Experimental Investigation on the Desiccation and Fracturing of Clay

Abstract

Waterways and lakes in low-lying delta areas require regular dredging for maintenance. Reuse of dredged sediments has been proposed for various applications. However, directly after dredging the physical characteristics of these sediments make them generally unsuited for immediate reuse. They are often placed on land, where they are allowed to ripen through a combination of drainage, consolidation and evaporation. At a certain stage during the desiccation process cracks will develop, affecting the physical properties of the material. These include its strength, stiffness and hydraulic conductivity, as well as drainage, water infiltration and evaporation. The soil composition can be likewise altered by biochemical processes, which are stimulated by the ingress of oxygen through the cracked surface. Consequently, identifying how cracks develop in the soil is essential for understanding the behavior of the material.
Much investigation has been performed studying desiccation cracks, but the underlying mechanisms are still not fully understood. It is known that the drying speed affects the final amount of cracks in a soil, which points out to the potential impact of rate effects in soil cracking. The effects of the drying rate might be related to variations in the tensile strength influenced by different shrinkage rates, as well as its impact on the suction development in the soil.
In this thesis different sets of experiments were carried out to study the phenomenon of desiccation fracturing in soil. The first series of tests were carried out in a controlled laboratory environment to study the crack development in drying clay slurries under different initial and boundary conditions. The outcomes of those tests indicated that cracks can propagate in a different way than commonly assumed (namely under the surface) and that fracture characteristics strongly depend on the initial and boundary conditions.
A second series of tests examined the combined effects of pull rates and high water contents on the tensile strength. Particle Image Velocimetry analysis was also carried out on pictures taken during the tests to examine the strains generated. It was found that the effect of pull rate on the tensile strength of the clay was negligible compared to the effect of the water content. Pull rate did affect the stiffness response of the soil. The findings revealed that the influence of the evaporation rate on soil fracturing might be related more to the rate dependency of the stiffness rather than to significant changes in tensile strength.
The last series of experiments investigated the drying behaviour of a clay with different initial water contents and under different evaporative conditions. The small scale evaporation experiments carried out using commercially available suction measuring equipment with an adjusted test procedure. The results showed that the initial conditions have great influence on the drying performance of a soil, which can be partly attributed to the influence of the surface texture and the pore structure. It was observed that under certain circumstances, the evaporation of a soil surface can be higher than that of open water. The different evaporation rates had a marked effect on the water distributions with depth within the soil. The evaporation rate also produced a dynamic response of the soil water retention curve.
At the end, a simple one-dimensional model was set up to try to capture the behavior observed during all the laboratory tests. It also served to analyze the consequences of different hypotheses about the material behaviour on the crack onset in a homogenous soil layer undergoing surface drying.