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Ingeokring Newsletter
THESIS ABSTRACTS
Abstract thesis Jeroen
van Nes
TU Delft
Application of computerized tomography to investigate
strain fields caused by cone penetration in sands.
Everywhere in the Netherlands, where a structure is
built, a study of the shallow subsurface is done. It is of
major importance to know the geotechnical properties
of the soils on which the structure is to be founded.
One of the most important methods to explore the
subsurface is the cone penetration test (CPT).
A large number of correlations between cone penetration
test parameters and several geotechnical soil properties
exist but have a low degree of repeatability. Empirical
relationships require local tuning. Analytical
relationships make strong assumptions on failure
structures, which are believed to be created by the CPT
test but have not been really investigated. Numerical
codes have problems in simulating cone penetration
correctly and their results are only partially validated
with cone resistance measurements.
The objective of this thesis is to visualize and
characterize the deformation patterns taking place during
cone penetration using X-ray Computerized
Tomography (CT). Its outcome can be used to improve
hypotheses made in analytical models or validate new
numerical models, either finite element models or discrete
element models.
For this purpose, homogenous dry sand samples of
various densities are prepared with dry sand pluviation.
Baskarp sand is selected due to the finesses, well sorting
and small amount of contamination by deviate minerals
and materials. Sand samples are created with a density
varying between 1,51 g/cm3 and 1,68g/cm3. Small-scale
penetration tests are conducted with a 6 mm diameter
aluminium cone in calibration chambers with varying
density, confining pressures and boundary properties.
Measured cone penetration resistances range from 5
MPa in loose sand under low confining pressure (40
kPa) up to18 MPa in dense sand under high confining
pressures (80 kPa). The measured lateral strains ranges
from almost zero in loose sand, up to 0,0035 in dense
sand. Taking in consideration the measurements of the
cone penetration resistance and lateral strain
measurements it can be concluded that the repeatability
of the small scale CPT test is adequate.
To investigate the deformation patterns and failure
Spatial distribution of the absolute density of the
initial sand sample (left) and penetrated sand sample
(right). The horizontal lines in the sand sample are
placed on purpose in order to obtain a better
visualization of the deformation occurring during the
CPT test.
structures, computerized tomography, a nonedestructive
investigation method, is used to analyse
the sand sample before and at several stages during the
CPT tests. By obtaining the linear relationship between
the Hounsfield Values observed in the CT-analyses and
the sand density it is possible to define the density
changes and volumetric strain in the sand samples.
Several scanning parameters are to be set before any
CT-scanning can begin: the X-ray intensity X-ray beam
strength and slice width. Also the diameter of the sample
influences the image quality obtained by the CT-scanner.
Several tests show that the optimum scanning
parameters are a high X-ray intensity (to decrease noise),
a moderate high X-ray beam strength (to decrease beamhardening
effects and to keep the loss of beam
sensitivity to a minimum), and a slice thickness of 1 mm
(to obtain an adequate resolution in the direction
perpendicular to the slices). The diameter of the sand
samples used during this thesis study is 10 cm but it is
also possible to use larger diameters up to 15 cm.
In general the test results showed that a dilating area is
formed ahead of the cone tip during the penetration,
creating a dilating zone around the probe. A very dilating
zone was found at 1-2 mm from the probe and shear
bands are believed to be present in these zones. Outside
this dilating area an area is observed which is not
influenced or slightly compacted by the penetration test.
In some cases some compaction takes place in front of
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