Ling Xu

Loess in the Chinese Loess Plateau (CLP) is a representative of aeolian sediments worldwide. It contains high volumes of voids which results in unique soil properties, yet the voids evolution in CLP remains unexplored. Here we identify that loess strata in CLP have higher void ratios ( e ) compared to other typical aeolian, glacial, alluvial, and residual sediments on a global scale. In a given profile, void ratio decreases with continuous gravity stress ( σ _v ) accumulation, satisfying a linear sedimentary compression line (SCL) in e ‐log ₁₀ σ _v space. For voids evolution in CLP, the variation of SCLs reveals that void ratios first increase and then decrease from northwest to southeast across the plateau. After eliminating the stress effect, we demonstrate strong correlations among void ratio, magnetic susceptibility, particle size distribution and bonding material content of loess, and identify the critical particle size distribution to achieve the highest void ratio. This research enriches aeolian sedimentation theory and brings potential for breakthroughs in accurate prediction of loess mechanical and hydraulic properties with physical indices. It also provides a soil voids basis for interdisciplinary understanding of soil ecological function and geological processes in CLP.

Clarifying the transient acceleration effects and dynamic fragmentation processes of rapid long‐runout loess landslides under seismic loading is essential for predicting and mitigating earthquake‐induced geological hazards in loess areas. In this study, a series of quantitative simulations were conducted using the finite‐discrete element method (FDEM) to investigate the motion and fragmentation characteristics of loess landslides subjected to dynamic loading. The numerical models were calibrated based on the depositional characteristics of the Sunjiagou landslide and relevant laboratory tests. The simulations successfully captured the transient motion of the sliding mass, the rapid transition from continuous to discontinuous deformation, and the coupled evolution between dynamic fragmentation and motion characteristics. Results indicate a pronounced vertical acceleration amplification effect and stratified motion, where the surface layer travels farther than the interior. Internal collisions and flow‐like movements also cause the rear portion of the sliding mass to move farther than the front. Maximum displacement and velocity decrease with increasing vibration frequency but increase with higher acceleration amplitude. Low‐frequency, high‐amplitude excitations produce the strongest motion responses and most extensive fragmentation, whereas high‐frequency loading yields denser and less expanded deposits. Overall, the combined influence of vibration frequency and acceleration amplitude governs the dynamic instability of loess landslides. The findings suggest that loess slopes are more sensitive to vibration frequency than to peak acceleration, emphasizing that seismic hazard mitigation in loess regions should focus on the effects of low‐frequency ground motions.

Sediment connectivity is an effective analytical concept that characterizes the connectivity between sediment sources and sinks. Analysing changes in sediment connectivity as a response to topographic and geomorphic features, as well as land use/cover change (LUCC), is of great significance for watershed water resource management and soil erosion prevention. This study quantified the spatiotemporal patterns of sediment connectivity and its driving factors over the past two decades (2000–2020) in a typical loess hilly and gully watershed, using a modified index of connectivity (IC) index weighted by RI (roughness index) and C factors (RUSLE). The results showed that sediment connectivity in the slope area was significantly higher than that in the check dam and terrace areas, with the proportion of sediment connectivity hot spot areas in the gully slope land being markedly greater than that in the gully bottom and inter‐gully areas. Furthermore, the IC in this region showed a decreasing trend from 2000 to 2020, declining from 0.37 (in 2000) to −0.18 (in 2020). Specifically, the effectiveness of different land use types in reducing sediment connectivity was ranked as follows: forest > settlement > cropland > grassland. Furthermore, validation using watershed sediment yield data from 2000 to 2020 revealed a positive correlation between sediment connectivity and sediment yield ( R ² = 0.71). This indicates that sediment connectivity can be used to characterize dynamic sediment changes in this region. These results provide a scientific theoretical basis for the formulation of soil and water conservation measures for this region.

The complex interplay between fractures and salt precipitation during gas injection in fractured porous media significantly impacts injectivity during subsurface gas storage and requires a deeper understanding of the underlying mechanisms. This study investigates salt precipitation and the clogging phenomenon in fractured micromodels under various injection conditions. Results indicate that precipitated salt may clog the gas injection channel and decrease the effective permeability. Pore‐scale observations reveal the correlation between salt forms and their impacts on injectivity. Aggregated salt grows toward the direction of gas inflow and exhibits negative impacts on injectivity, while bulk salt grows within residual brine and shows little impact on relative permeability. An increase in gas injection rate weakens the negative impacts of precipitated salt on injectivity due to the suppression of brine backflow and salt creeping along the wall surface of the channel. Results deepen the understanding of salt‐induced injectivity impairment in saline formations.

Haines jumps are abrupt movements of fluid interfaces accompanied by sudden changes in capillary pressure during multiphase flow in porous media. They are a fundamental pore‐scale mechanism influencing displacement patterns and efficiency. Haines jumps are controlled by the interplay of varying factors, including capillarity, pore geometry, and fluid properties. Previous studies showed that the elastic deformation of the space containing the liquid due to varying capillary pressure at pore constrictions creates a “soft” fluid system, which promotes Haines instability. Here, we use microfluidics to study the effect of system softness on Haines jumps and drainage in porous media by introducing an entrapped gas bubble with controlled volume in the liquid phase. A pore‐scale analytical model incorporating softness demonstrates that the entrapped gas bubble alters the pressure response during Haines jumps. A dimensionless number is proposed to characterize the system softness. Drainage experiments are conducted in single pore throat, pores‐in‐series, and pore network micromodels. Results show that larger elastic deformation of the entrapped gas bubble in response to the capillary pressure change causes longer jump distance and waiting time before the Haines jump event in a single pore throat. The Haines jump can fill one or multiple pores in pores‐in‐series micromodel, which is determined by the system softness. In pore networks, the final displacement pattern and saturation are barely affected, although the increase of softness results in more pronounced Haines jump events and changes the invasion process. This phenomenon may be due to the constraint of smaller pores on Haines jumps.

Saturated loess in earth structures is often subjected to cyclic loading under nonzero initial shear stress, for which liquefaction assessment remains uncertain. Undrained cyclic triaxial tests were conducted on saturated reconstituted loess with two initial void ratios and different initial static shear stress ratios ζ, representing symmetric stress reversal, non-symmetric stress reversal, and non-reversal loading. Results show that ζ controls the cyclic failure mode: cyclic mobility-type failure occurs at ζ = 0, residual deformation failure occurs at ζ = 0.5, and a plastic strain accumulation-dominated failure pattern develops when ζ ≥ 1.0. Conventional strain-based and R _u = 1.0 criteria become ambiguous under biased cyclic stress paths. A rheology-informed framework based on apparent viscosity is proposed, with the average flow coefficient used as a complementary descriptor. A phase-transition threshold R _u,th is defined from the peak of the apparent-viscosity gradient. R _u,th decreases with increasing ζ and remains below unity. Using R _u,th as the liquefaction criterion, a non-monotonic variation of cyclic resistance with ζ and a normalised cyclic stress ratio–cycle relation are obtained, providing a link between laboratory tests and liquefaction evaluation in loess areas.

Cementation plays a crucial role in stabilizing the open structures formed during aeolian deposition in loess. However, the specific effects of different cementation types on collapsibility and structural integrity remain unclear. This study constructed loess-like deposits under controlled conditions using air-fall deposition and selected cementation materials, and examines the influence of cementation on structure and collapsibility through one-dimensional compression and collapse tests, along with small-strain stiffness ( G ₀ ) measurements. Scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS) observations indicate that the prepared samples reproduce several collapse-relevant microstructural features, including an open pore framework and localized bonding forms. Initial G ₀ values demonstrate that cementation enhances small-strain stiffness, with CaCO ₃ showing the most significant effect. Despite its lower content, NaCl cementation also has a notable impact. Cementation type influences collapsibility differently: NaCl and clay cementation increase collapsibility, with compression curves merging to the normal consolidation line (NCL) after wetting, indicating structural degradation. In contrast, CaCO ₃ cementation reduces collapsibility, and G ₀ remains relatively stable post-immersion, suggesting its dominant role in structural formation. SEM observations and small-strain stiffness parameters provide further insights into particle-scale mechanisms. This study elucidates the role of different cementation types in sediment structure formation and stabilization, offering valuable implications for understanding and mitigating loess collapsibility.

In the investigation of the relationship between measured soil suction and the rate of saturation change, an increase in response time prevents the tensiometer from instantaneously reacting to changes in suction. Solving this problem requires a tensiometer response model, which is based on the measured data from partial drainage and adsorption curve to obtain the potential actual suction related to the flow rate. Although several models have been developed for the calibration of low‐capacity tensiometers in recent decades, these models have not been effectively practiced on high‐capacity tensiometers. In this paper, we investigated the physics mechanism of tensiometer seepage, established a two‐dimensional infiltration equation based on the Green–Ampt model. Soil column experiments were conducted to investigate the applicability of the developed model for the curves obtained from Jingyang loess, Dingbian loess, and Maowusu sand. The results indicated that the developed model could fit the delayed stage and equilibrium stage of the response curves. Compared with Klute's equation and Tsai's equation, the proposed model had achieved an average reduction of 36.57%, 34.23% and 85.00%, 89.47% in the mean absolute error and root mean square error, respectively. This optimized improvement confirmed that the retention of L ( t ), introduction of R , and appropriate selection of K ( h _SW ) together lead to better fitting performance. In addition, we found that the ceramic cup conductance ( C ) required consideration of the internal structure of the cup. This research contributes to refining infiltration theory of tensiometers and promoting the application of tensiometers in unsaturated soils.