Having read the text try to prove the idea that tunnel maintenance is much more expensive as compared to bridge maintenance. Give your reasons.

Railway, metro and motorway tunnels need a lot of effort to keep them safe, clean and reliable thus creating a well-coordinated, smoothly functioning transport system. Tunnels are dangerous places for track workers and passengers in an emergency, because they are often dark, dirty and difficult to escape from. There are some distinct operations in tunnel maintenance: removing rubbish, dust and other contaminants; upgrading the track sections; examining of a “third rail” electric system; after-inspection repairing of signaling equipment and signs; improving reduced light intensity; cleaning of air treatment filters, etc. Both coarse and fine dust particles are deposited in tunnels from the movement of trains, especially during braking and when negotiating curves. Track maintenance activities, particularly rail grinding, also produce much dust. On metros, passengers, waiting on station platforms, create dust and have to breathe it in if there lacks an efficient cleaning/filtering system.

Railway dust is insoluble in water, and if it exceeds legal limits, it can pose a health risk. The accumulation of dust, in railway tunnels, can cause obstruction of signs, obstruction of air treatment filters, both on trains and in technical rooms, increased risk of malfunction in telecommunication and signaling equipment, etc. These problems are far worse in metro tunnels because train frequency is much higher and because of the piston effect of trains moving air through the tunnels and into stations. Tunnel cleaning has to be repeated several times a year. Even if the technology and tools being used are the same, metro tunnels must be cleaned more often than mainline railway tunnels because of the high rate of dust generation.

Day-to-day tunnel maintenance is much more expensive as compared with the upkeep of bridgeworks and culverts. Needless to say, there are no negligible problems because safety, scheduled running and service reliability depend upon many factors. Maintenance supervisors and tunnel trackwalkers work side by side with a maintenance crew belonging to that track section of the railway. Due regard should be given to the track, rail fastenings and joints which are carefully examined for faults and necessary repairs are conducted daily. It prevents accidents and eliminates any possibility of derailment. In addition, two parallel rails of the gauge (1520 mm) must be supported at the same level. The next problem in maintenance is providing steady tunnel height and width clearances, which are tested by a measuring rod and a band or by a trolley, equipped with a frame matching the tunnel’s dimensions. In case a tunnel is built in a seismically active area, it’s lining is influenced by the shift of the Earth’s crust and other displacement processes within the massif. Permanent close inspection is aimed at examination of the shifts in lining elements and the incipient cracks in concrete and iron cast segments. The maintenance supervisors use special screeds attached to the lining surface to watch the dynamics of the displacement.

Water discharging is considered one more complicated problem for tunnel maintenance especially in icy conditions. For instance, the railway tunnels along the Abakan-Taishet mainline in Siberia face this situation. When the water in gutters freezes the ice blocks the trains because the gutters are built on both sides of the railway track. The gutter dimensions depend on the amount of water entering the tunnel. They call for extra maintenance cost, under low temperatures, because they need heating or special encasement to keep them warm.

There are some more problems facing the tunnel and above all is the ventilation and lightning. Ventilation is one of the most complicated problems, which vehicular tunnels face because of confined tunnel space. Air contamination resulting from toxic auto emissions calls for various types of ventilation facilities, which ensure a sufficient volume of fresh air under the tunnel roof. The simplest ventilation method proposes constructing the opposite tunnel portals, at different levels to provide natural ventilation, due to the difference between air pressure at both ends. Another type of natural ventilation is used in short tunnels, which have large cross-sectional areas, and takes advantage of the wind rose. However, usually the tunnels need a forced air ventilation system and we have to estimate the required volume flow.

Tunnel ventilation involves blowing, forced, exhaust and combined ventilation. The blowing ventilation method supplies fresh air from a pollution-free source usually from the portal. Using the exhaust method, the foul air is ejected through the pipe and fresh air enters from the portal. The combination of these methods uses fans with special filters to separate air from dust and auto emissions. Firstly, the fans function as blowing ventilation, and then the fans are put into reserve.

Many tunnels are designed with ventilation plants including the ventilation stack and machinery rooms equipped with fans, dust collectors, mufflers and ventilation control devices. The air is supplied alongside the overall road length reducing the volume of toxic agents within the tunnel environment. Diluted vehicle emissions are absorbed near the tunnel exit and discharged after the dust is removed from them. Then clean air is discharged from the tunnel, and detecting devices control the leakage of exhaust gas into the outside environment.

Tunnel lightning is very important for traffic safety. If an accident occurs in a tunnel within a city, it will have a destructive effect not only inside the tunnel, but it will influence the surface environment because vehicles may carry hazardous freight. For instance, some tunnels in Switzerland are equipped with solar batteries, which are used in remote areas in the mountains.