Seismic Restraint for the Marmaray Tunnel’s Ventilation Fans and MEP Services

The Marmaray rail tunnel links the European and Asian sides of Istanbul beneath the Bosphorus. Its tunnel section runs for about 13.6 km, and at its centre is an immersed tube 1.4 km long: eleven precast sections of up to 18,000 tonnes each, placed 60 m below sea level. It is the deepest immersed tube tunnel in the world, and it opened to passengers on 29 October 2013.

The tunnel lies about 18 km from the North Anatolian Fault. Its structure was designed to flex in an earthquake, and its MEP services had to do the same: stay in place, stay connected and keep working. That was our part of the project, and Marmaray remains the largest project in Acrefine’s history.

Our scope

The work came in two stages, for two clients.

Concept seismic design, for Taisei Corporation. Before construction, we prepared the concept seismic design for the tunnel’s electromechanical services for Taisei, the Japanese contractor leading the Turkish-Japanese consortium that built the tunnel. The design covered restraint for the mechanical equipment and seismic bracing for pipework, ductwork and cable trays. Calculations followed the Turkish seismic code, with FEMA guidance for the non-structural components.

Shop drawings, products and site supervision, for Anel. During construction, we worked for Anel, the MEP contractor. We turned the concept design into shop drawings that installers could build from, supplied almost all of the seismic restraint products on the job, and supervised their installation on site.

Heavy-duty seismic snubbers for the tunnel ventilation fans

A rail tunnel depends on its ventilation system in an emergency. If a fire breaks out, the fans have to move air and smoke along the tunnel while people get out. An earthquake that dislodges a fan, or tears its connection to the ductwork, takes that protection away at the worst possible moment.

The tunnel ventilation fans were supplied by Woods Air Movement, then trading as Fläkt Woods. They are large axial fans, installed across eight ventilation shafts: thirty double-impeller units of 2.5 m and 2.8 m in diameter, and eighteen single-impeller units of about 1.6 m.

The fans stand on spring isolators that keep their vibration out of the tunnel structure. That leaves them resiliently mounted, and a resiliently mounted fan of this size can move a long way in an earthquake unless something stops it. That is the job of the snubber.

For these fans we used our AS-AR-10000, a heavy-duty, tailor-made seismic snubber built for equipment far larger than standard snubbers are sized for. A snubber does not carry the fan in normal service; it leaves a small gap around the support so that the isolators can do their work. In an earthquake the gap closes and the snubber takes the load, through resilient contact faces, so that the fan is stopped without a hard steel-on-steel impact. The fans are resiliently mounted yet seismically restrained.

The snubbers were sized from the fan weights, the position of the centre of gravity above the supports and the seismic demand at the tunnel, and they were galvanised for a damp, corrosive environment below sea level.

Seismic cable bracing for pipework, ductwork and cable trays

Fire water, drainage, ventilation ductwork and power distribution run the length of the tunnel. Each of these services has to survive the earthquake, and each has to keep working when it is needed in the emergency that follows.

We designed and supplied seismic cable bracing for all of them: tens of thousands of braces in total. Each run was checked for its weight, its support spacing and its connections to the structure, with transverse and longitudinal braces placed so that pipework, ductwork and cable trays stay in place and do not strike each other or the tunnel lining.

Engineering challenges

Life safety systems. The ventilation fans, fire water pipework and emergency power are all life safety systems. Their restraint was designed for an earthquake at full seismic demand, not for the loads of normal service.

A marine environment. Below the Bosphorus, the air is humid and corrosive. Restraint hardware had to be protected for a service life measured in decades.

Coordination across services. In a tunnel, ductwork, pipework and cable trays share very little space. Bracing for one service could not clash with another, and movement in one could not be allowed to damage the next.

Anchoring to the tunnel structure. Anchor positions for the snubbers and braces were coordinated with the tunnel structure, so that restraint loads went into the concrete where the structure could take them.

Why seismic restraint matters in tunnels

In a building, people can usually leave by more than one route. In a tunnel they often cannot, and evacuation takes longer. The MEP services that protect them — ventilation, fire water and emergency power — have to keep working during and after an earthquake, which is exactly when they are most likely to be called on.

The same thinking applies wherever large equipment is resiliently mounted: in plant rooms, on rooftops and in data centres. Vibration isolation and seismic restraint have to be designed together.

We’d love to hear your thoughts. Join the conversation on our LinkedIn post about seismic restraint in undersea infrastructure.