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

Railway, metro and motorway tunnels need a lot of effort to keep them safe, clean and reliable. To ensure their faultless maintenance is a daily challenge to tunnel managers, supervisors, foremen and other specialists. To meet the highest reliability and safety standards the key personnel must keep tunnels in excellent technological condition, and provide minimal traffic disruption.

The Eurotunnel, being t he world’s longest undersea railway tunnel, pays much attention to its maintenance because it is handling the world’s highest density of traffic. The rails wear out rather quickly as they carry a load of around 120 million tonnes each year. It poses extreme demands on the track inspection, and during the years of operations, the Channel Tunnel’s specialists have already replaced the rails twice. Their approach to maintenance includes the “Maintenance Production Plan” that covers a detailed description of each maintenance operation for improving tunnel performance in terms of reliability.

Tunnel maintenance covers upgrading the track sections; after-inspection repairing of signaling equipment and signs; examining of a “third rail” electric system; improving reduced light intensity; cleaning of air treatment filters, removing rubbish, dust and other contaminants; etc (fig. 23.1b). Both coarse and fine dust particles, deposited in tunnels especially during the braking of the trains or rail grinding, cause malfunction in telecommunication and signaling equipment, etc. An efficient filtering system cleans the air because the passengers can inhale the dust, and if it exceeds legal limits, it can pose a health risk. These problems are far worse in metro tunnels because train frequency is much higher and air moves through the tunnels into stations. Metro tunnels are cleaned more often than railway tunnels because the confined tunnel space aggravates high rates of dust generation.

a – Inspection carriage	b – Maintenance crew removing rubbish, dust and other contaminants
c - Examining the track, rail fastenings and joints	d – Reconstruction crew examines the faults and conducts necessary repairs

Figure 23.1 Routine tunnel maintenance

Day-to-day tunnel maintenance is much more expensive compared to the upkeep of bridgeworks and culverts. Maintenance supervisors and tunnel trackwalkers work side by side with a maintenance crew belonging to a specific track section. Due considerations should be given to the track, rail fastenings and joints (fig. 23.1d). They are carefully examined for faults, and necessary repairs are conducted daily (fig. 23.1e). The workers supported two parallel rails with the gauge of 1,520 mm at the same level. It prevents accidents and eliminates any possibility of derailment.

The next maintenance concern is providing steady tunnel height and width clearances. The workers test them by a trolley, equipped with a frame matching the tunnel’s dimensions. In case a tunnel is built in a seismically active area, its lining is influenced by the shift of the Earth’s crust and other displacement processes within the massif. The aim of the permanent close inspection is to examine the shifts in lining elements and the incipient cracks in concrete and iron cast segments. The maintenance crews use special screeds attached to the lining surface to watch the displacement changes.

Water discharging is one of the complicated problems for tunnel maintenance especially in icy conditions. When the water in gutters, laid on both sides of the railway track, freezes, the ice blocks the trains. 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 require special encasement to keep them warm.

Water infiltration can cause tunnel degradation and safety risks due to settlement of eroded and weakened tunnel structures. Infiltrated water dissolves road salts carried by vehicles into tunnels. This concentrated solution increases deterioration of tunnel structures. Poor concrete cover cannot protect reinforcement steel that corrodes and causes concrete delaminating and spalls. Weakened and corroded tunnel structures and equipment including fans, lights, etc. pose danger to vehicles traveling through the tunnel during a seismic event.

Tunnel maintenance also deals with ventilation and lightning. Ventilation is one of the most complicated problems. Road tunnels face this problem because of its confined space. Air contamination resulting from toxic auto emissions calls for various types of ventilation facilities. The simplest ventilation method proposes constructing the opposite tunnel portals at different levels to provide natural ventilation. There is another type of natural ventilation in short tunnels with large cross-sectional areas. They take advantage of the prevailing winds influencing air movement. Moreover, moving traffic creates the “piston” effect by pushing the air through the tunnel. Bi-directional traffic reduces the effect of natural ventilation, and engineers can add a centre shaft to the tunnel ventilation system. Fresh air can enter the tunnel and smoke-filled or gas-polluted air can exit through this shaft.

Long or heavily traveled tunnels have large amounts of air with high levels of dust, exhaust gases and other contaminants. To replace the polluted air within the tunnel and to make its circulation more frequent, some tunnels are equipped with a separate plenum or ductwork for supply-air and exhaust-air systems. The ductwork is located either above a suspended ceiling or below a floor slab within a circular cross-section tunnel. The supply or exhaust ductwork also may be along both sides of the tunnel. Mechanical supply-air fans may be accommodated at both ends of the plenum, and push fresh air from a pollution-free source, usually from the portal, towards the centre of the tunnel. Some ventilation systems use reversible fans that can flow the air in opposite directions.

Rectangular-shaped tunnels with no extra space for a separate plenum above the ceiling or below the roadway employ longitudinal ventilation that is similar to natural one. There are additionally mounted mechanical fans of longitudinal ventilation inside the tunnel. The engineers may also install fans either in the portal buildings or in the centre shaft. Short circular tunnels also use the longitudinal system because the amount of air that needs replacement is not large.

Many tunnels are equipped with ventilation plants including the vent stack and machinery rooms with fans, dust collectors, mufflers and ventilation control devices. The air is supplied alongside the whole length of the road 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 safe maintenance and traffic. The magnitude of light level must provide such illumination that inspectors can clearly see the track elements without using additional light sources and flashlights. Tunnel illumination should also provide sufficient light for correct adaption of drivers from the bright portal conditions to the darker tunnel illumination. For this purpose, there are brighter lights, installed at a certain length at the tunnel entrances.