Undrained Cyclic Triaxial Tests Research Articles

Sheet-coating mitigation for membrane penetration in undrained triaxial tests and evaluation of comprehensive liquefaction resistance of crushed gravel

In undrained cyclic triaxial tests on crushed gravel, it is difficult to maintain undrained conditions because of the penetration of the rubber membrane (MP: membrane penetration) into the rough surface of the test specimens. A method has been developed in the present research to physically mitigate the MP-effect wherein the rough surface of each specimen is coated and smoothed by a prefabricated fines-soil sheet. This mitigation measure has been found very effective in minimizing the CR (compliance ratio for an undrained system) to achieve more strict undrained conditions. It has been further clarified that this method prevents not only the overestimation of the strength of loose specimens, as has already been recognized, but also the underestimation of the strength of dense specimens with highly positive dilatancy. The undrained cyclic strength of dense crushed gravel, comprised of various grain sizes and gradations, has been summarized in a series of undrained cyclic triaxial tests by employing the MP-mitigation method to find that, in place of the relative density governing the liquefaction resistance of loose soils, the void ratio or absolute density is a key parameter for uniquely estimating the strength with a simple empirical formula.

Liquefaction resistance and small strain stiffness of silty sand: Effects of host sand gradation and fines content

In this paper, a well designed experimental study comprising undrained cyclic triaxial and bender element tests was carried out on five sand-silt mixtures with three grain size ratios (Rd) and three sand uniformity coefficients (Cus). The results reveal that at given fines content (FC) and void ratio, the liquefaction resistance (CRR) of the mixtures decreases as Rd increases from 9.7 to 27.8 or Cus from 1.50 to 4.46, and the CRR of the mixture at the highest Rd or Cus becomes as low as nearly 50% of that at the lowest Rd or Cus. The equivalent relative density (Dr*) based on the binary packing theory can be utilized to capture the coupled effects of host sand gradation and FC on the CRR of sand-silt mixtures. A novel finding is that a universal relationship exists between the liquefaction resistance and the shear modulus normalized by the minimum void ratio function of host sand (GN') for different FCs, initial effective confining pressures, sample preparation methods (SPMs), and host sand gradations. Based on the comprehensive laboratory test data, the empirical CRR15-GN' correlation is proposed and further adjusted for the field liquefaction triggering analysis based on the shear wave velocity (Vs). Soil can be considered non-liquefiable when its equivalent shear wave velocity exceeds 225 m/s or the applied cyclic stress ratio (CSR) is below 0.088. Comparisons with the previously established liquefaction triggering boundary curves validate the applicability of the proposed curve for initial evaluation of the liquefaction resistance of sandy soils, indicating laboratory studies can complement in-situ investigations for Vs-based liquefaction assessment.

Small-strain shear modulus and liquefaction resistance of calcareous sand with non-plastic fines

Incorporation of nonplastic fines can dramatically affect the liquefaction resistance and stiffness of sands. This study aims to evaluate the influence of nonplastic fines on the liquefaction resistance and small-strain shear modulus of calcareous sand under cyclic loading. Forty-seven sets of undrained cyclic triaxial tests and companion bender element tests are conducted on reconstituted specimens. The cyclic behavior of clean sand and silty sand with varying fines content is examined with respect to the global void ratio, relative density, and granular skeleton void ratio. The findings demonstrate that the microscopic contacts between coarse and fine grains have a significant impact on the macroscopic behavior of sand-fines mixtures. The experimental findings are evaluated using the equivalent granular skeleton void ratio, which has been recognized as a suitable parameter to describe the overall effect of fines. The findings on calcareous sand with fines are supplemented and compared with published data in accordance with the semiempirical simplified approach for liquefaction triggering based on shear wave velocity.

Numerical modeling of LEAP-2017 liquefiable sloping ground centrifuge tests using the ISA-hypoplasticity model

This article evaluates the prediction capabilities and limitations of the ISA-hypoplasticity constitutive model for sands in the simulation of liquefiable sloping ground dynamic centrifuge tests from the LEAP-2017 project. For that purpose, the experimental results of three centrifuge tests performed at different centrifuge facilities are considered. The corresponding dynamic coupled predictions were carried out using the finite element software Abaqus Standard. The material parameters of the adopted constitutive model were determined based on a detailed laboratory characterization of the tested Ottawa F-65 sand, which included drained monotonic triaxial and undrained cyclic triaxial tests considering a wide range of initial conditions and testing characteristics. The comparison between measured and computed sloping ground accelerations and spectral response suggests reasonable predictions by the model. Furthermore, the results suggest that the constitutive model is only able to realistically reproduce the trends of the experimental excess pore water pressure when considering cyclic mobility effects.

Experimental evaluation of dynamic geotechnical parameters of tailings

The paper is focused on the testing of material from the "Lyulyakovitsa" tailings dam situated in proximity to the town of Panagyurishte in Bulgaria. The primary objective of this study is to investigate various laboratory techniques aimed at determining essential dynamic characteristics of the studied material. This research is conducted with the goal of subsequently assessing the seismic response of the existing tailings dam. In the context of performing an equivalent linear analysis, a widely used approach in geotechnical earthquake engineering, several crucial relationships and parameters come into play. These include the material's stiffness degradation, the damping ratio, and the small-strain stiffness, expressed through the shear wave velocity. The paper describes analysis of three experiments carried out in the geotechnical laboratory "Komaba" at the University of Tokyo. These experiments introduce undrained cyclic triaxial tests conducted under identical relative density (95%) of the tailings and executed at 50 kPa, 80 kPa, and 100 kPa effective confining stress, specifically, respectively. Additionally, two distinct testing methods were employed to measure the shear wave velocity in the specimens. The findings align with certain established directions in the existing literature for characterizing the dynamic properties of granular soils. Simultaneously, they underscore the practicality of the laboratory experiments adopted for acquiring a comprehensive set of relationships and parameters.

Evaluation of large axial strain of granular sand in post-liquefaction stage using DEM simulation

Field investigations in recent studies have revealed that significant damage to foundations often occurs in the post-liquefaction stage, during which sand undergoes substantial deformation. As a result, numerous researchers have dedicated considerable effort to studying the evolution of the axial strain at this stage. However, the accurate measurement of the exact value of this strain is challenging mainly owing to the limitations of the triaxial apparatus. In this study, the numerical method of discrete element method (DEM) is used to simulate undrained cyclic triaxial loading tests according to a large number of loading cycles. The mechanical behaviour of granular sand under different cell pressures was reproduced using the DEM method, wherein the liquefaction resistance increased with the cell pressure. The post-liquefaction axial strain exhibited a similar pattern of variation among the calculated cases at different cell pressures. The value of the double amplitude of the axial strain stabilised at a maximum value after a certain loading cycle. The microscopic characteristics derived from the DEM calculations, including the contact number and force chain network, were analysed to provide reasonable explanations. The sand fabric was re-established after a large number of loading cycles and controlled by cyclic loading, which helped stabilise the accumulated axial strain.

Undrained permanent deformation characteristics of Yellow River silt under one-way cyclic loads

As a new type of engineering construction filler, it is particularly important to know the shakedown and permanent deformation characteristics of the Yellow River silt under cyclic load. In this study, a series of undrained cyclic triaxial tests for Yellow River silt are carried out by using GDS triaxial apparatus, the effects of relative density, loading frequency, confining pressure and cyclic stress ratio on the development curve of permanent strain are explored. Based on the shakedown theory and combined with the data of permanent strain development curve of Yellow River silt at different stress levels, a calculation model for its critical dynamic stress and limit cyclic stress ratio under shakedown state is determined. Further analysis is conducted on the development law of the permanent strain curve of Yellow River silt under shakedown state, and a prediction model for permanent strain of Yellow River silt considering different parameters was established. Then the accuracy of the model is verified. Combining with splitting summation method, the settlement of foundation can be calculated.

Comparative analysis of two intergranular strain-based hypoplastic models through elemental and centrifuge testing

While hypoplastic models have demonstrated accurate predictions of sand behavior under monotonic loading, their accuracy diminishes when applied to cyclic loading conditions. To address this limitation, the intergranular strain approach is used as an extension to the model. The current investigation focuses on the analysis of two variants: the original Intergranular Strain (IS) approach proposed by Niemunis and Herle (1997) and the Intergranular Strain Anisotropy (ISA) by Fuentes et al. (2019). Although both models have the same objective, they present distinct mathematical structures and therefore different repercussions on the simulations. In this study, sand Hypoplasticity is enhanced with IS and ISA, and employed to simulate a series of experimental tests conducted on Fontainebleau sand. These tests encompass isotropic compression, drained monotonic triaxial, and undrained cyclic triaxial tests, while considering different initial densities and test characteristics. Furthermore, the calibrated models were applied to simulate a series of centrifuge tests, involving a pile embedded in the same sand, which is subjected to various episodes of monotonic and cyclic lateral loading. A comparison and discussion of the similarities and differences in elemental and finite element predictions, arising from the two intergranular strain formulations is presented.

The friction-weakening cause by localized wear-induced damage and its effect on liquefaction resistance of sandy soil using DEM

Localized wear-induced damage caused by stress concentration at the contact point can lead to the decrease of the frictional coefficient (μ) of sandy soil, further affecting the liquefaction resistance. In this study, a bi-linear the friction-weakening model is proposed to describe the decreasing trend of μ with the increase of normal contact force (fn). Through a series of undrained cyclic triaxial tests, the influence of localized wear-induced damage on the characteristics of cyclic liquefaction is investigated from both macroscopic and microscopic perspectives. The macroscopic results indicate that samples have weaker liquefaction resistance due to the friction-weakening. Under low confining pressure (P0), the influence of the friction-weakening on the liquefaction resistance is relatively limited. With the increase of P0, the decrease of liquefaction resistance gradually becomes significant. However, the effect of friction-weakening on the liquefaction resistance does not remain constant, but shows a decreasing trend with the accumulation of excess pore water pressure ratio (EPWPR). From the microscopic perspective, the reduction of liquefaction resistance can be explained by the coordination number (Z) and the contact sliding ratio. Although higher Z caused by the friction-weakening may lead to greater liquefaction resistance, higher sliding contact ratio have been found to weaken the liquefaction resistance, and its influence exceeds Z.

An extended hypoplastic model for sands with additions of highly plastic fines formulated under the ISA framework

In the present work, the hypoplastic model for sands by Wolffersdorff, (1996) coupled with Intergranular Strain Anisotropy (ISA) by Fuentes et al., (2020) is extended to account for the influence of fines content on the undrained cyclic response and liquefaction resistance of the material. A set of undrained cyclic triaxial tests on clean Ottawa C778 sand and on the same sand with additions of bentonite or laponite were simulated to evaluate the model’s capabilities. The comparison between experiments and element test simulation results suggest that the proposed extended model is able to accurately describe the behavior of clean Ottawa C778 sand and the significant influence of plastic fines in the response of the mixtures, such as the reduction in the accumulation rates of strains and pore water pressure under cyclic loading, and therefore, the improvement in the liquefaction resistance with increasing fines content.

Mechanism of the variation in axial strain of sand subjected to undrained cyclic triaxial loading explained by DEM with non-spherical particles

The mechanical response of sand is affected by the axial strain accumulated during undrained cyclic triaxial loading tests. However, the large deformation that occurs during the post-liquefaction stage is difficult to measure using testing apparatus. To address this limitation, this research utilizes the numerical method of DEM with non-spherical particles to simulate the undrained cyclic triaxial loading test on clean sand material. The validity of DEM is confirmed through laboratory test. A four-stage scheme for the development of axial strain in the liquefaction process is proposed and verified by analyzing the deformation features of sand specimens with different densities. Meanwhile, the macroscopic behavior is also described in conjunction with microscopic characteristics of sand particles, such as coordination number, force chain network and anisotropy degree of contact normal, which are calculated using DEM. In addition, the central value of variating axial strain at each cycle is analyzed to determine its variation feature via loading cycle. Due to the capability of DEM in describing the microscopic behavior of soil particle aggregation, it is found that the difference in the proposed four-stage scheme between compression and extension sides is the main cause of the central value of axial strain shifting monotonically.

Comparison of a tailings material and a natural sand in seismic site response analyses

This article investigates the seismic response of a tailings sand material in the context of site response analysis. The computed response is contrasted with that obtained for an equivalent deposit consisting of a well-characterised natural sand. Initially, the fundamental properties of both materials are used to interpret and compare their behaviour in the framework of Critical State Soil Mechanics (CSSM). Subsequently, both materials' monotonic and cyclic responses are calibrated for an advanced bounding surface plasticity model. Finally, the impact of different calibration approaches is examined through site response analyses under a strong motion. This process identifies fundamental differences between the two geo-materials, impacting the single elements simulations, especially the accuracy of undrained cyclic triaxial tests. The site response analyses examined herein show the impact of the calibrations adopted for both materials in terms of acceleration response spectra, displacements, and excess of pore water pressures (PWP). Although a relatively deep phreatic surface is adopted, the cyclic resistance and permeability of both materials plays a dominant role in the site response characteristics, controlling the excess PWP generation and hence the non-linear behaviour in the soil deposit. Finally, parametric studies are also conducted to explore the impact of permeability of the sands in the site response.

On the determination of cyclic shear stress for soil liquefaction triggering in centrifuge model test

As the dynamic shear stress acting on soil column could not be measured directly during earthquakes, the simplified procedure proposed by Seed and Idriss is used to calculate the earthquake-induced cyclic shear stress based on acceleration record of strong motion station for liquefaction triggering evaluation. However, when this procedure is applied to physical modeling such as centrifuge model test, it will result in overestimation of the cyclic loading because the acceleration is monitored in liquefiable soil deposit and the acceleration time history will be distorted due to the softening process, and the whole time history is counted regardless the triggering of liquefaction or not. To address this problem, a centrifuge model test with long duration shaking was conducted to observe the difference of acceleration response between the soil deposit and the sidewall at the same elevation. And a series of stress-controlled undrained cyclic triaxial tests of the same sand used in centrifuge model test were carried out to obtain the cyclic strength at different elevations in model ground. This study shows that the cyclic loading before the time of liquefaction occurrence instead of the whole time history corresponds to the cyclic strength well, and also the calculation based on acceleration record of sidewall is closer to the cyclic strength than that in the soil. Thus a method is proposed to determine the cyclic shear stress in dynamic centrifuge model test, which takes the acceleration record before the time of liquefaction triggering at the sidewall of laminar container instead of that of the liquefiable soil as the source data to calculate the cyclic shear stress ratio.

Density-Dependent Pore Water Pressure Evolution in a Simplified Cyclic Shear Test

When specimens of different sands are produced using the same preparation method and sheared under the same conditions (consolidation stress, loading, etc.), while simultaneously keeping the drainage closed, the resulting tendencies of these sands regarding the PWP build-up will be different. This research paper presents a simplified cyclic shear test, which is used to evaluate the accumulation of PWP in sands under defined specimen preparation procedure and testing conditions. In the proposed experiment, a comparison of different sands with this respect is easily achieved. The principle of this experimental method is based on the evolution of the PWP during cyclic shearing of a water-saturated sand sample. Undrained conditions during the experiment allow for the evolution of the PWP, which is quantified by the rate of the PWP build-up. The duration of a single cyclic shear test, including specimen preparation, is approximately 30 min. The evaluation of the rate of the PWP build-up for different densities resulted in an exponential dependence of the PWP build-up on the variation of the relative density. The results confirmed a higher generation of PWP in a fine sand compared to a coarse sand. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods. The basis for the comparison was the density-dependent evolution of PWP in these methods. The presented method delivers a value (index) that quantifies the PWP build-up in sands under the defined testing conditions.

Loading rate influence on high-cyclic loading of clays

Experimental observations reported in the literature, based on cyclic triaxial and cyclic simple shear tests, show that the loading frequency influences the excess pore pressure and strain accumulation for fine-grained soils under undrained conditions. A high-cycle accumulation (HCA) model for clay was introduced by Wichtmann (2016) and Staubach et al. (2022) to quantify the strain/pore pressure accumulation as a function of the number of applied cycles, amplitude, void ratio, stress ratio, OCR and loading frequency. This article explores the convenience of including the loading frequency factor in the HCA clay model based on the interpretation of a series of cyclic undrained triaxial tests performed on three different clay materials, namely Dorfner kaolin, Lower Rhine clay and Onsøy clay. Analysis of results shows that for those specific three clay materials, the normalization of the pore pressure accumulation rate u̇acc with the amplitude function fampl purifies the data from the influence of loading frequency and thus no explicit frequency function ff is required.

Experimental Study on Liquefaction Characteristics of Coral Gravelly Soils with Different Particle Size Distributions

Many laboratory studies have shown that particle size distribution (PSD) affects the liquefaction susceptibility of granular materials. However, few studies have focused on the impact of PSD on coral particles. In this study, two different soil families were prepared: one with three levels of mean particle size (D50) with identical uniformity coefficient (Cu)and the other with four levels of Cu with the same D50 for coral gravelly soils. In addition, a series of undrained cyclic triaxial tests were conducted on coral gravelly particles with two groups of PSDs at a relative density of 40% and an adequate confining pressure of 100 kPa. The test results indicated that D50 with identical Cu can affect the undrained cyclic behavior of coral gravelly particles. In contrast, Cu with identical D50 does not impact the undrained cyclic behavior of coral gravelly particles. The developing pore water pressure was uniform when the sample was subjected to the same cyclic loading. For samples with changing D50 values of 2.35, 4.70, and 7.05 mm, increasing D50 improved the cyclic liquefaction resistance. For samples with changing Cu, increasing Cu in the range of 1.06–5.00 first increased and then decreased the liquefaction resistance.

Particle shape effects on dynamic properties of granular soils: A DEM study

Particle shape is an essential aspect of granular soils that affects their mechanical properties. It is challenging to isolate the effect of particle shape from other mechanical properties such as inter-particle friction coefficient and particle shear modulus in laboratory tests. The discrete element method (DEM) makes it feasible to investigate the effect of macroscopic and mesoscopic particle morphologies including sphericity and roundness. This study provides a systematical DEM investigation of the particle shape effect on the dynamic properties of granular soils by simulating cyclic undrained triaxial tests at various initial confining pressures and void ratios. The results indicate that specimen consisting of angular particles generally exhibit a higher secant shear modulus G than that of rounded particles. Additionally, particle shape has no significant effect on modulus ratio G/G0 but a noticeable impact on damping ratio D at relatively large strains due to dilatancy. A weighted coordination number CN' was proposed to reveal the microscopic effect of particle shape on G0. Contact sliding was found to be the main cause of modulus degradation and damping increase with increasing shear strain. The tangential shear modulus ratio Gtan/G0 is inversely correlated to the weighted contact sliding ratio fs'.

Experimental investigation on cyclic behavior of geogrid-reinforced coral sand from the South China Sea

The present study provides insight into the cyclic behavior of unreinforced and geogrid-reinforced coral sand. To achieve these goals, stress-controlled undrained cyclic triaxial tests were conducted on medium-dense coral sand with and without geogrid reinforcements. The influence of geogrid reinforcement, cyclic stress ratios, and confining pressure on the deformation, pore pressure, liquefaction resistance, stiffness degradation, and particle breakage characteristics of saturated coral sand was investigated. The results indicated that coral sand reinforced with geogrid increased the liquefaction resistance and decreased the rate of pore pressure accumulation. The effectiveness of the geogrid inclusion on coral sand’s liquefaction resistance was found to be more pronounced under large effective confining pressure and a smaller cyclic stress ratio. Increasing the effective confining pressure increases the liquefaction resistance for both unreinforced and geogrid-reinforced coral sand, whereas increasing the cyclic stress ratios decreases the liquefaction resistance of the unreinforced and reinforced coral sand. The particle breakage was observed to be increased with increasing the effective confining pressure and cyclic stress ratio, and the breakage was more in the reinforced coral sand than the unreinforced coral sand.

Aging effects on liquefaction resistance of sand estimated from laboratory investigation

Liquefaction resistance is known to increase concomitantly with the increase in time after construction or sedimentation. Nevertheless, the mechanisms of its aging effect on liquefaction have not been completely elucidated. To clarify the mechanisms of aging in sandy soils, the liquefaction resistance (CRR), initial and secant shear moduli (G0 and Gsec), and laboratory penetration resistance of long-term consolidated sand specimens were examined using cyclic undrained triaxial tests, local small strain (LSS) tests equipped with bender elements (BEs), and penetration index tests, respectively. Based on the existing reports, the CRR was inferred from G0, Gsec, and the laboratory penetration resistance. In the case of Toyoura sand of Dr = 40%, the CRR increased by about 14% with a 360-day consolidated specimen in the cyclic undrained triaxial tests. However, increases in the CRR evaluated from G0 and the laboratory penetration resistance were nothing and only 2%, respectively. Gsec started to degrade at greater shear strain in the long-term consolidated specimens. An increase in the CRR, evaluated from the Gsec of 0.01% shear strain, had a much better agreement with that obtained from the cyclic undrained triaxial tests.

Energy-based approach to study liquefaction triggering in homogeneous and stratified soils under consolidated undrained cyclic loading

Pore water pressure development through seismic effect is one of the key reasons for liquefaction failures in the foundations, dams, bridges, slopes and other vulnerable geotechnical engineering structures. The energy dissipation theories have provided a remarkable understanding of the pore water pressure generation inside a soil skeleton. In the present study, a series of consolidated undrained cyclic triaxial tests have been performed to evaluate the normalized dissipated energy (Wn) required for triggering liquefaction in both homogeneous and stratified soil specimens (prepared using silt interlayer) to replicate the site conditions nearby marine or alluvial deposits. The generated hysteresis loops using cyclic loading facilitate the calculation of dissipated energy. Also, the test results indicated a significant variation in the dissipated energy of homogeneous specimens at different magnitudes of cyclic stress ratios (CSR) and relative densities (Dr). With an increase in the cyclic stress ratio from 0.14 to 0.20, the trend of dissipated energy was observed to be different when relative densitie of sand varied from 30% to 65%. In the stratified specimens, thickness (t) and location of the silt layer (d) in the given soil specimen were the major influencing factors for estimating the liquefaction potential in loose conditions. Additionally, the highest value of Wn,max was obtained as 0.51 for the stratified specimen with topt = 28 mm at a cyclic stress ratio of 0.14. Based on the trendline variations, empirical models have been developed to estimate the normalized dissipated energy (Wn) for both homogeneous and stratified conditions. The developed models for homogeneous specimens are functions of CSR and Dr, whereas, in the case of stratified specimens, t, d and σd/σ'c (σd = deviator stress, σ'c = effective confining stress) have been utilized to predict the Wn with an satisfactory R2 value above 0.90. From the analysis of liquefaction resistance curves, the energy-based approach used in this study has been observed to provide more realistic results than the pore pressure ratio and strain-based failure criteria.