Mechanics of Unsaturated Porous Media

The special collection on Mechanics of Unsaturated Porous Media is available in the ASCE Library (https://ascelibrary.org/page/jenmdt/unsaturated_porous_media).

The last few decades witnessed a great surge in research activities in variably saturated porous materials, particularly in the mechanical and hydrological behavior of earthen materials. This is in response to many emerging engineering applications, such as natural and engineered slope stability under rainfall conditions, municipal and high-level nuclear waste containments by clay barriers, geologic sequestration of ${\mathrm{CO}}_2$, and wastewater injection in deep wells, among others. The underpinning of all these engineering problems is the mechanics of porous materials. During the annual conference of the Engineering Mechanics Institute of ASCE in Stanford, California, in summer 2015, there were three minisymposia on mechanics of unsaturated porous media. A total of 22 presentations were given on various aspects of the mechanical behavior of porous media under variably saturated conditions. Because of the technical breadth and depth of the presentations that were presented by some of the most prominent researchers in the field, a special issue dedicated to the theme was determined to be the most beneficial outcome to expose a broader audience to the advances reported at the symposia. This special issue, cumulating the postsymposium effort, highlights 12 articles covering a range of cutting-edge areas of research frontier, which include advances in the characterization of solid–fluid interactions, analytical and computational methods for coupled hydromechanical processes, and constitutive modeling guided by effective stress principles.

The collection contains a series of papers addressing recent advances in the characterization of unsaturated soils at laboratory scale. The paper by Chen et al. (2017) tackles one of the most fundamental concepts for unsaturated soil behavior: the water retention curve. By recognizing the crucial role of water retention processes to quantify intensity and type of solid–fluid interactions, the paper proposes a new approach based on the latest technique for nuclear magnetic resonance that allows greatly reduced testing time for fine-grained soils while maintaining accuracy of the measurement. A new measurement technique for dynamic soil property is also presented by Dong and Lu (2016). The paper recognizes the remarkable importance of shear wave propagation for site monitoring and geotechnical design, thereby addressing the challenging characterization of the dynamic properties of unsaturated soils with a new laboratory setup to explore wave propagation in different classes of soils under a wide range of saturation conditions. The water retention behavior is again the focus of the paper by Khorshidi and Lu (2017), who explored the theoretical connection between water retention curve and fundamental soil properties, thereby touching the still unsolved topic of soil characterization through fundamental scientific principles. Specifically, the paper recognizes that the cation exchange capacity (CEC) of a soil can be directly linked to the water content at low relative humidity and presents a methodology to determine the CEC by using measurements at high suction range and accounting at the same time for the effect of cation hydration. Fundamental solid–fluid interactions are addressed also by the work of Akin and Likos (2017), who draw our attention to the fundamental differences between such interactions in the presence of different suction regimes, as a result, elucidating the variability of the mechanical properties of soils at the transition from regime dominated by surface hydration to other regimes controlled by capillary condensation. Fundamental microscopic solid–fluid interactions are also addressed in the paper by Puppala et al. (2017), who addressed the richness and complexity of volume change in swelling clays by offering an insightful interpretation of volume change measurements, which is inspired by theoretical models for the diffuse double layer of clay minerals.

Similar depth was displayed by the contributions focusing on analytical and computational modeling techniques. An example is provided by the work of Vo and Russel (2017), who addressed the concept of suction-dependent strength in the context of geotechnical structures. The problem was tackled through an integrated experimental and analytical study, which relied on frictional failure criteria and slip line methods to critically examine deviations between evidence and theory. Failure processes are also at the core of the work of Mihalache and Buscarnera (2016), who tackled the challenging analysis of coupled fluid-deformation processes in near-instability conditions. For this purpose, the paper studies from a mathematical standpoint the link between the instability of unsaturated soils and the illposedness of coupled flow-deformation equations, thereby providing theoretical guidance to assess the robustness of simulations tools widely used for engineering applications. Coupled phenomena are also the focus of the work presented by Pedroso et al. (2017), who discussed the ever-growing richness of numerical tools for coupled analyses in unsaturated soils by recognizing their strategic role for the solution of a wide variety of engineering problems. For this purpose, the authors present a series of computational strategies able to efficiently handle processes ranging from dynamic effects to hydraulic hysteresis and transitions between states of saturation.

The special collection consists of another series of papers that are closely related to coupled solid-fluid interactions in unsaturated porous media. A common theme across these papers is the fundamental importance of effective stress concepts as a platform to understand deformation processes and coupled phenomena. Along these lines, the work of Duriez and Wan (2017) tackled the intuitive—yet not yet fully understood—connection between soil microstructure and unsaturated soil behavior. For this purpose, the authors used both analytical tools and discrete element methods to explore how the soil fabric influences the nature of stress interactions within granular soil samples in the pendular regime. The work by Pasha et al. (2017) addresses the fundamental link between pore structure and water retention processes by recognizing the role of volume change on the water retention behavior of deformable porous media. For this purpose, the authors present a new approach to evaluate the void ratio dependency of the water retention curve (WRC) solely on the basis of the WRC measured at a reference void ratio, thereby simplifying the characterization of couplings at the local material scale. The paper presented by Mašín (2017) offers new insights about the growing area of coupled process in porous materials by focusing on the complex feedback generated by multiple levels of porosity. Specifically, the paper explores thermohydromechanical couplings in expansive soils by taking into account separate effective stress definitions for each level of porosity. Finally, effective stress principles are at the core of the conclusive paper by Ma et al. (2016), who presented a constitutive model for unsaturated soils accounting for physicochemical effects through a new intergranular stress definition able to quantify the influence of the pore fluid chemistry in processes such as osmosis, capillarity, and adsorption.

In summary, we hope that this collection provides the reader with a fresh sample of the vibrant field of the current global research in Mechanics of Unsaturated Porous Media. Challenges in experimental techniques, analytical and computational methods, and constitutive modeling continue to be the frontier in academic research and engineering applications of mechanics of unsaturated porous media. We trust that this collection of papers will stimulate new scientific advances and continued growth in this area of engineering mechanics.