Cut and cover tunnels or enclosures of existing highways in urban areas are typically rather short (up to 600 m) and have a high number of lanes. Due to high traffic frequency in urban areas such tunnels have a high probability for congested traffic. In case of incident the number of trapped persons in the tunnel is high and the risk for incidents with high severity grows. A scenario analysis, investigating self-rescue process and self-rescue conditions due to ventilation in such tunnels has been performed. The results of this scenario analysis reveal that common ventilation and self-rescue strategies, as defined in various normative documents should not be applied to these tunnels. A better safety level can be obtained by increasing the number of emergency exits, which can be built cheaply in cut and cover tunnels, and choosing a passive ventilation strategy in case of congestion. This paper showed that normative requirements for road tunnels are not always applicable in a straightforward manner to cut and cover tunnels in urban environments. These are usually characterized by high traffic loads and frequent congestion. A scenario-based approach was presented for analyzing the requirements in terms of ventilation and emergency exits for any specific configuration. The results and conclusions are case dependent, but a few general indications clearly emerge from the analysis of several cases: (1) A distance of 300 m between emergency exits is likely to be insufficient in case of short tunnels with natural or longitudinal ventilation; (2) A door width of about 1.2 m is likely to be sufficient for tunnels with high number of lanes, as egress time is mainly determined by self-rescue distances; (3) In case of congestion and short tunnels the risk of smoke de-stratification is high and the best possible ventilation strategy is likely to be natural ventilation (no activation of the jet fans in the fire tube) complemented by providing short self-rescue distances; (4) Ventilation design must take into account smoke recirculation and over-pressurizing the opposite tube (used for self-rescue); (5) Congestion detection must be fast and accurate and thus needs a dense net of point sensors or support from CCTV; (6) Fire detection needs special attention in case of large numbers of lanes and high tunnel aspect ratios.
Ventilation and escape facilities for short cut-and-cover urban tunnels
2014
8 Seiten, Bilder, Tabellen, 5 Quellen
Conference paper
English
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