A History of Research and a Review of Recent Developments

Blast loads in tunnels and shafts 103
There were two problems, the first resulting from an explosion in the tunnel,
the second due to the entry of blast from an explosion outside. A selection of
Philip’s results for explosions within a long straight tunnel was given by
Christopherson [4.6] and some are shown in Figure 5.8. They refer to bricklined
tunnels, in which pressure decay is likely to be quicker than in smoother
tunnels of concrete or steel. The rate at which the peak pressures decayed was
influenced by the parameter W 1/3 /S 1/2 , where S is the cross-sectional area in
square feet, and W is the weight of the charge of TNT in lb. Although the
peak pressures gradually decayed along the tunnel it was interesting to note
that at short distances from the explosion the blast impulse had increased
slightly, and for very small charges this increase could still be measured at
long distances from the point of explosion, as indicated in Figure 5.9.
It was clear that in long straight tunnels the decay was slow enough to suggest
that damage due to flying debris might occur at long distances from the point of
the explosion, and this led to the policy of interrupting long straight runs with
‘blast traps’. Christopherson reported estimates of the effect of a variety of trap
shapes based on trials at the Road Research Laboratory, and a selection of these
estimates is shown in Figure 5.10. The factor K p should be applied to the pressures
in straight tunnels given in Figure 5.8, if a trap of the geometry indicated is
interposed between the charge and the point under consideration. The factor
K 2 gives similar information for impulse, and is applied to the impulses from
Figure 5.9. The estimates from the Road Research Laboratory suggested that a
simple right-angle bend reduced the peak pressure by 30%, and a uniform
Figure 5.9 Blast impulse from a charge exploding inside a tunnel (from Philip, ref.
5.10).