A History of Research and a Review of Recent Developments

Blast loads in tunnels and shafts 107
Figure 5.14. The distance aR is substituted for R in calculating z, used in
expressions relating pressure to scaled distance, as in Eq. (1.3) for example.
The consequences of nuclear blast entering underground railway systems were
assessed by a number of test programmes in the UK and USA during the 1960s,
and the results are still in limited circulation. However, Henrych has quoted
experimental studies on the decay of peak pressure along a pipeline due to the
roughness of the surface. He suggests that the pressure decrease is exponential
and that at a distance x along the tunnel, the peak pressure p x is given by
(5.21)
where f is the hydraulic coefficient of friction, r is the pipe radius, and there is
no reflecting surface at the inlet opening. The value of f can be found from the
approximate formula
f=(2 log r/2h+1.74) –2 ,
(5.22)
where h is the height of the roughness.
From the work of Henrych it is clear that in comparison with ‘free field’
conditions, the peak instantaneous pressure at a given range along the tunnel
is much less than would be measured in air at the same range from an explosion,
Figure 5.14 Entry of blast into a tunnel from an explosion in line with the tunnel
mouth (from Henrych, ref. 5.1).