The Sand Compaction Pile (SCP) method is a widely utilized ground improvement technology that enhances the density of the ground by constructing sand piles through penetration and repeated withdrawal/re-driving of a casing pipe. This method is the most widely used liquefaction countermeasure method in Japan. While the improvement effect of SCP is predominantly attributed to the resultant increase in soil density, recent studies have suggested that the stress history (such as increased lateral pressure and shear history) induced during the SCP work process also contributes significantly to its effectiveness. In order to more accurately reproduce the behavior of the ground during the construction of Sand Piles, the stress history simulating the SCP work process was applied to specimens in hollow cylindrical torsional shear tests, and the effects of the stress history were observed. The specimens were initially consolidated with a lateral stress ratio of 0.5 (K0 = 0.5). Subsequently, a stress history including increased lateral stress and cyclic shear stress was applied. Finally, liquefaction resistance was assessed through cyclic loading. After applying the stress history, an increase in liquefaction resistance was observed in these specimens. This increase was larger than that of specimens subjected only to a lateral stress increase without the shear stress history. This increasing trend persisted even after the lateral stress was reduced following the application of stress history. Finally, these test results were analyzed to assess the impact of stress history on liquefaction resistance by comparing them with the relationship between relative density and the liquefaction resistance. (c) 2025 Japanese Geotechnical Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

The dynamic deformation characteristics of saturated sands are considerably influenced by the loading frequency (f). Nevertheless, the effect of f on the deformation behavior of saturated coral sand (CS) has not been comprehensively investigated. This study aims to investigate how frequency (0.01-4Hz) affects the shear modulus (G) and damping ratio (lambda) characteristics of CS through a series of cyclic shear tests. The experimental results demonstrate that, under consistent initial conditions, both the strain-dependent G and lambda increase as f increases. Moreover, there is a linear relationship between the maximum shear modulus (G0) and small strain damping ratio (lambda min) with ln(f). Specifically, the regularized G of CS remains unaffected by variations in f. To facilitate the prediction of G in CS at different f, we propose a prediction equation that integrates the revised Hardin's model and Davidenkov skeleton curve. Besides, a power function expression is suggested for lambda-lambda min versus G/G0 to predict lambda in CS at different f. The revised equations for G and lambda are validated using experimental data from natural sands in the literature, confirming their suitability for evaluating strain-dependent G and lambda values of natural sandy soils over a wide strain range.

The major principal stress direction angle (ota) experienced by granular soils varies widely in engineering, causing different strengths. However, how particle morphology affects the strength anisotropy behavior under different ota remains unclear. To address this gap, this study performed drained hollow cylinder torsional shear tests under different ota on six granular materials with distinct morphologies. Results highlight the significant dependence of peak strengths of granular materials on both particle morphology and ota. Increasing particle shape irregularity and surface roughness leads to a considerable enhancement in peak strength, while this peak strength significantly degrades with increasing ota. Materials with more irregular shapes were found to have a more pronounced strength anisotropy. Furthermore, the initial fabric of particle packings, derived from three-dimensional X-ray microtomography, was used to interpret microscopic mechanisms behind the morphologydependent strength anisotropy. Irregular-shaped materials display broader preferred particle orientations and higher initial fabric anisotropy compared to relatively regular-shaped materials. This higher morphology-induced fabric anisotropy contributes to strength anisotropy, and a correlation was established for describing this trend. Additionally, an anisotropic failure criterion incorporating fabric anisotropy was developed to characterize the strength envelope for granular materials with diverse shapes.

Soil instability and potential failure under principal stress rotation require greater attention than ever before due to increased operation of heavier and longer high-speed trains. This study focuses on the interplay between cyclic vertical stress and torsional shear stress on the failure condition of a low-plasticity subgrade soil, facilitated by a hollow cylinder apparatus. Combined vertical and torsional loading significantly influences strain response, with increasing torsional stress leading to higher strain accumulation. Moreover, the data indicate that an increase in torsional shear stress is generally accompanied by a swift rise in the EPWP and a corresponding decrease in the soil stiffness. In view of this, a novel parameter, the overall stiffness degradation index (delta o) that simultaneously captures both the vertical and torsional shear effects under principal stress rotation is proposed as an early indicator of instability. In addition, a normalised torsional stress ratio (NTSR), which is the ratio of the amplitude of torsional shear stress to the confining pressure, is introduced to assess the impact of torsional shear stress. Whereby, higher NTSR values correlate with premature inception of failure. These experimental results provide new insights for a better understanding of soil instability under simulated railway loading.

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (a) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress-strain curves in cases with different a and b values. The distribution of force chains is affected by both a and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either a orb varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The geotechnical behavior of residual soil differs essentially from that of sedimentary soil because of the weathering pedogenesis of the former, thereby posing significant difficulties in predicting soil response. In this study, the shear strength and stiffness of natural granite residual soil are evaluated through systematic monotonic and cyclic simple shear tests performed using a hollow-cylinder apparatus. Simple shear testing provides critical information about soil behavior under plane-strain conditions and involves principal stress rotation, which is beyond the scope of triaxial shear tests. The mechanical properties of granite residual soil measured in monotonic simple shear are found to be different from those obtained through other routine laboratory tests such as triaxial shear and resonant column tests. Whereas the conventional triaxial compression test gives unconservatively high soil strength parameters, those from simple shear testing appear more reasonable than the triaxial results. The cyclic behavior of residual soil in simple shear is dominated by the cyclic stress ratio, a higher value of which results in more significant deformation and pore water pressure build-up as well as more rapid stiffness degradation. This is particularly the case when the cyclic stress ratio exceeds a critical value in the range of 0.125-0.1875. No consistent pattern can be established for how the loading frequency influences soil responses within the range of 0.01-1.0 Hz. This study enriches the techniques for characterizing residual soil and provides new data sets about its mechanical behavior.

The effect of geotextile inclusion on the shear modulus and damping ratio of sands is evaluated in a wide range of shear strain amplitudes, from very small to fairly large, using the results of several resonant column and hollow cylinder torsional shear tests. The resonant column test results are utilized to characterize the reinforced soil behavior at the range of small to medium strains whereas the hollow cylinder torsional shear test results are exploited to assess the medium to large strain dynamic properties. The results demonstrate that the inclusion of geotextile sheets in the soil medium would increase both its shear stiffness and damping ratio in the whole range of shear strain amplitudes, thus rendering a perfect composite to resist dynamic forces applied on geo-structures in earthquake prone areas. Empirical equations are proposed to estimate the small strain and strain-dependent shear modulus and damping ratio of geotextile-reinforced sands. The effect of scaling is also accounted for by a simple analysis so that the results obtained in the current study in the element scale could be extended to the prototype scale in the field. Finally, the accuracy of the proposed scaling approach is verified against a finite element model of a geotextile-reinforced embankment.

Understanding the characteristics of the development of excess pore water pressure during cyclic loading is important to evaluating the dynamic behavior of soils. Many researchers have proposed experimental models to estimate pore water pressure. However, existing experimental models are mainly based on experimental results obtained under isostatic and constant amplitude loading. In this study, K0-controlled cyclic loading is undertaken by simulating a horizontal stratified ground, and the development of excess pore water pressure is evaluated by measuring the accumulated shear strain; a modified accumulated shear strain is proposed based on these results. The results show that the excess pore water pressure can be predicted from the modified accumulated shear strain, regardless of soil type, initial soil pressure coefficient, initial shear stress, or the shape of input waveform.

Although extensive investigations have been performed on soil anisotropy, little information is available regarding its quantification. The quantification of anisotropy reflects the extent to which the inherent anisotropy of soil controls its mechanical behavior and so it is crucial for connecting comprehensive experimental research with soil constitutive models and engineering practices. The present study evaluates the level of shear strength anisotropy in various types of soil. A database is compiled of the variations of shear strength with the direction of major principal stress (alpha) as established via hollow-cylinder torsional shear tests, covering more than 20 types of soil. This study reviews critically the existing methods for strength anisotropy, finding none of them quantifies anisotropy satisfactorily. The present study proposes using the bracketed area by the S(alpha)/S(0)-alpha curve and the line S(alpha)/S(0) = 1 to measure the level of strength anisotropy, where S(alpha) is the soil strength as a function of alpha. The proposed method in this study can measure the strength anisotropy levels of sands, silts, clays, residual soil, calcareous sand, mudstone, and glacial till, among others. Additionally, the anisotropy degree measured using the proposed method appears to be consistent with the results of microstructural anisotropy evaluations. This study enhances the understanding of soil anisotropy by providing a comprehensive database of soil strength anisotropy and proposing a general method for its quantification.

This paper aims to comprehensively analyze the influence of the principal stress angle rotation and intermediate principal stress on loess's strength and deformation characteristics. A hollow cylinder torsional shear apparatus was utilized to conduct tests on remolded samples under both normal and frozen conditions to investigate the mechanical properties and deformation behavior of loess under complex stress conditions. The results indicate significant differences in the internal changes of soil particles, unfrozen water, and relative positions in soil samples under normal and frozen conditions, leading to noticeable variations in strength and strain development. In frozen state, loess experiences primarily compressive failure with a slow growth of cracks, while at normal temperature, it predominantly exhibits shear failure. With the increase in the principal stress angle, the deformation patterns of the soil samples under different conditions become essentially consistent, gradually transitioning from compression to extension, accompanied by a reduction in axial strength. The gradual increase in the principal stress axis angle (alpha) reduces the strength of the generalized shear stress and shear strain curves. Under an increasing alpha, frozen soil exhibits strain-hardening characteristics, with the maximum shear strength occurring at alpha = 45 degrees. The intermediate principal stress coefficient (b) also significantly impacts the strength of frozen soil, with an increasing b resulting in a gradual decrease in generalized shear stress strength. This study provides a reference for comprehensively exploring the mechanical properties of soil under traffic load and a reliable theoretical basis for the design and maintenance of roadbeds.