the dowry provided for London & Continental Railways (LCR) included substantial areas of under-developed land at St Pancras and King's Cross, Stratford and Ebbsfleet. These areas are being regenerated as part of the Thames Gateway vision, and will help secure the region's post-industrial vitality.

The project is being carried out under the twin spotlights of rail privatisation and increasing environmental controls. It is a catalyst for clear thinking on rail safety and maintenance and waste minimisation, and is a model for environmental good management.

By any standards, the project is large, costing a total £5.2bn, including land acquisition - about £2.5bn of which is associated with civil engineering activities. The project has been divided into individual contracts, each with its particular character:

open earthworks and bridges, tunnels and bridges, tunnels and vent shafts, railway remodelling of new railway.

In June 1996, after the government accepted LCR's bid to design, construct and operate the CTRL, Rail Link Engineering (RLE) - a consortium which comprises Arup, Bechtel, Halcrow and Systra - was appointed designer and project manager.

One of RLE's first tasks was to take Union Railways' work and set up the Ground Engineering (GE) team to turn a route alignment into detailed engineering. The team is multi-disciplinary, encompassing geotechnical and earthworks engineers, geologists, hydrogeologists, data analysts, chemists and contaminated land specialists.

The team is drawn mostly from consortium members and has been encouragingly stable: evolving in composition, rather than having been subject to continuous role change and varying population. Geotechnical engineers have moved from engineering design into the construction team as their work proceeds to site, resulting in a unity of purpose and approach.

The CTRL contracts use the New Engineering Contract form, with target cost and risk of under or overrun of the project shared between employer and contractor.

Natural ground risk is borne by the contractor unless the ground conditions are such that a change is necessary to the works information - broadly speaking - the 'design'. The contractor is also obliged to provide a quality management system that enables self-certification of the works throughout the supply chain.

The absence of a conventional resident engineer places new demands on both design and construction. Geotechnical design has to go beyond mere specification and sizing of structural foundations or earthworks. It has to identify the parameters needed for the contractor's staff to self-certify, in effect, to ensure that the designer's intent has been achieved.

Very often, the way in which the works are built affects performance: this has led to close working, facilitated by partnering workshops between RLE and contractors' site staff.

The practical benefit of self-certification is the increasing responsibility placed down the supply chain to provide a product of desired quality. At its best, it can lead to an increase in the skills base and broadening of expertise within the construction industry:

the fruits of this are being seen in Section 2 now that the lessons ofSection 1 have been learned.

A vital part of the work is the provision of trackbed support for trains travelling at 300km/h, at a defined quality of ride. The dynamic behaviour of structural foundations and earthworks requires a detailed understanding of soil/structure interaction. This differentiates a high speed railway from more conventional highways or lower-speed railways.

Specification of the earthworks for the route was therefore the GE team's responsibility. The ownership of the engineering specification enabled detailed analysis of the probability of acceptance of the various materials to be excavated. In turn, materials could be identified for structural earthworks or mitigation fills - the aim being to minimise waste.

A crucial milestone was identifying the need for a land raise at Stratford, so that the 2.5M. m 3ofspoil from the London Tunnels could be used as regeneration enabling works for a development on a podium above the surrounding non-CTRL track layout.

The project is following in the footsteps of earlier railway building eras. The difficulties encountered in the mid-1800s in constructing the tunnels at Saltwood in the variably cemented and water bearing Folkestone Beds, were relived in the analysis of the brickwork lining, and in the eventual remodelling of the landscape around the live railway. The new Thameslink station box widens an existing cut and cover tunnel under the St Pancras station complex.

Trial and error - and the winnowing of experience that typified much of the Victorian railway age - cannot be part of the risk management philosophy today. This puts increasing pressure on ground engineers to learn from the past and predict future behaviour.

Nowhere is soil/structure interplay more important than in bridge design and construction.

High speed railway bridges are characterised by their low tolerance to movement during train braking and accelerating, and the required rigidity can result in high forces generated by longterm thermal cycling of the bridge deck. Thus, the foundation system can be neither too soft nor too stiff.

Full-scale lateral and vertical load tests on foundations for critical structures are a necessary part of design verification. Numerical analysis enables such field tests to be interpreted so that future behaviour can be predicted.

Ground investigations have been carried out over successive phases as the alignment and detailed design have developed.

All data has been accumulated using Association of Geotechnical & Geo-environmental Specialists (AGS) format.

Early on in the RLE design process, groundwater quality and water levels were incorporated into a baseline monitoring programme. Again, the data is held in AGS format. The baseline monitoring database is accessed and used routinely in the office and in the field to support temporary works and groundwater protection activities.

Groundwater control around cut and cover tunnels has been a significant part of design and construction. Clear spans between permanent propping slabs are very large - around 8m - to provide sufficient clearance for trains, trackbed, power and other services. Maximum depth of excavation is normally well over 12m.

Cut and cover tunnels are located in up to 15m of soft alluvium over gravel and chalk at the Thames Tunnel, in variably weathered Gault Clay at Boxley, and in Atherfield and Weald Clay at Ashford. In each case, the permeability of the ground was found to be sufficiently high so that water pressures would dominate both short and long term performance of the walls and props.

This meant that assumptions of groundwater pressures during and after construction were explicitly addressed to be incorporated into the construction sequence and programme. Fullscale pumping trials and insitu permeability testing figured high in ground investigations.

The challenge to provide a 21st century railway at lowest first cost, consistent with defined reliability, availability, maintainability and safety (RAMS) criteria, has led to the pioneering use of dry deep soil mixing in the UK.

This has built on the experience of embankment stabilisation for the Swedish high speed railway at Ledsgard. This technique has been used for short lengths of embankment on soft clays and peats adjacent to the live Ashford to Folkestone railway, and similar treatment may be adopted for non high-speed works in Section 2.

Off-line, a trial of wet soil mixing was carried out on a methaneproducing, unlined landfill at Runham Lane in Kent, where the alignment passed in a 10m deep cutting. The technique was used in an attempt to produce a cemented block that would support the landfill to the north of the railway. This would have saved the wholesale removal of the landfill contents to a new, fully engineered site. In the event, the trial did not produce a sufficiently robust product, but enabled design and verification tools to be developed that were later used for soil mixing and stabilisation processes elsewhere on the project.

With collaboration of designers from a variety of backgrounds and organisations, there has been a major effort to harmonise design standards and to develop a uniform design philosophy.

Deficiencies in existing standards and guidance have been addressed, particularly in relation to the design of the trackbed support, retaining walls and propping slabs, the engineering properties of chalk and waste minimisation.

The CTRL project has made and - as it reaches completion - will continue to make, significant contributions to new industry guidance documents and, through collaboration between the RLE GE team and contractors and subcontractors in the field, to industry practice.

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