Temperature effects become important in a number of geotechnical applications, such as nuclear waste disposal facilities, buried high-voltage cables, pavement, energy geostructures and geothermal energy. On the other hand, soft soils act time- and strain rate dependent. Both temperature and strain rate influence soil behavior, affecting stiffness, strength, and deformation even under constant stress levels. A model to predict temperature and loading rate effects on soil behavior is presented in this article. The model is based on a simple visco-hypooplastic model for clays and encompasses key aspects of coupled rate- and temperature-dependent soil behavior such as (partially irreversible) thermal expansion, heating-induced irreversible compression, stress history, drained heating/cooling cycles, as well as mechanical and thermal creep, incorporating isotachs, and isotherms.

Accurate prediction on long-term settlements of soft soils is challenging. One of the reasons is the time-dependent soil behaviours. How to extrapolate such behaviours from thin laboratory testing with limited time scales to in situ thick soil layer with large time scales is still an ongoing debate, which can be dated back to Ladd et al. (1977). Many experimental results previously used to advocate Hypothesis A have been re-analysed and found to align with Hypothesis B. However, discrepancies remain in some historical data, particularly in the case of Osaka clay retrieved from the seabed under the Kansai International Airport Islands. This study begins by introducing the main implications and selected existing models for the time-dependent behaviours of soft soil. It then focuses on the re-examination of selected testing results from Watabe et al. (2008b), supplemented by numerical simulation using isotache models. The analysis emphasises the importance of considering equal initial conditions when comparing data from samples with varying thicknesses. Furthermore, the good agreement observed between measured data and the simulation results using Hypothesis B methods demonstrates the validity of Hypothesis B for predicting the long-term consolidation behaviour of soft soil in both laboratory and field scales.

Computation of end-of-primary compression for clayey soils is an important step undertaken to meet the serviceability criteria of geotechnical engineering structures. But for soils that exhibit secondary compression behaviour, computing the end-of-secondary compression values is more significant. One of the prevailing methods suggested for its computation is the c(alpha) (coefficient of secondary compression) method. However, without sufficient data, interpretation of the end-of-secondary compressions could be challenging, if not erroneous. This paper presents the computation of the end-of-secondary compression for a set of five soils from India namely Red soil, Bombay marine clay, Kuttanad clay, Cochin marine clay and Peat by conducting long-term consolidation tests to the order of 10 days for each pressure increment. A non-linear creep function was used to determine the limiting creep condition of the soils. The curves obtained are more realistic compared to the c(alpha) concept which overestimates secondary compression settlements.

A viscoplastic constitutive model is developed to describe the viscoplastic behavior of unsaturated soils. The proposed model accounts for the stain-rate and suction effects on yield by adopting an unsaturated isotach concept. The nonstationary flow surface theory (NSFS) is applied for modeling the viscoplastic behavior, with the yield surface which can evolve with the viscoplastic strain, viscoplastic strain rate and suction. Meanwhile, the progressively hardening concept is adopted for reproducing the viscoplastic behavior of soil at an over-consolidated state. For the validation, a series of loading conditions are considered based on the data from the literature. Results show that the proposed model is able to reproduce the main viscoplastic behaviors of unsaturated highly compacted Gaomiaozi (GMZ) bentonite, Glenroy silt and Qianjiangping landslide (QL) soil, including CRS compression tests, rate-dependent triaxial shear tests, and triaxial creep tests.

Current overstress typed elastic viscoplastic models fall short in describing some time-dependent mechanical behaviors of anisotropically overconsolidated clays comprehensively. This paper presents a rigorous fractional order anisotropic elastic viscoplastic two-surface model for such clays, based on the principles of fractional consistency viscoplasticity and bounding or subloading surface theory. First, a three-dimensional formulation of isotach viscosity is proposed and integrated into two rate-dependent surfaces, i.e., the loading surface and yield surface. Then, by incorporating the stress-fractional operator of the rate-dependent loading surface into isotropic, progressive, and rotational hardening rules, the incremental form of stress-strain-time model with a fractional order viscoplastic flow rule is developed by meeting the consistency condition on the loading surface. Accordingly, the proposed model cannot only maintain the predictive capabilities of a classic bounding surface model but also describe the general features of the time-dependent behavior under various stress conditions. Validation and versatility of the proposed fractional order elastic viscoplastic model are successfully evaluated against constant strain-rate and stress relaxation tests on anisotropically overconsolidated resedimented Boston Blue clay.