Contractors employed Meyco Fireshield 1350 in the 15.9kmlong Bodio section (southern portion) running tunnels of the Gotthard project, the longest railway tunnel in the world. Fireshield 1350 is a special fire protection mortar that can be applied by hand, as with other mortars, or by the same spraying methods used for sprayed concrete lining support.

The engineering consortium that designed the Bodio section, as well as the Faido and Sedrun sections, totalling about 38km in length, is the Engineering JV Gotthard Base Tunnel South comprising of Lombardi Engineering, Amberg Engineering and Poyry Infra, all of Switzerland.

In a similar way to Channel Tunnel safety procedures (see previous article), it is known that a potential train fire in the tunnel could cause damage to the structure, putting users at risk and resulting in the need for time-consuming and expensive rehabilitation to make the structure safe for operations again. The aim of fire protection is therefore to ensure tunnel availability during the event (fire) for the safety of individuals, and to promote availability after the event for cost effectiveness in returning the train services to full operation. The first requirement implies that the affected bore must be adequately structurally safe for 45 minutes, and the neighbouring bore for 90 minutes, so that users can be rescued safely.

The project owner, AlpTransit Gotthard, set up a fire protection task force in 2003 to indentify scenarios associated with a freight train fire and a passenger train fire (sizes of 250 MW and 40 MW respectively) and to develop damage and risk assessments for both. This was coupled with the assessment and recommendation of protective measures to meet the availability requirements described earlier.

The passive fire protection measures identified were:
• Addition of polypropylene fibres to the concrete mix to avoid heat spalling,
• Where these fibres were used, increase the concrete cover to protect reinforcement steel and to ensure no reduction in tensile strength due to increased temperature,
• Sacrificial fire protection layers to insulate the lining from the fire’s heat.

The Fireshield 1350 fire protection mortar manufactured by BASF protects the concrete tunnel lining of the Bodio tunnel cut-and-cover section. This is near the southern portal in Switzerland Ticino canton, linking it with the Faido multifunctional station further north. The section comprises the two bores, each 400m long, and one cross-passage about 260m from the portal. Application of this special fire protection mortar was required since temperatures exceeding 300 degrees C lead to concrete spalling and crumbling, and when concrete is exposed to temperatures above 1000 degrees C it loses its load-bearing capacity altogether and the tunnel may collapse.

A minimum thickness of 31mm of Fireshield was required to meet fire protection requirements, applied direct to the structural concrete lining. The specification was increased to 35mm thickness to cater for application tolerances. To maximize a high-tensile bond strength between the Fireshield layer and the concrete lining, a fully-bonded design including stainless-steel fine-mesh reinforcement was chosen. Before application the concrete surface was hydromilled at 2000bar pressure by a robot to achieve the specified minimum roughness depth of 5mm. Stainless steel rockbolts were also used at a rate of five piece per square metre to hold the mesh and provide additional security against delamination.

The Fireshield 1350 was applied by similar robotic spray equipment to that used for sprayed concrete, with a total applications area of about 13 500 m2. The correct application thickness was aided by coloured markers on specially designed Fischer spaces, used with Fischer Nail Anchors to hold the mesh.

“Being covered with a layer of fire protection mortar the tunnel walls withstand temperatures of up to 1400 deg Celsius for a minimum of 90 minutes. This way we gain some precious time for firefighting,” says Frank Clement, BASF expert for underground fire protection solutions.

A Meyco Suprema spraying unit and nozzle system were used by contractor Walo Bertschinger to apply a layer of a specified thickness onto the inner concrete lining of the tunnel.

Spraying Fireshield through and onto the stainless steel mesh by remote spraying unit