At the heart of the new flat transalpine rail route being constructed by the Swiss Federal Railways is the Gotthard Base Tunnel. The tunnel is part of the Swiss Alp Transit project, also known as New Railway Link through the Alps (NRLA).
With its planned length of around 57.1km, and a total of 153.5km of tunnels, shafts and passages, once finished, the Gotthard Base Tunnel will be the longest tunnel in the world.
The tunnel cuts through the Gotthard massif at nearly ground level. The two portals will be near the villages of Erstfeld, Canton Uri (Northern Portal) and Bodio, Canton Ticino (Southern Portal).
The dual purpose of this project is to provide a high speed link for passengers between southern Germany in the north and Italy in the south of continental Europe and to transfer freight traffic from the roads to rail, as required by the ‘Alpine Protection Act’ of 1994. It represents also an essential step to actively protect the sensitive region of the Alps and to get an important contribution to preserve the environment in general.
The tunnel system mainly consists of two single track tunnel tubes that are interconnected approximately every 325m by cross passages. The horizontal distance between the tube axes varies between 40 and 70m. Furthermore, at the one third and two third points along the tunnel, there are two multi-functional stations with track changes, emergency stations, technical equipment rooms and ventilation systems.
In order to optimise the total construction time, the tunnel was divided into five sections: Bodio, Faido, Sedrun, Amsteg and Erstfeld.
The design consortium of the Bodio, Faido and Sedrun Sections (total length about 38km) is the engineering joint venture Gotthard Base Tunnel South (Lombardi Engineering Ltd/ Amberg Engineering Ltd / Pöyry Infra Ltd.
The Bodio section, at around 15.9km, connects the Southern Portal with the Faido multi-functional station. The two tunnel tubes and 51 cross-passages of this section were excavated between 2000 and 2006. Following the initial cut-and-cover and loose ground sections approximately 400m each in length, the 1.7 km in the eastern tube and 0.8 km in the western tube were excavated by drill and blast. The remaining section up to the boundary with the adjacent Faido section was then driven by the main contractor using two open face TBMs.
General fire protection
It is important to the operator that the tunnel remains in use at all times. Therefore, reasonable effort should be put into allowing continued operation of a tunnel bore in the event of a passenger and freight train fire in the other tunnel bores.
A passenger or freight train fire in the GBT could potentially cause damage to the structure putting tunnel users at risk. This would lead to time-consuming and expensive rehabilitation works to bring the structure to its original state.
For these reasons the following two requirements must be met:
• Availability during the event (individual safety): the main and mandatory criterion is to ensure the safety of people and full operation of the tunnel until all users are safe. In order to assure this, the availability of the tunnel has to be maintained as long possible until users are able to reach a safe place. This implies that the affected bore must be structurally safe and adequate for 45 minutes, and the neighbouring bore for 90 minutes.
• Availability after an event (cost effectiveness). In the case of a fire on a train reasonable effort should be put into ensuring that the service interruption is minimized. This is a cost optimization problem. The reopening of the railway line has immediate priority following the rescue of users. In order to ensure this, constructive fire-protection measures have to be taken to exclude major damages or collapse of the neighbouring bore and also to reduce resulting damages, time and costs for the repair of the damaged bore. In 2003 the “fire protection” task force was founded by Alp Transit Gotthard AG with the following goals:
• Identification of fire scenarios for “freight train fire” and for “passenger train fire” • Elaboration of a damage and risk assessment (without protection) for the different fire scenarios
• Assessment and recommendation of protective measures, provided both availability requirements are fulfilled
For the evaluation of the design fire scenarios of the tunnel structures, fire sizes of 250MW (freight train) and 40MW (passenger train) were assumed. A computer simulation of freight and passenger trains (with a train length of: 750m, 32 wagons) catching fire inside a tunnel bore was undertaken. In the simulation, each wagon burns for 45 min with the fire source hopping from wagon to wagon every 6 minutes (moving source). The duration of the fire is 4 hours.
Different temperature-time diagrams have been developed around Europe, however a new time-temperature curve was adopted for the Gotthard Base Tunnel, to better consider the particular fire-event scenarios and the effective tunnel design.
The need for fire protection was identified through a damage and risk assessment conducted for the entire length of the tunnel. This meant evaluating, for “individual safety” and “cost effectiveness”, the probability and extent of damage and consequences to the safety and to the structure of the tunnel system without any fire protection.
The following passive fire protective measures were identified:
• Addition of polypropylene fibres (especially in unfavorable geological formation) to avoid spalling
• In combination with polypropylene fibres, increased concrete cover, to protect the steel and ensure no reduction in tensile strength with increased temperature
• Sacrificial fire protection layers to insulate the tunnel lining.
Fire protection in Bodio
The cut-and-cover section of Bodio started in 2000 and was completed two years later. It consists of two bores each of 400m in length and one cross-passage, which is situated about 260m from the southern portal.
Following the investigations of the fire protection task force it was decided that to comply with the individual safety criterion a fire protection layer on the existing tunnel lining was necessary on the whole length of the cut-and-cover section of Bodio. This is because in the event of a fire, it could not be excluded that damage or collapse of one bore could cause damage to the other making it impossible to evacuate the tunnel.
Many fire protection systems were analysed and rated for their technical and economical performance. A cement based fire protection layer resulted as the best solution for the cut-and-cover section of Bodio.
The following are requirements for the passive fire protection layer:
• Fire protection of the existing tunnel lining according to the RABT/ZTV standard design fire curve (90 minutes at 1200°C and the following cooling phase of 110 minutes) with respect of the following two conditions: temperature at the interface £ 400°C, temperature at the reinforcement (with a concrete cover of 4cm) 250°C.
• After an event the fire protection layer can be partially or completely replaced
• Good tensile bond strength with the existing concrete lining
• High frost and freeze-thaw resistance.
• Dead load resistance and resistance against stresses caused by the train service. The assumed amplitude of air pressure fluctuation is ±10 kN/m2.
• Resistance against variations in temperature between -10°C and +40°C and against fluctuations of relative humidity between 20% and 100%.
• Resistance against cleaning by high pressurized water
• Resistance against local perforations and against stresses induced by fixation of railway infrastructures
• The thickness of the fire protection layer has to be as thin as possible because of the tight space available
• Service life of 50 years
Bodio solution
Under the prescribed circumstances and requirements the application of a layer of MEYCO Fireshield 1350 mortar was chosen. The product distributor (BASF Construction Chemicals) defined a minimum thickness of 31mm for the mortar to meet the fire protection requirements of the project. The fire protection layer was applied on the concrete lining in both tunnel sections (see Figure 1). To give some tolerance in the layer thickness of ±4mm a standard thickness of 35mm was defined – the effective layer thickness therefore varies between 31mm and 39mm.
A fully bonded solution combined with stainless mesh reinforcement was chosen to ensure the tensile bond strength with the existing layer met the requirements.
The existing surface of the cut and cover section was pre-treated using robotic hydro milling (2’000 bar) of the concrete surface to ensure full bonding of the mortar to the substrate. A minimum depth of roughness of 5mm was prescribed. (see Figure 2).
A stainless fine-meshed mesh reinforcement (with a diameter of 1.5mm each 5cm in both directions) was installed on the structure substrate with a minimum of 5 stainless bolts per m2 to provide additional safety against delamination. Afterwards, the fire protection mortar was applied in one step with a sprayed concrete robot (see Figure 3).
A total area of about 13,500 m2 of fire protection mortar was spray-applied in the Bodio Section of the Gotthard Base Tunnel. Figure 4 shows the detail of the fire protection layer construction.
For fixing stainless steel reinforcement matting to the inside walls of the tunnel a new spacer (mesh holder) developed by Fischer has been used in combination with the Fischer Nail Anchor FNA II 6×30/20 A4 (see Figure 5). This new spacer system has been tested according to the RWS fire curve. The coloured markings show the user the necessary minimum thickness of the mortar layer to be sprayed over the matting.
Fig 1 – Cross-section of the cutand- cover portion Bodio with thermal barrier Fig 2 – Hydromilling Fig 3 – Fire protection mortar is applied in one step Fig 4 – Fire protection layer construction Fig 5 – The Fischer spacer used for fixing reinforcement matting
