Page 21 - Numerical Analysis and Modelling in Geomechanics

2 JOHN W.BULL AND C.H.WOODFORD
Detonations at the runway surface and at shallow depths destroy parts of the
runway and eject subgrade such that a crater is formed. These craters are
clearly visible and research by the authors has been published into how to repair
these craters [1–8]. For detonations below the air—ground interface, where no
surface disturbance can be seen, the level of subgrade support remaining to the
runway is difficult to assess and methods to detect the detonation are required. In
this research it is assumed that the detonation takes place in the subgrade such
that a camouflet is formed, as shown in Figure 1.1. A camouflet is a void
surrounded by a shell of compacted subgrade, zones 5 and 6 of Figure 1.1. The
subgrade between the void and the runway, zones 2, 3, 4 and 5, may or may not
be affected by the detonation. There is an inherent weakness in the subgrade due
to the void, but if the air-ground interface is undisturbed, then compaction of the
subgrade must also have occurred, giving the compacted subgrade additional
strength. However as the distance from the detonation point increases beyond the
outer diameter of the void, the compaction of the subgrade reduces. This
reduction is related to the proximity of the air—ground interface and that
interface’s ability to reflect the compression waves from the detonation. It is for
this reason that it is difficult to determine the exact strength of the subgrade
above the camouflet and why 17 subgrade material sets are considered for
numerical analysis later in this chapter.
It is assumed that the subgrade between the void and the runway is disturbed
and that the runway does not heave or crack. A damaged or displaced runway
would be examined and repaired, not so an apparently undamaged runway. In
time the void would collapse with the subgrade falling into the void, thereby
reducing subgrade support to the runway and presenting a considerable danger to
aircraft using the runway. The shape of the disturbed subgrade above the void is
assumed to be that of a cone extending to the underside of the runway, as shown
in zones 5, 4, 3 and 2 of Figure 1.1. At the centre of the camouflet is a spherical
void. Around the void is a shell of compacted subgrade, zones 5 and 6, with the
point of the cone being at the detonation point.
Previous research by the authors has been carried out in a number of parts. In
part one, the camouflet, the disturbed subgrade and the undisturbed subgrade
surrounding the detonation was modelled. In part two, a uniform pressure was
applied to the air-ground interface to assess the resulting subgrade displacement,
subgrade stress and inherent camouflet instability. Parts one and two assumed
that no runway overlaid the subgrade [9]. Part three was similar to part two but
introduced a runway overlaying the subbase and the camouflet [10]. Part four
considered a series of possible materials that could be used to fill and stabilise
the void to effect a rapid and long-term runway repair [11]. Part five introduced a
subsurface barrier layer immediately beneath the runway and suggested a means
of determining the presence of a camouflet [12]. Part six considered the fatigue
life of the runway, with the void being either filled or unfilled [13]. Part seven
considered the effect of tension in the subgrade on the fatigue life of both the
subgrade and the runway [14]. The present research is part eight and considers