Due to the closed space of a tunnel, fires can have very serious effects on users. The main dangers are the production of gas and smoke, and even low concentrations of carbon monoxide are very toxic. For example, in the 2001 Gotthard tunnel fire, 11 people died, all of them from smoke and gas inhalation. More than 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive stopped in a long tunnel. Carbon monoxide poisoning was the leading cause of death. In the 1982 Caldecott Tunnel Fire, most fatalities were caused by toxic smoke and not the initial crash. Similarly, 84 people died in the 1904 Paris metro fire.
Motor vehicle tunnels often require ventilation shafts and electric fans to remove toxic exhaust gases during routine operation.[5].
Rail tunnels typically require fewer air changes per hour, but may still require forced air ventilation. Both types of tunnels usually have provisions to increase ventilation in emergency conditions, such as a fire. Although there is a risk of increasing the combustion rate" by increasing the airflow, the main objective is to provide breathable air to people trapped in the tunnel, as well as firefighters.
The aerodynamic pressure wave produced by high-speed trains entering a tunnel[6] is reflected at its open ends and changes sign (a compression wave front "Compression (physics)") changes to a rarefaction wave front and vice versa); When two wave fronts of the same sign meet in the train, a rapid and significant increase in atmospheric pressure occurs [7] which can cause hearing discomfort[8] to passengers and crew. When high-speed trains leave tunnels, a loud "tunnel boom" can occur, which can disturb residents near the tunnel mouth, and is worse in mountain valleys, where the sound can echo.
Where a separate parallel tunnel is available, airtight but unlocked emergency doors are often installed to allow trapped personnel to escape from a smoke-filled tunnel into the parallel tube.[9].
Larger, heavily used tunnels, such as the Big Dig Tunnel in Boston, Massachusetts, may have a dedicated operations center staffed 24 hours a day that monitor and report on traffic conditions, and respond to emergencies. Video surveillance equipment is often used, and the public can view real-time images of traffic conditions on some highways via the Internet.
A database of seismic damage to underground structures using 217 case histories shows that the following general observations can be made about the seismic behavior of underground structures:.
• - Underground structures suffer much less damage than surface structures.
• - Reported damage decreases with increasing overburden depth. Deep tunnels appear to be safer and less vulnerable to seismic shaking than shallow tunnels.
• - Underground facilities built in soil can be expected to suffer more damage than openings built in competent rock.
• - Lined and cemented tunnels are safer than unlined rock tunnels. Shaking damage can be reduced by stabilizing the ground around the tunnel and improving contact between the lining and the surrounding ground by grouting.
• - Tunnels are more stable under symmetrical loading, which improves the interaction between the ground and the lining. Upgrading the tunnel lining by placing thicker, stiffer sections without stabilizing the surrounding ground can result in excess seismic forces in the lining. Backfilling with non-cyclically mobile material and rock stabilization measures can improve the safety and stability of shallow tunnels.
• - Damage may be related to the maximum ground acceleration and velocity depending on the magnitude and epicentral distance of the affected earthquake.
• - The duration of strong shaking during earthquakes is of utmost importance because it can cause fatigue failure and therefore large deformations.
• - High frequency movements can explain local spalling of rock or concrete along planes of weakness. These frequencies, which attenuate rapidly with distance, can be expected mainly at small distances from the causative fault.
• - Ground movements can be amplified when impacting a tunnel if the wavelengths are between one and four times the diameter of the tunnel.
• - Damage at the mouths of the tunnels and their surroundings can be significant due to the instability of the slopes.[11].
Earthquakes are one of nature's most fearsome threats. A magnitude 6.7 earthquake struck the San Fernando Valley in Los Angeles in 1994. The quake caused extensive damage to several structures, including buildings, freeway overpasses, and road systems throughout the area. The National Center for Environmental Information estimates that total damages amounted to $40 billion.[12] According to an article published by Steve Hymon of TheSource - Transportation News and Views, the Los Angeles subway did not suffer serious damage. Metro, the owner of the Los Angeles subway, issued a statement through its engineering staff about the design and consideration that goes into a tunnel system. Engineers and architects carry out exhaustive analyzes on the intensity with which they expect earthquakes to occur in the area. This is all part of the overall design and flexibility of the tunnel.
This same trend of limited subway damage after an earthquake can be seen in many other places. In 1985, an 8.1 magnitude earthquake shook Mexico City; The subway system was not damaged and, in fact, served as a lifeline for emergency personnel and evacuations. In 1995, a 7.2 magnitude earthquake struck Kobe, Japan, without causing damage to the tunnels. The entrance portals suffered minor damage, but this damage was attributed to inadequate earthquake design originating from the original construction date, 1965. In 2010, an 8.8 magnitude earthquake, massive on any scale, struck Chile. Entrance stations to the subway systems suffered minor damage, and the subway system was out of service for the rest of the day. The next afternoon, the subway system was operational again.[13].