Softly, softly under the city

David R Bennett, BSc (Hons) CEng MIEE, is Implementation Director, Union Railways (North) Ltd, David J Bush, BSc (Hons) CEng MICE CPEng FIE(Aust) MHKIE, is Project Engineering Manager, Systemwide, Rail Link Engineering, and Richard J Greer, BSc (Hons) MIOA, is Noise & Vibration Manager, Rail Link Engineering

THE Channel Tunnel Rail Link will be the UK’s first major new railway for almost a century. Comprising a 109 km high speed line between the Channel Tunnel and London St Pancras station, it is being opened in two stages. Commercial services will be launched over Section 1 on September 28 between the Channel Tunnel and Fawkham Junction near Gravesend (p557). Section 2 will dive under the River Thames, and then penetrate east London in nearly 20 km of tunnel to the terminus at St Pancras. This phase is on schedule for opening in early 2007, and will permit non-stop timings of 2h 15min between London and Paris. London - Brussels will take just 2h.

With a projected out-turn cost of £5·2bn, CTRL is a massive project with a demanding programme. To ensure work is completed to schedule all risks must be strictly controlled. This has resulted in a philosophy which, although simple, is probably unique within the UK railway industry.

Risk has been minimised by taking an existing high performance system, proven in service elsewhere in Europe, and implementing it in the UK. We have no desire to be on the leading edge of railway development, and in practical terms the technology being implemented is that used on the French TGV network. This is a known product, with an almost impeccable service history.

This design philosophy has been successfully applied to civil engineering works and structures, plain surface track and the overhead electrification, and to a very large degree it also applies to the tunnels, signalling and controls.

Tunnels are unique

There is, however, one particular feature unique to the CTRL that challenges this approach. This is the 18 km of high speed tunnel under the densely-populated urban approaches to London. High speed railways have successfully run in tunnels, and railways in tunnels have been successfully designed to control groundborne noise, but no high speed railway in tunnel has previously been designed to control groundborne noise and vibration to such a degree and over such a long length.

‘Incremental innovation’ has been the approach adopted to cope with this unique constraint, and it is consistent with the project’s underlying risk-averse philosophy.

Vibration generated by steel wheels rolling over steel rails in tunnels can filter through the track, tunnel and ground into overlying buildings where it is either directly perceptible by the occupants, or it might cause the walls, floors and ceilings to radiate a low frequency rumbling noise. Significant groundborne noise and vibration is regularly experienced in properties above underground railways, particularly on older lines.

The CTRL Environmental Statement (ES) identified groundborne noise and vibration from the London tunnels as a potential cause of significant adverse environmental effects. There was little existing objective guidance on groundborne noise and vibration from high speed railways. Given this situation, and the implications of mitigation through realignment and provision of low-vibration track, in 1993 Union Railways identified groundborne noise as the most significant environmental risk faced by the project.

Union Railways secured powers to build CTRL by committing to the highest level of mitigation consistent with the operational constraints of a high speed railway and reasonable cost that did not require changes in alignment or floating slab track. The route had already been designed to pass under existing transport corridors as much as possible, and floating slab track, which is unproven for high speed lines, would have required larger and considerably more expensive tunnels.

The cornerstones underpinning these commitments were an accurate prediction method and a detailed understanding of the levels of mitigation that could be provided with different track designs.

UR funded the development and validation of a prediction method for groundborne noise and vibration specifically for high speed rail. This method is empirical, although it includes a theoretical design tool for the vibro-acoustic performance of the train-track system (Fig 1), and it has been an essential tool supporting both the environmental assessment and the detailed design stages.

The empirical backbone of the methodology was developed by statistical analysis of a database of thousands of vibration measurements, including French and German high speed lines and British heavy rail and underground services. The procedure was subject to peer review by leading railway noise and vibration specialists, and was scrutinised by local authorities in London and their specialist advisors.

This prediction method was used to assess potential adverse effects arising from trains in the London tunnels. The results were in turn used to identify a trackwork acoustic performance specification (Vibration Reference Case) reflecting the highest level of mitigation considered possible with current track technology, but without using floating slab track.

Both parliamentary committees accepted that the predicted low level of adverse impact associated with the reference design alignment and the VRC was an unavoidable consequence of building a project of national importance.

Developing the design

Union Railways and Rail Link Engineering are committed to ensuring not only that CTRL is designed to produce groundborne noise and vibration effects no worse than those agreed at the time of the parliamentary processes, but also to a number of more onerous undertakings affecting specific sites.

As RLE developed and finalised the tunnel alignment design, levels of groundborne noise were reassessed and compared with those commitments. Based on the assessment, three levels of mitigation performance (insertion gain) have been defined for the track relative to the VRC (Fig 2).

Level S1 is equivalent to the best slab track currently in operation on high speed lines. S2 is equivalent to the VRC, and represents a significant incremental improvement on S1 and current technology. S3 is the highest mitigation level at the reasonable limit of current technology.

RLE carried out detailed evaluation of potential suppliers and analysed the reliability, availability, maintainability and safety (RAMS) of their products, as well as the implications of the resulting performance specifications before they were transferred to the trackwork contactor. We are currently investigating options for reducing the number of track types to simplify maintenance.

RLE had already made a number of reviews of generic track systems used throughout the industry, comparing them to the various CTRL criteria (Fig 3). We found that options worthy of further study were limited to resilient baseplates (Fig 4) and booted sleepers (Fig 5).

These reviews confirmed that slab track systems have been proven at high speed, albeit with stiffer rail support than required for CTRL, and that the levels of resilience required had been achieved, although at lower speeds. The incremental innovation is therefore to take proven high speed slab track and tune it to provide the resilience required whilst ensuring compliance with the RAMS criteria.

Trackwork contractor ACT (a consortium of Alstom Transportation Projects Ltd, Carillion Construction Ltd and Travaux du Sud-Ouest appointed by URN in August 2002) validated the RLE review, producing a final review taking account of the most recent technological developments.

The final review selected 12 systems for detailed analysis. These included eight sub-types of resilient baseplates and booted blocks, and four originally rejected by RLE but which had subsequently been developed further. They were assessed against weighted criteria including vibration mitigation, proven design and life, constructability, RAMS, system integration factors, capital cost and development risk.

Final validation modelling

ACT needed a model which could predict the level of mitigation of the proposed track designs to ensure compliance. The Trackwork Acoustics Performance Specification for the London tunnels allowed the contractor to use a train-track model of their choice, but required calibration against the earlier modelling.

The contractor employed an international expert to undertake acoustic modelling. In January 2003, a 3D finite element track insertion gain model from D2S International was used for the initial assessment of the design parameters required to meet the different London tunnel mitigation levels.

To provide assurance to the local authorities and for CTRL procurement, RLE commissioned a peer review of ACT’s vibro-acoustic track model. This involved workshop reviews of the methods used and validation of the model’s output against results from the peer reviewers’ prediction methods. The review led to some refinement of the contractor’s models of the train and its damping characteristics but concluded that, once modified, the ACT model demonstrated good consistency with the reviewer’s models. This provided confidence in the revised designs, paving the way for RLE’s acceptance of the contractor’s proposals.

Trackforms selected

At this stage it was possible to finalise performance specifications for the S1, S2 and S3 track form types.

Broadly, the ACT modelling results showed that with Eurostar the S1, S2 and S3 levels of mitigation are likely to be achieved using booted blocks or sleepers, a rail pad acoustic stiffness of 150MN/m, and sleeper soffit pads with acoustic stiffness of 30MN/m, 18MN/m and 10MN/m respectively per block (half sleeper). With high-quality resilient pad materials these values are approximately equivalent to static stiffness of 20 to 25MN/m, 12 to 15MN/m and 7 to 8MN/m respectively. For reference, the sleeper soffit pads installed in the Marseille tunnel on TGV Méditerranée have an approximate acoustic stiffness of 30MN/m and a static stiffness of 20MN/m.

This information was passed to suppliers considered capable of undertaking the detailed design and testing of a conforming system. An exhaustive range of tests has been set up by ACT that will cover the RAMS performance, especially fatigue and ageing, and the acoustic performance for both tangent track and curves. Acoustic performance is particularly important given the mixed traffic that CTRL is being designed to accommodate, and hence the significant lateral forces associated with excess cant and lower speed engineering (and possibly freight) traffic, and cant deficiency for high speed services.

Based on the calibrated modelling and the final comparison of each system in the ACT shortlist, the tender process has been launched with proposed suppliers of twin block, single block and mono-block systems. At time of writing, the supplier’s proposals are being evaluated, with the aim of placing the contract during August.

The development risks in terms of procurement and acoustic performance are minimal for most of the trackwork. However, the risk is higher for those sections that have to provide the highest levels of noise and vibration control in the most sensitive locations. The length of these sections has had to be minimised and the risk has been further controlled by the development of an exhaustive test programme for the prototype.

The design process continues to involve open and proactive discussion with all interested stakeholders, particularly the London local authorities. This will lead to a track designed to meet the demanding RAMS requirements for a 230 km/h line while generating the lowest levels of groundborne noise and vibration in properties over the tunnels yet achieved by trains travelling at high speeds.

 

• CAPTION: Contract 250 covers construction of the 7·5 km section of the running tunnels between the eastern portal at Ripple Lane and the intermediate shaft at Barrington Road Photo:RLE/QA Photos
• CAPTION: Fig 1. A theoretical design tool was used to assess and predict the vibro-acoustic performance of the train-track system in the London approach tunnels
• CAPTION: Fig 2. Groundborne noise and vibration mitigation requirements in the London approach tunnels between St Pancras and Ripple Lane
• CAPTION: Fig 3. RLE compared ballasted and slab track systems with the CTRL criteria, concluding that slab track with resilient baseplates and booted sleepers warranted further study
• CAPTION: Fig 4. Rail pads and base plate pads provide two levels of resilience in the various types of track that use resilient base plates. These include double-clip (above), floating (top right), and bolted (right) designs
• CAPTION: Fig 5. Three types of track with booted sleepers are available

Softly, softly under the city

Track in the long tunnels forming the approach to London of Section 2 of the Channel Tunnel Rail Link will have to meet a demanding specification to minimise the transmission of noise and vibration from high speed trains passing below east London. An approach of ‘incremental innovation’ and risk-averse philosophy has been adopted throughout the project, and the choice of track design was based on the same principles. As no existing line has the same requirements, the design team decided to taken proven types of slab track used on high speed lines and tune them to meet the CTRL’s specific requirements for long tunnels

Tout doux, tout doux sous la ville

Sur la section 2 de la ligne Londres - Tunnel sous la Manche (Channel Tunnel Rail Link - CTRL), la voie des longs tunnels à l’approche de Londres devra répondre à une spécification contraignante afin de minimiser la transmission de bruits et de vibrations venant des trains à grande vitesse passant sous l’Est de Londres. Une approche basée sur ‘l’innovation progressive’ et une philosophie du moindre risque ont été adoptées pour l’ensemble du projet, et le choix de conception pour la voie a fait appel aux mêmes principes. Comme il n’existe aucune ligne avec le même cahier des charges, l’équipe de conception a décidé de faire appel aux types éprouvés de voies sur dalles utilisées sur les lignes à grande vitesse et de les ‘accorder’ afin de répondre au cahier des charges spécifique des longs tunnels de la CTRL

Sachte, sachte unter die Stadt

Die Gleise in den langen Tunneln der Zufahrt nach London des zweiten Abschnitts des Channel Tunnel Rail Links müssen sehr strengen Spezifikationen entsprechen, um die Übertragung von Lärm und Vibrationen von den Hochgeschwindigkeitszügen unterhalb Ost-Londons auf ein Minimum zu reduzieren. Eine ‘inkrementelle Innovation’ genannte Vorgehensweise und eine risikoreduzierende Philosophie kommen in diesem Projekt zum Zuge, und die Wahl des Gleisaufbaus wurde aufgrund derselben Prinzipien vorgenommen. Da derartige Anforderungen noch nirgendwo sonst gestellt worden sind, hat sich die Konstruktionsgruppe entschieden, bewährte Typen des Festen Oberbaus, wie er auf Hochgeschwindigkeitsstrecken benutzt wird, zu übernehmen und auf die CTRL-spezifischen Anforderungen für die langen Tunnel abzustimmen

Suave, suave bajo la ciudad

Las vías de los largos túneles que forman el acercamiento a Londres de la Sección 2 del enlace al Túnel del Canal (CTRL) tendr? n que cumplir con una norma exigente para minimizar la transmisión de ruidos y vibraciones provenientes de los trenes de alta velocidad pasando por debajo del Este de Londres. Partiendo de una nueva filosofía de ‘innovación incremental’ y evitación de riesgos que ha empapado todo el proyecto, la elección del diseño de la vía se basó en los mismos principios. Debido a que no hay ninguna línea existente con los mismo requisitos, el equipo de diseño decidió tomar tipos probados de vías usados en las líneas de alta velocidad y ‘adecuarlas’ para que cumplan con los requisitos específicos de la CTRL para los túneles largos