SAFETY COMES FIRST

Tunnel schemes are increasingly required to demonstrate compliance with air quality and fire safety standards. The article discusses the implication of such standards on the design of tunnels using two case studies to illustrate the use of modelling techniques to solve potential problems and assess the possible effect of fires. Scale wind or water tunnel models can be used to produce an analogue of tunnel structure but the model is very much smaller than full size and often is not a realistic representation. Quantitative risk assessment involves the estimation of the frequency of occurrence of an incident based on statistical evidence and the use of mathematical models to evaluate the consequences of the incident. An example is given of a study to assess vehicle exhaust emission in the planning of an autoroute around Paris which changed from bored tunnel to trench to cut-and-cover tunnel. Studies of the effect of geometry, different traffic flows and ventilation conditions showed that carriageway separation could prevent substantial build up of pollution at the portals. Calculation to estimate liquid outflow, pool spread, evaporation, gas dispersion, explosion overpressure and fire in a twin tunnel planned for the city of Rouen showed that dangerous goods that vaporise rapidly can quickly affect large portions of the tunnel.