Acta Geotechnica最新文献

Leakage at the bottom of an underground structure can compel ground water to form a converging flow, under which soil erosion may occur, thereby threatening the safety of the structure. Previous studies have shown that converging flow may induce backward erosion, and unravelling the mechanism behind this type of erosion was pivotal to mitigating disasters in practical engineering. Existing studies on this topic have been limited to monolayer erodible medium, while the mechanism behind the backward erosion of a multilayer erodible medium under converging flow remains unclear. In this study, both experimental tests and numerical simulations based on the validated computational fluid dynamics and the discrete element coupling method (CFD–DEM) were conducted to investigate the backward erosion of a multilayer sample under converging flow. The results demonstrated that the multilayer sample was eroded layer-by-layer, whereby residual layers could be observed at the bottom of the sample. The eroded regions in different layers were similar in shape but smaller in size for lower layers. In addition, particle exchange occurred among different layers during the erosion process. In general, lower-layer particles could directly ascend to upper layers in the eroded regions, whereas upper-layer particles mainly settled on the periphery of the eroded regions.

We propose a constitutive model for both the solid-like and fluid-like behavior of granular materials by decomposing the stress tensor into quasi-static and collisional components. A hypoplastic model is adopted for the solid-like behavior in the quasi-static regime, while the viscous and dilatant behavior in the fluid-like regime is represented by a modified (mu (I)) rheology model. This model effectively captures the transition between solid-like and fluid-like flows. Performance and validation of the proposed model are demonstrated through numerical simulations of element tests.

Reinforced calcareous sand is increasingly recognized as a promising roadbed filler. This study evaluated the effects of different reinforcement methods, rubber content, dynamic stress amplitude, and loading frequency on calcareous sand through stress controlled undrained triaxial tests, and studied the dynamic characteristics and particle crushing of reinforced calcareous sand. The results showed that geogrid reinforcement increased the occurrence of particle fragmentation in calcareous sand and had the ability to resist deformation of rubber–calcareous sand mixtures, but the addition of rubber retarded particle fragmentation. In the case of geogrid reinforcement, the increase in rubber particle content from 0 to 10% corresponded to a 23.9% decrease in relative particle fragmentation. In the case with rubber particle reinforcement only, after 1000 cumulative loading cycles, the cumulative axial strain of the specimens increased 2.9 times when the rubber content was increased from 10 to 30%; the cumulative axial strain increased 60.7% when the amplitude of the dynamic stress was increased from 40 to 80 kPa; and the cumulative axial strain decreased 1.7% when the loading frequency was increased from 0.5 to 2 Hz, from 2.46 to 1.825%. The results of the study can provide reference and guidance for practical engineering.

This paper presents a consolidation model for stone column-reinforced soft ground subjected to time-dependent loading under free strain condition. Smear effects and three types of loadings, namely, constant loading, ramp loading, and sinusoidal loading, are considered in the developed consolidation model, which is solved by a numerical method based on a partial differential equation solver. The applicability of the proposed consolidation model and the reliability of the numerical method are demonstrated and verified by well-predicting the consolidation behaviors of two practical engineering cases and one laboratory experiment. The verified model and the numerical method are then employed to investigate the effects of smear zone and time-dependent loading on consolidation characteristics of stone column-improved soft ground. The results indicate that the excess pore water pressure undergoes a sharp change at the interface between the smear zone and the undisturbed zone due to smear effects. The smaller the range of the smear zone, the faster the settlement of the composite foundation develops. The faster the loading rate, the faster the dissipation of excess pore water pressure and the faster the settlement develops. In addition, for the foundation subjected to sinusoidal loading, the higher loading frequency results in a larger amplitude corresponding to the excess pore water pressure and a smaller amplitude corresponding to the settlement of the soil.

Numerous geotechnical applications are significantly influenced by changes of moisture conditions, such as energy geostructures, nuclear waste disposal storage, embankments, landslides, and pavements. Additionally, the escalating impacts of climate change have started to amplify the influence of severe seasonal variations on the performance of foundations. These scenarios induce thermo-hydro-mechanical loads in the soil that can also vary in a cyclic manner. Robust constitutive numerical models are essential to analyze such behaviors. This article proposes an extended hypoplastic constitutive model capable of predicting the behavior of partially saturated fine-grained soils under monotonic and cyclic loading. The proposed model was developed through a hierarchical procedure that integrates existing features for accounting large strain behavior, asymptotic states, and small strain stiffness effects, and considers the dependency of strain accumulation rate on the number of cycles. To achieve this, the earlier formulation by Wong and Mašín (CG 61:355–369, 2014) was enhanced with the Improvement of the intergranular strain (ISI) concept proposed by Duque et al. (AG 15:3593–3604, 2020), extended with a new modification to predict the increase in soil stiffness with suction under cyclic loading. Furthermore, the water retention curve was modified with a new formulation proposed by Svoboda et al. (AG 18:3193–3211, 2023), which reproduces the nonlinear dependency of the degree of saturation on suction. The model’s capabilities were examined using experimental results on a completely decomposed tuff subjected to monotonic and cyclic loading under different saturation ranges. The comparison between experimental measurements and numerical predictions suggests that the model reasonably captures the monotonic and cyclic behavior of fine-grained soil under partially saturated conditions. Some limitations of the extended model are as well remarked.

Mixing discrete flexible fibres into sand may improve its liquefaction resistance during cyclic loading. Here, the benefits are demonstrated by performing undrained cyclic triaxial tests on fibre-reinforced samples in very loose and loose states. The development of a liquified state may be delayed when fibres are present. Here, the strain energy dissipation during loading, and liquefaction development, is focused on. The results show that strain energy continuously dissipates as undrained cyclic loading proceeds. The capacity energy, which coincides with a double amplitude axial strain of 5% or the unity of excess pore pressure ratio (({r}_{u})), whichever occurs first, is increased by the inclusion of fibres. Under the two-way symmetrical cyclic loading, with a cyclic stress ratio of 0.2, the inclusion of fibres with a fibre content of 0.5% leads to the capacity energies of the samples in very loose and loose states increasing by 86.8 and 158.8%, respectively. The generation of pore pressure is closely related to the dissipated energy. The fibres alter the liquefaction responses of a sand skeleton in ways that depend on the applied loading conditions, and this depends on the extent to which the fibres are mobilized in tension during loading. When unities of ({r}_{u}) are attained for fibre-reinforced sand samples, their states may vary greatly and remain far from liquefaction. A newly defined pore pressure ratio (({{r}_{u}}^{*})) proves to be a better indicator of liquefaction in fibre-reinforced sand. A possible energy-based method, intended for practical use to assess liquefaction resistance of fibre-reinforced sand, and the margin of safety against liquefaction, is also presented.

The effect of breakable particle corners is often overlooked in research on pile–soil interaction, which hinders the understanding of the uplift pile behaviors in calcareous sand. This research examines the breakable corner effects on uplift pile–soil interaction in calcareous sand from macro to micro, through model tests and coupled discrete element method–finite difference method. Results revealed that compared to that in silica sand, the higher bearing capacity and relatively abrupt failure behavior of uplift piles in calcareous sands were attributed to the corner interlocking effect and corner breakage effect, respectively. The unstable load transmission along piles in calcareous sand was thoroughly explained by a coupled effect of corner interlocking and breakage. Furthermore, the reduction in effective contacts and alterations in soil skeletons were identified as critical factors contributing to the distinctive soil behaviors in calcareous sand. Moreover, the relative sliding distance of particles was found to be the key factor in determining the amount of corner breakages due to stress concentration at corners. Lastly, a positive feedback loop involving corner breakage effects was proposed, successfully explaining the distinctive phenomenon of uplift piles in calcareous sand. This study provides new perspectives to clarify distinctive pile–soil interaction behaviors in calcareous sand.

Under in-situ conditions, natural hydraulic fractures (NHF) can occur in permeable rock structures as a result of a rapid decrease of pore water accompanied by a local pressure regression. Obviously, these phenomena are of great interest for the geo-engineering community, as for instance in the framework of mining technologies. Compared to induced hydraulic fractures, NHF do not evolve under an increasing pore pressure resulting from pressing a fracking fluid in the underground but occur and evolve under local pore-pressure reductions resulting in tensile stresses in the rock material. The present contribution concerns the question under what quantitative circumstances NHF emerge and evolve. By this means, the novelty of this article results from the combination of numerical investigations based on the Theory of Porous Media with a tailored experimental protocol applied to saturated porous sandstone cylinders. The numerical investigations include both pre-existing and evolving fractures described by use of an embedded phase-field fracture model. Based on this procedure, representative mechanical and hydraulic loading scenarios are simulated that are in line with experimental investigations on low-permeable sandstone cylinders accomplished in the Porous Media Lab of the University of Stuttgart. The values of two parameters, the hydraulic conductivity of the sandstone and the critical energy release rate of the fracture model, have turned out essential for the occurrence of tensile fractures in the sandstone cores, where the latter is quantitatively estimated by a comparison of experimental and numerical results. This parameter can be taken as reference for further studies of in-situ NHF phenomena and experimental results.

Precise stratigraphic characterization and assessment of soil parameters are essential for agricultural and geotechnical engineering. The cone penetration test (CPT) has become one of the most extensively used techniques for soil site assessment, because of its reproducibility, robustness, accuracy, and simplicity. The existing DEM (discrete element method) simulations on CPT are only applicable to dry soil, which cannot consider fluid phase (i.e., pore water) and its interaction with the soil particles. The combined DEM and CFD (computational fluid dynamics) approach is developed to model CPT testing on saturated soils in this study. Several sets of CPT simulations at various penetration rates have been performed by using CFD–DEM coupled analysis. The variation of penetration velocity leads to different magnitudes of fluid force, and the variation in fluid force, in turn, affects the CPT measurement of soil’s characteristics. Furthermore, the study extends beyond the properties of the soil itself to explore the complex interplay among soil particles, the surrounding fluid environment, and the penetrometer. The cumulative interactions among these elements highlight the intricate nature of CPT and underline the importance of comprehensive computational models in enhancing our understanding of these dynamics.