Cracks are prevalent in natural and engineered soils and provide preferential pathways for fluid flow and contaminant transport. A clear understanding of the behavior of cracked soils is vital for the safe and economical design of slopes, dams, clay liners, and other engineering structures related to these soils. The objectives of this research are: (1) to investigate the crack development and to characterize crack geometrical parameters under natural atmosphere conditions by field tests; (2) to study the permeability tensor and REV for saturated soils containing random crack networks through numerical simulation; and (3) to develop the hydraulic property functions for deformable cracked soils under unsaturated conditions....[ Read more ]

Cracks are prevalent in natural and engineered soils and provide preferential pathways for fluid flow and contaminant transport. A clear understanding of the behavior of cracked soils is vital for the safe and economical design of slopes, dams, clay liners, and other engineering structures related to these soils. The objectives of this research are: (1) to investigate the crack development and to characterize crack geometrical parameters under natural atmosphere conditions by field tests; (2) to study the permeability tensor and REV for saturated soils containing random crack networks through numerical simulation; and (3) to develop the hydraulic property functions for deformable cracked soils under unsaturated conditions.

Comprehensive field tests including crack surveys, double ring infiltration tests and two-stage infiltration tests have been performed to characterize the development, pattern and geometry of cracks in soils, and the influence of cracks on the hydraulic properties of soils. A digital imaging method was used to document the cracks without disturbing the soils. The method is capable of obtaining a large amount of crack geometrical data with high accuracy. Therefore, the cracks in soils can be surveyed periodically and the crack development process can be studied in the field. The desiccation cracks generally develop in three stages: initial stage, primary stage, and steady state stage. The critical water content at crack initiation is predicted using a stress criterion. When the water content approaches the shrinkage limit, the crack development approaches a steady state. The probability distributions for crack aperture, length, orientation, and location are analyzed and suggested for numerical simulation.

A mathematical model and a computer code are developed to simulate water flow through saturated random crack networks. The anisotropic permeability tensor and the representative elementary volume (REV) for a random crack network can be obtained. A rigorous procedure for generating random crack networks is developed, with a requirement of a minimum number of cracks in an analysis domain to ensure the probability distributions for the crack parameters. The size of the REV for a random crack network and a cracked soil is determined using two proposed criteria in terms of relative error and central-difference gradient error. The REV is also determined based on the crack geometrical parameters using the proposed density of degree of freedom of the crack network. A continuum approach is then presented to obtain the permeability tensor and REV of a soil mass containing a random crack network. A parametric study is conducted to investigate the influence of crack geometry and soil matrix type on the anisotropic permeability tensor and REV of the cracked soil.

A method is proposed to predict the soil water characteristic curve (SWCC) and permeability function for an unsaturated cracked soil considering crack volume changes during drying-wetting cycles. First, the cracked soil at a certain time is analyzed without considering the crack change. The cracked soil is viewed as an overlapping continuum of a crack network system and a soil matrix system. The pore size distributions can be established for the two pore systems. Then the water retention curves and permeability functions are estimated for the two pore systems using their pore size distributions. The estimated water retention curves for the two pore systems are combined to give the SWCC of the cracked soil at that moment. Similarly, the permeability function for the cracked soil at that moment can be obtained. Secondly, the SWCCs and permeability functions for the cracked soils at different moments along a crack development path can be obtained. Finally, the permeability functions or SWCCs for the cracked soils at different moments can be combined to give the permeability function or SWCC for the cracked soil considering crack volume changes. An example is presented and the results show that different kinds of permeability functions can be obtained for a cracked soil following different crack development paths. A numerical analysis is conducted to illustrate the use of these proposed SWCCs and permeability functions in the study of rainfall infiltration in a soil slope containing cracks and the stability of the slope during rainfall infiltration.