Undrained residual strength, s(ur), often termed remolded or postcyclic strength, is a critical input into embankment dam numerical deformation analyses. There are multiple methods available to assess s(ur) for fine-grained soils, each with advantages and disadvantages. Field tests, such as the vane shear test and the cone penetration test, can provide reliable in situ measurements of s(ur). In the laboratory, s(ur) can be estimated by measuring the shear stress mobilized at high strains in monotonic tests such as direct simple shear or triaxial shear. s(ur) is also frequently determined from postcyclic monotonic testing; however, the postcyclic stress-strain curves can be difficult to interpret because of high excess pore water pressure existing at the start of monotonic shear due to the sample being previously subjected to cyclic loading. Such analyses often have a significant amount of uncertainty. The work described here presents two new methods developed to quantify s(ur) through lab testing, namely, analysis of stress paths from postcyclic monotonic tests and iterative strain-controlled cyclic loading. This paper introduces the new approaches and presents results from testing performed on five fine-grained soils from the foundations of embankment dams. Values of s(ur) from the new approaches are compared with those from VST and monotonic and postcyclic monotonic direct simple shear testing. The paper details the new approaches and presents results and conclusions from five fine-grained soils from various sites across the western United States.

This paper presents the results of 3D discrete element modeling of monotonic constant volume simple shear test on Pea gravel. 3D DEM simulations were validated using results from large-scale stacked-ring simple shear laboratory tests on real soils, where each particle was accounted for and was characterized by size and shape using the translucent segregation table (TST) test. To acknowledge and incorporate both the irregularity and non-uniformity of particle shapes in real soil specimen and providing a realistic representation of soil assembly in the numerical simulations, a non-uniform distribution of rolling resistance (obtained from the particle shape characterization using TST) was assigned to the spherical particles in the simulated specimens. Different aspects of soil behavior at micro- and mesoscale such as non-coaxiality, stress-induced fabric anisotropy and validity of boundary measurements in evaluating the soil response were investigated. It is shown that boundary measurements (as generally done in laboratory) lead to a conservative estimate of the soil strength and generated pore pressure inside the specimen.

In an effort to enhance engineering infrastructure and reduce environmental waste, the use of COVID-19 face-mask chips (FMC) in sand reinforcement is experimentally explored through drained hollow cylinder torsional shear tests, including monotonic stress paths with different fixed orientation of the principal stress axes and cyclic tests with traffic load and pure principal stress rotation. Fujian sand and Hong Kong CDG sand were used. The monotonic test results indicate that both sands exhibit a strong strength anisotropy, however, although the addition of FMC increases the peak stress ratio to failure of the composites for all tests, the strength anisotropy trends with alpha(sigma) are not changed. Results from x-Ray CT scanning analyses conducted on FMC-reinforced and unreinforced cylinder sand specimens supported the interpretation of experimental data. Furthermore, the inclusion of FMC induces increased plastic deformation under cyclic loads in all tests, however, the level of these plastic strains is sand-type and stress-level dependent. It was also observed that both sands exhibit non-coaxial characteristics, but the presence of FMC inclusions do not change the non-coaxial trends observed for the pure sands.

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/).

Variations in excavation construction periods for fissured soil transportation engineering lead to differing unloading rates, which affect the soil's mechanical properties. This study utilizes a triaxial testing system to conduct monotonic and cyclic loading undrained shear tests on undisturbed fissured samples as well as remolded samples subjected to three distinct unloading rates. The K0 consolidated samples are regarded as soil mass that undergoes no unloading during testing. The findings indicated that the initial unloading rate influences the reloading shear mechanical properties of undisturbed and remolded specimens. The effects of unloading rates differ between undisturbed and remolded soil, a discrepancy attributed to inherent fissures. Specifically, undisturbed soil exhibits significant damage at low unloading rates due to fissures, while remolded soil experiences strength augmentation due to compaction with decreased unloading rates. Similarly, unloading will cause a loss of strength. Structural disparities result in the monotonic loading strength of undisturbed specimens being higher than that of remolded ones. In contrast, remolded specimens demonstrate greater dynamic strength under cyclic loading, likely because fissures deform, diminishing overall dynamic strength. Subsequent microscopic analysis, utilizing SEM images, along with a discussion of macroscopic inherent fissures, elucidated the impact of unloading rate on soil damage mechanisms, advancing the understanding of fissured soil behavior post- unloading. The study of mechanical properties of fissured soil following varying unloading rates is crucial for comprehending its damage mechanism and determining post-unloading soil strength parameters, providing valuable insights for practical applications in soil engineering.

An important drawback of the hypoplastic model is the inaccurate prediction of the sand behavior under undrained monotonic loading conditions. The model is not able to reproduce the limited liquefaction type response widely observed in undrained tests on loose sand, and it often underestimates the initial stiffness and hardening rate of sand during the shearing. To address these issues, three novel modifications are introduced into a basic hypoplastic model to enhance its undrained predictive capability. Firstly, a new factor is added to the nonlinear term of the model, allowing the simulation of a purely elastic response at the beginning of loading. By doing so, the model can accurately capture the initial stiffness and undrained effective stress path of sand. Secondly, the characterized void ratios are related to an evolving state variable, enabling the model to reasonably reproduce the limited flow response and quasi-steady state. Furthermore, a new term is incorporated into the deviatoric part of the strain rate to adjust the hardening rate of the model. The model performance for undrained loading is significantly improved through the above modifications, as evidenced by the good agreement between simulation results and experimental data for tests with varying densities and confining pressures.

Geogrid stabilization has gained significant attention in recent years as an effective method for enhancing the performance of subgrade soils. However, the reinforcement effect of the geogrids under different loading conditions has not been thoroughly investigated, which hinders a comprehensive understanding of subgrade stabilization. Therefore, this paper aims to investigate and compare the behavior of a stabilized subgrade with geogrids reinforcement under cyclic loading and monotonic loading conditions. The experiments were conducted within a steel model box measuring 1.0 m (length), 1.0 m (width), and 1.2 m (height). The subgrade layer was consistently maintained at a thickness of 500 mm and strength of 2.5% California Bearing Ratio (CBR). A granular layer of high-quality material with a thickness of 200 mm was applied on top of the weak subgrade and geogrid was placed at the interface between the granular layer and subgrade. The tests were conducted in a controlled laboratory setting, specifically measuring vertical displacement in response to monotonic and cyclic loading. The results were then analyzed to evaluate ultimate bearing capacity, stiffness and rutting thereby estimating the effect of geogrids on stabilization of weak subgrades. These findings are anticipated to contribute significantly to the development of design guidelines for stabilized subgrade with geogrids reinforcement. By incorporating these insights, the design, and optimization of geogrid reinforcement systems for subgrade stabilization can be enhanced, ultimately resulting in improved performance and increased longevity of transportation infrastructure.

Shape of soil grains has a major role in the characterization of its behavior. This important feature has an appreciable impact on all mechanical properties of granular soils. In order to accurately capture such an important contribution, the grain morphological features should be carefully analyzed and described in a systematic manner. The present study aims to review the literature concerning the qualitative and quantitative characterizations of the soil particle shape and its substantial impact on the mechanical behavior of granular soils. Qualitative characterization of the particle shape involves descriptive assessments of the level of angularity or roundness, such as the terms rounded, sub-rounded, sub-angular, and angular, while quantitative characterization refers to measurable parameters, like aspect ratio, regularity, sphericity, and circularity, which provide numerical data on the shape of soil grains. This study specifically examines the influence of particle shape on the compression characteristics, monotonic shear response, shear strength properties, and critical state behavior of granular soils, as well as the mechanical behavior of soil-solid interfaces and stabilized earthen materials. The potential challenges associated with the investigation of the effect of particle morphology on the mechanical characteristics of granular soils are also summarized, and a roadmap is outlined for future research. The findings of this study show that as the soil grains become more angular, the at-rest coefficient of earth pressure (K0) and the maximum dilation angle ( psi max ) decrease whereas the peak friction angle (phi p), critical state friction angle (phi cs), intercept of critical state line (e Gamma), and slope of critical state line (lambda) all increase. With this broad perspective and in an attempt to put such crucial effects into practice, comprehensive sets of experimental data records are compiled from the previous studies in the literature based upon which new practical machine learning (ML) models are developed for the prediction of various mechanical properties of granular soils by accounting for the substantial contribution of particle shape. The study provides practicing geotechnical engineers with a profound insight into the macro- and micro-scale impacts of grain morphology on the mechanical characteristics of granular soils while offering new horizons in the incorporation of particle shape into predictive models for such properties. With these practical models, engineers can readily estimate the compression and strength-related parameters of granular soils by simply examining their particle shape, grain size distribution, density, and overburden pressure.

Geosynthetics-soil interfaces are exposed to varying temperatures coupled with complex stress states. Quantifying the mechanical response of the interface considering this combined influence of temperature and complex stress is always a huge challenge. This study proposes a new displacement and stress-loading static and dynamic shear apparatus that is capable of testing the geosynthetics-soil interfaces with high and low-temperature controlling function. The apparatus satisfactorily simulates monotonic and cyclic direct shear tests, and creep shear tests on geosynthetics-soil interfaces at temperatures ranging from -30 degrees C to 200 degrees C. To validate the functionality of this device, a series of temperature-controlled experiments were conducted on different types of interfaces (sand-geogrid interfaces, sand-textured geomembrane interfaces, sand-smooth geomembrane interfaces). The experimental results indicate that the apparatus can simulate static, dynamic, and creep shear loading on geosynthetics-soil interfaces in high and low temperature environments, and these can be measured reliably. It also manifests that temperature has a non-negligible influence on all mechanical interface responses. These findings highlight the significance and potential of the proposed apparatus and its practical implications.

To assess the stability of coral sand foundation in complex environments, the undrained monotonic and cyclic shear tests were conducted in the laboratory. The test results indicate that the coral sand exhibits pronounced inherent anisotropy in the vertical direction. Under complex consolidation conditions, significant stress-induced anisotropy can also be observed. With increasing generalized shear strain (gamma g), both the generalized monotonic and cyclic shear modulus (Ggm, Ggd) exhibit a decreasing trend irrespective of consolidation ratio (kc) and inclinations of major principal stress (alpha c). Additionally, a strong linear relationship is evident between Ggm and Ggd, suggesting a consistent reduction pattern of Gg for various loading modes. The investigation on the inclination of the failure line (phi FL) for monotonic and cyclic shear is also conducted. The test results show that consolidation conditions have minimal influence on phi FL during monotonic shear, but exert a significant impact on phi FL during cyclic shear. A novel index called the consolidation parameter (eta) is proposed to quantitatively assess the relationship between kc, alpha c and phi FL. The average values of phi FL for cyclic shear increase with increasing eta, indicating the non-failure zone of coral sand during undrained cyclic shear will shrink with higher values of kc and alpha c.