Accurate characterization of soil dynamic response is paramount for geotechnical and protective engineering. However, the impact properties of unsaturated cohesive soil have not been well characterized due to lack of sufficient research. For this purpose, impact tests using the Split Hopkinson Pressure Bar (SHPB) were elaborately designed to investigate the dynamic stress-strain response of unsaturated clay with strain rates of 204 similar to 590 s(-1). As the strain rate increased up to 500 s(-1), a lock-up behavior was observed in the plastic flow stage, which can be explained as the breakage and rearrangement of soil gains under a high level of stress. Further, the strain rate dependency of the dynamic strength was quantitatively characterized by the Cowper Symonds (CS) model and the CS coefficients were determined to be the intercept of 55 and slope of 0.8 in the double logarithmic scale of Dynamic Increase Factor (DIF) and strain rate space. Furthermore, the SHPB test was reproduced using a modified Material Particle Method (MPM), which involves an improved dynamic constitutive model for unsaturated soil considering the strain rate effect. The simulated stress-strain curves basically agree with the experimental results, indicating the feasibility of MPM for investigating the dynamic properties of unsaturated soil under SHPB impact loading.

This study investigates the geotechnical properties of soft Pak Phanang marine clay, prevalent in Nakhon Si Thammarat province, Thailand, where rapid economic development demands a comprehensive understanding for sustainable construction. Triaxial tests on undisturbed marine clay specimens with various stress histories and strain rates were conducted, focusing on over-consolidation ratios (OCRs) of 1, 2, 4, and 8. Shearing was performed at rates of 0.020%, 0.075%, 1.000%, and 8.500% per minute after K0 consolidation. The strain rates selected for this study represent specific values that have been chosen for a comprehensive exploration of Pak Phanang clay behavior under different loading conditions. The effects of stress histories on the marine clay behavior at various strain rates under K0 conditions were investigated. It is indicated that the greater strain rates under K0 conditions potentially lead to the larger undrained shear strengths and reduce pore water pressure for varied over-consolidation ratios. On the other hand, the greater over-consolidation ratios commonly result in lower shear strengths at all strain rates. Examination of pore pressure parameter at failure (Af\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${A}_{f}$$\end{document}) and secant Young's modulus reveals significant strain-rate-dependent behavior and OCR influence on the marine clay's response. Undrained shear strength increases with higher OCRs, emphasizing OCR's pivotal role. Rate effect analysis confirms undrained behavior, with a consistent 28% strength increase, regardless of OCR variations. Pore pressure responses exhibit a transition at OCR 4. Secant Young's modulus decreases with rising OCR, establishing a linear correlation with undrained shear strength.

Stiff clay exists widely in the world, but its significant time- and temperature-dependent mechanical features have not been fully modeled. In the context of fractional consistency viscoplasticity and bounding/subloading surface theory, this study proposes a novel nonisothermal fractional order two-surface viscoplastic model for stiff clays. First, by proposing a generalized plastic strain rate, the isotach viscosity is modified and extended to both over-consolidated and nonisothermal conditions that take into consideration the effects of temperature and OCR on thermal accelerated creep. Then, two strain rate and temperature-dependent yield surfaces are proposed with isotropic and progressive hardening rules to consider thermal collapse, strain rate effects, and smooth transition from elastic to viscoplastic behaviors. Next, the stress-fractional operator of the loading surface, according to the principle of fractional consistency viscoplasticity, is introduced to describe the nonassociativity of stiff clays. Finally, the predictive ability of the model is validated by simulating triaxial tests on Boom clay with various stress paths considering the temperature- and time-dependent features of stiff clays.

The influence of strain rate on the mechanics of particles is well documented. However, a comprehensive understanding of the strain rate effect on calcareous particles, particularly in the transition from static to dynamic loading, is still lacking in current literature. This study conducted 720 quasi-static and impact tests on irregular calcareous particles to investigate the macroscopic strain rate effect, and performed numerical simulations on spherical particles to explore the underlying microscopic mechanisms. The strain rate effect on the characteristic particle strength was found to exhibit three regimes: in Regime 1, the particle strength gradually improves when the strain rate is lower than approximately 102 s-1; in Regime 2, the particle strength sharply enhances when the strain rate increases from 102 s-1 to 104 s-1; and in Regime 3, the particle strength remains almost constant when the strain rate is higher than 104 s-1. The three-regime strain rate effect is an inherent property of the material and independent of particle shape. The asynchrony between loading and deformation plays a dominant role in these behaviors, leading to a thermoactivation-dominated effect in Regime 1, a macroscopic viscosity-dominated effect in Regime 2, and a combined thermoactivation and macroscopic viscosity-dominated effect in Regime 3. These mechanisms induce a transition in the failure mode from splitting to exploding and then smashing, which increases the energy required to rupture a single bond and, consequently, enhances the particle strength. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Dynamic properties of sandy soil under medium-high strain rates are of great significance for protection engineering, pile penetration, ship anchoring, aircraft landing, and so on. This paper reviews the current research status of split Hopkinson pressure bar (SHPB) impact tests and numerical simulations on sandy soil. The key issues in the research of sandy soil impact characteristics are summarized as follows: (1) The SHPB test still faces uncertainties for granular materials, such as the lack of standardized test sample size, difficulties in controlling boundary conditions, and the immaturity of triaxial testing methods. Future triaxial SHPB tests need to address issues related to measuring radial deformation of the samples and maintaining consistent confining pressure. (2) Due to uncertainties in gas and water discharge under test conditions and the presence of inertial effects, the accurate determination of strain rate effects becomes challenging. (3) The impact characteristics of granular materials are influenced by moisture content, which is correlated with changes in pore water pressure and pore air pressure. However, measuring these related variables is difficult, making it challenging to analyze the results. It is necessary to develop a device that completely eliminates the effects of gas and water discharge to mitigate the influence of boundary conditions. (4) To study the impact characteristics of sandy soils, it is necessary to overcome computational limitations and establish numerical models that account for complex mechanisms such as water content and particle fragmentation. Existing methods such as the finite element method, discrete element method, and coupled methods are unable to uniformly simulate the continuity of wave propagation and particle fragmentation. (5) It is crucial to develop constitutive models that consider the strain rate effects and can simulate complex mechanisms such as water content and particle fragmentation. This will refine the theoretical framework of soil mechanics at medium to strain rates.