The project Corredor Bioceanico Aconcagua (CBA) is a future rail line between Chile and Argentina with a length of about 215km. An important component of the CBA project is the 53km long base tunnel (BT), which will allow crossing the Andes most directly and with moderate inclination (Figure 1).

The construction of the BT is divided into two different phases. First a single-tube, single-track tunnel will be constructed and operated. In a second phase, a second parallel tube will be added, so that a twintube, single-track rail tunnel will be available. While in the first phase the BT will be used almost exclusively for freight trains, in the second phase passenger trains may pass as well. The future doubletrack system of Phase Two will allow the traffic of more than 160 freight trains every day corresponding to 70Mt of goods in both directions each year, and an appropriate number of passenger trains.

Introduction
High rock temperatures and a substantial height difference of the portals of the BT require particular measures to assure an appropriate climate, reasonable aerodynamic conditions and a sufficient air-exchange by natural and mechanical ventilation. Here these aspects are clarified for all modes of operation (normal, emergency, maintenance and congested). The design objectives and the proposed solutions are presented. Onedimensional numerical simulation models are used for the analysis of the aerodynamic and thermodynamic issues of the CBA-BT. A major outcome is the important role of tunnel climate and tunnel ventilation in the CBA-BT project.

The project involves restoring a former rail line between Chile and Argentina. The system will connect the cities of Los Andes (Chile) and Lujan de Cuyo (Argentina). The future railway network will be of importance as it will offer a direct rail connection between the Atlantic (Buenos Aires) and the Pacific Ocean (Valparaiso). The BT will also guarantee higher train speeds.

The cities of Los Andes and Lujan de Cuyo will be provided with terminals for shuttle trains, as well as for the management of customs operations.

With the planned traffic the CBA may become the most important railway line for goods transportation of South America in the future.

Due to its exceptional length, the BT requires particular measures to ensure:

• An appropriate climate,
• Reasonable aerodynamic conditions and
• Sufficient air-exchange by ventilation.

This paper aims to analyse the aerodynamics and the climate for the CBABT and to present a ventilation concept. It shall confirm the feasibility of the tunnel ventilation system (TVS) for Phase One and Phase Two of the project regarding the normal, maintenance, congested and emergency modes of operation.

Tunnel system
The 53km-long CBA-BT has an inclination of up to 17 per thousand (17:1,000). There is no peak point in the middle part of the tunnel but a mostly continuous positive gradient from the Chile portal in direction of the Argentina portal. The different heights of the two portals generate a pressure difference, which depends on the difference between the tunnel and the outdoor air temperatures. This results in a predominant air flow along the tunnel from portal Chile to portal Argentina (natural draft effect).

Existing international norms, guidelines and recommendations shall be followed to plan the civil layout of the CBA-BT. Some safety related standards are:

• Union International des Chemins de Fer UIC 779-9 (2003).
• United Nations (2003).
• European Community Technical Specifications of Interoperability (2008).

At its final stage (end of Phase Two), the CBA-BT will consist of:

• Two single-track rail tunnels.
• Cross passages every 500 m connecting the rail tunnel tubes.
• Two multifunction stations (MFS) with a distance of 17 km between them.

An illustration of the CBA-BT for the two construction phases is provided in Figure 2.

Rolling stock
Phase One of the CBA-BT is foreseen for the operation of freight trains only. The use of passenger trains is planned for Phase Two when the second tube will be in use.

Two different types of freight trains are considered:

• Shuttle trains: transport of road vehicles, i.e., trucks.
• Qualified freight trains: mixed stock for transport of goods.

Velocities of 100km/h are foreseen for freight trains and 120km/h for passenger trains.

The magnitude of a possible fire on a train may vary a lot depending on the type of goods that the freight train is transporting. As indicated in Figure 3 (above), fires on freight trains could reach a maximal heat release rate (HRR) of approximately 250 MW. Due to the extreme release of energy, comparing different reference tunnel projects is not relevant for the ventilation equipment needed to handle fire on freight trains. This consideration is generally planned by considering a maximum HRR of 20MW (expected power on typical passenger trains).

Methodology
The following methodology has been considered to study the TVS:
• Setting of the concept.
• Prove the feasibility of the designed TVS by numerical simulations.
• Identify the consequences for civil design of the specified TVS.

Numerical tools
For the analysis of the aero- and thermodynamic issues of the CBA–BT, two numerical tools with their latest versions are used: Thermotun and Thermo.

Thermotun is a program for aerodynamic simulations of rail tunnels developed by A.E. Vardy. It is based on a one-dimensional method of characteristics and is validated by several easurements. The program allows for modelling the performance of the TVS, including fans and dampers, or to calculate the dispersion of pollution and heat. Thermotun is an internationally-approved tool and is recognised for its excellent prediction capabilities.

Thermo is a one-dimensional programme designed for thermodynamic short- and long-term computations. It has been developed by HBI Haerter and considers the thermodynamic interaction between the train, the tunnel air as well as the tunnel wall enabling the program to include heat transfer from rock and trains. The output from Thermo contains the temporal and spatial distribution of temperature and humidity as well as the heat balance of the trains in the tunnel system. Thermo has been validated against other thermodynamic programs (e.g., a program of the SNCF [French Railways] for the Alpetunnel Project between Lyon and Turin) and with measurements for the Furka tunnel. Thermo has been applied for numerous tunnel systems.

TVS functional requirements
In normal operation design, normal trains pass through the CBA-BT according to schedule. When a train enters the tunnel, a pressure wave is generated. This propagates at the speed of sound as a compression wave (+) along the tunnel (piston effect). When the wave hits the exit portal, it is partially reflected and travels back as an expansion wave (-). Further reflections at portals, cross-sectional area changes or trains might lead to various compression or expanding waves oscillating in the tunnel. These pressure waves together with the pressure differences along the moving train act on the train structure, the tunnel wall, the installed equipment and the people travelling on the train. The magnitude of the pressure fluctuations is a result of the speed, the cross-section, the shape and the roughness of the train and the length, roughness and the civil construction of the tunnel and portals.

The tunnel temperature and humidity are influenced by several factors such as:

• Rock thermodynamic properties.
• Heat load from the rolling stock and
• electrical installatons in the tunnel.
• Water through the tunnel walls.
• Meteorological conditions at the entrance portal.

During normal mode of operation, the air quality, temperature and humidity are controlled by the train-induced piston effect or (if necessary) by forced ventilation provided by the TVS.

In congested or occasionally disturbed operation, trains need to stop due to track blockage, power cut-off or other reasons. Staying stationary for several minutes or even hours might lead to an unacceptable temperature rise because of release of waste heat (from air-conditioning or cooling facilities). During the congested mode of operation, the TVS is used to supply ambient air and remove (with respect to exhaust) waste air. The amount of fresh air that has to be supplied (and removed) is much lower than the one needed during the emergency mode of operation. The congested mode of operation is irrelevant for sizing the ventilation equipment.

During maintenance work in the CBA-BT, train traffic is restricted or stopped. Maintenance works can affect the singlebore tunnel (in use during Phase One) more than the twin-bore considered for Phase Two, as no alternative route is possible when people are working in the tube. Generally, rail tunnel doors are employed in order to separate the tunnel segments under maintenance and in order to provide controlled ventilation conditions.

Tunnel parts with maintenance operation have to be provided with a longitudinal air flow. Fresh air supply is needed to remove polluted air and to provide sufficient working conditions. The amount of fresh air that has to be supplied is much lower than the one needed during the emergency mode of operation. Thus, the maintenance mode of operation does not determine sizing of ventilation equipment.

In emergency operation, it is most important to control the propagation of smoke. While natural ventilation allows the removal of smoke only under certain boundary conditions, mechanical ventilation allows full control of the smoke dispersion. For the CBA-BT, specific ventilation requirements must be considered for Phase One (single tube) and Phase Two (double tube).

Concerning the emergency ventilation, the following principles must be adopted:

• Longitudinal flow of air/smoke in incident tunnel: The objective is to achieve the critical velocity, i.e., no back layering of smoke, which will protect escaping passengers upstream of the fire from the effects of the smoke (see Figure 4, and which provides a defined access for rescue and fire fighting services. This principle will be applied for Phase One of the CBA-BT. Longitudinal ventilation inherently implies the risk of moving smoke in the direction of escaping passengers and rescue services. Even if the position of a fire is known exactly, this dilemma cannot be eliminated unless local smoke extraction is applied. However, operating the ventilation system in a proper manner can moderate the harmful effects of this ventilation principle, e.g., a moderate air velocity at the beginning of an incident will support the smoke stratification. The advantage of providing defined conditions outweighs the disadvantages of the longitudinal ventilation.

• Pressurisation of the escape routes: The objective is to provide a flow of air towards the incident tube when crosspassages are open (see Figure 5). This will be applied for Phase Two of the CBA-BT and in general for handling emergency operations in an MFS.

Phase One TVS concept
One ventilation station will be located at each portal of the access tunnels of every MFS. In order to reach 100 per cent redundancy, each consists of two fans for supply and two fans for extraction (with associated dampers). By configuring the dampers at the axial fans it is possible to direct air via a number of paths and enabling system flexibility to provide the required level of redundancy. Possibly additional jet-fans may be installed near the tunnel portals. The main features of the MFS and of the TVS for Phase One are presented in Table 2.

In part two of this paper, to appear in the next issue of T&TI, the basic concepts for the ventilation during the four operation modes are described.


Figure 1, the Corredor Bioceanico Aconcagua base tunnel (CBA-BT). Detailed view beneath Table 1: Tunnel ventilation objectives Table 2: Features of TVS for Phase One Figure 5, MFS during Phase One (numbers refer to feature in Table 2) Figure 4, twin-tube tunnel with ventilation station and cross passages Figure 3, heat release rates on freight trains Figure 2, Construction design for the two operating phases