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8 Secondary Ventilation
8.4.5 Estimation as a Function of the Tunnel Transverse
Section
By way of reference, in workings where no CH 4 is present, airflow rates from 0.25
to 0.3 m
3 s
−1 per m
2 of gallery cross section can be used for the dilution of blasting
gases in development ends (Expilly 1960). That is to say (Eq. 8.16):
Q = k S = 0.25 S
(8.16)
where
• Q: Airflow rate required at the face (m
3 s
−1 ),
• k: Wind speed (0.25 − 0.3
m
s ·s
m 2 ), and
• S: Drive cross section (m
2 ).
Note that Eq. 8.16 can be compared with the definition of airflow rate (Q = v S),
thus if v = k, we have wind speeds of 0.25–0.3 m s
−1 . For tunnels in which works
are not undertaken in conditions of positive pressure, BS 6164 requires an airflow
rate of 9 m
3 per min per m
2 of gallery cross section (0.15
m
3 ·s
m 2 ).
If it is assumed that the gases propagate to a distance of about 20 m from a blast
site, Eq. 8.17 can be used (Vutukuri and Lama 1986, p. 101):
Q =
20 S N
t
(8.17)
where
• Q: Required airflow rate at the face (m
3 s
−1 ),
• S: Tunnel cross section (m
2 ),
• t: Time in which the blast fumes are to be evacuated (s), and
• N: Number of air renovations (usually 5).
8.4.6 Dilution Factor Method
This expression (Eq. 8.18), is usually included in Chilean methodological guides
(e.g. Andrade 2008):
t =
100 M exp a
d Q
(8.18)
where
• t: Time for the dilution of the gases (min).
• M exp : Explosive mass detonated (kg).