This study explores the effectiveness of soft viscoelastic biopolymer inclusions in mitigating cyclic liquefaction in loosely packed sands. This examination employs cyclic direct simple shear testing (CDSS) on loose sand treated with gelatin while varying the gelatin concentration and the cyclic stress ratio (CSR). The test results reveal that the inclusion of soft, viscoelastic gelatin significantly reduces shear strain and excess pore pressure during cyclic shear. Liquefaction potential, defined as the number of cycles to liquefaction (NL) at an excess pore pressure ratio (ru = Delta u/sigma ' vo) of 0.7, is substantially improved in gelatin-treated sands compared to gelatin-free sands. This improvement in liquefaction resistance is more pronounced as the inclusion stiffness increases. Furthermore, the viscoelastic pore-filling inclusion helps maintain skeletal stiffness during cyclic shearing, resulting in a higher shear modulus in gelatin-treated sand in both small and large-strain regimes. At a grain scale, pore-filling viscoelastic biopolymers provide structural support to the skeletal frame of a loosely packed sand. This pore filler mitigates volume contraction and helps maintain the effective stress of the soil structure, thereby reducing liquefaction potential under cyclic shearing. These findings underscore the potential of viscoelastic biopolymers as bio-grout agents to reduce liquefaction risk in loose sands.

This study examines the behavior of anisotropically consolidated granular assemblies under undrained cyclic true triaxial loading paths. To achieve this, the Discrete Element Method (DEM) is conjugated with the Coupled Fluid Method (CFM) to account for fluid-solid interaction in undrained conditions. The examined loading paths include two phases: anisotropic consolidation and undrained cyclic true triaxial loading. During consolidation, samples are sheared at various Lode angles to reach a spectrum of initial static shear stress levels. In the second stage, undrained cyclic loading is applied with constant shear stress amplitudes at various Lode angle values. The results indicated that the monotonic and cyclic Lode angle, initial static shear stress, and amplitude of deviatoric stress have pronounced effects on the secant shear modulus degradation and the rate of excess pore water pressure generation of granular assemblies. In tandem with macro-scale observations, the evolution of the microstructure within assemblies is analyzed using the coordination number, redundancy index, inter-particle contact fabric tensor, and particle orientation fabric tensor. The micro-scale findings confirm that the anisotropy induced by changes in the loading direction significantly impacts the shear strength of the assemblies. Additionally, the fabric of assemblies aligns along the preferential direction corresponding to the major principal stress, influencing the dilative response.

Soil liquefaction caused by earthquakes is a devastating occurrence that can compromise the foundations of buildings and other structures, leading to considerable economic losses. Among the new remedies against liquefaction, Induced Partial Saturation (IPS) is regarded as one of the most promising technologies. In order to improve liquefaction resistance and the fluid phase's compressibility, gas or air bubbles are introduced into the pore water of sandy soils. This article deals with the general laboratory evaluation of a sand under partially saturated conditions and under cyclic loading to assess if this technology is applicable for a ground improvement of the examined soil. The use of the Axis Translation Technique for sample desaturation and diffusion-stable butyl membranes significantly influences the laboratory results. Additionally, it is found that the trapped air bubbles of the partially saturated samples act like a damping mechanism, which are reflected in the stress paths of the deviator stress q over the mean pressure p with an inclination of 1 : 3. Zum Verfl & uuml;ssigungsverhalten von teilges & auml;ttigtem SandDie durch Erdbeben verursachte Bodenverfl & uuml;ssigung ist ein verheerendes Ereignis, das die Fundamente von Geb & auml;uden und anderen Bauwerken gef & auml;hrden und zu erheblichen wirtschaftlichen Verlusten f & uuml;hren kann. Die induzierte partielle S & auml;ttigung (Induced Partial Saturation, IPS) gilt als eine der vielversprechendsten Technologien unter den neuartigen Baugrundverbesserungen gegen Verfl & uuml;ssigung. Um den Verfl & uuml;ssigungswiderstand und die Kompressibilit & auml;t der fl & uuml;ssigen Phase zu verbessern, werden dabei Gas- oder Luftblasen in das Porenwasser sandiger B & ouml;den eingebracht. Dieser Beitrag besch & auml;ftigt sich mit der generellen labortechnischen Evaluierung eines Sandes unter teilges & auml;ttigten Verh & auml;ltnissen und unter zyklischer Beanspruchung zur Beurteilung, inwiefern sich diese Baugrundverbesserung f & uuml;r den untersuchten Boden eignet. Die Verwendung der Axis Translation Technique zur Probenentw & auml;sserung und die Verwendung von diffusionsstabilen Butylmembranen haben einen erheblichen Einfluss auf die Laborergebnisse. Au ss erdem ist festzustellen, dass die eingeschlossenen Luftblasen der teilges & auml;ttigten Proben wie eine D & auml;mpfung wirken und sich in den Spannungspfaden der Deviatorspannung q & uuml;ber dem mittleren Druck p mit einer Neigung 1 : 3 widerspiegeln.

The computational cost of discrete element modelling is high owing to the limitations of particle size and contact in fibre modelling. This paper proposes an optimised discrete element method (DEM) for a hybrid model of soil and fibres based on the fibre influence range. First, a relative velocity state function is established based on the relative motion state between the fibres and soil particles under undrained cyclic loading. Subsequently, the influence range of the fibres is determined using the relative velocity function based on the first few cycles of the undrained cyclic loading numerical tests. Cluster and clump models of the fibre are then generated based on the influence range of the fibre. Finally, a symmetrical shape of the optimised model is developed by extracting the distribution length of the edge curve of the influence range along the vertical direction of the axis. In this study, the proposed optimised DEM was validated through a series of undrained cyclic loading numerical tests on fibrereinforced soil. The results of the optimised model were highly consistent with those of the traditional model, and the computational time was significantly reduced. The cyclic loading timing for determining the range of influence of the fibre was analysed. The optimised model based on the influence range of the 15th cycles not only restored almost the same results but also saved the calculation cost by nearly eight times. The optimised model established based on the influence range after the 15th cycles had a slight influence on the results. In addition, the applicability of the optimised model is discussed. This paper provides new insights into the establishment of a hybrid model of soil and fibres.

This paper addresses the cyclic behaviour and stiffness degradation of subgrade soils subjected to stress-controlled cyclic loading, with particular emphasis on soils that are prone to mud pumping or subgrade instability. With continuous passage of trains over weak, saturated, low-plastic subgrade foundations, the finer fraction of the soils tends to fluidise (i.e., behave like a fluid) and migrate upwards, thereby, fouling the ballast and hindering the long-term performance of the rail track infrastructure. This leads to significant costs associated with annual track maintenance. Through a series of undrained cyclic triaxial testing varying the cyclic stress ratio (CSR, representing the axle loads) and loading frequency (simulating train speeds), the authors noted a significant upward migration of finer fraction coupled with internal moisture redistribution within the failed specimens. Further analysis revealed the instability of specimens was caused by early softening behaviour, and it is accompanied by a sharp reduction in the specimen stiffness. To tackle this, the stiffness was evaluated in terms of axial dynamic modulus and strain energy per cycle was evaluated to better understand the fluidisation behaviour. A novel quasi-linear relationship between threshold residual strain and number of cycles is proposed to serve as a practical guide.

Liquefaction and dynamic response of granular materials under dynamic loading has been studied intensively in field and laboratory tests. However, theoretical modeling and analytical solutions on liquefaction are still lagging and investigations are mostly restricted to laboratory observations. To investigate undrained liquefaction shear deformation and fluidity of granular material, the updated state evolution model is proposed by introducing an excess pore water pressure ratio parameter. A series of undrained cyclic triaxial tests and DEM simulations are conducted to verify the proposed model. The result indicates that the liquefaction behavior of granular materials can be captured by the updated state evolution model both at constant and varying loading frequency. Furthermore, the state parameter based on the deviatoric strain and excess pore water pressure ratio is determined to quantify assess the fluidity of granular materials. It facilitates the refinement of the discriminative criteria for cyclic liquefaction of granular materials. This parameter increases slowly at the beginning of loading, followed by a rapid and fluctuating rise, and reaches the peak before the initial liquefaction. Another significant finding is that the turning point of the state parameter range from 0.89 to 0.95 in the theta - t/t0 plane and between 0.84 and 0.94 in the theta - ruplane, as affected by the cyclic loading conditions.

Accurate simulation of laboratory undrained and cyclic triaxial tests on granular materials using the Discrete Element Method (DEM) is a crucial concern. The evolution of shear bands and non-uniform stress distribution, affected by the membrane boundary condition, can significantly impact the mechanical behavior of samples. In this work, the flexible membrane is simulated by using the Finite Element Method coupled with DEM. In addition, we introduce a hydro-mechanical coupling scheme with a compressible fluid to reproduce the different undrained laboratory tests by using the membrane boundary. The evolution of pore pressure is computed incrementally based on the variation of volumetric strain inside the sample. The results of the membrane boundary condition are compared with more classical DEM simulations such as rigid wall and periodic boundaries. The comparison at different scales reveals many differences, such as the initial anisotropic value for a given preparation procedure, fabric evolution, volumetric strain and the formation of shear bands. Notably, the flexible boundary exhibits more benefits and better aligns with experimental data. As for the undrained condition, the results of the membrane condition are compared with experimental data of Toyoura sand and rigid wall boundary with constant volume. Finally, stress heterogeneity during undrained monotonic and cyclic conditions using the membrane boundary is highlighted.

The undrained cyclic behavior of rubber-sand mixture (RSM) is usually investigated under the cyclic loads with unidirectional shear stress. However, bidirectional shear stress exists in many engineering practices subjected to complex loads, under which the liquefaction resistance of soil may be overestimated. Furthermore, the soil behavior under bidirectional shear stress exhibits quite differently from that under unidirectional shear stress. Therefore, undrained cyclic behavior of RSM under bidirectional shear stress should be further investigated. In this study, several specimens made by RSM with different rubber contents (from 10 % to 30 % by volume) are consolidated under two conditions, K0 consolidation and the combination of K0 consolidation with consolidation shear stress (CSS). Subsequently, numerous tests are conducted under the unidirectional and bidirectional cyclic loading paths to investigate the cyclic undrained behavior of RSM. The results show that the bidirectional shear loads incur a larger normalized pore water pressure (PWP) than unidirectional shear loads. In addition, an energy-based method is employed to understand the relationship between cumulative energy and normalized PWP. During the stage of rapidly accumulating PWP, the dissipated energy required to generate the same normalized PWP is identical, and it is independent of the shapes of loading paths.

There are many geotechnical applications involving dams, embankments and slopes where the presence of an initial static shear stress prior to the cyclic loadings plays an important role. The current paper presents the experimental results gathered from undrained cyclic simple shear tests carried out on non-plastic silty sand with fines content in the range 0-30% with the consideration of sustained static shear stress ratio (alpha). Two distinct parameters, namely the conventional state parameter Psi, and the equivalent state parameter Psi*, are introduced in the context of critical state soil mechanics (CSSM) framework to predict failure mode and undrained cyclic resistance (CRR) of investigated soils. It is proved that the failure patterns for silty sands are related to (a) the initial states of soils (Psi or Psi*) and (b) the combination of initial shear stress with respect to cyclic loading amplitude. At each alpha, the CRR-Psi (or Psi*) correlation can be well represented by an exponential trend which is practically unique for both clean sands and silty sands up to a threshold fines content (f thre congruent to 24.5%). Varying alpha from low to high levels simply brings about a clockwise rotation of the CRR-Psi (or Psi*) curves around a point. This CRR-Psi (or Psi*) platform thus provides an effective methodology for investigating the impact of initial shear stress on the cyclic strength of both clean sands and silty sands. The methodology for estimating Psi (or Psi*) state parameters from in-situ cone penetration tests in silty sands is also discussed.