The rapid progress of urbanization and industrialization has led to the accumulation of large amounts of metal ions in the environment. These metal ions are adsorbed onto the negatively charged surfaces of clay particles, altering the total surface charge, double-layer thickness, and chemical bonds between the particles, which in turn affects the interactions between them. This causes changes in the microstructure, such as particle rearrangement and pore morphology adjustments, ultimately altering the mechanical behavior of the soil and reducing its stability. This study explores the effects of four common metal ions, including monovalent alkali metal ions (Na⁺, K⁺) and divalent heavy metal ions (Pb²⁺, Zn²⁺), with a focus on how ion valence and concentration impact the soil’s microstructure and mechanical properties. Microstructural tests show that metal ion incorporation reduces particle size, increases clay content, and transforms the structure from layered to honeycomb-like. Small pores decrease while large pores dominate, reducing the specific surface area and pore volume, while the average pore size increases. Although cation exchange capacity decreases, cation adsorption density per unit surface area increases. Monovalent ions primarily disperse the soil structure, while divalent ions induce coagulation. Macro-mechanical tests reveal that metal ion contamination reduces porosity under loading, with compressibility rises as the ion concentration increases. Soils contaminated with alkali metal ions shows higher compression coefficients at all loads, while heavy metal ions cause higher compression under lower loads. Shear strength, the internal friction angle, and cohesion in metal-ion-contaminated clay decrease compared to uncontaminated field-state clay, with greater declines at higher ion concentrations. The Micropore Morphology Index and hydro-pore structural parameter effectively characterize both micro- and macrostructural properties, establishing a quantitative relationship between HPSP and the engineering properties of metal-ion-contaminated clay.

Artificial ground freezing (AGF) is widely applied in complex geotechnical environments; however, in situ evaluation of frozen soil strength remains limited. This study employs a self-developed embedded four-electrode system to investigate the strength and electrical properties of water-rich silty clay from the Yangtze River floodplain. Results indicate that unconfined compressive strength (UCS) increases with decreasing temperature in a "slow-sharp-slow" pattern and rises with dry density. A critical water content of 22% produces peak strength, beyond which frost-heave cracking reduces strength, while near-saturation (~ 30%) promotes the formation of an ice skeleton that partially restores it. Resistivity exhibits stepwise growth with decreasing temperature, primarily due to free water freezing and increased tortuosity, with sharper conductivity losses at higher water contents. A conduction shift occurs at a critical dry density, transitioning from pore water dominated to bound water and soil matrix dominated pathways. Resistivity follows a power-law relationship with saturation, controlled by bound water freezing and frost-induced deformation. Both UCS and deformation modulus correlate strongly and linearly with resistivity. A resistivity-based model is proposed to link strength with microstructural and moisture changes. Brunauer-Emmett-Teller (BET) and Scanning Electron Microscopy (SEM) analyses reveal that higher water content enlarges pores, while freeze-thaw cycles transform flaky aggregates into honeycomb structures and convert micropores into macropores, highlighting the coupled influence of phase transitions and pore evolution on strength. These findings enhance understanding of the frozen soil-resistivity coupling mechanism and provide a basis for accurate strength evaluation using resistivity.

Elliptical fractures are often used to characterize the deformation of real fractures; however, their deformation responses do not accurately align with those of actual fractures. Nonelliptical fractures shorten by closing near the fracture tips under compression, demonstrating significant advantages in accurately capturing fracture closure characteristics. This warrants investigation of the elastic response of rocks containing nonelliptical fractures. Focusing on single-fracture closure, this study verifies the accuracy of the numerical simulations for elliptical and nonelliptical fractures. A dynamic numerical simulation for crack deformation under compression is proposed to simulate crack closure. The stress-dependent fracture parameters are collected for models consisting of the two fracture types. Then, the pressure-dependence of wave velocities is fitted using an empirical relationship. Under initial compression (<10 MPa), the velocity increase in the elliptical fracture model is just 14.45% of that in the nonelliptical fracture model, displaying a three-stage stress-dependent behavior. The nonelliptical model follows a more realistic two-stage trend, which is more consistent with empirical observations. Moreover, the differences between microscopic parameters are negligible for the models containing elliptical and nonelliptical fractures with a small aspect ratio (≈0.003). This research lays a theoretical foundation for future inversion of fracture distributions using nonelliptical fracture models.