*2.7. Dead Zone Delimitation*

The dead zone is the area in which the ventilating flow is not capable of sweeping the blasting gases. For practical reasons, several authors and legislations have defined the dead zone as that where the air velocity is at most 0.5 m/s [19] or, less restrictively, at most 0.15 m/s [18,25]. This definition, for a long period, implies that any air clearance is still possible until the air velocity is zero. We can see in the discussion section that this definition does not fit into reality.

For this reason, two alternative methodologies were tested in order to design the dead zone, which are: (a) air jet velocity analysis through the tunnel axis; (b) analysis of the area-weighted mean vertical speed in sections transverse to the tunnel axis.

For this first method, the control surfaces 1.25 times as large as the duct diameter and coaxial with it, were generated every half meter, covering the distance between the duct and the face. In the second approach, control surfaces were generated every half meter, covering the entire section of the tunnel.
