Special Issue on Experimental and Computational Geomechanics for Unsaturated Soils

Unsaturated soils are three-phase porous media that consist of a solid skeleton, pore water, and pore air. They are quite prevalent in nature and have significant influence on the performance of civil infrastructures. Compacted soils used in highway embankments and landfill clay liners, as well as many near-surface soils that support foundations for civil engineering structures, are mostly under unsaturated conditions. Swelling, shrinking, and collapsible unsaturated soils cause billions of dollars in damage to civil infrastructure all over the world, and rainfall-induced slope failures that involve unsaturated soils are a major problem in many countries.

The mechanics of unsaturated soils is a very complex problem involving coupled water/air flows and solid-skeleton deformations. In addition, thermal flows have to be considered when studying geothermal systems that involve unsaturated soils. Experimental techniques required for validating unsaturated soil constitutive models are both complex and rather time-consuming because of the requirement to control both pore-water and pore-air pressures/flows. In contrast, full-scale field tests and small-scale tests in unsaturated soils, such as centrifuge model tests necessary for validating finite-element computer codes at the boundary-value problem level, are scarce.

In the past two decades, significant progress has been made in understanding the thermo-hydro-mechanics of unsaturated soils. Constitutive models have been developed and validated, experimental techniques have been further refined, and full-scale and centrifuge model tests are being successfully conducted. Unsaturated soil mechanics is also being applied to ever-more-complex problems such as the behavior of unsaturated soils under dynamic loading conditions. This special issue of the International Journal of Geomechanics, sponsored by the Unsaturated Soils Committee of the Geo-Institute (G-I) of the ASCE and cosponsored by the G-I Soil Properties and Modeling Committee, brings together 23 papers that capture the significant progress that has been made in recent years in the areas of experimental and computational geomechanics for unsaturated soils.

Papers that address experimental geomechanics include laboratory tests to determine shear and tensile strength, stress-strain behavior, critical state parameters, wave velocities, dynamic excess pore-water pressures, microstructure, and soil–structure interface properties. Experiments made use of existing and novel testing devices, including the triaxial apparatus, the direct shear device, the resonant column apparatus, X-ray tomography equipment, and a custom-made device for testing soils undergoing desiccation. Saturated soils to soils with very high suction values were used in these tests. Many papers also address hydro-mechanical coupling in unsaturated soils. Papers on small-scale tests include reinforced soil embankments and friction piles.

Papers addressing computational geomechanics vary from theoretical aspects on effective stress equations for unsaturated soils to simulations of thermo-hydro-mechanical field-scale problems. Several papers cover constitutive modeling of unsaturated soils, and the authors use these models to simulate hydro-mechanical behavior of sands, silts, and clays under monotonic, cyclic, and dynamic loading conditions. Papers also include derivations of governing equations for a variety of unsaturated soil problems and finite-element implementation of these equations for solving boundary-value problems, including those that involve double-porosity soils. The boundary-value problems solved include laboratory-scale triaxial tests and drying of soil columns to field-scale slope stability and nuclear waste depositories.

The efforts of all the authors and the peer reviewers of these papers, many of whom are from the G-I Unsaturated Soils Committee and the G-I Soil Properties and Modeling Committee, and the dedication of the publication staff in producing this special issue are gratefully acknowledged.