With the increasing number of tunnels under construction or being planned in the country and the growing volume of traffic in existing tunnels, safety issues are becoming increasingly important. A wide range of factors need to be considered when it comes to the development of safe tunnels. From the tunnel design, ventilation system and lighting to actual firefighting – every element is critical in saving lives.
The fire load in tunnel traffic is increasing as flammable and combustible materials are being transported across regions. This exposes the tunnels to fire risks, thereby increasing the demand for measures to mitigate the danger. Often, wet fire hydrant systems are provided inside and outside tunnel portals, including pumps and pipelines for filling water tanks and connections to fire mains. The hydrant pipe runs through the escape tunnel and connects to the main tunnel through cross passages. Safe areas are also made in tunnels, with sufficient standing surface relative to the time passengers are expected to wait until they are evacuated to a final place of safety.
Tunnel ventilation is an important aspect of passenger comfort during passage of locomotive trains inside tunnels. The choice of the type of ventilation system for a tunnel depends on different key elements such as length, unidirectional or bidirectional traffic, gradient, cross-section, and peak traffic flow rates.
Ventilation systems in tunnels operate in three modes. In the normal mode, adequate air quality is maintained for all tunnel users, particularly in terms of pollutant level, oxygen level, temperature level and visibility. In the fire emergency mode, which the tunnel ventilation control system automatically switches to, when a fire alarm is activated, the system can manage the jet fans of the escape tunnel as per the ventilation scenario inside the main tunnel in order to avoid smoke propagation. The last mode is the congested/maintenance mode, which is activated when a train stops because of a breakdown or faults inside the tunnel. A tunnel ventilation system is essential to maintain temperature during normal operations and remove smoke during emergency conditions.
Another essential feature to ensure safety in tunnels is the design of tunnel lights. It should take into account a number of factors, such as the black hole effect at the entrance, the glare effect at the exit, and sufficient reaction time/stopping distance when a hazard is identified. Three types of lighting systems are used for tunnels. Normally, there is regular lighting inside the tunnel for service, and maintenance works and for rescue reasons. In case of an emergency when normal supply fails, 30 per cent of the total lighting is continued as emergency lighting. Emergency lighting allows safe walking on the walkway throughout a tunnel in case of evacuation. During fire conditions, the lights are switched on automatically. In escape tunnels, the lighting is switched on automatically when there is a fire scenario, or manually by the tunnel operator or maintenance personnel (via a switch on-site) otherwise.
Best practices and the road ahead
The Dr Syama Prasad Mookerjee (formerly Chenani Nashri) Tunnel is India’s first tunnel equipped with a fully transverse ventilation system (including exhaust and fresh air ducts). The system is suitably regulated to extract smoke and facilitate the safe evacuation of users through the escape tunnel. In order to combat fire outbreaks, smoke and heat dampers have been deployed, which react quickly in such a scenario. Fire hydrants have been placed every 150 metres, and there are 129 fire doors. At either portal of the tunnel, there is a pump room with a jockey pump, a main electrical pump and a backup diesel pump for supply of water at high pressure to hydrants installed inside the tunnel. Further, 6,000 LED multiple colour lights have been installed in the tunnel.
The recently inaugurated Atal Tunnel, also known as the Rohtang Tunnel, has deployed many new technologies to monitor local and remote connectivity, ventilation, lighting, fire safety systems and power distribution. The tunnel ventilation system includes a point extraction system with fire dampers evenly spaced along the overhead ventilation duct, leading back to ventilation stations located at the north and south portals. It also has an emergency egress tunnel ventilation system located under the main road tunnel, supplying air to the egress tunnel and electrical niches. In a bid to ensure fire safety, fire hydrants have been placed every 60 metres.
A safe tunnel can be created through the optimised and balanced interaction of all aspects influencing safety, including infrastructure, equipment, user behaviour, operational practices and emergency response procedures. Going forward, the tunnel sector should look into deploying more digital and automated mechanical solutions to complement safety audits and comply with security protocols.