384
M. Hattab et al.
et al. (2014) showed that the tensile strength generally increases with the decreasing
water content and eventually reaches a maximum value at a critical water content
named w c . Then, it decreases significantly on the dry side of optimum water content.
These results highlighted different behaviors under tensile loading, which the authors
related to microstructure organization of aggregates as well as the different regimes of
unsaturated soils (pendular, funicular, and capillary regimes). Flexure tests (Fig. 19.3)
performed by Ighil Ameur and Hattab (2017) on little-beams of one-dimensionally
consolidated clays submitted to different levels of initial imposed suction showed
a maximum peak (maximum strength) increasing with suction. This corroborates
quite well the Tang et al. (2014) observations in tensile tests. The marked drops after
the peak, showing rupture of brittle type in Fig. 19.3a, are due to the high suctions
imposed to the clay samples before the tests. Using the digital image correlation at
the peak of maximum strength, one can observe in the tension zone, at the bottom
of the beam (Fig. 19.3b), a red color zone showing large extension with an average
of horizontal local strains ε xx equal to +0.2%. This zone defines the crack initiation
by indirect tensile strength.
19.3 Characterization of Free Shrinkage Development
19.3.1 Material and Methods
The used material is kaolin K13, a commercial synthetic clay from Sibelco, France.
Table 19.1 summarizes the geotechnical and mechanical parameters. The grain size
distributions of kaolin K13, obtained by laser analysis, show that the clay fraction
(<2 µm) is about 83%. The mineralogical analysis, carried out by X-ray diffraction,
reveals the presence of various secondary minerals, in addition to kaolinite, such as
illite and quartz. SEM image in Fig. 19.4a (Hammad et al. 2013) shows kaolinite
particles in the form of hexagonal flat and rigid platelets.
Desiccation tests have been carried out in the laboratory under controlled temperature (T = 20°) and relative humidity (RH) conditions, using the salt solution technique. The tests were performed on a thin square layer of clay slurry (prepared
at a water content w 0 = 1.2 w L ) of 5 mm thickness, and 200 mm of length and
width (Fig. 19.4b). The sample is spread on a smooth support and placed in the RHcontrolled chamber. Notice that the little samples of slurries for the determination of
the SWCC were prepared in the same way as this larger square slurry sample. The
experimental device presented in Fig. 19.5a ensures the control of relative humidity
in the chamber (for instance at RH = 5.8% when the equilibrium is reached as shown
in Fig. 19.5b). This setup allows to control the kinetic of the drying process.
To precisely follow the displacements and strains field during the deformation
of the clay, digital image correlation (DIC) technique was used. Using this method
requires a great rigor through each step of the test, from the preparation of the sample
surface to observe, to the interpretation of the results. The calibration (i.e., choice
M. Hattab et al.
et al. (2014) showed that the tensile strength generally increases with the decreasing
water content and eventually reaches a maximum value at a critical water content
named w c . Then, it decreases significantly on the dry side of optimum water content.
These results highlighted different behaviors under tensile loading, which the authors
related to microstructure organization of aggregates as well as the different regimes of
unsaturated soils (pendular, funicular, and capillary regimes). Flexure tests (Fig. 19.3)
performed by Ighil Ameur and Hattab (2017) on little-beams of one-dimensionally
consolidated clays submitted to different levels of initial imposed suction showed
a maximum peak (maximum strength) increasing with suction. This corroborates
quite well the Tang et al. (2014) observations in tensile tests. The marked drops after
the peak, showing rupture of brittle type in Fig. 19.3a, are due to the high suctions
imposed to the clay samples before the tests. Using the digital image correlation at
the peak of maximum strength, one can observe in the tension zone, at the bottom
of the beam (Fig. 19.3b), a red color zone showing large extension with an average
of horizontal local strains ε xx equal to +0.2%. This zone defines the crack initiation
by indirect tensile strength.
19.3 Characterization of Free Shrinkage Development
19.3.1 Material and Methods
The used material is kaolin K13, a commercial synthetic clay from Sibelco, France.
Table 19.1 summarizes the geotechnical and mechanical parameters. The grain size
distributions of kaolin K13, obtained by laser analysis, show that the clay fraction
(<2 µm) is about 83%. The mineralogical analysis, carried out by X-ray diffraction,
reveals the presence of various secondary minerals, in addition to kaolinite, such as
illite and quartz. SEM image in Fig. 19.4a (Hammad et al. 2013) shows kaolinite
particles in the form of hexagonal flat and rigid platelets.
Desiccation tests have been carried out in the laboratory under controlled temperature (T = 20°) and relative humidity (RH) conditions, using the salt solution technique. The tests were performed on a thin square layer of clay slurry (prepared
at a water content w 0 = 1.2 w L ) of 5 mm thickness, and 200 mm of length and
width (Fig. 19.4b). The sample is spread on a smooth support and placed in the RHcontrolled chamber. Notice that the little samples of slurries for the determination of
the SWCC were prepared in the same way as this larger square slurry sample. The
experimental device presented in Fig. 19.5a ensures the control of relative humidity
in the chamber (for instance at RH = 5.8% when the equilibrium is reached as shown
in Fig. 19.5b). This setup allows to control the kinetic of the drying process.
To precisely follow the displacements and strains field during the deformation
of the clay, digital image correlation (DIC) technique was used. Using this method
requires a great rigor through each step of the test, from the preparation of the sample
surface to observe, to the interpretation of the results. The calibration (i.e., choice