Page 20 - Numerical Analysis and Modelling in Geomechanics

Chapter 1
The effect of increasing the depth of an

underground void on the surface
displacements of an airfield runway

John W.Bull and C.H.Woodford

Abstract
This research considers the effect of the detonation of a 213 kg mass of explosive
in a subgrade at depths of 8.354m, 9.354m, 10.354m, 11.354m, 12.354m, 15.
354m and 18.354 m beneath a cement concrete runway. The detonation produces
a void in the subgrade and disturbs the subgrade beneath the runway. Seventeen
computational models of the void and the disturbed subgrade [camouflet] are
considered ranging from where all the detonation-affected subgrade zones are
increased in strength to where all the detonation-affected subgrade zones are
reduced in strength. Consideration is given to the possibility of using the
diameter of the runway deflection bowl as a means of determining the size, depth
and position of the void. The results from the computational models are
compared with empirical data that states that for the void to have no effect at the
air–ground interface, the depth of detonation must exceed a specified value. From
this comparison it is found possible to determine the material sets that can be
generated feasibly from the detonation. It is also found that for some material
sets it is possible to identify the size, depth and location of the void.

Introduction
This research considers the effect of the detonation of a chemical explosive
beneath a cement concrete runway on the deflection of the runway when a
uniformly distributed downward load is applied to the runway surface. Later
research will consider the effect of the detonation on the fatigue life of the
subgrade and the runway. When chemical explosives damage a runway, it is a
typical requirement that the runway is repaired and operational within three
hours and that it will remain operational for thirty days.
When a detonation takes place it may be above ground, at the runway surface
or below the air-ground interface in the subgrade. Detonations above the air-
ground interface are not considered here, but in previous publications by the
authors, research was undertaken into detonations at or below the runway surface
[1–14].