Page 41 - Numerical Analysis and Modelling in Geomechanics

22 JOHN W.BULL AND C.H.WOODFORD
354 m beneath a cement concrete runway on the deflection of the runway surface
when a uniformly distributed downward load is applied to the runway. As there
is no other published research that details the changes in the subgrade cone
above a camouflet detonation, this research computationally modelled 17
material sets as shown in Table 1.1, by changing the Young’s modulus of zones
2, 3, 4, 5 and 6 of Figure 1.1. These material sets cover a range of subgrade
possibilities, from material set 1 where zones 2, 3, 4 and 5 were all increased in
strength to material set 17 where zones 2, 3, 4 and 5 were all reduced in strength.
What is known is that for the detonation to have no effect at the air-ground
interface, the depth of detonation must exceed a value of between 8.286 m and
16.572 m. For computational reasons, a value of 18.354 m was chosen as the depth
at which no air—ground effect could be detected.
Consideration was given to the possibility of using the diameter of the
deflection bowl as a means of determining the size, depth and position of the
camouflet void. The computational modelling showed that for material sets 1 and
8 to 17 inclusive, the deflection bowl extended beyond the zone 1–8 interface.
Thus for these material sets, the deflection bowl will identify a point above the
centre of the camouflet void, but the depth and diameter of the void will be
overestimated. For the remaining material sets, 2 to 7 inclusive, the size, depth
and location of the void can be determined.
It is necessary to consider the validity of the empirical data relating to the
depth of the detonation required to produce no surface rupture. Ignoring the
empirical data would mean accepting that all 17 material sets are possible
outcomes of the detonation. However, the empirical data, although unconfirmed
by any other published work, has to be accepted until modified by further data. This
means that material sets 1 and 12 to 17 inclusive have to be rejected as possible
outcomes of a camouflet-producing detonation. Thus a detonation that produces
a camouflet will produce one of material sets 2 to 11 inclusive.
Considering further the material sets that are rejected as being infeasible,
material set 1 is the only material set that reduces runway deflections. This is
unlikely to occur in practice. Inspecting the remaining infeasible material sets 12
to 17 inclusive, the common theme is that zone 2 has been weakened to 7 MPa.
Further research is required to determine the minimum value of the zone 2
Young’s modulus of these material sets to make them feasible. An indication is
given for material set 13, when it is compared with material set 6. The only
difference between the two material sets is that zone 2 of material set 6 has a
Young’s modulus of 95 MPa while that of material set 13 is 7 MPa. A similar
consideration applies to material set 11 when compared with material set 17.
The feasible material sets 2 to 11 inclusive all have a zone 2 Young’s modulus
of 95 MPa. This suggests that as the depth of the camouflet detonation increases,
the arching effect of zone 2 in the base of the cone makes the identification of the
camouflet almost impossible. If the zone 2 Young’s modulus remains at 95 MPa,
then inspection of the subgrade will not reveal the existence of a camouflet. For