Washington Center for X-ray and Imaging Technology (WACXIT)

Department of Civil and Environmental Engineering

Project Description

This collaborative project was supported by NSF grants CMMI-0856793 to Washington State University and CMMI-0856276 to University of Missouri.

The Research Goals

Even though a significant portion of geotechnical engineering problems involve unsaturated soils (also referred to as “three-phase media” or “partially saturated soils” in this study),  mechanics and the characterization of these materials has often been limited to a two-phase system composed of solid and water or air. Unsaturated soil behavior is greatly influenced by the co-existence of air and water in pore spaces which result in phenomena that have significant behavioral differences from two-phase media. Such behavior is manifested in engineered earth systems and natural earth processes, including problems associated with precipitation-induced landslides expansive or collapsing soils, near surface contaminant transport, bearing capacity and settlement of shallow foundations. Proper understanding of these processes requires considerations that must go beyond those available for saturated soil.

The presence of negative pore water in partially saturated media is one of the main causes for the significant difference between saturated and unsaturated soil behavior. This variable defined as soil suction replaces the pore pressure, and blurs the effective stress concept associated with the stress carried by the solid matrix. The majority of partially saturated soil research introduces soil suction as a separate stress variable to provide a practical solution for engineering applications. This approach is not as fundamentally sound as the concept of effective stress principle in saturated soil mechanics because suction is inherently coupled with parameters like saturation, wetting direction, and intergranular stress. As highlighted by many studies direct adoption of the effective stress concept of saturated soils to partially saturated soils  in search of a unifying framework has failed to bring success in capturing unsaturated behavior in a robust way.

This collaborative research between Washington State University and University of Missouri seeks to advance the applicability of effective stress concepts for unsaturated soils and hopes to transform the practice of unsaturated soil mechanics in the same way that Terzaghi’s effective stress transformed the practice of saturated soil mechanics. The following are the specific objectives:

  • Scanning of unsaturated soil microstructure under controlled suction and saturation directions.
  • Development of new automated imaging algorithms to quantify the evolution of unsaturated soil microstructure.
  • Integration of microstructural features and measurements into a new effective stress formulation for unsaturated granular soils.

The Research Results

Technological advances, experimental procedures and analytical approaches were integrated in this project to investigate the behavior of soils in a state of partial saturation at microstructural level. This study made use of advanced imaging techniques to image and analyze three-phase media. A novel experimental apparatus was developed and utilized to control saturation direction and suction during simultaneous imaging of specimens. Analytical approaches were combined with digital image processing techniques to quantify parameters of interest. Algorithms and macros were developed and applied to quantify the degree of saturation and the fabric tensor of the liquid phase. A standalone image processing program was developed and used as an integral part of the scanning and image pre-processing. Results from statistical approaches were combined with digital image processing to automate the quantification of the fabric tensor of the liquid phase in partially saturated specimens. The study concludes by proposing a new microstructure based effective stress formulation for partially saturated granular soils.

By modifying the Tempe type cell and hanging column method experimental setup for real time monitoring of microstructure was designed. This novel sample scanning apparatus was produced from acrylic material. The apparatus was integrated with X-ray CT scanner to generate the three dimensional images of soil microstructure while saturation direction and suction are controlled. The developed apparatus was used to image the three phases, distinctively, of partially saturated granular soils.

The techniques of digital image processing ware applied to automate the calculation of the degree of saturation, for partially saturated specimens, from X-ray computed tomography (CT) images. Similarly the concept of fabric tensor as defined, statistically, for solid particles was modified and a computer code was written to automate the calculation of the fabric tensor for the liquid phase from digital images.

Void fabric and liquid bridge vectors change dramatically with wetting and deformation and their characterization is important towards achieving a good understanding of unsaturated soil behaviors. Accordingly, an investigation on microstructural evolution of the anisotropic liquid fabric in unsaturated soils was presented in this work. A method to compute fabric tensor was given and second order fabric descriptors of the anisotropic liquid fabric were quantified from X-ray CT scanned images of unsaturated specimens.

It was found that the components of the fabric tensor vary randomly and significantly throughout the specimens as the saturation direction and suction change. The results: including principal values, principal directions, and vector magnitudes confirm that the liquid fabric is anisotropic and that the components satisfy the basic tensorial properties such as symmetry. These parameters can be used by researchers in developing advanced theories for modeling the behavior of unsaturated soils.

This study proposed a fabric based effective stress formulation for unsaturated soils using the principle of virtual work. It was shown that fabric tensor of the liquid phase has an important role in the evolution of effective stress in unsaturated soils. The fabric tensor of the liquid phase was shown to vary throughout the saturation and desaturation process. Thus it was intrinsically shown that the liquid distribution influences the effective stress and therefore, the consideration of soil microstructure in effective stress formulations is imperative.

Contact Information

Acknowledgment b>:  This facility was realized by a grant from the US National Science Foundation (NSF), and a gift from the Murdoch Trust Foundation.
Department of Civil and Environmental Engineering, PO Box 642910, Washington State University, Pullman WA 99164-2910,(509) - 335 1917, Contact Us