A History of Research and a Review of Recent Developments

Blast loads in tunnels and shafts 105
Figure 5.11 Four right angle bends with blast pockets (from Christopherson, ref.
4.6).
bends (shown dotted), the reduction was 92%. The Road Research Laboratory
figures were 65% and 77%, but the latter was increased to 86% if the pocket
depth was double the pipe diameter. The introduction of a rectangular pipe
system (Figure 5.12) was said by the US Army Technical Manual to lead to a
reduction of 85%, whereas the Road Research Laboratory figure was 62%.
Later research suggested that the above figures were over optimistic, and
more recent publications in the USA and elsewhere propose more conservative
design rules. Much of this information still seems to be restricted, but open
recommendations are given in a book by Henrych [5.1] published in English in
1979. He gives information on the propagation of a shock wave in cranked and
branched channels in which he notes that the factor K p changes as the peak
entering pressure increases. Figure 5.13, taken from [5.1], gives a range of values
of K p for several branch geometries. At the higher pressures the reduction at a
simple right angle bend is only 10%, so that K p=0.9. For successive right-angle
bends his results show that the same coefficient is applied each time, so that for
three successive bends the overall value of K p is 0.9 3 =0.73. His results suggest
that for n successive bends the overall factor is 0.9 n , but there is no analytical
proof of this. What is very apparent from a study of all the experimental
information is that the smoothness of the inside of the tunnels has an important
bearing on the results, and as this information is not always given in experimental
reports it is dangerous to correlate the conclusions of separate experimenters.
Figure 5.12 Rectangular pipe system blast trap (from Fundamentals of Protective
Design, TM5–855–1, 1965).