Special Issue on Time-Dependent Stress–Strain Behavior of Geomaterials

The Second International Symposium on Constitutive Modeling of Geomaterials: Advances and New Applications (IS-Model 2012) was successfully held in Beijing, China, October 15–16, 2012. This symposium was organized by Q. Yang (chairman), C. S. Desai (cochair), and J. P. Carter (cochair) with organizations of Tsinghua University and the International Association for Computer Methods and Advances in Geomechanics, together with a few other sponsoring organizations. This IS-Model 2012 followed the first successful International Workshop on Constitutive Modeling held in Hong Kong, organized by J. H. Yin in 2007. The main scope of IS-Model 2012 was to provide a forum for researchers and engineers working or interested in the area of constitutive modeling to meet and share new ideas, achievements, and experience through presentations and discussions. This symposium included a workshop focused on “Time-Dependent Stress–Strain Behavior of Geomaterials,” jointly organized by J. H. Yin, Z.-Y. Yin, and T. M. Bodas Freitas. Eight papers from this workshop have been selected, revised, and extended for peer review for publication as a special issue with the same name as the workshop in this journal.

The time-dependence of the effective stress–strain behavior of geomaterials includes phenomena such as creep, relaxation, swelling, aging, etc., developing with time, excluding the time effects due to dissipation or flow of pore water. The time-dependence is one of the viscous phenomena of geomaterials. Other viscous aspects include strain-rate effects and stress-rate effects. The rate effects and time effects are related to each other and shall be considered by an appropriate constitutive model. The study on the time-dependent stress–strain behavior of geomaterials and constitutive modeling has been a hot research topic in geomechanics and geotechnical engineering in recent years. The main reason is that more and more projects have encountered geomaterials in which the time effects or rate effects must be considered. The eight papers in this special issue present the latest studies and findings on the time-dependence of the stress–strain behavior of geomaterials and their constitutive modeling.

The paper by Yin addresses a number of fundamental issues, such as the differences between a true instant-compression line and a normal consolidation line; the uniqueness of viscoplastic strain rates with a stress–strain state; misunderstanding of creep compression smaller than the instant compression; separation of the total strain rates, etc. Understanding these issues will help us to develop better constitutive models for the time-dependence of the stress–strain behavior of geomaterials.

The work by Yin, Xu, and Yu focuses on nonlinear creep behavior, considering decreasing creep coefficient $C_{\alpha \epsilon }$ with the increase of the soil density. Data from oedometer tests on reconstituted samples of several natural soft clays were selected to clarify and to fit the evolution of the nonlinear creep coefficient with soil density. The nonlinear creep function is then incorporated into a newly developed elasticviscoplastic model to take into account the nonlinear creep of natural soft clays. Wang and Yin present their study on the stress–dilatancy relationship of a natural soft clay during undrained creep. Several typical stress–dilatancy relations for soils are discussed and compared with experimental data. Kimoto, Khan, Mirjalili, and Oka propose a cyclic elastoviscoplastic constitutive model for clayey soils depending on the nonlinear kinematic hardening rules concerning asphalt pavement experimental data.

The comparison of different constitutive models and their application to settlement calculation is done by Nash and Brown. They discuss the breakdown of clay structure owing to loading and influences on the time-dependent stress–strain behavior of clays in one-dimensional compression. Four elasticviscoplastic models are used to compute the time-dependent stress–strain behavior and are compared. The comparison shows that despite presentational differences, all calculated creep rates are in fundamentally the same manner. Nash and Brown have used a number of models to calculate settlements of a benchmark full-scale problem. It is shown that the predicted long-term settlement may vary by factors of two or more depending on the assumptions made. Karstunen, Rezania, Sivasithamparam, and Yin compare two anisotropic rate-dependent models and their applications to consolidation analysis of two natural clays.

Watabe and Leroueil present an isotache concept for long-term consolidation behavior of clays. The isotache concept introduces a unique relationship between the strain and the consolidation pressure corresponding to the strain rate in association with viscosity. In fact, this isotache concept is the same as the statement of the uniqueness of viscoplastic strain rates with a stress–strain state discussed in the paper by Yin.

The behavior of asphalt and asphalt pavement is dependent on time and temperature. The constitutive modeling of the time and temperature-dependent stress–strain behavior of asphalt and asphalt pavement is needed and challenging. Solanki, Zaman, Adje, and Hossain investigate the thermal cracking resistance of hot-mix asphalt mixes containing recycled asphalt pavement. A total of four mixes containing different percentages of recycled asphalt pavement were designed and tested. They have found that different mixes have different temperature effects and creep-compliance values.