The 16.9km long Gotthard Road Tunnel, in Switzerland, is the key element of one of the most important road connections through the Alps. The single-bore tunnel with bi-directional traffic on two lanes was constructed between 1968 and 1980 and officially opened on 5 September 1980. The tunnel’s safety record was excellent before the tragic fire on 24 October 2001, which claimed 11 victims and caused a two-month closure of the tunnel. None of the 67 fires (7 of which involved HGVs) registered between 1992 and 2001 had serious consequences. This has been mainly due to three key safety elements:

  • 64 shelters, located every 250m, connected to a safety tunnel (originally constructed as a pilot for a second tunnel tube), pressurised and ventilated independently from the main tunnel

  • a powerful transverse ventilation system

  • an efficient intervention structure, with intervention teams at both tunnel portals

    Traffic volumes have increased steadily, now approaching 7M journeys yearly, with about 20% of vehicles being HGVs. This corresponds to an average of about 19,000 journeys/day, with peaks of the order of 35,000-40,000 (or a peak monthly average of about 27,000-28,000), with an average of over 5,000 HGV journeys on weekdays (now reduced to 4200, by means of traffic regulation). The increasing risk level needed to be compensated with adequate measures, including procedures as well as technical improvements.

    A significant upgrade of the powerful ventilation system was already well under way when the fire of October 2001 occurred. The works became the highest priority after this tragic event. The main improvement is an enhanced smoke-extraction system with dampers, which allows for a concentrated extraction and smoke confinement within the fire zone.

    Powerful transverse ventilation

    The main characteristics of the Gotthard road tunnel’s transverse ventilation system are (Figure 2):

  • 6 ventilation sections (including cut and cover stretch)

  • 6 ventilation stations

  • 4 ventilation shafts (height up to 844m);

  • 22 fans (including 4 fans for the cut and cover stretch)

  • fresh air – up to 2,150m³/s (additional 30% reserve)

  • exhaust – up to 2,150m³/s

  • installed power – 23MW (fresh-air reserve excluded)

    The ventilation system was originally designed to achieve a peak fresh-air and exhaust capacity of 125-130m³/s per km. The corresponding flow-rates through the shafts are high, between 345m³/s and 560m³/s. Additionally, a reserve of 30% in the fresh-air supply was introduced for a future second tube. Fresh-air injection is achieved in a conventional manner, with nozzles every 8m. Exhaust extraction was achieved (before the installation of the dampers) by way of fixed openings of 0.8×0.8m³ every 16m, with a uniform exhaust distribution.

    The ventilation is operated in a full transverse mode. A semi-transverse mode was used in the past, with forced fresh-air injection and free exhaust through the open shafts, in order to exploit the powerful natural ventilation through the high stacks, in particular during the wintertime. In case of fire, the highest smoke-extraction rate and a reduced fresh-air rate are used in the fire section, while the transverse ventilation is operated at about 50% elsewhere.

    Improving the fire ventilation

    The main drawback of this powerful ventilation system is the difficulty of confining smoke propagation to short stretches. In effect, the smoke arising from a HGV fire could be expected to propagate over stretches of 1km or more, as illustrated by numerical simulation. The particular results presented in Figure 3 apply to the following conditions:

  • traffic: 500 vehicles (hr/direction), 80km/hr, 20% HGV

  • ventilation: before fire 50% transverse ventilation in the whole tunnel

  • 3 minute delay for fire detection and ventilation equipment response

  • Fire: standard HGV 30MW fire

    Proper confinement of smoke propagation requires the average longitudinal velocity in the tunnel, on both sides of the confinement zone, to be sufficiently high and directed towards the fire. The new Swiss guidelines require a velocity of at least 1.5m/s on both sides. This can be achieved by the utilisation of high concentration smoke extraction rates, coupled with proper control of the longitudinal air velocity in the proximity of the fire.

    It had therefore already been decided, in 2000, to install remote-controlled dampers along the exhaust duct and to establish a proper system for mastering the longitudinal velocity in the case of fire. Figure 4, which relates to the same conditions as Figure 3 but for the smoke-extraction mode, shows a drastically improved fire ventilation performance with dampers. The average longitudinal velocity distribution, represented for two fire locations in Figure 5, illustrates the fundamental difference between the two systems.

    Smoke-extraction dampers

    In order to attain concentrated smoke extraction, the installation of 178 dampers, with a typical distance of 96m, and new infrastructure for damper management and power supply was required. The system also had to integrate well with the existing equipment. The new dampers have a double function:

  • In normal operation (transverse mode) they are partially open to allow for uniform exhaust distribution – at a typical angle of between 5?, near the ventilation stations, and 15?-20?, at intermediate locations

  • In case of fire, the 3 dampers (or more, depending on the particular conditions and fire location) closest to the fire are opened completely, while all other dampers in the fire section are closed. This allows for concentrated smoke extraction over roughly 200m. Outside the fire section the ventilation is operated in transverse mode.

    None of the fans have been replaced. From an aerodynamic point of view, the key issues were therefore mostly related to stability and operating conditions of the existing fans.

    The new dampers are managed by an entirely new control system. The key issues, at design, installation and verification level have been related to the necessity for a smooth integration into the complex structure of the tunnel, with equipment from different generations. The main components of the system are: two main computers with appropriate man-machine interfaces in the control rooms, for the supervision of the whole system; over 30 Programmable Logic Controllers (PLC) in the tunnel, organised in two hierarchical levels; and 52 electrical cabinets (power supply and control). Roughly 200km of electric cables and 18km of fibre-optic cable have also been required.

    For enhanced protection and maintenance most of the electrical equipment has been installed in the technical rooms, with all connection cables and motors in the fresh-air ducts. The damper-control system offers the best guarantee for reliable performance in the case of an emergency.

    Longitudinal velocity

    The longitudinal velocity requirements in the Gotthard road tunnel are less stringent than in other tunnels, due to the high smoke-extraction rates available (Figure 6). As a comparison, smoke-extraction rates in the Mont Blanc tunnel, after the 1999-2001 renovation, are in the order of 150m³/s through 6 dampers (600m). A proper system for mastering the longitudinal velocity in the Gotthard road tunnel in case of fire is nevertheless necessary, mainly because of the high atmospheric pressure differences between the two portals (up to 400Pa-500Pa). Due to the high ventilation flow-rates and to the flexibility of the system, with 22 fans installed in 6 ventilation stations, the longitudinal velocity in the Gotthard road tunnel can be mastered by adjustment of the fresh air and exhaust rates, without jet fans. This is still in progress.

    Installation and verification

    The ventilation system upgrade was achieved with a minimal impact on the tunnel’s operation. This was particularly difficult because of the high traffic volumes. Tunnel closures are especially critical during the winter period, as the Gotthard pass is closed due to the snow. The main schedule was as follows: Design = 2000; tendering = January-August 2001; start of tunnel works = September 2001; electrical installation = November-December 2001 (during tunnel closure after fire); damper installation = February-March 2002 (closure nights); verification = April-September 2002.

    Only the first construction phase (cutting of the false ceiling and installation of the main cables) was completed before the fire of October 2001. All project phases essential to tunnel safety were accelerated after the fire and were completed well ahead of the original schedule. All system verification was carried out during night-time closures, while guaranteeing the full availability and safety of the tunnel during the day.

    Testing of all the system components was carried out at several levels, ranging from low-level installation tests, to global performance tests of the whole system. Particular attention was given to a reliable system performance. This included the systematic verification of every individual damper function and of the automatic response to all fire scenarios. A fundamental issue was the verification of the system’s performance in the case of reduced availability of some components.

    The aerodynamic performance of the new system was investigated by way of a number of measurement campaigns. They allowed identification of the allowable operational range and measurement of the system’s key performance parameters.

    Conclusions and outlook

    The ventilation system upgrade is now approaching its conclusion. Under normal conditions the system performs as before. However, the modifications allow for a significantly improved smoke containment in case of fire. Fine-tuning of the new system is still in progress.

    This is only one of a number of improvements to the tunnel’s equipment, including new lighting and fire-detection systems. Further improvements to the ventilation system are also in progress. These include: a new control system; improvements to the fire ventilation in the southern section, Airolo – Motto di Dentro, in terms of increased smoke-extraction rates, reduction of leakages in the exhaust ducts; and improvements to the safety tunnel’s ventilation.

    Related Files
    Figure 1
    Figure 6
    Figure 3
    Figure 2
    Figure 5
    Figure 4