Innovative passive smoke control in Copthall Tunnel

This paper describes a ventilation concept for Copthall tunnel which allows this 600m long tunnel to be naturally ventilated to control pressures, temperatures and control smoke in the event of a fire.

A significant benefit of the tunnel solution for Copthall (when compared with the cutting previously planned at the hybrid Bill stage) is that land over the tunnel can be returned to a useable function, allowing for flora and fauna and the possibility of wider public land use. There are also opportunities to integrate excavated material within the wider landscape design. Usually a 600m long tunnel would require the provision of mechanical ventilation systems for temperature and smoke control. The associated plant and equipment for this mechanical ventilation would have a visual impact and would add to both capital and operational cost.

The proposed natural ventilation approach is based on ventilation openings in the tunnel soffit and through the earthworks above that allow smoke to disperse to atmosphere. The openings are spaced at regular intervals along the length of the tunnel to allow for effective smoke control for any fire location. Like all natural ventilation systems, their effectiveness is strongly influenced by outside wind which might otherwise cause smoke to be blown along the tunnel rather than pass up the chimneys. To mitigate this, a concept was developed that allowed the wind to create a suction pressure at the top of the chimneys and thus help draw smoke out of the tunnel on windy days.

Three-dimensional modelling is used to account for the complex effect of the wind field over the ventilation openings and at the tunnel portals on smoke movement from a train fire in the tunnel. This analysis predicts that the relatively large tunnel cross section allows smoke to rise to high level within the tunnel before dispersing to atmosphere through the openings, thereby keeping escape routes tenable in the event of passenger evacuation, and allow the size and height of the vents to be optimised to create enough suction force.

This innovative passive smoke control concept is a key part of a cost-effective tunnel design that creates green infrastructure and connects to the wider landscape and ecological context.