Pioneering ventilation for High Speed One tunnels

Phil Doyle of HVAC contractor Senior Hargreaves explains the technical and logistical challenges in the ventilation systems employed on High Speed One - the final link between London and Europe.

The Infrastructure.

The final phase of the Channel Tunnel Rail Link - High Speed One - Fawkham Junction to St Pancras International required a total investment of GBP3.3 billion.

More than half the route is below ground and this accounts for 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.

This is at the centre of a huge new development called 'Stratford City', where the new international station will service the Olympic Village in 2012 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 then 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 minimised disturbance to residents and businesses in the capital.

The need to ensure safety in the twin bore tunnels created technical and logistical challenges for which unique solutions were developed based on ventilation and controlled airflow.

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.

The key principle is 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 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.

This also gives greater flexibility and control over airflow.

Adopting powered ventilation, rather than natural aspiration, 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 were used to achieve the same goal.

Sinking ventilation shafts in the soft earth close to the banks of the Thames would have been expensive and posed technical problems.

A new solution has therefore been adopted using powerful fans and novel engineering at both ends of each tunnel.

At both the Essex and Kent ends of the Thames tunnels, plant rooms top the tunnel portals.

Each of these is fitted with a bank of four 1.6m diameter fans rated at 155Kw.

The system uses only three fans 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.

Flakt Woods specially engineered these fans, and the fire-rated fans in the tunnel ventilation shafts.

The fans drive air through a tapered chamber, fabricated by Senior Hargreaves, known as a Saccardo nozzle.

Air discharges 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, give the airflow/pressurisation essential to contain smoke from an incident in the adjoining tunnel.

A single Saccardo system at either end of the incident tunnel can provide cover for emergency services to gain access to an incident with less risk.

The London Tunnels On the London tunnels, there is a different solution.

These deep tunnels are located up to 40 metres below ground 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 that can inject or extract air from the corresponding tunnel as directed by the control system.

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 necessary to deal with daily needs or overriding emergency requirements.

As the line passes through built-up areas, noise break out from shafts and tunnel ends demanded special attention.

Attenuators fitted above and below the fans take care of this.

In addition, fan units are sprung and have flexible connecting skirts to avoid resonance of the ductwork due to vibration.

Special Engineering Considerations.

The long term, low maintenance performance of equipment is vital and Hargreaves is able to guarantee fixed equipment for 25 years.

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 large police-notifiable loads at peak times.

To complicate matters further, the A12, a key route to 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 at the time required.

Conclusion.

Breakfast in Edinburgh, lunch in London and ample time to see the sights of Paris before dinner is now a reality.

The fact that this is achieved in a relaxed and civilised manner has only been made possible by 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.

Back to top