The gallerie They are complex infrastructural systems that allow for overcoming orographic obstacles, on a road or railway route, in territories with a predominantly mountainous development. They are engineering works capable of literally hold up the mountains above them and prevent these from collapsing inside them, thus allowing continuity of passage along the route, limiting travel times from a given origin to a given destination.
Although their technical implementation essentially concerns aspects structural and geotechnicalassociated with the stability of the excavation face and the interaction that the excavation has with the surrounding buildings present, the design of a tunnel actually touches multidisciplinary aspects related to fire safety design, management of exhaust fumes and escape routes, internal signage and everything needed to manage any emergencies and/or broken down vehicles. This article outlines a specific focus on these characteristics, describing the main requirements of these infrastructure works.
What’s inside a gallery
A gallery is in all respects a drilling with predominantly horizontal development of the soilnecessary for connecting two points with respect to which an orographic discontinuity is interposed. The removal of the soil necessary to create the passage area for vehicles or trains requires the insertion of a support structure that allows the lateral and overlying soil to be supported. To do this, elements are usually used in concrete, even prefabricated, with a curved shape and such as to recreate a arch inside the hole: This geometric configuration ensures good use of the materials and the definition of a regime of internal forces favourable to the overall stability of the work.
In addition to the surrounding structural works and the necessary waterproofing to ensure that there is no water infiltration inside the structure, the tunnel is equipped with a series of systems mainly serving:
- Interior lighting;
- Ventilation for the removal of exhaust fumes;
- Disposal of accumulated platform water.
In addition to this, inside the gallery we find the location of various points of emergency exitescape routes, usable in emergency cases and mandatory when the length of the work is considerable. Let’s see below, case by case, how these system parts work.
Emergency exits and escape routes in tunnels
In tunnels with significant longitudinal development, exit areas must be explicitly provided for in the event of an emergency. They may be direct exits to the outside of the gallery or corridors communicating with a parallel gallery (for example in the case of motorways) or with emergency tunnels. The entrance and exit portals of the tunnel are classified as emergency exits. In tunnels length less than 500 m There are no emergency exits, except in some special cases (such as crossings in urban areas). Above 500 m, however, Every 300 m an emergency exit will be required that converges onto an escape route. In some cases, emergency exits converge into a real emergency tunnela pedestrian or possibly vehicular structure separate from the operating tunnel.
The escape route can be, in some cases, a temporary safe place, that is, a parking place that is physically separated from the tunnel by an appropriate fire compartmentation and with a dedicated ventilation system that introduces air directly from outside the tunnel.
In general, in reality all these exits must guarantee an appropriate compartmentalization, therefore they must prevent the propagation of smoke and any fires from the “accidented” points of the tunnel. Therefore, the closing devices must have a predetermined technological and fire-prevention design characteristic. This characteristic is expressed with the acronym REI and with a number associated with it. The acronym is representative of an acronym: stability (R), estate (And and thermal insulation (I) in case of fire. The number instead represents the quantification of the minutes for which this element is certificate to guarantee these three requirements. In the case of tunnels, we are talking about a compartmentalisation at least equal to REI 120.
The internal lighting system of the tunnels
The tunnel lighting is divided into three types:
- Ordinary lightingthat is, the permanent one, which is active both during the day and at night and allows for safe passage through the tunnel, as it outlines the spaces and identifies the carriageway and lanes;
- Emergency lightingthat is, the one that is activated when the electricity supply fails. It is designed in such a way that it remains in operation for a minimum time of at least 30 minutes;
- Security lightingthat is, the part that allows the indication of escape routes and the position of any fire extinguishers and SOS zones, always present within the tunnel section.
The ventilation system
The ventilation system represents a fundamental part of the plant design of a tunnel. Its construction must take into account various parameters, including first and foremost the length of the tunnel and the area of the travel sectionthe altimetric trend of the tunnel and the estimate of the traffic volume which runs through the infrastructure during night and day hours, for various periods of the year. The system must be designed to ensure:
- Sanitary ventilation: that is, the removal of exhaust fumes from vehicles travelling along the stretch during tunnel operating conditions. In essence, sanitary ventilation ensures that the concentration of pollutants inside the tunnel remains at levels that do not compromise visibility and the health of users.
- Emergency ventilation: that is, the management and control of fumes in the event of major accidents that may develop at any point along the tunnel section. It must also ensure the safe escape of users and prevent the formation of any explosive mixtures that may be generated during an involuntary spill.
Although ventilation is naturally carried out by the recirculation of air generated by the entrance and exit area through the natural currents that develop, in many cases it may be necessary to use mechanical ventilation, that is, to equip the tunnel with real ventilation elements that allow the movement of air in the internal areas of the duct. Mechanical ventilation in tunnels is mandatory if they exceed a length of 1000 m. In cases of shorter length, the need to install a ventilation system arises from a preliminary analysis of the tunnel’s structural and traffic conditions. This can happen, for example, when dealing with small travel areas (the hole area, to be clear) and large traffic volumes.
The drainage system
Although it may seem superfluous at first glance, the drainage system is also a fundamental part of the tunnel system for the correct functioning of the work and the increase in safety conditions in the event of extreme events. In fact, the drainage system in these cases must be even more more efficient than in the open sections of the track: the drainage system also acts as a safety system inside a tunnel, as it is explicitly sized to also ensure the removal of flammable fluidsaccidentally spilled on the road surface for example following an accident! The pipes supplying the drainage system run parallel to the road axis and are positioned in the areas to the side of the carriageway, appropriately protected but inspectable periodically, to avoid any problems related to involuntary blockages.
