400
M. Hattab et al.
19.6 Conclusions
In this experimental work, mainly based on the observations of a clay slurry during
drying, shrinkage phenomenon has been placed in the formal framework of continuous media mechanics and assumed to evolve independently from the external
imposed stresses. Cracking propagation has been considered from the macroscopic
point of view. Without external mechanical stresses, the capillary pressure becomes
similar to suction. The properties of the unsaturated kaolin were precisely characterized by establishing the drying curves, taking into account both the suction, water
content and volume changes.
Digital image correlation (DIC) technique has been used on a saturated slurry clay
sample during drying, to precisely follow the changes in displacements and strains
field.
In the shrinkage phenomenon, for a given clayey material, the geometric boundary
conditions seem to be an important factor having a large impact on the anisotropic
strains development. In the current study (square and rectangular samples), shrinkage
that appears anisotropic with ε xx
sh
≈ ε yy
sh and ε zz
sh /ε xx
sh
= η. η is the coefficient
of anisotropy. During desiccation, restriction to the shrinkage process can appear,
leading to the development of local stress concentration. Accordingly, when the
stresses locally developed reach the maximum strength, the soil fails.
The stress concentration and its development can be regarded as the result of the
evolution of a local “mechanical” strain field ε ij
* , obtained by difference between the
local total strain field ε ij , measured by digital image correlation, and the estimated
anisotropic shrinkage strain field ε ij
sh . The current observations show that stress
concentration often concerns the tensile stress development. Consequently, crack
occurs when a concentrated tensile stress exceeds the material capillary cohesive
strength, the latter increasing with the capillary stress development. The direction of
the crack is perpendicular to that of the principal extension strains in opening mode.
The extension strains remain very important in the positions where the opening of
the crack is not large enough to relax them. One of the key issues, that deserves to
be thoroughly analyzed in the perspectives of this study, relates to the local stress
development inducing the local strains that can be captured by the DIC method
(see for instance works of Hild and Roux 2006, and dell’Isola et al. 2019). In our
problem, the point is of going back to the capillary stresses, which are rather difficult
to measure directly in the material during the drying process. A possible method is
to perform complementary relevant tests allowing to couple the measurements, that
is to say a simultaneous measurement of stresses and strains during the test such as
Ighil Ameur and Hattab (2017), and El Hajjar et al. (2020).
M. Hattab et al.
19.6 Conclusions
In this experimental work, mainly based on the observations of a clay slurry during
drying, shrinkage phenomenon has been placed in the formal framework of continuous media mechanics and assumed to evolve independently from the external
imposed stresses. Cracking propagation has been considered from the macroscopic
point of view. Without external mechanical stresses, the capillary pressure becomes
similar to suction. The properties of the unsaturated kaolin were precisely characterized by establishing the drying curves, taking into account both the suction, water
content and volume changes.
Digital image correlation (DIC) technique has been used on a saturated slurry clay
sample during drying, to precisely follow the changes in displacements and strains
field.
In the shrinkage phenomenon, for a given clayey material, the geometric boundary
conditions seem to be an important factor having a large impact on the anisotropic
strains development. In the current study (square and rectangular samples), shrinkage
that appears anisotropic with ε xx
sh
≈ ε yy
sh and ε zz
sh /ε xx
sh
= η. η is the coefficient
of anisotropy. During desiccation, restriction to the shrinkage process can appear,
leading to the development of local stress concentration. Accordingly, when the
stresses locally developed reach the maximum strength, the soil fails.
The stress concentration and its development can be regarded as the result of the
evolution of a local “mechanical” strain field ε ij
* , obtained by difference between the
local total strain field ε ij , measured by digital image correlation, and the estimated
anisotropic shrinkage strain field ε ij
sh . The current observations show that stress
concentration often concerns the tensile stress development. Consequently, crack
occurs when a concentrated tensile stress exceeds the material capillary cohesive
strength, the latter increasing with the capillary stress development. The direction of
the crack is perpendicular to that of the principal extension strains in opening mode.
The extension strains remain very important in the positions where the opening of
the crack is not large enough to relax them. One of the key issues, that deserves to
be thoroughly analyzed in the perspectives of this study, relates to the local stress
development inducing the local strains that can be captured by the DIC method
(see for instance works of Hild and Roux 2006, and dell’Isola et al. 2019). In our
problem, the point is of going back to the capillary stresses, which are rather difficult
to measure directly in the material during the drying process. A possible method is
to perform complementary relevant tests allowing to couple the measurements, that
is to say a simultaneous measurement of stresses and strains during the test such as
Ighil Ameur and Hattab (2017), and El Hajjar et al. (2020).