19 Mud Shrinkage and Cracking Phenomenon Experimental …
381
through the evolution of a local strain field of mechanical type, obtained by difference
between the measured total local strain field (using digital image correlation) and
the estimated anisotropic shrinkage strain field. This approach permits to evaluate
the risk of crack initiation during drying.
19.2 Basic Notions—Hypotheses and Soil–Water
Characteristic Curve
During drying with no external mechanical stresses, the volume of a soil sample
changes (shrinkage) because the water in the soil becomes subjected to a tension state
(the air being at the atmospheric pressure). More generally, the difference between
the air (u a ) and the water (u w ) pore pressures, or capillary pressure (u c ), is the main
macroparameter causing the movements of the fluids in the porous medium:
u c = u a − u w
(19.1)
Thus, the shrinkage in clayey soils is strongly related to local mechanisms that
induce water movements between pores, quite similar to that defined in granular
materials (Scholtès et al. 2009; Yuan and Chareyre 2017). At the scale of the pore,
the difference in pressure (Eq. 19.1) results in meniscus forming between air and
water, characterized by the surface tension of the water and the interface curvature
(Laplace’s law). As mentioned for instance by Biarez et al. (1987), the concept of
capillary pressure may be defined as a global notion, at larger scale than the molecular
scale. The concept includes both capillary properties of the medium through the pore
dimensions, and the adsorption properties (in clays cases) through the shape of the
solid–liquid–air membrane. Considering the electrical forces (of van der Waals type)
in the vicinity of the clay particles, the connection angle itself may be considered as
a macroscopic parameter involving the wetting concept of the soil material (Fleureau
et al. 1988; Santamarina 2001; Lu et al. 2010).
Let us consider in this study macroscopic behavior, which we know is strongly
dependent on the microscopic mechanisms. The shrinkage phenomenon may be
placed in the formal framework of continuous media mechanics. Without external
mechanical stresses, the capillary pressure becomes similar to suction. The shrinkage
in clayey mud during drying can be addressed in different steps as illustrated on
Fig. 19.1.
(i) Drying of the initially saturated clay provokes a progressive loss of mass caused
by water evaporation. Hence, the water content changes, leading to (ii) suction development in the material, which induces (iii) volumetric strains and the decrease of the
volume of the specimen. During the process, one can assume that the saturation of the
material is maintained as long as the point of air entry is not reached yet. Meanwhile,
381
through the evolution of a local strain field of mechanical type, obtained by difference
between the measured total local strain field (using digital image correlation) and
the estimated anisotropic shrinkage strain field. This approach permits to evaluate
the risk of crack initiation during drying.
19.2 Basic Notions—Hypotheses and Soil–Water
Characteristic Curve
During drying with no external mechanical stresses, the volume of a soil sample
changes (shrinkage) because the water in the soil becomes subjected to a tension state
(the air being at the atmospheric pressure). More generally, the difference between
the air (u a ) and the water (u w ) pore pressures, or capillary pressure (u c ), is the main
macroparameter causing the movements of the fluids in the porous medium:
u c = u a − u w
(19.1)
Thus, the shrinkage in clayey soils is strongly related to local mechanisms that
induce water movements between pores, quite similar to that defined in granular
materials (Scholtès et al. 2009; Yuan and Chareyre 2017). At the scale of the pore,
the difference in pressure (Eq. 19.1) results in meniscus forming between air and
water, characterized by the surface tension of the water and the interface curvature
(Laplace’s law). As mentioned for instance by Biarez et al. (1987), the concept of
capillary pressure may be defined as a global notion, at larger scale than the molecular
scale. The concept includes both capillary properties of the medium through the pore
dimensions, and the adsorption properties (in clays cases) through the shape of the
solid–liquid–air membrane. Considering the electrical forces (of van der Waals type)
in the vicinity of the clay particles, the connection angle itself may be considered as
a macroscopic parameter involving the wetting concept of the soil material (Fleureau
et al. 1988; Santamarina 2001; Lu et al. 2010).
Let us consider in this study macroscopic behavior, which we know is strongly
dependent on the microscopic mechanisms. The shrinkage phenomenon may be
placed in the formal framework of continuous media mechanics. Without external
mechanical stresses, the capillary pressure becomes similar to suction. The shrinkage
in clayey mud during drying can be addressed in different steps as illustrated on
Fig. 19.1.
(i) Drying of the initially saturated clay provokes a progressive loss of mass caused
by water evaporation. Hence, the water content changes, leading to (ii) suction development in the material, which induces (iii) volumetric strains and the decrease of the
volume of the specimen. During the process, one can assume that the saturation of the
material is maintained as long as the point of air entry is not reached yet. Meanwhile,