Like a living organism, the Gotthard tunnel must “breathe” constantly once it is in operation. Airflow is vital for train movements, maintenance and for emergency safety.
People need air of course but the crucial problem is the rock heat of up to 50 degrees C combined with equipment and train generated heat. “In air above 40 degrees C, the trains could have problems operating,” says Simon Peggs, AlpTransit Gotthard project manager for the mechanical systems.
During construction, forced ventilation has been used with large fans pushing in fresh air through ducts, and used air and pollutants exhausting along the tunnel bores. In later stages the twin bores have helped make air circuits with the crosspassages sealed with temporary airlocks.
Cooling was done by separate cold water systems feeding cooler units but in the operating phase the airflow will remove the heat.
Huge ventilation systems will be sited at the two multifunction stations, each one-third of the way along the tunnel. Fans will have a 200m3/sec capacity at each, with a reserve fan giving 100 per cent redundancy.
But these systems will mainly be on standby, with the fans gently idling at around 35m3/sec. Trains themselves will create the flows, forcing air along the tunnels by the piston effect as they pass every five minutes. If there is a schedule break, the generated airflow will persist for a short while.
“To achieve this, the main bores must be isolated,” says Peggs. Each of the 178 cross-passages is sealed by massive doors at each end, no easy task given that the high-speed trains generate a 10t pressure wave. “As they pass, the doors must resist a 10t negative pressure too” he says.
But the doors must also glide open quickly in an emergency even for ‘a frail old grandmother’. Developing the right equipment for this has needed ‘quite a bit of engineering development’ with the five manufacturers who tendered for the supply contract.
The multifunction station fans will be triggered if temperatures in the tunnel rise too much. This will be on one side only, venting the last third of the tunnel which is where the gradually heating air may pass a critical 35 degrees C threshold in summer.
But the fans’ main function is to vent the tunnels during routine maintenance and especially for emergencies.
Maintenance will be done weekly, on Saturdays in one bore and Sundays the other. “The tunnel will be divided into twothirds and one-third using a large mobile ‘door’ mounted on a locomotive,” explains Peggs. Air from one station will blow into each part, exhausting at the portal.
The division is needed because a maximum 200m3/sec flow is possible. “Even that creates a 4m/sec wind for the crews to be working in”. It will be quite a challenge for the federal rail, who will operate the tunnels, to find crews.
For emergencies, the ventilation is linked to several possible scenarios, mostly to deal with fire on a train. If something does happen, the driver will keep the train running a maximum of one third of the tunnel length, to either an exit portal or one of the multi-function stations. Freight trains will be instructed to keep going until exiting the tunnel.
The ventilation system will blow air into the emergency concourses at the multifunction stations to prepare them. The concourses are connected to the main tunnel by lateral passages every 86m and the air pressure will ensure smoke does not enter the station as passengers evacuate.
Inside the main tunnel, ceiling vents at the station location will extract any smoke from positions between each of the connection passages. The hot gases will be exhausted via a separate duct tunnel to the second shaft at Sedrun and then along an exit vent tunnel to the mountainside above, or along a partitioned vent system at the Faido access tunnel.
Passengers will leave the concourse meanwhile along a connecting passage to the emergency station serving the other, unaffected, tunnel. Here they will be picked up.
In the event of a disaster preventing the train from reaching the MFS the main cross-passages will provide the escape route. In this case the ventilation will operate differently. “All the air will be blown into the unaffected tunnel to put it into overpressure to blow smoke and air back when an escape route through a cross passage is opened” explains Peggs. “Additional longitudinal fans will operate at the tunnel ends too, to maintain overpressure at the portals.”
These scenarios have been kept simple, he says, to avoid confusion in a disaster. “We do not want the danger of a ‘Mont Blanc’ effect where efforts to blow air in to help people, actually fed the fire.”
There is further complication for the ventilation. Each cross-passage must have its own independent ventilation fan systems because the sealing doors prevent airflow in normal operation. There is electronic, signalling and power equipment in every one that has to be kept cooled.
Air will be drawn from one of the main tunnels by small fans to circulate the rosspassage space and then back to an exhaust fan into the same tunnel bore. The air flow inlet will switch sides about half way along the tunnels, according to the direction of the main bore, each of which is cool close to its entrance and hot at the exit.
In an emergency, the cross-passage ventilation must guarantee a 90-minute workability for the equipment inside the passages. A complex scenario operates to open vents on the untouched side of the cross-passage if the fire is on the usual ventilation inlet side.
Further complications arise if an emergency occurs while maintenance is underway, particularly if the deep shafts at Sedrun are being inspected or worked upon. Special shaft elevator work platforms with vent gaps have been needed to ensure they are not pistoned down the shaft if the fans should turn on.
“However there is a sequencing procedure to draw out the platform first before the fans turn on,” says Peggs “so that should not happen at all.”
Simon Peggs, AlpTransit Gotthard project manager for the mechanical systems Cross passages require individual fan ventilation from the main tunnel to keep important equipment cooled The tunnel will be only fan ventilated during rail track installation and maintenance periods at weekends or in emergencies Forced ventilation will be used in critical conditions in the last third of the tunnels where there is a chance of the threshold temperature to be exceeded
