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Throughout the world, millions of people travel each day with perfect confidence in transport reliability. Bridges and tunnels play an important role within the transport network, but they are at risk everywhere. Risks are serious because most old bridges and tunnels do not meet modern traffic demands, which have significantly changed since the old structures came into service.

a - Bridge damage caused by train derailment	b - Bridge collapse caused by the pier washout
c – The collapce of the Road Bridge in the city of Mount-Vernon (the State of Washington) on May 24,2013	d – The collapse of the flyover and main pipe running along it in Cyberjaya (Malaysia) on February 28, 2013.

Figure 24.1 Bridge failures

Much attention should be given to bridge and tunnel accidents to avoid train derailments and other tragedies (fig.24.1a). Engineers must provide assessment of bridges and identify structures with poor structural conditions or deficiencies. They must thoroughly review train accident data and adopt stringent safety assurance protocols to prevent deterioration or degradation of railway bridges for reducing the risk of structural failures and casualties.

The main reasons causing the failure and destruction of Railway and Motorway Engineering Structures refer to the following four groups: natural disasters (earthquakes, hurricanes, floods and avalanches); various blunders in engineering and technical decisions; negligence and ignorance in construction and maintenance recommendations; and, last but not least, traffic accidents.

More than 100,000 earthquakes occur each year, and no place on the Earth is safe from tectonic pressures. Sudden, abrupt and violent shifts in the Earth’s crust result in vertical and horizontal displacements, which can put the piers into a slanted position and crush or throw off the spans. However, it is worth noting that during quakes bridges and tunnels have not been damaged as much as other civil engineering structures. An earthquake crushed about 85% of dwellings in Tokyo on September 1, 1923 but only 337 out of 1,028 bridges went out of service. Moreover, the Great Tashkent Earthquake in 1966 affected no Railway and Motorway Engineering Structures.

The impact of ice can also be dangerous for bridges. In 1938 ice accumulation on the Niagara River cut the abutment of the arch span, and the 256-m span collapsed. One more reason for bridge collapse is the scour of pier foundations. The current in the Russian River Uvod increased due to its channel constriction in 1881. It produced a 5-m hole in the soft riverbed. The pier foundations lost stability, and the bridge collapsed (fig.24.1c).

The next challenge for bridges is their construction and maintenance under harsh freezing conditions. The superficial knowledge of metal characteristics and the behaviour of metal structural elements resulted in sudden and unexpected collapse of the Hasselt Road Bridge in Belgium in 1938. When the air temperature dropped abruptly below freezing, some of the metal arch elements broke without any additional loads. The collapse occurred due to the high carbon content in steel elements that caused metal increased brittleness under cold conditions.

Each division of Railway Company should maintain an accurate inventory of its bridges by conducting detailed comprehensive bridge inspections at least once per year. Competent engineers must determine the capacity and condition of each bridge. They must accurately record the inspection information in detail.

Tunnels also play an important role within the transport infrastructure. If an accident occurs in a tunnel, it will have a destructive effect not only inside the tunnel, but it will also influence the surface environment because vehicles may carry hazardous freight. Safety Standards guarantee a high level of safety, but tunnels face problems with large water inflow, which can cause tunnel collapse.

The Kirov-Vyborg Line (Red Line) in the St Petersburg Metro passes through the centre of the city. Its running tunnels and stations are at a depth of 60-70 m under several rivers and at the proximity to the Gulf of Finland. Due to poor geologic conditions, the builders faced a water leakage, and more than 10,000 cubic meters of water mixed with soil flooded into the tunnel. The builders managed to complete the tunnel, and the line was open to traffic, but in 1995, the 500-meter tunnel section at this very station-to-station block was flooded again and collapsed. Water and sand were falling from the tunnel roof and walls but fortunately no one was hurt. Water pressure against the dam located nearby the tunnel caused the repeated washout at the same tunnel section. Additional load resulted into cracks spreading in the tunnel lining. When the tunnel was crumbling, technical and emergency crews were among the first on the scene. To relieve pressure on the tunnel walls and to avoid the further collapse and damages on the surface, the opened dams flooded the tunnel. That segment of the line was out of sevice during nearly ten years. Normal service started in 2004 after having built a set of tunnels along a new alignment. The Slurry Tunnel Boring Machine performed the excavation work of this section and could assure the face stability towards a water pressure in the most difficult water bearing soils.

Owing to the enclosed space of a tunnel, accidents, particularly those involving fires and collisions, can have dramatic consequences leading to the traffic disruption, and destroy a region’s economy. The main dangers are highly toxic accumulations of gas and smoke. The tunnel fire accidents with the highest number of fatalities occurred in tunnels with narrow profiles and with a single-track line (fig. 24.2a,b).

a – A fire accident in a vehiclular tunnel	b – A truck and car collision in a vehiclular tunnel
c – A fire accident in a rail tunnel	d - The collapse of the Interstate 35W Bridge over the Mississippi River in the USA

Figure 24.2 Collapses of Tunnels and Bridges

For instance, the fire in the Baku metro in 1995 killed most of the passengers because the concentration of heat and smoke became unbearable. The narrow cross-sectional tunnel area (28m²) contributed significantly to the severity of the accident and did not allow evacuation. In the Euro tunnel fire in 1996, the train stopped next to an emergency exit but the concentration of smoke and fire gases, following the train was very high and prevented people from using the exit. The passengers could escape into the parallel tunnel due to a bubble of fresh air, injected into the tunnel through the emergency exit. Railway convention throughout the world demands that, if a train is caught on fire in a tunnel, people are evacuated at the first confirmation of a fire. The installation of fire suppression stations in each running tunnel is also a strict demand to stop costly fire events.

Security is the watchword in the modern world, which makes the transport system authorities more watchful because of dramatic terrorist attacks. A suicide bomb attacks destroy metro trains resulting in fatalities and casualties. Therefore, railway and motorway operators must increase their preparedness, detect potential threats and take the correct and immediate action. Public transport staff must be well trained to prevent dangerous situations. Installed video-surveillance systems, along with recording observe tunnels, turnout tracks at the endings of the lines, etc. Video images constantly analyse and detect abnormal situations. The alert systems, based on the video records, work automatically. Security-related measures can help to prevent and handle incidents, and passengers can avoid panic and confusion. The designers of Railway and Motorway Engineering Structures must plan facilities for reducing the tragedies because the daily security is Job Number One.