Page 34 - Numerical Analysis and Modelling in Geomechanics

SURFACE DISPLACEMENTS OF AN AIRFIELD RUNWAY 15
5 of material set 17. Zone 5 of material set 15 has a strengthened Young’s
modulus of 950 MPa.
Downward vertical deflections were recorded along the top of the runway from
point 1, directly above the detonation point to point 29, 14.32 m from point 1. For
the purposes of this research, the deflections for the points numbered 1, 4, 8, 12,
16 and 20 are shown. Points 1, 4, 8, 12 and 16 are on the surface of zone 1, the
part of the concrete runway that overlays the subgrade affected by the
detonation. Point 20 is in zone 8, the part of the runway overlaying the subgrade
not directly affected by the detonation. For some material sets, the effect of the
detonation extended to increasing the deflections as far as point 22 in zone 8 if
subgrade zones 2, 3, 4 or 5 were weakened.
For no surface rupture for the 213 kg mass of explosive used for this
computational modelling, the depth of the detonation must be between 8.286 m
and 16.572 m [23, 33]. Consequently, it is expected that the computational
results will show that as the depth of detonation exceeds 8.354 m and reaches 16.
572 m the surface disturbance will tend to 100% and that when the detonation
depth reaches 18.354 m, the 100% deflection will be obtained. That is, no effect
of the detonation will be detectable at depths of detonation of 16.572 m and
greater.

Discussion of the numerical results
The 17 material sets are considered in three groups. Considering first group 1,
material set 1, as shown in Table 1.3, to obtain the 100% value over the
undisturbed subgrade, the average of points 21 to 29 inclusive was used. This
indicates that the surface deflection detonation effects extended as far as point 20.
That is, the deflection due to the detonation extended over the apparently
undisturbed subgrade points 18, 19 and 20. When the depth of the detonation
was 8.354 m, the deflections at points 1, 16 and 20 were 89.1 %, 96.4% and 99.1
% respectively. Thus increasing the Young’s modulus in zones 2, 3, 4 and 5
reduces the deflections as far out as point 20. As the depth of detonation increases,
the change in the displacements at points 16 and 20 is small, but there are still
reduced surface deflections. Further, for all the remaining deflection points, the
surface deflection continues to reduce. For material set 1, the camouflet can be
detected for all depths considered, but the size and depth of the camouflet will be
overestimated as its deflection bowl exceeds the zone 1–8 interface. The results
for material set 1 can be interpreted in two ways. Firstly the experimental work
underestimates the depth required for no surface disturbance, or, if the range of
depths found experimentally is correct, material set 1 is infeasible.
Group 2 is considered in two parts. In part 1, material sets 2, 3, 4, 5, 6 and 7,
Tables 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9 respectively, all the deflections at point 1 for
the camouflet depth of 8.354 m are less than 103.7%. Material sets 2 to 7
inclusive have Young’s moduli in zones 2, 3, 4 and 5 of 95 MPa or above. For
material sets 2, 3 and 4, the deflections for camouflet depth 8.354 m are less than