The Stonehenge Tunnel is to form part of the proposed upgrade of the existing A303 to dual carriageway between Berwick Down and Amesbury. The twin-bore scheme is approximately 2.1 km long, with each bore accommodating a uni-directional two-lane carriageway and paved verges within a total cross-sectional area of ~ 70 m2.

The tunnels require ventilation during normal operation to dilute vehicle emissions such as CO, NOx and particulates to acceptable levels (e.g. as stipulated in the applicable Codes & Standards). In the event of a fire, ventilation may be required both to control smoke spread in order to facilitate both evacuation of the tunnel occupants and subsequent emergency services operations.

In recent years, the favoured ventilation system for twin bore tunnels has become the longitudinal type, comprising reversible jet fans suspended from the roof of the tunnel – in this design, no ducts or external vents are required (see diagrams on left).

The critical design data for normal and emergency ventilation states are the projected peak hourly traffic flow (together with the relevant emissions data) and the ‘design fire’, respectively. Once these inputs have been defined, the proposed ventilation system is modelled using a PC-based aerodynamic analysis package which takes into account the various flow resistances in the system (e.g. tunnel wall friction, vehicle body drag, meteorological effects etc.) to arrive at the optimum fan power and distribution pattern.