Page 30 - Numerical Analysis and Modelling in Geomechanics

SURFACE DISPLACEMENTS OF AN AIRFIELD RUNWAY 11
analysis was completed in less than two hours elapsed time using a workspace of
approximately 400 Mb on a SUN workstation connected to the university
network. Three-dimensional isoparametric finite elements from the PAFEC
37110 element library were chosen. These elements have curved faces and are
provided in a 20-node brick shape and a 15-node wedge shape. Element nodes
have three translatory degrees of freedom and the PAFEC analysis provides
nodal displacements, nodal stresses, stresses at element centres and at the centres
of element faces. The authors were satisfied that this was a realistic model and that
any further refinement would not have made a significant impact on the analysis.
Figure 1.3 shows a quarter finite element model of the camouflet for a detonation
depth of 12.354 m.

Camouflet determination
The requirement to define the extent and the size of a camouflet under a runway
is similar to the determination of a void under a highway. Voids under highways
have implications for the maintenance, repair and safety of vehicles and people
using the highway [42]. Using current technology, some voids in the subgrade
under a highway are easily detected, while others are problematic [42]. To
improve the detectability of highway voids, the effectiveness of various
geophysical and non-destructive testing methods such as ground probing radar, has
been proved [42]. Other geophysical techniques such as seismic methods, which
use the spectral analysis of surface waves, microgravity surveys and non-
destructive test methods such as the falling weight deflectometer have yet to be
evaluated.
For highways, the number and types of load applications and vehicular speed
are usually unknown, as is the time of the initiation of the void. For runways,
construction details and maintenance data, types and frequency of loading are
readily and accurately known. Further, the time at which a camouflet occurs is
precisely known. What is required is unambiguous evidence to identify that
a camouflet is present and to be able to determine its size and its effect on
runway support.
Laboratory tests have shown that for a saturated clay subgrade, subjected to
static and superimposed cyclic load of low frequency, there is an initial rapid
settlement constituting between 60% and 80% of the total permanent settlement
and it is completed within the first ten cycles of loading [43]. This is followed by
secondary settlement at a slower rate, which continues for up to 15,000 to 20,000
load cycles until equilibrium is reached. Settlement is then practically negligible
for all other load cycles. For runways, it is possible to obtain deflection and
settlement measurements as heavy aircraft traverse the length of the undamaged
runway. These deflection and settlement records allow the determination of
whether the runway, prior to the introduction of a camouflet, was in its initial,
secondary or equilibrium settlement stages.