This study investigated how soil properties affect levee erosion and foundation scouring by evaluating the behavior of loose and cohesive (mixed) soils beneath a rigid crest under overflow conditions and analyzing flow dynamics within the scoured hole to understand the scouring mechanism. Four cases were examined with varying overtopping depths (Od): LS-FS, LS-FM, and LM-FS, at Od = 2 cm, and LS-FM at Od = 3 cm, where 'L' stands for levee, 'F' for foundation, 'S' for sand (#8), and 'M' for mixed soil (20% silt + 80% sand #8). The results revealed distinct differences among the cases. Notably, erosion of the back slope in the LM-FS case was delayed fourfold compared to LS-FS. In the LS-FM case, breaching of the levee body was delayed by 1.6 times compared to the LS-FS case with a 2 cm overtopping depth. Moreover, different scour hole geometries with complex flow patterns occurred in different timespans. Particle image velocimetry (PIV) was utilized on two physical scoured hole models to analyze the flow behavior within these scoured holes. The PIV analysis revealed the formation of twin eddies, moving in opposite directions and shaped by the nappe flow jet, which was instrumental in the development of the scour holes. This study found that foundation cohesion is more essential than the levee body in delaying levee breaches under rigid crest. Additionally, it revealed the role of twin eddies, especially the levee-side eddy, in increasing the size of the scoured hole upstream and causing levee breaches.

A series of dynamic centrifuge modeling tests were conducted to evaluate the volumetric threshold shear strain of loose gravel-sand mixtures composed of various ratios of gravel and sand by weight. The maximum and minimum void ratios of the mixtures were evaluated, and the optimum packing condition was determined when the mixture contained approximately 60-70 % gravel by weight. A total of six centrifuge modeling tests were performed at 50-g centrifuge gravitational acceleration. Each centrifuge model was subjected to six shaking events consisting of uniform sinusoidal motions with various amplitudes and numbers of cycles. During the entire duration of the test, the development of excess pore water pressure and settlement was monitored. Empirical relationships of pore water pressure ratio and shear strains were developed for these mixtures. The development of excess pore water pressure in the mixtures with greater than 60 % gravel exhibits transient behavior, while residual excess pore water pressure was observed in the mixtures with less than 60 % gravel. Based on the results, the volumetric threshold strain evaluated from the generation of pore water pressure and volume change during shaking is similar. The values were found to be in a range of 0.03-0.10 % and are influenced by soil composition. The threshold strain increases as the amount of gravel in the soil mixture increases.

Suffusion, a process whereby water gradually carries away fine particles from soil, is thought to be one of the possible reasons for the settlement or inclination of bridge piers after a major flood (delayed displacement). The aim of this study is to offer fresh insights into suffusion and its mechanical impact on the affected soil, with a specific focus on how it relates to bridge pier failures. Riverbed material replicated with relatively larger fine particles than those used in past studies which focused on soil in embankments or dikes. Through both monotonic and cyclic loading tests on soil samples with varying initial fines contents, while maintaining a constant relative density of 79%, several important discoveries are made. The small strain stiffness of suffused soil fluctuates as erosion occurs, along with a decrease in shear strength and an increase in soil contraction under monotonic stress. Furthermore, the research simulates the train loading exerted on the base soil of bridge piers susceptible to suffusion by subjecting the soil samples to cyclic loading both before and after erosion, mirroring practical conditions. The key findings of this study reveal that the stiffness of soil drops during erosion with no significant deformation of the soil. This leads to a large strain accumulation in the soil specimens under subsequent cyclic traffic loading. These findings highlight that the delayed settlement or inclination of bridge piers under cyclic or train loading after major flood is possibly due to suffusion in the base soil of the piers. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The state of Pinus cultivation in southern Brazil, from the perspective of potential environmental damage, is controversial. Little is known about naturally regenerated forests after clear-cutting Pinus, particularly regarding ferns, forest structure, and soil composition. This study evaluated differences in fern community structure, forest attributes, and soil properties in a regeneration area 15 years after P. elliottii removal, compared to an Araucaria Forest in the Canela National Forest, Brazil. A 1-hectare plot was divided into 100 m2 subplots, with 12 units randomly selected at each site. In each subplot, all fern species were recorded, and their coverage was measured. Forest and soil parameters were also collected. In the Pinus removal area, forest regeneration shows a fern community with lower species richness, greater floristic homogeneity, and a simplified structure compared to the reference area. Tree species structure differs between sites, with a high density of Pinus and absence of Araucaria angustifolia in the regeneration area. These trees are taller, form a more closed canopy, and create thicker leaf litter. The soil at the regeneration site has lower nutritional quality, greater acidity, and higher aluminum concentration compared to the natural forest. Assisted regeneration is recommended to accelerate and improvide forest recovery.

Previous studies investigating dynamic responses of gravelly soil were limited to high-strain conditions, in which a high level of pore-water pressure is developed, leading to a significant reduction in shear strength and subsequent liquefaction. This paper presents a series of dynamic centrifuge modeling tests performed on loose gravel-sand mixtures to evaluate progressive response under various shear strains. The centrifuge models simulated a uniform soil profile of gravel-sand mixtures with gravel contents of 20%, 40%, 65%, 80%, and 100% that were subjected to incrementally increasing shaking amplitudes from 0.01 to 0.40 g. Due to the influence of composition on the void ratio of the specimens, the results were analyzed in terms of their dominant behaviors (i.e., sandlike, gravellike, or transition soil). Although the soils had comparable initial relative densities, the sandlike soils had the lowest void ratio, and the void ratio increased when the gravel content was greater than 65%. Resonant column testing results indicated that the soils had comparable dynamic properties because of their loose condition. The results showed that dynamic shaking generates comparable shear strains ranging from 0.03% to 3.8% in all models, but the accumulation of pore pressure leads to upward flow in sandlike soils, whereas transient pore-pressure behavior leads to oscillatory flow in gravellike soils. Differences in the stress-strain response and the effects of the number of shaking cycles were observed in different soil mixtures depending upon the level of excess pore pressure. At low shaking amplitude and low excess pore pressure, stiffness degradation was observed while the stress-strain loop was symmetric. At high shaking amplitude and high excess pore pressure, significant stiffness degradation was observed followed by shear-induced dilation resulting in an asymmetrical stress-strain loop. This study clarifies the differences in the dynamic responses and behaviors of sandlike, gravellike, and transition soil over a wide range of strains.