The Infrastructure

The Fawkham Junction to St Pancras International link represents a total investment of £3.3 billion. Tunnelling more than half the route absorbed much of this cost. From St Pancras International the line passes through the first London tunnel of 7.5km to the Stratford box, a walled cutting of 1.1km. At the centre of a huge new development called Stratford City, which is where a new international station will service the Olympic Village and a revitalised Lea Valley.

From Stratford International, the line passes through the second London tunnel of 10.5km before travelling overland and passing the M25 to the Thames. A tunnel of 3.2km takes the line under the Thames from where it continues on the surface to Fawkham Junction.

Placing most of the urban section of the railway below ground and following the line of a surface rail line has minimised disturbance to residents and businesses in the capital. The need to ensure safety in the tunnels, recognising the new potential risks, however has created technical and logistical challenges for which unique solutions have been required.

Safety principles

Consultants Parsons Brinkerhoff undertook the tunnel ventilation design and modelling, devising a system of ventilation to meet everyday needs and to give protection in an emergency. This ensures that if there is a serious incident in any of the tunnels, then the adjoining tunnel – linked by cross passageways and protected by fire doors – will become a place of safe refuge and an exit route.

The primary role of ventilation is to pressurise the safe area in order to exclude smoke and fumes. Additionally, airflow in the tunnel affected by the incident is regulated to remove fumes and products of combustion to contain the hazard. It will also introduce fresh air remotely to allow emergency services to approach the incident with greater visibility and safety.

In non-emergency situations, like a train held in a tunnel or while maintenance work is in progress, the ventilation system can deliver fresh air for general comfort. During the later phases of fit-out, the ventilation systems operated this way to improve working conditions for the contractors installing track, signals and other equipment.

Powered ventilation throughout the scheme ensures the required air volumes can be delivered reliably and give flexibility and control over airflow. This has also reduced the number of service shafts required. The longer 10.5km London tunnel has three service shafts and the shorter has two.

The Thames tunnel ventilation

The requirements of the Thames tunnel differ from those of the longer London tunnels and so different techniques are used to achieve the same goal. Sinking ventilation shafts close to the banks of the Thames would have been expensive and posed technical problems. A novel solution has therefore been adopted using powerful fans at both ends of each tunnel.

At both the Essex and Kent ends of the Thames tunnels, plant rooms top the tunnels. Each of these is fitted with a bank of four 1.6m diameter fans rated at 155Kw. The system requires only three fans to be operational at any one time to deliver the required air volume of 175 cubic metres per second. The remaining fan provides back-up and continuity during maintenance. Fläkt Woods specially engineered these fans, and the fire-rated fans in the tunnel ventilation shafts.

Air is driven from the fans through a tapered chamber, known as a Saccardo nozzle. This is discharged through a narrow slot at high level in the tunnel reaching an exit velocity of 34 metres per second. These systems, operating at both ends of the evacuation tunnel, will give the pressurisation required to contain smoke from an incident in the adjoining tunnel. A single Saccardo system operated at either end of the incident tunnel can provide cover for emergency services to gain access to the incident with less risk.

The London tunnels

On the London tunnels, there is a different solution. These tunnels are located up to 40 metres below ground level and service shafts provide ventilation, power and personnel access. Head-house buildings close to the top of each shaft contain plant rooms for power and switchgear. The control rooms have conventional heating and ventilation systems to ensure comfort for personnel and efficient plant operation. These HVAC systems were also part of the Hargreaves contract with EMCOR Rail.

Each shaft houses twin ducts with reversible fans to inject or extract air from the corresponding tunnel. As an incident could arise anywhere on this underground network, all fans and ductwork are fire rated to withstand extended exposure to high temperatures.

Within the tunnels, reversible jet fans are installed that can induce airflow in either direction supporting the flow pattern induced by the fans in the ventilation shafts. A sophisticated monitoring and control system, supplied by Johnson Controls, determines the direction of airflow required to deal with daily or emergency requirements.

Because the line passes through built-up areas, noise break out from shafts and tunnel ends requires special attention. Attenuators fitted above and below the fans take care of this. In addition, fan units are sprung and have flexible plastic connecting skirts to avoid resonance of the ductwork due to vibration.

Special engineering considerations

The long term, low maintenance performance of equipment is essential and contractors have been required to guarantee fixed equipment for 25 years. Hargreaves ductwork, working platforms and support steelwork is therefore galvanised or hot metal sprayed to give lasting protection.

A further consideration has been the design needs for equipment to withstand air pressure from trains travelling at up to 230km/h (143mph) through the tunnels. This induces large positive pressures ahead of the train and a negative pressure behind it with a near instantaneous swing between the two extremes. To withstand these immense forces, Hargreaves has fabricated the ductwork in heavy gauges with additional stiffening and reinforcement where appropriate. This is particularly evident on the Saccardo nozzles in the roof of the Thames tunnels.

Senior Hargreaves and the other contractors faced considerable logistical problems. The ventilation systems and associated structural steelwork in each shaft serving the tunnels weigh around 100 tonnes. Structural steelwork and platforms alone account for 30 tonnes, attenuators a further 30 tonnes and pressure relief dampers almost 20 tonnes. The system comprised hundreds of individual elements, all of which needed to be built and moved up to site in the precise assembly sequence. Storage space at many locations was limited. Access was also limited to specific times of the day because of the need to avoid disturbance to residents and local businesses by the transport of the police-notifiable loads at peak times.

To complicate matters further, the A12, a key route for many of the locations, was being re-built during the tunnel construction and consequently imposed strict limitations on oversized loads. Close coordination between the Hargreaves factory at Bury, galvanising and coating contractors in Bolton, transport operators and on-site management, was essential and ensured precise delivery times to site were kept.

Conclusion

Soon you will be able to enjoy breakfast in Edinburgh, have lunch in London and still have ample time to see the sights of Paris before dinner. Trans-continental rail travel will transform both leisure and business travel. Building this final link has been a massive effort of civil and mechanical engineering.

As most passengers speed below London’s suburbs at over 140mph, they will be unaware of the sophisticated systems that support their safety. However, the engineers and contractors such as Hargreaves that have worked together to create these pioneering tunnels, setting new technical standards along the way, know they have paved the way for the next phase of ground breaking rail projects.

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