Global climate change has caused frequent extreme weather events, leading to the degradation of soil engineering properties. Eco-friendly biopolymer has been considered for soil reinforcement under extreme climate. This study investigates the effects of biopolymer amendment on soil mechanical properties under freeze-thaw (F-T) cycles. Direct shear tests were conducted on plain soil (PS) and biopolymer reinforced soil (BRS) under varying water contents (5 %, 15 %, and 25 %) and F-T cycles. Microstructural analysis and numerical simulation were carried out to reveal the influence of biopolymer on the evolutions of microstructure, shear band and particle interaction. The results showed that biopolymer significantly enhanced soil strength, particularly at lower water contents, with strength increases of up to 3.6 times as water content decreased from 25 % to 5 %. BRS exhibited better resistance to strength deterioration under F-T cycles, with an average strength loss of 25.5 % compared to 35 % for PS after 10 cycles. SEM and MIP analyses demonstrated that biopolymer reduced porosity and pore size by filling voids and cementing particles while mitigating F-T damage. DEM simulations revealed that F-T cycles increase the shear band area and reduce the average contact force. However, the addition of biopolymer effectively mitigates the adverse effects of F-T cycles. Biopolymer is demonstrated to be effective in enhancing soil strength and durability in seasonally frozen ground region.

Artificial ground freezing (AGF), widely employed in subway tunnel construction, significantly alters the microstructure of surrounding soils through freeze-thaw processes. These changes become critical under subway operation, where traffic-induced dynamic loading can lead to progressive soil deformation. Understanding the dynamic behavior of freeze-thaw-affected soils is therefore essential for predicting and mitigating deformation risks. This study investigates the microstructural evolution of soil subjected to a single freeze-thaw cycle-representative of AGF practice-and subsequent dynamic loading. Dynamic triaxial tests were conducted under a fixed dynamic stress amplitude of 10 kPa and loading frequencies of 0.5 Hz, 1.5 Hz, and 2.5 Hz, simulating typical subway traffic conditions. Microstructural analyses were performed using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Results show that the freeze-thaw cycle leads to a denser yet more disordered particle arrangement, with sharper and more angular particles, as reflected by increased probability entropy and reductions in surface porosity, form factor, and uniformity coefficient. Dynamic loading further causes particles to flatten and align in a more directional manner, accompanied by decreased surface porosity and form factor, and an increased uniformity coefficient. Pore structures become more uniform and less complex. Among various microstructural indicators, total intrusion volume from MIP displays a strong correlation with cumulative plastic strain, suggesting its potential as a micro-scale predictor of soil deformation. These findings enhance our understanding of the coupled effects of freeze-thaw and dynamic loading on soil behavior and offer valuable insights for improving the safety and durability of subway tunnel systems constructed using AGF.

Studying the rheological properties of deep-sea shallow sediments can provide basic mechanical characteristics for designing deep-sea mining vehicles driving on the soft seabed, providing anchoring stability of semi-submersible mining platforms, and assessing submarine landslide hazards. Shallow sediment column samples from the Western Pacific mining area were obtained, and their rheological properties were studied. A series of rheological tests was conducted under different conditions using an RST rheometer. In addition, conventional physical property, mineral composition, and microstructure analyses were conducted. The results showed that shallow sediments have a high liquid limit and plasticity, with flocculent and honeycomb-like flaky structures as the main microstructure types. The rheological properties exhibited typical non-Newtonian fluid characteristics with yield stress and shear-thinning phenomena during the shearing process. In contrast to previous studies on deep-sea soft soil sediments, a remarkable long-range shear-softening stage, called the thixotropic fluid stage, was discovered in the overall rheological curve. A four-stage model is proposed for the transition mechanism of deep-sea shallow sediments from the solid to liquid-solid, solid-liquid transition, thixotropic fluid, and stable fluid stages. The mechanism of the newly added thixotropic fluid stage was quantitatively analyzed using a modified Cross rheological model, and this stage was inferred from the perspective of mineralogy and microstructure. The results of this study can be useful for improving the operational safety and work efficiency of submarine operation equipment for deep-sea mining in the Western Pacific Ocean. (c) 2025 Japanese Geotechnical Society. 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/).

The complex phenomenon of suffusion is the selective erosion of the fine fraction under the effect of seepage flow within the matrix of coarser particles. Three processes are involved simultaneously: detachment, transport, and partial filtration of the fine particles. With the objective to characterize the influence of the stress state on suffusion-related parameters, downward seepage flow tests were conducted under hydraulic-gradient controlled conditions. Four stress states are investigated: triaxial isotropic, triaxial compression, triaxial extension and rigid vertical boundaries. Also, four different cohesionless gap-graded soils were tested, from underfilled to overfilled microstructures. The entire erosion process can be divided into four phases: onset, self-filtration, blow-out and steady state. The definitions of several suffusion-related parameters are given for each suffusion phase, in terms of hydraulic gradient, hydraulic conductivity variation, cumulative expended energy, erosion resistance index and Darcy velocity. The results demonstrate that the suffusion kinetics of soils in transition between underfilled and overfilled microstructures are more affected by the stress state than others.