Undrained Cyclic Triaxial Tests Research Articles

Analysis of Liquefaction in Tailings Deposits by Fem Modeling of Undrained Cyclic Triaxial

In this article, a numerical calibration procedure for undrained cyclic triaxial tests is presented to evaluate the liquefaction potential in sand and silt samples from mining tailings in northern Chile. The numerical modeling of an axisymmetric specimen involved two stages: isotropic consolidation using the Hardening Soil Small (HSS) model and a cycling phase employing the UBC3D-PLM model to simulate the onset of liquefaction using the criterion that the excess pore pressure ratio Ru should exceed 0.8. The results demonstrate that the UBC3D-PLM modeling calibrated with experimental data from cyclic triaxial tests effectively represents the excess pore pressure in both sandy and silty soils from mining tailings. The accuracy of the modeling decreases when a single set of parameters is applied to the same soil at different cyclic stress ratios (CSR), highlighting the need for specific calibrations for each loading.

Predicting cyclic liquefaction behavior of saturated granular materials using an updated state evolution model

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 θ−t/t0 plane and between 0.84 and 0.94 in the θ−ru plane, as affected by the cyclic loading conditions.

Nonlinear seismic response analysis of liquefiable sites based on effective stress method

In this study, the one-dimensional nonlinear Matasovic model was extended to the two- and three- dimensional stress space by replacing the shear strain with the generalized shear strain. The extended Matasovic model was subsequently combined with Dobry’s excess pore water pressure generation model, and the reliability of the proposed loosely coupled effective stress analysis model was verified through undrained cyclic triaxial tests and a one-dimensional site response analysis. Finally, this method was applied to an engineering site in China to evaluate the nonlinear seismic response of liquefiable sites. The results show that the proposed effective stress analysis method can capture the dynamic behavior of the soil and the generation of pore water pressure during strong shaking. Moreover, there are differences between the proposed effective stress analysis method and the total stress analysis method for liquefiable sites, which indicates the need for careful selection in practical applications.

Enhancing sand liquefaction resistance through microbial-induced partial saturation: An experimental study

The liquefaction potential of saturated sand can be significantly reduced by inducing partial saturation in the soil. Conventional soil liquefaction mitigation methods, namely soil densification, drainage, cementing, and groundwater lowering, pose environmental concerns and are challenging to apply to pre-existing structures. However, the microbially induced partial saturation (MIPS) method is emerging as a novel and eco-friendly approach to mitigate liquefaction. The MIPS method involves microbial denitrification, which produces nitrogen gas and results in a desaturating effect in the saturated soil. The current study conducted a series of stress-controlled undrained cyclic triaxial tests on saturated sandy soil and microbially-desaturated sandy soil under different relative densities and loading conditions. In addition, the study systematically analyzed the effects of temperature and pH on bacterial activity and the denitrification process. Batch experiments were conducted to establish a relationship between the initial nitrate concentration in the bacterial media and the resulting desaturation.Comprehensive analyses of cyclic resistance curves were performed to gain a thorough understanding. Additionally, the study conducts detailed analyses of the accumulation of excess pore pressure and the resulting axial strains and deformation patterns in both treated and untreated sand. This study demonstrates that the MIPS treatment considerably enhances the liquefaction resistance of treated sand.

The improved cyclic resistance of bio-treated sands with various gradations for liquefaction mitigation: Density increase and/or cementation?

MICP (microbial induced calcium carbonate precipitation) method shows a huge potential in solving geological and geo-environmental engineering problems, such as liquefaction mitigation, soil remediation, erosion control, slope stabilization and dust control., due to its low-carbon, sustainable and undisturbing character. For the liquefaction mitigation, previous studies showed the remarkable enhancement in cyclic behavior of bio-cemented soils. Little attention has been paid to how soil gradation affects the effectiveness of MICP method in enhancing soil properties, which is one of the key factors of soil mechanical behavior. MICP method has been mainly used in fine sands (mostly <1 mm) with narrow gradation and a certain number of particles smaller than 75 μm. As a result, few papers discussed the application of bio-cementation to soils with larger grains. In addition, the respective contributions of density increase and cementation on cyclic resistance enhancement is still not clear. In this study, a fine sand (0.1–1 mm), a coarser sand (1–5 mm), and a mixture of both, were used, featuring a larger mean size (d50 mm) and uniformity coefficient (Cu) than in most previous studies. In this study, cyclic undrained triaxial tests were performed to analyze the liquefaction potential of loose and dense untreated specimens, and initially loose treated specimens. Results show that sand gradation has a great influence on the cyclic behavior and MICP effectiveness. For all the sands, the MICP treatment gives rise to a significant increase in cyclic resistance and mitigates liquefaction. This enhancement is due to the formation of cementitious precipitates in the pores or on the surface of particles, which creates (1) bridges between particles and/or change in grain characteristics (i.e., size, roughness, angularity), and (2) an increase in density. For sands with various gradations, these two effects differ in magnitude: in the coarsest sand, the effect of density increase appears to be predominant whereas, in the finest and medium sands, the formation of bonds seems to play the major part. These results are important to evaluate the possibility of using this method to improve cyclic resistance in various sands, and to compare the treatment effect with other techniques.

Effects of bacterial strains on undrained cyclic behavior of bio-cemented sand considering wetting and drying cycles

The microbial-induced calcite precipitation (MICP) technique has been developed as a sustainable methodology for the improvement of the engineering characteristics of sandy soils. However, the efficiency of MICP-treated sand has not been well established in the literature considering cyclic loading under undrained conditions. Furthermore, the efficacy of different bacterial strains in enhancing the cyclic properties of MICP-treated sand has not been sufficiently documented. Moreover, the effect of wetting-drying (WD) cycles on the cyclic characteristics of MICP-treated sand is not readily available, which may contribute to the limited adoption of MICP treatment in field applications. In this study, strain-controlled consolidated undrained (CU) cyclic triaxial testing was conducted to evaluate the effects of MICP treatment on standard Ennore sand from India with two bacterial strains: Sporosarcina pasteurii and Bacillus subtilis. The treatment durations of 7 d and 14 d were considered, with an interval of 12 h between treatments. The cyclic characteristics, such as the shear modulus and damping ratio, of the MICP-treated sand with the different bacterial strains have been estimated and compared. Furthermore, the effect of WD cycles on the cyclic characteristics of MICP-treated sand has been evaluated considering 5–15 cycles and aging of samples up to three months. The findings of this study may be helpful in assessing the cyclic characteristics of MICP-treated sand, considering the influence of different bacterial strains, treatment duration, and WD cycles.

Exploring the Macroscopic Behavior and Microstructure Evolution of Lightly Cemented Sand in the Post-Liquefaction Process Using DEM.

This study investigates the post-liquefaction monotonic undrained shearing behavior of cemented sand at the macro- and microscales, using the discrete element method. A series of cyclic undrained triaxial tests with different stress amplitudes and post-liquefaction monotonic undrained triaxial tests were simulated on cemented sand with diverse cement contents (CCs). For comparison, a series of monotonic undrained triaxial tests on cemented sand without liquefaction (virgin cemented sand) were also modeled. The macroscopic behavior was analyzed in conjunction with the microscopic characteristics of the assembly, such as the deviator fabric of contact normal orientation, mechanical coordination number, energy components, and bond breakage. The results show that the DEM model can capture the effect of CC and cyclic stress ratio (CSR) on the undrained shear strength, stiffness, and pore pressure observed in laboratory experiments. Referring to the virgin specimen, with an increase in CC, the mechanical coordination number and the input work increment increase, while the deviator fabric for total contacts changes irregularly, leading to a greater initial stiffness and shear strength. In the case of the liquefied specimen, the smaller initial mechanical coordination number results in a very low initial stiffness regardless of CC. Contrary to the uncemented sand, both the mechanical coordination number and the input work increment decrease with an increasing CSR for the cemented sand. The microstructure evolution governs the effect of cementation level and liquefaction history on the macroscopic post-liquefaction behavior.

Results of the cyclic triaxial testing on mechanically-biologically treated waste.

Accurate assessment of the dynamic strength characteristics of mechanically-biologically treated (MBT) waste is crucial for the construction and safe operation of landfill sites. Herein, samples of MBT waste from the Hangzhou Tianziling landfill were collected and subjected to consolidated undrained cyclic triaxial tests under four confinement levels and six cyclic stress ratios (CSRs). Under cyclic loading, the MBT waste exhibited a critical CSR. If the CSR exceeds the critical value, the MBT waste specimen rapidly undergoes deformation and failure. Dynamic strength of MBT waste decreases with an increase in the number of cyclic vibrations and increases with an increase in confining pressure. Considering the influence of cyclic vibrations and confining pressure, a formula for dynamic strength in terms of cyclic vibrations and confining pressure has been established. The dynamic shear strength parameter ranges for MBT waste were obtained under different seismic magnitudes. We compared the dynamic and static shear strength parameters of MBT waste and municipal solid waste. These study findings can serve as a reference for the dynamic stability analysis of MBT waste landfills.

On the influence of the shearing rate on the monotonic and cyclic response of Malaysian kaolin

This paper experimentally evaluates the influence of shearing rate on the monotonic and cyclic response of isotropically consolidated samples of Malaysian kaolin. On the one hand, a series of undrained monotonic triaxial tests were performed with varying shearing displacement rate. On the other hand, undrained cyclic triaxial tests were conducted considering different deviatoric stress amplitudes and loading frequencies. The well-known soil rate dependency under monotonic loading was confirmed up to a displacement rate threshold. The experimental results under cyclic loading suggest that for the given loading frequency, the variation of the deviatoric stress amplitude remarkably influences the strains and pore water pressure accumulation rates. In addition, the results suggest that, depending on the loading frequency, different shapes of mobilised effective stress loops are obtained. Larger loading frequencies lead to banana-shaped effective stress loops, while smaller frequencies reproduce eight-shaped effective stress loops. Furthermore, higher loading frequencies result in a larger number of cycles required to reach failure conditions. The reasons for the observed differences in the behaviour are thoroughly analysed and discussed.

Experimental study on the dynamic properties of soft clay with sand interlayers under cyclic loading

Unlike uniform soils, soft clays with sand interlayers in coastal soft clays, can affect their mechanical properties, potentially impacting underground-construction safety and stability. Consolidated undrained cyclic triaxial tests were conducted to study the dynamic properties and deformation behavior of clay, focusing on how the thickness ratio between the sand and clay layers and the cyclic-stress ratio influence the pore pressure, axial strain, shear-modulus ratio, and normalized damping ratio. The results indicate that higher thickness ratios and cyclic-stress ratios lead to a faster decay of the shear-modulus ratio, quicker increases in pore pressure, faster strain accumulation, and fewer cycles to failure. The normalized damping ratio has three different forms: decreasing, decreasing then increasing, and increasing. However, at a cyclic-stress ratio of 0.2 and thickness ratio of 0.25, the samples exhibit better dynamic characteristics. Soft clay with sand layers exhibits characteristics in line with the stability theory. At low thickness and cyclic-stress ratios, purely elastic and elastically stable phases are observed. As the thickness and cyclic-stress ratios increase, it transitions to plastic stability and incremental failure.

Shear wave velocity-based evaluation of cyclic mobility type of liquefaction and validation by centrifuge model tests in LEAP

Existing study shows that small-strain shear modulus is a promising parameter for characterizing soil liquefaction resistance, while liquefaction resistance is liquefaction type dependent. In this study, firstly a revisit of laboratory tests on saturated sands was conducted to explore the relationship between liquefaction resistance CRR and small-strain shear modulus Gmax with emphasis on cyclic mobility type of liquefaction. The investigation indicates that in the stress-normalized space of cyclic resistance and small-strain shear modulus, the fitting curve of soil testing data does not always approach to the coordinate origin and there will be an intercept on the abscissa axis. Based on this finding, a more general form of the power function between liquefaction resistance and small-strain shear modulus is proposed. Then, undrained cyclic triaxial tests were conducted in conjunction with the data available from LEAP to establish the power function between CRR and Gmax for medium dense to dense Ottawa F-65 sand, which leads to an updated characterization model on the basis of Zhou-Chen model. Finally, centrifuge model tests of submerged gently sloping ground of Ottawa F-65 sand conducted at Zhejiang University (i.e., ZJU) for LEAP were used to compile liquefaction and non-liquefaction case histories with shear wave velocity measurement to investigate the validity of the updated model for evaluating the cyclic mobility type of liquefaction. The test results show that the updated model could classify the case history datasets produced by centrifuge model tests well. And the further consideration of initial shear stress acting on the slope could enhance the evaluation performance of this characterization model, which is worth of further research.

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

Incorporation of non-plastic fines can dramatically affect the liquefaction resistance and stiffness of sands. In this study, the aim is to evaluate the influence of non-plastic 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 behaviour 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 behaviour of sand–fines mixtures. The experimental findings are evaluated using the equivalent granular skeleton void ratio, which has been recognised 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.

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.

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.

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.

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.