Acta Geotechnica最新文献

This study presents a fully resolved numerical method coupling framework for capturing the interaction between debris flows and flexible barriers. The proposed coupling method combines a simplified flexible wire representation, smoothed particle hydrodynamics (SPH), and discrete element method (DEM). Specifically, the flexible wire representation is used to represent a flexible barrier that intercepts debris flows. The coupled SPH-DEM simulates a two-phase debris flow containing arbitrarily shaped boulders. Additionally, a modified dynamic boundary method (mDBC) is employed to couple SPH with DEM. These three methods are strongly coupled to simulate the behavior of boulders in debris flows and improve the efficiency and accuracy of the interaction between flexible barriers and debris flows. The proposed coupling framework is evaluated through a series of verification instances, which are consistent with experimental observations. Subsequently, the impact dynamics of the two-phase debris flow on the flexible barrier is modeled and analyzed. Overall, the framework holds great potential for the design and optimization of flexible barriers for debris flow mitigation in engineering practice.

O’Kelly et al. critique the Wang et al. undrained shear strength prediction model paper in terms of its utility and accuracy. Both criticisms suffer from the miscomprehension of the model comparison method. Utility of the three-parameter liquidity index (I_L)—undrained shear strength (S_u) models has been enhanced through optimizing some empirical equation.

In this study, we automatically estimated the parameters of the modified Cam-Clay model, a representative constitutive model for soil. The estimation was carried out by minimizing the objective function using the dynamic multiswarm particle swarm optimization (DMS-PSO) algorithm, which is an improvement over the original PSO. The objective function was newly defined by quantifying the discrepancy between the targeted results and the model calculations in q-p′-v space. DMS-PSO divides particles into several islands to search globally and prevent local solutions, and even particles that fall into a local solution can be relocated. To evaluate the automatic estimation performance of DMS-PSO, we examined whether model parameters could be correctly estimated from the calculation results (Consideration (1)) and whether the DMS-PSO algorithm could consistently obtain the same parameter values when reproducing the experimental results (Consideration (2)). Regarding Consideration (1), the objective function was consistently smaller than 1.0 × 10^–6 when the number of particles was greater than 400 and the number of islands was greater than 40. At this time, the parameter values could be estimated to the fifth decimal place. When two experiments were conducted, the estimation was obtained approximately 1.5 times faster than when only one was conducted. Regarding Consideration (2), the coefficient of variation of the parameters obtained from 100 estimations was at most 1%, and the parameter values were estimated to be approximately the same each time. In addition, narrowing the solution search range based on soil physical properties could reduce the variation in parameters by approximately 10%. Additionally, the parameters could be accurately estimated by data from at least two mechanical experiments.

A coupled hydro-mechanical field-enriched finite element method (HM-FE-FEM) is developed for simulating the hydraulic fracture process of rocks subjected to in situ stresses. The staggered iterative scheme based on the Newton–Raphson iterative algorithm is utilized to address the coupled hydro-mechanical problems. The opening process of the hydraulic fracture and natural fracture subjected to in situ stresses can be reproduced by the proposed method. The accuracy of the proposed method is verified through stress intensity factor, fracture opening and experiment results. For coalescence patterns between hydraulic and natural fractures, the influences of horizontal stress difference and intersection angle are analyzed and discussed. Additionally, for coalescence patterns between hydraulic fracture and natural karst caves, the influences of cave location, cave radius and horizontal stress difference are investigated and discussed. The numerical results illustrate the effectiveness of HM-FE-FEM in handling the complex coalescence behaviors in the hydraulic fracture process of underground rock reservoirs with different defects. This study has a certain guiding significance for optimizing the fracturing operation parameters.

The stiffened deep cement mixing (SDCM) pile is a new composite pile type, which is a stiffened core inserted into the cemented soil, compared to the DCM pile structure. The SDCM pile not only combines the advantages of DCM piles, but also improves the corresponding bearing capacity. In this paper, the bearing characteristics (the ultimate bearing capacity, the pile end resistance and the pile side friction) of PHC pipe pile and the long-core SDCM piles with number of different piles are investigated through field tests. The ultimate bearing capacities of PHC pipe pile, the long-core SDCM pile and the double long-core SDCM pile are 5.82, 8.23 and 14.67 MN. Combined with finite element analysis, it was found that the long-core SDCM pile is a friction pile in sandy soil, the cemented soil can provide lateral friction resistance and its role in bearing the vertical load is cannot be ignored. Parametric analyses were carried out for parameters: the length ratio of the cemented soil and PHC pipe pile (L_C/L_P), the radius ratio of the cemented soil and PHC pipe pile (R_C/R_P) and the modulus of elasticity of PHC pipe pile (E_P) on the long-core SDCM pile with the objective of ultimate bearing capacity. The ultimate bearing capacity will be increased with increasing in L_C/L_P and R_C/R_P, and changes in E_P will not affect the ultimate bearing capacity. Finally, the bearing capacity equations of the long-core SDCM pile were adjusted with the results of the study.

The mechanical properties of clay minerals are largely dependent upon the chemical compositions and the mesoscale fabrics of the constituent particles. This paper describes results of a series of mesoscale molecular dynamics simulations of the hydrostatic compression and shear strain behavior for initially randomly oriented assemblies of 10³ illite primary particles. The particles are simulated as rigid-body ellipsoids that interact through the single-site, Gay–Berne potential function. This corresponds to a coarse-grained model based on prior atomistic scale computation of the potential of mean force for water-mediated interactions between pairs of particles through free energy perturbation method. We investigate the mesoscale fabrics of the NPT-equilibrated assemblies for confining pressures ranging from 1.0 to 125 atm, including path dependence associated with unloading and reloading. We analyze and quantify the geometric arrangement including particle orientation, specific surface area, properties of particle stacks/aggregates, and interstack pair correlation functions. The compression of each particle assembly is associated with large irrecoverable changes in void ratio, while unloading and reloading involves much smaller, largely recoverable volumetric strains. The results are qualitatively similar to macroscopic compression behavior reported in laboratory tests. We simulate the uniaxial and shear behavior at each of the equilibrated pressure states through a series of strain-controlled steps, allowing full relaxation of the virial stresses computed at each step. The simulations investigate directional and path dependence of the shear behavior for strain deviations up to 0.2%. The results show the onset on nonlinear stiffness properties at strain levels (sim)0.01% and hysteretic behavior upon unloading and reloading. Small-strain stiffness properties of the particle assemblies are qualitatively in good agreement with quasi-static, elastic stiffness properties reported for illitic clays.

Fugitive emissions of methane (CH₄), carbon dioxide (CO₂), and hydrogen sulfide (H₂S) from municipal solid waste landfills are major environmental concerns. To address this, a biogeochemical cover (BGCC) system is developed to mitigate these emissions. This study evaluates the effectiveness of the BGCC in comparison with a conventional soil cover (SC) system using a new large-scale laboratory setup that simulates near-field scale conditions. Both cover systems were exposed to synthetic landfill gas (LFG) across five phases, featuring varying gas compositions and influx rates. Surface emission rates and gas concentrations were continuously monitored. Post-termination of the experiments, both cover systems were dismantled, and samples were collected from different depths and locations to analyze spatial variations in physico-chemical properties. Select samples from the biocover layer of both the cover systems and basic oxygen furnace (BOF) slag layer of BGCC were subjected to batch tests to measure potential CH₄ oxidation rates and residual carbonation capacity, respectively. The results showed that both cover systems achieved their highest CH₄ removal efficiency at moderate influx rates (23.9–25.5 g CH₄/m²-day), with BGCC's CH₄ removal ranging from 74.7 to 79.7% and SC's from 83.5 to 99.8%. Complete H₂S removal occurred in the biocover layer of both systems. The highest average CH₄ oxidation rates were 277.9 µg CH₄/g-day at 50 cm below-ground surface (bgs) in BGCC and 260.2 µg CH₄/g-day at 70 cm bgs in SC, with the lowest oxidation rates observed at deeper regions (at 85 cm bgs) of both covers. The breakthrough of CO₂ occurred after 156 days of continuous exposure and could be attributed to the desiccation of the BOF slag layer. Overall, the BGCC system effectively mitigated CH₄, CO₂, and H₂S emissions, whereas the SC system only mitigated CH₄ and H₂S at moderate flux rates, indicating that BGCC provides a comprehensive solution for LFG mitigation.

The discrete element method (DEM) is proving to be a reliable tool for studying the behavior of granular materials and has been increasingly used in recent years. The accuracy of a DEM model depends heavily on the accuracy of the particle property parameters chosen which is of vital importance for studying the mechanical properties of granular materials. However, the existing DEM parameter calibration methods are limited in terms of applicability, and the trial-and-error method remains the most common way for DEM parameter calibration. This paper presents a novel calibration method for DEM parameters using the multi-objective tree-structured parzen estimator algorithm based on prior physical information (MOTPE-PPI). The MOTPE-PPI does not rely on the training datasets and may optimize with every single test, significantly reducing the computational efforts for DEM simulation. Moreover, MOTPE-PPI is suitable for a variety of contact models and damping parameters in DEM simulation, showing robust applicability and practical feasibility. Taking an example, the DEM parameters of sandy gravel material collected from Dashixia rockfill dam in China are calibrated using MOTPE-PPI in the paper. The prior physical information is obtained through a series of triaxial loading–unloading tests, single-particle crushing tests, and literature research. Seven parameters in the rolling resistance linear contact model and breakage model are considered, and the optimization process takes only 25 iterations. Through quantitative comparison with existing parameter calibration methods, the high efficiency and wide applicability of the DEM parameter calibration method proposed in this study. The calibrated DEM parameters are used to investigate the hysteretic behavior and deformation characteristics of the granular material, revealing that the accumulation of plastic strain and resilient modulus is related to confining pressure, stress level, and the number of cycles.

The inclusion of calcite precipitates (CaCO₃) in soft soil can improve the mechanical properties. Understanding the variability in sand stiffness due to heterogeneous precipitates is crucial for stiffness evaluation and prediction. A novel discrete element-Monte Carlo (DE-MC) method was proposed to quantify the sand stiffness variability induced by stochastic distributions of calcite precipitates, specifically focusing on shear wave velocity (V_s) as an indicator of soil stiffness. A total of 1972 samples were constructed to simulate stochastic spatial distributions of calcite precipitates. Through joint stochastic analysis, the preferential paths formed by calcite clusters were identified as significant contributors to V_s variability. The normalized connectivity per unity distance contact weight (C_d,n) exhibited the most correlated relation with V_s. Two weight selection methods were applicable for using C_d,n to characterize and predict V_s. The results suggest that the DE-MC method has the potential to assess the variability in sand stiffness quantitatively.

Anhydritic claystones are widely distributed in the Gypsum Keuper formation. Their swelling is associated with the chemical process of anhydrite to gypsum transformation and has caused extensive damages in tunnels. Even though this problem has attracted great scientific interest, an adequate mathematical description of the swelling of anhydritic rocks is still missing. The present paper contributes towards closing this gap by formulating a coupled chemo-mechanical constitutive model, which considers anhydritic rock as an elastoplastic porous medium according to the principle of effective stresses, with a Mohr–Coulomb yield criterion, a non-associated flow rule and an additional, chemically induced strain component. The volumetric chemical strain is equal to the sum of the changes of the volume of the solids and of the pore volume. The change of the volume of the solids depends on the stoichiometry of the chemical reaction and is proportional to the mass of the transformed anhydrite. The pore volume may increase or decrease during the anhydrite to gypsum transformation, depending on how gypsum grows. The pore volume increases if the gypsum crystals crack and expand the matrix, and decreases if the gypsum crystals precipitate within the available pore space. The proposed model considers experimental results according to which the higher the stresses and porosity, the lower the increase in pore volume. In addition, the model assumes that the chemical strains are coaxial with the principal stresses and that the volumetric chemical strain in each principal direction is inversely proportional to the corresponding principal stress. The model is calibrated with results of tests on artificial anhydrite-kaolin specimens and achieves a very high correlation degree (R² = 0.92).