Under normal operation, each bore of the proposed twin bore tunnel at Stonehenge carries unidirectional traffic flow from one portal to the other. The ventilation scheme within each bore is arranged so that in a fire emergency the direction of flow produced by the ceiling-mounted jetfans can be set so as to control smoke and move it in a direction away from motorists.

During maintenance operations when one tunnel is closed however, the traffic flow in the available bore is bi-directional and so this smoke control strategy cannot be employed; this is because the longitudinal displacement of smoke would cause a rapid deterioration of conditions downstream of the fire and potentially dangerous conditions for drivers in that area. Hence, for a fire emergency during bi-directional traffic flow a passive smoke control strategy (i.e. no positive displacement of smoke – jet fans off) is recommended, in line with international guidelines. This aims to achieve stratification of smoke and hot gas beneath the tunnel crown, as the result of buoyancy and natural convection. The layering of hot gas and smoke beneath the tunnel crown increases the available evacuation time, before the onset of untenable conditions.

This strategy was studied using CFD and four fire scenarios were simulated, considering both unidirectional and bi-directional traffic flow, with a range of fire heat release rates:

	5 MW - car
	30 MW - Heavy Goods Vehicle (HGV)
	100 MW - HGV

The computational domain simulates a 296 or 500 metre section of tunnel, including cross passages and two jet fans. Individual cars and an HGV are represented in some detail by the computational mesh. Transient simulations were conducted, modelling the first 15 minutes of fire development. In each case, the calculations were used to produce animations of the developing smoke cloud and temperature field for discussion within the project’s Tunnel Design and Safety Consultation Group (TDSCG).