Acta Geotechnica最新文献

Numerous incidents and failures of bank slopes are caused by the creep behavior of sliding zone soil. During reservoir regulation, the pore water pressure in the sliding zone undergoes cyclic changes. Under such complex cyclic hydraulic conditions, the creep behavior may differ from that under the monotonic seepage condition, which is still poorly understood. In this paper, the Majiagou landslide in the Three Gorges Reservoir area is taken as a case study. Triaxial creep tests were first carried out to study the creep behavior of the sliding zone soil specimen under cyclic seepage pressure. Then, the nonlinear Burgers creep model was proposed to characterize the observed creep behavior of the sliding zone soil specimen, and the secondary development was performed based on FLAC3D software. Finally, the proposed model was applied to the Majiagou landslide to simulate its deformation under fluctuating reservoir water levels. The following results were obtained: (1) Under low deviatoric stress levels, cyclic seepage pressure causes the creep strain curve to fluctuate significantly. The decrease of seepage pressure leads to a reduction in pore pressure, resulting in a sharp increase in the strain rate of sliding zone soil. (2) The proposed model can well reflect the creep characteristics of sliding zone soil under cyclic seepage pressure. (3) During reservoir operation, the landslide deformation exhibits a step-like growth, and the proposed creep model can effectively simulate the retrogressive deformation characteristics of the Majiagou landslide. The research results provide the theoretical basis for the long-term stability of reservoir landslides under fluctuating water levels.

In this paper, a new approach for rotational hardening in elastic-plasticity is formulated. After discussing the standard yield criteria employed for geomaterials and the rotational hardening models proposed in the past, the authors introduce the concept of pure rotational hardening, that is a rigid rotation of the yield surface not implying any distortion of it. In the second part of the paper, a new approach for rotational hardening, based on Householder transformations, is proposed. The method, that allows to reflect vectors with respect to a given hyper-plane, is briefly described since not usually employed in geomechanics. Moreover, the authors clarify that any yield surface or plastic potential rotation, not being a rolling, is a transformation keeping unaltered first and second invariants, but not the third. As a consequence, when rotational hardening is introduced, the use of the third mixed invariant, for defining in the deviatoric plane the yield surface shape, is not appropriate. Finally, the application of the proposed approach in the formulation of anisotropic elastic–plastic strain hardening constitutive models is briefly discussed for the classes of uncoupled and hybrid yield criteria that include a dependence of the yield surface on Lode angle.

While the effect of combined loading on the load carrying capacity of skirted foundations on horizontal ground surfaces is well documented in the literature, comprehensive studies for skirted footings situated on or near slopes remain lacking. This study aims to investigate the influence of various factors, including slope angle (β), angle of internal friction (ϕ), setback distance (B′/B), and skirt length (L/B), on failure loads and capacity envelopes for skirted foundations. The performance of a skirted footing system under combined vertical and lateral loadings in cohesionless soils was analysed using finite-element limit analysis (FELA) and small-scale experimental tests. The results indicate that the optimal skirt length depends on the specific conditions of the slope and setback distances. As the setback distance increases, the optimal skirt length tends to decrease, while an increase in slope steepness corresponds to an increase in the optimal skirt length. Furthermore, this study presents integrated capacity envelopes that incorporate a wide range of realistic soil slopes. The calculated capacity envelopes are subsequently compared to those on flat ground, highlighting that the unique characteristics of these envelopes are based on different combinations of vertical load (V) and combined vertical and lateral load (VH). Skirted footings on slopes exhibit distinct behaviour compared to those on level ground, with multiple factors influencing their capacity envelopes. Specifically, skirted foundations demonstrate lower capacity than equivalent block foundations, particularly when placed near steep slopes due to the formation of an inverted circular scoop failure surface for larger L/B ratios. In contrast, the formation of an inverted circular scoop is less pronounced for moderate slopes and footings with a smaller L/B ratio. The numerical study provides a comprehensive explanation of the failure mechanisms associated with the design of skirted footings located in close proximity to slopes, especially when subjected to lateral loads. Based on the results obtained, capacity envelope diagrams are proposed for estimating the capacity of skirted footings on slopes. Small-scale experiments were also conducted to explore the effects of varying skirt lengths placed on both level and sloping ground within a testing tank. The outcomes of this study yield valuable insights into the behaviour of skirted foundations on slopes, as well as their overall response to both vertical and lateral loadings.

Extensive research has been conducted on the impact behavior of unsaturated sand at high strain rates. However, achieving the undrained boundary condition remains a persistent challenge, leading to an inconsistent understanding of the dynamic responses of sand with varying saturation degrees. In this study, a novel sleeve designed to conduct split Hopkinson pressure bar (SHPB) tests under undrained boundary conditions. Furthermore, drained SHPB tests were carried out by using the conventional steel sleeve as references. The absolute particle crushing distributions within various size ranges were investigated by utilization of dyed calcareous sand. Results revealed that, for the conventional drained sleeve, the locking-up phenomenon of full saturation sand was only observed at strain rate of 750 s⁻¹. However, locking-up occurs at all strain rates for undrained sleeve. The locking-up stiffness at strain rate of 1000 s⁻¹ was 1.6 times larger compared to that at strain rate of 500 s⁻¹ and 750 s⁻¹, respectively. It means that the locking-up stiffness increases with strain rates under the fully undrained boundary conditions. Furthermore, for the drained sleeve, negligible reductions up to 10.8% were observed in measured B_r if saturation degrees change from 0 to 100%. In contrast, for the undrained sleeve, the maximum reduction on B_r was 47.6% and increases rapidly with increasing strain rates. The particle crushing was more sensitive to saturation degree at higher loading strain rates under undrained boundary conditions. Meanwhile, the particle crushing probability ({P}_{text{c}}) of medium-sized (1.0–1.5 mm) particles decreases with decreasing saturation degrees and increasing strain rate. It leads that the probabilities of particle crushing across various size ranges become more uniformly distributed with lower saturation degree and higher strain rates.

Transformation models have been widely used in geotechnical engineering to relate data from lab or field tests (e.g., cone penetration tests, CPT) to design parameters required in geotechnical analysis and design. Proper selection of transformation models is crucial but challenging for accurate prediction of geotechnical responses (e.g., reclamation-induced settlement) in practice. This study proposes a general machine learning framework that accommodates a wide variety of existing CPT-based transformation models and uses field monitoring data (e.g., settlement data observed from a specific project) to select suitable transformation models for improving prediction of spatiotemporally varying reclamation-induced settlement. The proposed approach takes advantage of sparse dictionary learning (SDL) and achieves prediction of settlement by a linear weighted sum of dictionary atoms that are constructed using outputs from finite element models (FEM) of reclamation-induced consolidation. Input parameters of the FEM models are determined using existing transformation models in literature. A transformation model database that relates multiple soil consolidation parameters with CPT data is also compiled for consolidation analysis and dictionary construction in SDL. The proposed approach is illustrated using a real reclamation project in Hong Kong. Results show that the proposed approach provides an effective and transparent vehicle to leverage existing abundant transformation models, identify appropriate transformation models using field monitoring data, and improve prediction of spatiotemporally varying reclamation-induced settlement, with greatly reduced prediction uncertainty. The transformation model selection and settlement prediction are also improved continuously as more field monitoring data are obtained.

Given the importance of drag model in solving fluid–particle interactions in unresolved numerical methods, this study proposed a machine learning (ML)-based drag model for irregular sand particles in transition flow, aided by spherical harmonic (SH) analysis and a resolved computational fluid dynamics-discrete element method (CFD-DEM). Initially, realistic particle shapes were reconstructed by the SH function, and their multi-scale shape features were quantified by the energy spectrums of SH frequencies. A developed fictitious domain method, particularly for irregularly shaped clumps, was proposed to solve fluid–solid interactions within resolved CFD-DEM. Subsequently, the fluid flow past a fixed particle test was repetitively simulated by the resolved CFD-DEM for 270 realistic sand particles, and a dataset consisting of 4220 drag coefficients was finally established. A classic ML algorithm, namely the multi-layer perceptron (MLP) neural network, was then utilized to train a drag model associated with the multi-scale shape features, particle orientations, and flow conditions. Compared with the results from the resolved CFD-DEM, the trained MLP model demonstrates both efficiency and accuracy in predicting the drag coefficients of natural sand particles with irregular shapes. This work provides a more reliable drag model for granular soils and shows its potential for application in large-scale modeling using the unresolved CFD-DEM framework.

It is well perceived that the shear strength properties of natural soil stratum are inherently anisotropic and spatially heterogeneous due to the depositional, geological and environmental factors. In this study, the concurrent effect of inherent anisotropy and spatial heterogeneity of undrained shear strength of clay on stability of sloped ground is examined by using a numerical lower bound limit analysis approach. The inherent anisotropy of the undrained shear strength of clay is incorporated into the numerical model by the application of an iterative process, and the random field technique is utilized to account for clay’s spatial variability. The numerical tool for slope’s safety factor determination is based on the combination of lower bound theory, finite element method, second-order conic optimization and the strength reduction method. Probabilistic analyses of slopes with/without external tractions showed a remarkable effect of concurrent consideration of anisotropy and heterogeneity of soil on the evaluated safety factor of slope.

This paper proposes a coupled hydro-mechanical constitutive model for unsaturated clay and sand (CASM-U) in a critical state framework. The mechanical behaviour of unsaturated soils is modelled by modifying the unified clay and sand model (CASM) with Bishop’s effective stress, bounding surface concept and loading collapse (LC) yield surface. The hydraulic behaviour is described by a soil–water characteristic curve (SWCC) with nonlinear scanning law, considering the coupled effects of soil deformation and hysteresis. CASM-U is implemented into a commercial finite element software through the user-defined material subroutine (UMAT), and the implementation is benchmarked by a new semi-analytical cavity expansion solution adopting CASM-U. Finally, the performance of CASM-U in predicting hydro-mechanical behaviour of unsaturated clays and sands is examined by comparing with experimental data from tests along various loading paths, including isotropic compression, cyclic drying–wetting, triaxial shearing, and their combinations. It is shown that CASM-U can provide reasonable predictions for hydro-mechanical behaviour of unsaturated soils with a total of 15 material parameters.

Suffusion is a critical issue in geotechnical engineering. Despite extensive studies, the effect of soil specimen dimensions on suffusion remains unclear. In this paper, a coupled computational fluid dynamics and discrete element method (CFD-DEM) approach is employed to study the suffusion of gap-graded soils with varying aspect ratios, and the underlying physical mechanisms are discussed. The results indicate that as the aspect ratio increases, erosion degree, mechanical coordination numbers, and unevenness in the fines distribution decrease, while the likelihood of fine particles integrating into the soil skeleton rises. Before suffusion, specimens with lower aspect ratios show higher peak strengths. After suffusion, peak strength decreases with erosion degree. However, all specimens exhibit comparable residual strengths. The mechanism behind different suffusion behaviors in specimens with varying aspect ratios is primarily governed by their unique suffusion boundary conditions. Accounting for suffusion boundaries significantly modify erosion laws and eroded soil mechanics behaviors. A standardized specimen size is proposed to account for suffusion boundary effects, thereby minimizing errors attributed to variations in outlet sieve aperture sizes and inconsistencies in specimen dimensions. The results obtained highlight the influence of specimen size on suffusion, advancing our precise understanding of eroded soil behavior.

The determination of the factor of safety (FoS) of slopes during seismic excitation can be complex if the relevant effects of pore water pressure accumulation, nonlinear material response and variable shear strength are duly accounted for. A rational two-step approach to tackle this task based on a hydro-mechanically coupled dynamic simulation and finite element limit analyses is henceforth introduced. To ensure accurate transfer of the hydro-mechanical soil state, a mapping concept is presented, accounting for spatial distributions of stresses, excess pore water pressures, inertial forces and shear strength. The proposed approach is compared to limit equilibrium method (LEM) for the case of a large-scale water-saturated open cast mine slope subjected to seismic loading. In comparison with LEM, the new approach to assess seismic slope stability proves to be simpler in its implementation and straightforward, which could be an important asset for practitioners.