At this time of uncertainty it has given a number of us time to think and ponder some of the more interesting or controversial areas of tunnelling and life. The ATS will be presenting these pieces of writing from our membership and we welcome submissions.
The conditions of the article publication are:
Author must be an ATS member
Writing must have at least a tenuous link to tunnels or tunnelling
Pieces shall be reviewed and chosen by the ATS Executive
Please submit your article to firstname.lastname@example.org with the subject line Tunnel Thoughts.
Critical velocity and the significance of the imminent retraction of 2020 NFPA 502’s Annex D critical velocity equations
Michael Beyer, Conrad Stacey, Arnold Dix This note summarises what was discovered in a review of the technical background for the change to the critical velocity values resulting from Annex D of (NFPA 502, 2020), as well as the recent developments regarding the critical velocity in Annex D of (NFPA 502, 2020), and why a retraction of this formula is imminent. It is very significant for road tunnel design in many parts of the world. The everyday objective of a tunnel ventilation system is to provide a safe environment for all tunnel users by diluting vehicle exhaust gases and particulates. In case of a fire, the focus becomes the control of the smoke, to provide tenable escape conditions for the tunnel users and access for attending emergency services. With vehicles becoming cleaner, the fire case now generally controls design, in terms of the required ventilation power, the costs, and the tunnel safety. Different countries have some very different approaches and philosophies on the control of smoke in tunnels. A brief overview will be provided here but a more comprehensive discussion about the different ventilation philosophies during a fire, and the different applied velocity approaches, is given in (Sturm, Beyer, & Rafiei, 2015). European approaches do not rely upon high tunnel air velocities to prevent any upstream propagation of a hot smoke layer. Especially for bidirectional traffic and tunnels with high congestion, European standards refer more to a ‘low velocity’ philosophy to preserve any smoke stratification, reduce the potential risk [...]