THE USE OF THE STEREO-PHOTOGRAMMETRY METHOD
IN GEOTECHNICAL ENGINEERING WORK
Abdelmalek Bouazza
Charge de Recherche, CNERIB, Souidania 42380,
Algeria
Commission V
ABSTRACT:
The stereo-photogrammetric technique is a powerful method for obtaining incremental
displacements for any point of a plane object subjected to a transformation.
In this
paper the principle and the method giving the associated displacement fields are
described. As an example the method has been applied to geotechnical model tests to
study the behaviour of a shallow plate anchor embedded in a dry sand and subjected to
an uplift force.
and the sand deformations.
KEY WORDS:
Stecometer.
1- INTRODUCTION
Engineering testing often involves the
measurements and analysis of displacements
or deformation in materials under various
types of loading conditions. For many
problems in soil mechanics it is by no
means trivial to establish the shape of
the displaced field. It is especially
important to know the characteristics of
the displacement field in order to relate
it to the relevant soil mechanics
parameters.
In the literature several succesful
methods have been described for performing
such measurements, e.g. optographic trace
recording (Feeser, 1984), X-ray technique
(Bourdeau & Recordon, 1988), speckle
interferometry (Pater & Niewenhuis, 1987),
tomodensimetry (Desrues et al, 1991).
In the present investigation, pseudo
stereo-photogrammetry, a technique by
which geometrical information is derived
from a pair of negatives or photographs,
has been used. Although its best known
application is in topography mapping, it
is being applied to an increasing extent
to a variety of scientific and engineering
measurements, such as concrete
deformations (Torrenti et al, 1989),
expertise work (Adamou et al, 1991). In
this paper, the utilisation of this
technique to monitor the zone of sand
displaced by a plate anchor subjected to
an uplift load is presented and the
results are discussed.
Z2- PSEUDO STEREO PHOTOGRAMMETRY
This method is based on the fundamental
principles of conventional stereo-
photogrammetry. Close range photography
techniques yield global information, on
both components of displacement,
throughout cross sectional areas; i.e. the
number of possible measurements is limited
Several interesting observations are made about the mode of failure
Anchor, Geotechnique, Model Tests, Pseudo Stereo-Photogrammetry, Sand,
only by the resolution of the photographs.
High density displacement information
allows the comprehensive analysis of
strains. Also since the photographs
represent a permanent record of the event,
information may be obtained from them at
any time following their exposure.
Typically, a test material is housed in a
rectangular container and is visible, in
cross section, through transparent glass
which constitute one side of the
container. Successive photographs of the
model are taken by a fixed camera as the
test progress. When two photographs,
including a relative displacement of the
soil particles with respect to a fixed
object, are viewed together as a
stereoscopic pair, a three dimensional
image with distinct topography will be
perceived. The test material may or may
not contain targets for measurements at
discrete points. If targets are not
present, it becomes necessary for the
materials to have sufficient texture to
form a discernable surface for
photogrammetric measurement. This method
has a number of inherent features which
can be advantageous when considering the
technique for specific applications.
Andrawes (1976) and Welsh (1986) listed
the following advantages:
-Remote, non contacting measurement can
be made.
-No embedded markers are used.
-The photographs provide a permanent
record which can be remeasured at a later
date.
-Measurement can be made with the object
in-situ.
-Conventional standard equipment available
for stereo photogrammetry can be used
directly.
-Efficiency of measuring the displacements
in the areas of large discontinuities or
high displacement gradients since every
particle can be individually traced.
