Riverbank instability in the seasonally frozen zone is primarily caused by freeze–thaw erosion. Using the triaxial freeze–thaw test on the bank of Shisifenzi Bend in the Yellow River section of Inner Mongolia, we investigated the changes in the mechanical properties of the soil at different freezing temperatures and freeze–thaw times, and analyzed the bank’s stability before and after freezing based on the finite element strength reduction method. The results showed that the elastic modulus, cohesion, internal friction angle and shear strength of the soil tended to decrease with the increase in the number of freeze–thaw cycles and the decrease in freezing temperature. After 10 freezing cycles at − 5 ℃, − 10 ℃, − 15 ℃ and − 20 ℃, the modulus of elasticity of soil decreased by 40.84 ~ 68.70%, the cohesion decreased by 41.96 ~ 56.66%, the shear strength decreased by 41.92 ~ 57.32%, respectively. Moreover, the stability safety coefficient of bank slope decreased by 18.58% after freeze–thaw, indicating that the freeze–thaw effect will significantly reduce the stability of bank slope, and the bank slope is more likely to be destabilized and damaged after freeze–thaw.

The Inner Mongolia section of the Yellow River is a seasonal frozen soil area, where the freeze–thaw effect can alter soil strength and compressibility, affecting bank stability. This study takes the banks sandy silt of the Inner Mongolia section of the Yellow River as the research object. It systematically investigates the relationship between shear strength parameters and compression index of sandy silt and the initial dry density, water content, and freeze–thaw cycles of the soil. It analyzes the order and significance of influencing factors, establishes prediction models of shear strength and compression index, and evaluates the effects of freeze–thaw cycles on soil cohesion and shear strength. The results show that the shear strength index of sandy silt is proportional to changes in initial dry density and inversely proportional to changes in water content. After 10 freeze–thaw cycles, the cohesion of the soil decreases by 22.53 to 58.85%, and the shear strength decreases by 22.67 to 58.91%. The internal friction angle is less affected by freeze–thaw and tends to be stable overall. The smaller the initial dry density and the greater the water content, the greater the compression index and compressibility of the soil, but freeze–thaw has little effect on compression index. The factors affecting sandy silt shear strength and compression index are ranked as dry density > moisture content > freeze–thaw cycles. The stepwise regression model of soil shear strength and compression index based on initial dry density, water content, and freeze–thaw cycles is effective, providing technical guidance for engineering practice.

Ice cover is a natural phenomenon unique to rivers in cold regions, and its existence is one of the reasons for the collapse of structural foundations of bridge piers across rivers. In order to understand the influence of different scouring environments on the hydrodynamics and sand bed morphology in the local scour holes around bridge pier foundations, this paper simulates the dynamic evolution process of the local scouring of tandem combination piers under open-flow and ice-cover environments, based on a turbulence model using the Reynolds-averaged Navier Stokes (RANS) method and a sediment transport model considering the slope collapse effect, respectively. This study also takes the vortex flow and shear stress distribution at different characteristic moments of the pier perimeter section as the penetration point to analyze the effect of the influence law of the scouring environment on the morphology and relative time scale of the scour hole, and makes a detailed comparison with the results of the indoor flume test. The results of this study show that: for local scouring in open-flow conditions, sediment initiation is doubly inhibited and the hydrodynamic forces in the scouring hole are weakened; the local scouring caused by ice cover contributes to the total scouring of the submerged pier within its coverage area, which significantly increases the depth and range of the local scouring hole; and, although the interaction of turbulent eddies and shear stress on the pier side is the main dynamic mechanism of the scouring generated around the pier, the two have a strong correlation. The results of this study, obtained by accurately quantifying the amount of eddies and shear stress around the pier, are the basis for the reasonable estimation of the maximum local scouring depth, which can provide a reference for the study of the evolution of the riverbed around submerged structures in cold areas and is essential to avoid structural damage to the piers and reduce the economic loss of traffic..

Ice cover is a common phenomenon in rivers in cold regions during the winter freeze-up period, leading to the formation of unsteady bypass structures around underwater piers. To reveal the variation law of the flow field around a pier under ice, a numerical calculation method is proposed to obtain the spatial and temporal characteristics of the fluid flow environment around the pier. The verification of flow conditions and convergence showed that the numerical model constructed in this study is reliable and can meet research requirements. The simulation results showed that the ice-cover condition considerably impacted the extent of a scour hole, and in the horizontal plane Z of −0.02 m, the lateral influence of the scour hole was approximately 2.6 times the diameter of the pier, which was approximately 42% wider than that of a scour hole under open-flow conditions; in the area on the side of the pier, there was a peak in longitudinal section y/D of −0.6, and the relative turbulence intensity was 0.4 and 0.51 under open-flow and ice-cover conditions, respectively, indicating that ice cover made the peak more significant in the area.

Hydrodynamics is a common manifestation that causes natural scouring of riverbeds, and it is one of the factors that exacerbate the natural disasters of local scouring of bridge piers, causing sustainability of environmental changes in the water. The evolution pattern and scour characteristics of the bed surface around the submerged structures under different scouring conditions vary greatly. In order to investigate the scour mechanism, the reformed group (RNG) turbulence model in computational fluid dynamics (CFD) simulation software (v11.2) was used to simulate the scour under the clear-water scour and live-bed scour environments, and different scour morphology characteristics around the tandem piers under the clear-water scour and live-bed scour environments were obtained in the final simulation. By capturing the cross-sectional vortex and bed shear stress during the scouring process, the characteristic pattern of scouring topography around the pier and the relationship between the scour hole structure scale were analyzed, and the relationship equation between the development of scour depth and time scale was established. The study shows that: under the clear-water scouring environment, the sediment transport rate lags behind, but the contribution time is superior; under the live-bed scouring environment, by the shading and reinforcement influence of the upstream piers, the extent and development of the downstream pier surrounding the scour hole is small; the development trend of the maximum sediment transport rate of the scour hole and the great value of the shear stress is more synergistic, and the peri-pier eddy is positively correlated with the bed shear stress; through the regression equation to compare the relevant test and simulation results, the two are in good agreement, indicating that the simulated local scour evolution law is consistent with the actual law.