During normal operation, the piston effect of the moving trains lead to an air movement inside the tunnel and controls the air quality, temperature and humidity levels. Thus, the fans for the tunnel ventilation and the shaft tunnel ventilation are not used to supply fresh ambient air to the tunnel during normal operation. If the climate inside the tunnel reaches a nonacceptable level (for example, too high temperature), forced mechanical ventilation will be applied during normal mode of operation as well.
Design verification: Only freight trains will pass through the tunnel during Phase One, with a maximum speed of 100 km/h. Achievement of tunnel ventilation objectives during the normal mode has been verified for both summer and winter. Only the results related to the summer (December, January, and February) are presented here but similar results can be observed in winter.
Pressure deviation and air velocity: The trains running in the tunnel cause temporal variations of the pressure along the tunnel. The history of the deviation from normal pressure in the tunnel is shown in figure 6 for the two positions along the tube with the maximum positive and negative deviations from normal pressure.
Figure 7 shows the maximum positive and negative deviations from normal pressure along the tunnel. It is derived from the pressure v time curves for every location along the tunnel. Due to the limited train velocities, the maximal positive and negative traininduced pressure fluctuations are small compared to high-speed rail tunnels and range between two and 3kPa.
Similarly to the pressure deviations Figure 8 and Figure 9 indicate the maximal and minimal train-induced air velocities along the BT. In Figure 8 the abrupt air velocity changes from negative to positive velocities and vice versa within a short period of time are due to passages of trains at that position. The mean velocity indicated in Figure 8 shows a predominant air flow from the portal Chile in direction of the portal Argentina. This behaviour is linked to the natural draft effect and leads to a full air-exchange within 24h for typical meteorological situations in summer (as sshown in the specifications in Table 1.
Local air velocities in the immediate vicinity of trains might be substantially larger due to three-dimensional effects. Dry bulb temperatures: A prognosis of the tunnel climate was undertaken using numerical simulations in order to examine the impact of the factors of the tunnel climate. In order to exclude the long-time climate the results are given for simulations after six years of tunnel operation.
The predicted maximum dry-bulb temperature in summer reaches 32°C, which is below the limit value of 35°C (see Table 1).
Congested operation
The specific ventilation requirements for the congested mode of operation need to be specified at a later planning stage. These are considered to be covered by the tunnel ventilation system (TVS) as designed for the emergency mode of operation.
Maintenance operation
Fan operation has to provide every tunnel portion under maintenance with a longitudinal air flow. Fresh air has to be provided in order to remove polluted air and provide adequate working conditions. In order to control the air flow in the maintenance zone, rail tunnel doors must be installed at different tunnel locations. For example, the characteristics of the maintenance operation mode for a tunnel segment located between the Chile Multifunctional Station (MFS) and the Argentina MFS are illustrated in the original paper; in particular, the positions of open and closed rail tunnel doors. Tunnel stretches between a portal and an MFS are treated similarly by correctly opening or closing the rail tunnel doors and operating the axial fans at the ventilation stations.
The amount of fresh (with respect to exhaust) air that has to be supplied (as opposed to removed) in case of maintenance is smaller than the one needed during the emergency mode of operation. Thus, no specific ventilation requirements are specified for the maintenance mode of operation.
Emergency operation
In an emergency a train shall always try to leave the tunnel. As a second option, the train should try to reach the nearest MFS and passengers evacuated through the access tunnel. If the train cannot reach an MFS, it stops within the tunnel. In any case, the TVS design must take all possible emergency scenarios into account.
At every MFS two extraction points located above the main and the passing track are foreseen. Due to the larger number of escape ways, a train on fire shall always be directed along the main track. However, the occurrence of a fire-related incident on the passing track shall not be excluded, so two extraction points located above the main and the passing track are considered. In this way all possible fire locations are covered.
During Phase One, only freight train traffic is foreseen. This aspect must be taken into account for the TVS design in order optimise the ventilation.
In an emergency the fire location must first be identified. During a second phase, the switching damper corresponding to the appropriate extraction point must be activated. The ventilation system should be kept as simple as possible in order to minimise the risk of incorrect operation.
In case of fire in a stretch of tunnel, the ventilation system must always be able to achieve the critical velocity in both directions. In each case the evacuation of passengers will be arranged in theupstream direction of the fire.
A summary of the designed ventilation system during emergency operation for a fire in different tunnel sections is given in Table 3 (overleaf). For every train location, the TVS is designed to carry the smoke in either direction, if required. The proposed values of the flow rates are derived from the knowledge acquired from other base tunnel projects with comparable length (for example, Gotthard Base-tunnel (GBT), Lotschberg Base-tunnel (LBT), Brenner Base-tunnel (BBT)). In addition, a typical freight train fire has been taken into account with a maximal heat release rate (HRR) of 250MW.
As an example Figure 10 illustrates the emergency ventilation system for a train on fire located between the MFS Chile and the MFS Argentina. A ‘push-pull’ ventilation system is considered with a longitudinal airflow from portal Argentina to portal Chile. Rail tunnel doors must be appropriately closed in order to counteract the predominant natural draft effect from portal Chile to portal Argentina. The supply fan is set to 250m3/s and the exhaust fan to 300m3/s. The rail tunnel door in the direction of portal Argentina is closed in order to counteract the predominant natural draft effect from portal Chile to portal Argentina.
In case of an accident in a MFS, the TVS must be able to maintain the escape ways, such as the cross-passages and the central safety tunnel, free of smoke, for which air velocities of 2-11m/s shall be achieved. In Figure 11 (far right) the emergency operation in an MFS is illustrated, with thetrain on fire located on the main track. Fresh air is guided through the access tunnel and toward the MFS. Here, the fres h air shall be distributed along the safety tunnel and carried through the open escape gallery doors into the rail tunnel. The access tunnel shall be provided with one intermediate ceiling in order to separateexhaust and supply air. Smoke/air is evacuated through the duct at the top of the tunnel cross-section, and fresh air is kept at the bottom part of the access tunnel.
Design verification: Numerical simulations have been adopted to verify that the designed TVS is able to meet the objectives specified.
Results from the emergency operation in a tunnel portion resulted in the following main findings:
• The critical velocity can always be achieved by operating the supply and exhaust fans as specified in table 3
• If the mechanical ventilation acts in the same direction as the natural draft effect (from portal Chile to portal Argentina), the critical velocity can always be achieved without closing the rail tunnel doors.
• If the mechanical ventilation acts in the opposite direction as the natural draft effect (from portal Argentina to portal Chile), rail tunnel doors must be appropriately closed in order to achieve the desired critical velocity.
Results from the emergency operation in an MFS indicated that velocities of 4.0-5.5m/s are achieved by setting 250m3/s for supply and 300m3/s for exhaust ventilation. Thus, the fulfilment of the ventilation criteria is guaranteed.
The proposed ventilation design allows handling freight train fires with maximum HRR of 250MW. In case of fire the safety of passengers will be increased and the damages on the civil structures will be limited.
Phase Two TVS
The final configuration of the MFS is illustrated in Figure 12 showing different construction phases.
An additional extraction point is added in order to handle fire-related incidents in the northern tunnel. The main elements to be added to the MFS for Phase Two are described in Table 4.
Normal operation
Similarly to Phase One, the fans for the tunnel ventilation and the shaft tunnel ventilation are not used to supply fresh ambient air to the tunnel since the airexchange, as well as the temperature and humidity levels, are controlled by the piston effect. Mechanical ventilation shall be employed only if necessary.
For safety reasons and in order to preserve the construction, the North running tunnel and the South running tunnel shall be aerodynamically separated during normal operation. All the cross-passage and crossover doors shall be closed.
Design verification: Freight and passenger trains travel the tunnel during Phase Two with a maximum available speed of 100km/h for freight trains and 120km/h for passenger trains. The achievement of the tunnel ventilation objectives during the normal mode of operation has been verified for both summer and winter conditions. Only the results related to the summer are presented here but similar behaviours can be observed for winter.
Similarly to the South running tunnel analysed for Phase One, the natural draft effect will also generate a predominant air flow from Chile to Argentina in the North running tunnel. Thus a full air exchange can be guaranteed within 24h for typical meteorological situations in summer.
Due to the limited train velocities, the maximum positive and negative traininduced pressure fluctuations are small compared to high-speed rail tunnels. It has been verified that they range at 2-3kPa.
Examples of special equipment which might be designed for the tunnel are:
Doors in the rail tunnel and crosspassages:
• The doors resist pressure differences of 30kPa working in both directions. During opening and closing, these elements resist corresponding air velocities, such as, they are permanently guided. They do not swing open or closed in an uncontrolled manner due to wind forces.
• Signs: Being exposed to the longitudinal air flow in a tunnel, these are designed solid enough to resist the wind forces.
Summer dry-bulb temperatures: Similarly to Phase One, a prognosis of the climate was undertaken for the North running tunnel. Simulation results indicated that the maximum dry-bulb temperature in summer does not reach 30°C and is consequently below the limit value of 35°C specified.
Congested operation
No specific ventilation requirements are specified for the congested mode of operation.
Maintenance operation
During Phase Two maintenance may be carried out in a stretch of the North or South tunnel, during which train traffic shall not be stopped. The position of the rail tunnel doors and the crossovers shall be planned in order to separate the maintenance zone from the rest of the tunnel and (at the same time) enable trains passage.
A set of 10 rail tunnel and crossover doors is foreseen in order to cover all the possible tunnel stretches that may be put under maintenance. The position of the rail tunnel doors in the northern and the southern tunnels, as well as the configuration of the crossovers, are planned in order to let the trains pass from one tunnel to the other. Thus, train traffic is not stopped during the maintenance of a particular tunnel stretch.
Selected rail tunnel and crossovers doors are appropriately closed in order to separate the tunnel stretch under maintenance and maintain the train traffic.
The amount of fresh air that has to be supplied in case of maintenance is smaller than that needed during emergency operation, so no specific ventilation requirements are specified for any maintenance operations.
Emergency operation
TVS emergency operation is similar to that considered for Phase One. One extraction point is added above the North running tunnel in order to handle possible fires on trains. Similarly to the Phase One, in case of fire in an MFS, the smoke extraction points are used to control smoke propagation through the tunnel. At the same time fresh air is supplied to the cross passages to enable passenger evacuation.
During a fire incident in a stretch of tunnel, the TVS must provide a longitudinal air flow in the opposite tube. Selected cross passages shall be opened to enable passengers evacuation from the incident to the non-incident tube. In the non-incident tube a rescue train must be provided for the evacuation of passengers out of the tunnel. The other cross passages, as well as the crossover doors, remain closed. The figure 13 illustrates the TVS in case of fire in the North running tunnel in the vicinity of the Chile portal.
Similar ventilation procedures are adopted with the train on fire located in the other stretches of tunnel segments, both in North and South running tunnels.
Design verification: The results for Phase One indicated that in the case of a fire incident in an MFS, the TVS provides air velocities in the escape galleries of 2- 11m/s. Thus meeting the specifications.
As indicated in Figure 13, in case of fire in a stretch of running tunnel, selected cross passages shall be opened and maintained with an overpressure in order to allow evacuation of passengers.
Similarly to a fire in the MFS the air velocity along the opened cross passages must range between 2-11m/s. Following the values specified during Phase One, this ventilation objective has been verified by considering a supply ventilation of 250m3/s (see figure 13) at the MFSs of both Chile and Argentina. It has been verified that, due to the natural draft effect, the ventilation objectives cannot be achieved without closing the rail tunnel doors. In addition, the desired air velocity along the cross passages cannot be obtained by considering only one MFS supplying 250 m3/s. The two axial fans for the supply of fresh air must be operating at once.
Civils design
The supply capacity of the TVS shall be set to 250m3/s and the exhaust capacity to 300m3/s. In order to limit pressure losses at tunnel walls, air velocity in the ventilationshafts should range at 10-15m/s. The ventilation ducts for the extraction of hot air shall have a cross-sectional area in the range 20-30m2.
Due to the important length of the access tunnels (about 4km), the axial ventilators have to overcome a high pressure loss, estimated at about 7500Pa. The necessary power of the ventilators is 3.5MW each with the first in operation while the second is in standby guaranteeing 100 per cent redundancy.
The area of each extraction point shall be of about 20m2. Four dampers having an area of 5m2 each shall be considered, installed at a distance of approximately one metre from each other.
Conclusions
A ventilation concept has been developed for the CBA-BT. Both, Phase One and Phase Two have been analysed and specific ventilation objectives established according to the normal, emergency, maintenance and congested operational modes referencing other major tunnel projects. The ventilation system has been designed for handling a fire on a freight train, which represents the major traffic of the CBA-BT. In addition, passenger traffic has also been considered.
The maintenance and congested mode of operations are not relevant for defining the dimensions of the TVS. Train passage during the maintenance mode of operation can be managed by properly opening and closing the tunnel and crossover doors. Rail tunnel doors are also necessary during the emergency mode of operation in order to reduce the impact of the natural draft in direction of the Argentina portal.
Figure 6, pressure history at two locations along the tunnel with maximum positive and negative deviation from normal pressure. Distances are rail chainages and not distance in tunnel Figure 7, maximum positive and negative traininduced deviations from normal pressure along the tunnel during a complete cycle of the train timetable Figure 8, velocity history at two positions along tunnel with maximal positive and negative velocity based on one-dimensional simulations Figure 9, maximum positive and negative train-induced air velocities along the tunnel Table 3: Description of the TVS for a train fire in a stretch of tunnel Table 4: Elements of the MFS to be considered for the construction of Phase 2 Figure 11, ventilation procedure with the train on fire on the main track of the MFS. The supply fan is set to 250m3/s and the exhaust fan to 300m3/s. Figure 12, MFS at Chile and Argentina during Phase Two Fig 13: Phase 2 emergency mode with train on fire in running tunnel North near Portal Chile
