Undrained Cyclic Shear Research Articles

Experimental Study of the Dynamic Shear Modulus of Saturated Coral Sand under Complex Consolidation Conditions

The shear modulus is an essential parameter that reflects the mechanical properties of the soil. However, little is known about the shear modulus of coral sand, especially under complex consolidation conditions. In this paper, we present the results of a multi-stage strain-controlled undrained cyclic shear test on saturated coral sand. The influences of several consolidation state parameters: effective mean principal stress (p0′), consolidation ratio (kc), consolidation direction angle (α0), and coefficient of intermediate principal stress (b) on the maximum shear modulus (G0), the reference shear strain (γr) and the reduction of shear modulus (G) have been investigated. For a specified shear strain level, G will increase with increasing p0′ and kc, but decrease with increasing α0 and b. However, the difference between G for various α0 and b can be reduced by the increase of shear strain amplitude (γa). G0 shows an increasing trend with the increase of p0′ and kc; on the contrary, with the increase of α0 and b, G0 shows a decreasing trend. To quantify the effect of consolidation state parameters on G0, a new index (μG0) with four parameters (λ1, λ2, λ3, λ4) which is related to p0′, kc, α0, b is proposed to modify the prediction model of G0 in literature. Similarly, the values of γr under different consolidation conditions are also evaluated comprehensively by the four parameters, and the related index (μγr) is used to predict γr for various consolidation state parameters. A new finding is that there is an identical relationship between normalized shear modulus G/G0 and normalized shear strain γa/γr for various consolidation state parameters and the Davidenkov model can describe the G/G0–γa/γr curves. By using the prediction model proposed in this paper, an excellent prediction of G can be obtained and the deviation between measured and predicted G is all within ±10%.

Modeling seismic compression of unsaturated soils in the funicular regime

A semi-empirical elasto-plastic constitutive model with a hyperbolic stress-strain curve was developed with the goal of predicting the seismic compression of unsaturated sands in the funicular regime of the soil-water retention curve (SWRC) during undrained cyclic shearing. Using a flow rule derived from energy considerations, the evolution in plastic volumetric strain (seismic compression) was predicted from the plastic shear strains of the hysteretic hyperbolic stress-strain curve. The plastic volumetric strains are used to predict the changes in degree of saturation from phase relationships and changes in pore air pressure from Boyle’s and Henry’s laws. The degree of saturation was used to estimate changes in matric suction from the transient scanning paths of the SWRC. Changes in small-strain shear modulus estimated from changes in mean effective stress computed from the constant total stress and changes in pore air pressure, degree of saturation and matric suction, in turn affect the hyperbolic stress-strain curve’s shape and the evolution in plastic volumetric strain. The developments of the new mechanistic model developed in this study will play a key role in the future development of a holistic model for predicting the seismic compression across all regimes of the SWRC.

Effects of physical properties and undrained cyclic shear conditions on the pore water pressure responses of saturated sands and clays

For clarifying the effects of relative density (Dr) and Atterberg’s limits on the cyclic shear-induced pore water pressure properties of soils, sandy soils with similar index properties and clayey soils with different Atteberg’s limits were collected from Vietnam and Japan and used for this study. Specimens at Dr = 50% of Nam O sand and Dr = 70% of Toyoura sand, and those of Hue clay and Japanese Kaolin clay were consolidated under the vertical stress of σvo = 49 kPa. They were then subjected to undrained cyclic shear for various cyclic shear directions and wide ranges of the number of cycles and shear strain amplitudes. Under the same cyclic shearing conditions, specimens of sand at higher Dr (Toyoura sand) and clay with higher Atterberg’s limits (Kaolin) show a lower pore water pressure ratio. The number of cycles and the cumulative shear strain at the starting point of pore water pressure generation were observed for different soils and testing conditions. In addition, using the cumulative shear strain, a new strain path parameter, the effects of shear strain amplitude and cyclic shear direction can be captured, resulting in a unique uacc/σ’vo - G* relation on each soil. Based on this, fitting lines can be drawn and referred to promote a prediction of the cyclic shear-induced pore water pressure accumulation for the used soils under different cyclic shear conditions.

Cyclic liquefaction resistance of sand under a constant inflow rate

Cyclic resistance curves, derived through undrained element tests, are central to the study of liquefaction. Their use implies that water drainage is negligible during an earthquake. Nevertheless, a growing body of evidence suggests that this hypothesis is not realistic. In proximity to the interface of a liquefiable layer with an overlying lower permeability layer, upwards water flow can lead to localised, co-seismic pore volume increase. The aim of this paper is to quantify the effects of pore volume increase on cyclic resistance curves. Cyclic triaxial experiments are presented, performed under undrained conditions and under conditions of volumetric expansion. A constant inflow rate is chosen for the sake of simplicity. Results show that even small inflow rates have a detrimental effect on cyclic resistance. A simplified methodology is developed, which can estimate cyclic resistance under constant water inflow rates, by assuming a superposition of isotropic unloading and undrained cyclic shearing. The results presented add to increasing evidence on how current liquefaction susceptibility assessments might not be conservative for layered deposits. In addition, they highlight significant aspects of soil response under partially drained conditions.

Influence of sloping ground conditions on cyclic liquefaction behavior of sand under simple shear loading

An experimental program of undrained cyclic simple shear tests was undertaken on Ticino sand, covering a broad range of initial relative densities and static stress levels, to investigate the effect of an initial static shear stress on failure mechanism, cyclic resistance, and pore water pressure generation. The results indicated that two types of failure mechanisms occurred, namely cyclic mobility and plastic strain accumulation, depending on the magnitude of the initial static shear stress with respect to the applied cyclic shear stress. Test results showed that the cyclic resistance of Ticino sand may either increase or decrease with an increasing initial static shear stress depending on the density state of the sand. The Kα correction factors measured under cyclic simple shear loading were compared with the values reported in the literature based on both cyclic triaxial and simple shear tests, suggesting that Kα could be dependent on the particle gradations and grain shape for a given depositional mechanism. Finally, it was verified that the initial static shear stress has a remarkable impact on the development of cyclic pore-water pressures (PWP) of the sands so that a modified PWP generation model has been effectively proposed in the present study.

Experimental study on the cyclic behavior of silty sands reinforced by disposal of shredded facemask

Due to the COVID-19 pandemic, personal protective waste generation has increased worldwide. Given this problem,urgent waste management methods are required to reduce waste surcharge to the environment. The present study tries to clarify the question using particle-level modeling combined with 36 undrained cyclic triaxial shear tests. The physical properties of the base materials and the face mask (FM) are first investigated and scanning electron microscopy is used to image the soil particles.This study used two kinds of sand with different median grain (D50) sizes.In doing so, shredded face mask (FM) contenthas been modifiedfrom 0 % to 1 %. In a Small silt system (fine content ≤ 40 %), liquefaction resistance decreased with silt content addition and had an opposite behavior in a Large silt system (fine content ≥ 40 %) for both sands. FM addition to silty samples leads to sustainable improvements such as more dilatative behavior and dissipation of excess pore water pressure, and enhanced liquefaction resistance. The effectiveness of FM reinforcement diminished with increasing the median grain size (D50). Also, the shear modulus of clean and silty sands improved with FM addition.

Improvement in dynamic behaviors of cement-stabilized soil by super-absorbent-polymer under cyclic loading

Silty clay with a high water content is a kind of soft soil widely distributed in the middle and lower reaches of the Yangtze River, China. Based on environmental protection and engineering requirements, cement combined with super-absorbent-polymer (SAP) stabilized soil is often used. However, the dynamic behaviors of cement-stabilized soil with SAPs are not well studied at present. In this study, a series of consolidated undrained cyclic triaxial shear tests were conducted on cement-stabilized soil with and without SAPs, and the effects of the effective stress, cyclic stress ratio (CSR), and cyclic number of cycles on the dynamic behaviors were studied. The results show that the SAP can increase the dynamic elastic modulus (about 10.5%) and damping ratio (about 11.2%) of cement-stabilized soil, and improve the brittle failure and negative impact on superstructures of cement-stabilized soil under cyclic loading. The influence of the CSR on the dynamic elastic modulus and damping ratio of cement-stabilized soil with SAPs has thresholds, and relevant empirical formulas are established. These can be used to predict the distribution of dynamic stress and evaluate the influence of environmental vibrations.

Liquefaction potential of reinforced sand with plastic wastes

Granular soil liquefaction, due to developed pore water pressure during undrained cyclic shear of saturated soils, is regarded as a common phenomenon under earthquakes loading. This phenomenon results in the huge damage to infrastructures. Various reinforcement materials have been successfully implemented with particular attention to use waste materials to satisfy design specifications and also reducing the adverse environmental effects. This paper investigates the possibility of using waste plastic fibers as a reinforcement material to mitigate liquefaction potential and pore pressure generation of reinforced sand under cyclic loading. For this purpose, 42 stress-controlled cyclic triaxial tests were conducted on Babolsar sand, reinforced by polyethylene terephthalate (PET) and polypropylene (PP) fibers with fiber contents of 0.25%, 0.5% and 1%, under confining pressures of 50, 100 and 200 kPa, and with cyclic stress ratios (CSR) of 0.2 and 0.35. Results revealed that the addition of these waste plastic fibers could significantly increase the liquefaction resistance of Babolsar sand, and also, with an increase in the waste content, the number of cycles leading to liquefaction increased. Adding wastes decreased the pore water pressure generation, and this effect was more pronounced with an increase in the waste content.

Visualized liquefaction behavior of sandy soil deposited in water under undrained cyclic shearing

Visualized liquefaction behavior of sandy soil deposited in water under undrained cyclic shearing

Pore water pressure responses of saturated sand and clay under undrained cyclic shearing

In this study, changes in the pore water pressure were observed for saturated specimens of a loose fined-grain sand (Nam O sand) and a soft silty clay (Hue clay) subjected to undrained cyclic shearing with different testing conditions. The cyclic shear tests were run for relatively wide range of shear strain amplitude (g = 0.05%-2%), different cycle numbers (n = 10, 50, 150 and 200) and various shear directions (uni-direction and two-direction with phase difference of q = 0o, 45o and 90o). It is indicated from the experimental results that under the same cyclic shearing condition, the pore water pressure accumulation in Hue clay is at a slower rate, suggesting a higher cyclic shear resistance of Hue clay than that of Nam O sand. Liquefaction is reached easily in nominally 50% relative density specimens of Nam O sand when g ³ 0.4%, meanwhile soft specimen of Hue clay is not liquefied regardless of the cyclic shearing conditions used in this study. The threshold number of cycles for the pore water pressure generation generally decreases with g meanwhile, the threshold cumulative shear strain for such a property mostly approaches 0.1%. In addition, by using this new strain path parameter, it becomes more advantageous when evaluating the pore water pressure accumulation in Nam O sand and Hue clay subjected to undrained uni-directional and two-directional cyclic shears.

Effect of Phase Difference on the Liquefaction Behavior of Sand in Multidirectional Simple Shear Tests

AbstractMultidirectional cyclic shearing induced by earthquakes involves variations not only in amplitude but also in direction. To investigate the undrained cyclic shear responses of sand under th...

Influence of stress history on undrained cyclic shear strength evolution

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long-term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in kaolin and reconstituted Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

The Influence of Apparatus Stiffness on the Results of Cyclic Direct Simple Shear Tests on Dense Sand

Abstract A series of undrained cyclic direct simple shear (CDSS) tests on dense Toyoura sand has been performed with the aim to investigate the influence of the stiffness of the DSS device on test results. To this end, springs were installed to reduce deliberately the stiffness of the apparatus. It is shown that the cyclic resistance of the sand depends strongly on the rigidity of the apparatus frame. In particular, as the stiffness of the DSS device increases, the number of loading cycles required to reach liquefaction decreases. This pronounced apparatus–stiffness dependence is of great practical concern in geotechnical engineering because it directly implies that the CDSS response of a soil sample can be predominantly controlled by the stiffness of the apparatus and not by the soil behavior alone. In addition, the test results indicate that the effect of equipment compliance in cyclic undrained DSS testing can be minimized when the ratio of the stiffness of the tested sand sample to the stiffness of the apparatus has a significantly low value.

Liquefaction Susceptibility of Saturated Coral Sand Subjected to Various Patterns of Principal Stress Rotation

The influences of consolidation conditions and rotation patterns of principal stress on the liquefaction susceptibility of saturated coral sand have emerged as an interesting problem in recent years. This paper presents results from a comprehensive experimental study comprising undrained monotonic shear tests and undrained cyclic shear tests subjected to various patterns of principal stress rotation. A remarkable finding is that a virtually unique correlation exists between a generalized shear strain amplitude γga and the excess pore water pressure ratio ru irrespective of consolidation conditions and cyclic loading patterns. A simple formulation is then proposed to relate γga and ru. Another significant finding is that the liquefaction susceptibility of coral sand and the correlation between the conventional cyclic stress ratio (CSR) and the number of cycles to failure Nf (corresponding to γga=2.5%) are strongly affected by the couplings of consolidation conditions and cyclic loading patterns. By introducing a generalized unit cyclic stress ratio (USRg) as a new proxy for liquefaction resistance, a strong correlation is found between USRg and Nf for all data sets from the experiments. An explicit relationship is then proposed for practical application. The wide applicability of this relationship is well demonstrated using the literature data for various undrained cyclic laboratory tests and different sands.

Deformation and cyclic resistance of sand in large-strain undrained torsional shear tests with initial static shear stress

This paper presents the findings from an experimental study focusing on the undrained cyclic behavior of sand in the presence of initial static shear stress. A series of undrained cyclic torsional shear tests was performed on saturated air-pluviated Toyoura sand specimens up to single amplitude shear strain (γSA) exceeding 50%. Two types of cyclic loading conditions, namely, stress reversal (SR) and stress non-reversal (SNR), were employed by changing the amplitude of the combined initial static shear and cyclic shear stresses. The tests covered a broad range of initial states in terms of relative density (Dr = 20–74%) and the initial static shear stress ratio (α = 0–0.30). The following five distinct modes of deformation were identified from the tests based on the density state, the transient undrained peak shear stress, and the combined cyclic and static shear stresses: 1) static liquefaction, 2) cyclic liquefaction, 3) cyclic mobility, 4) shear deformation failure, and 5) limited deformation. Of these, cyclic liquefaction and static liquefaction are the most critical. They occur in very loose sand (Dr ≤ 24%) under SR and SNR, respectively, and are characterized by abrupt flow-type shear deformation. Cyclic mobility occurs under SR in loose to dense sand with Dr ≥ 24%. Contrarily, shear deformation failure typically occurs under SNR in sand with 24 < Dr < 65%, and limited deformation may take place in dense sand with Dr ≥ 65%. In this paper, a stress-void ratio-based predictive method is proposed to identify the likely mode of deformation/failure in sand under undrained shear loading with static shear. Furthermore, the cyclic resistance is evaluated at three different levels of γSA (i.e., small, γSA = 3%; moderate, γSA = 7.5%; and large, γSA = 20%). The results show that, independent of the density state, the cyclic resistance continuously decreases with an increase in α at the small γSA level, while it first decreases and then increases for both loose and dense sand at the moderate and large γSA levels.

Cyclic Behavior of K0-Consolidated Soft Clay under Stress Paths with Different Major Principal Stress Directions

AbstractTo study the deformation response of soft clay under cyclic loads, five groups of undrained cyclic torsional shear tests were conducted under a stress path with different principal stress d...

Seismic Compression of Unsaturated Silty Sands: A Strain-Based Approach

The vast majority of surface structures are located on or surrounded by unsaturated soil deposits and may suffer excessive settlements during earthquakes. However, the fundamental understanding of the mechanisms by which the degree of saturation impacts the volumetric deformation of soils during seismic loading is still not mature. Consequently, it is critical to develop and calibrate seismic compression models while considering these mechanisms. The objective of this paper is to experimentally investigate the impacts of the degree of saturation, fines content, and desaturation technique on the seismic compression of sand and silty sands. The experimental program involved undrained cyclic direct simple shear tests on specimens prepared using suction control and wet-compaction techniques with similar relative densities but different levels of saturation. A strain-based predictive model was adapted and modified to capture the observed trends in the seismic compression of soils with different degrees of saturation. The suitability and applicability of the model were verified by comparing the measured and estimated compression values in this study with ones reported in the literature for other soils and desaturation approaches.

Comparisons on liquefaction behavior of saturated coral sand and quartz sand under principal stress rotation

A series of undrained cyclic axial-torsional shear tests on saturated coral sand and quartz sand under linear, circular, and heart stress paths are performed. The test results show significant differences in liquefaction characteristics for coral sand and quartz sand. A unique relationship between deviatoric strain amplitude and excess pore water pressure ratio (R u) can be established regardless of cyclic stress paths. The termed cyclic strain path is introduced to describe the generation features of strains, and different failure patterns for various cyclic stress paths can be easily identified. Moreover, the dissipative energy required to liquefaction triggering (E a1) of coral sand is much larger than that of quartz sand. The generation patterns of dissipative energy with R u for both sands are almost independent of cyclic stress paths, but the correlation between the cyclic stress ratio (CSR) and the number of cycles required to liquefaction triggering (N f) is dependent on cyclic stress paths. Using the proxy of the unit cyclic stress ratio (USR) proposed by Chen et al. (2020) instead of the CSR, the correlation between USR and N f is independent on cyclic stress paths. A remarkable finding is that, a unique correlation between USR15 and E a1 exists for all sands considered.

Undrained cyclic behavior of granular materials considering initial static shear effect: Insights from discrete element modeling

In-situ soil deposits often have an initial shear stress that significantly affects their subsequent cyclic behavior and liquefaction susceptibility. This is closely related to the internal structure of the soil but is very difficult to study thoroughly through conventional laboratory tests. In this study, a numerical procedure that can impose arbitrary two-dimensional stress or strain paths was developed using the discrete element method (DEM); this procedure was then used in simulations to investigate the behavior of granular polygonal samples under an undrained cyclic simple shear with various densities and stress conditions. Two types of cyclic responses could be identified from the simulation results: “cyclic liquefaction” and “residual deformation failure.” It was found that the initial static shear could either enhance or weaken the cyclic resistance of sand to liquefaction failure, depending on the extent of the shear stress reversal. The internal structure of the granular materials was quantified using a contact-normal-based fabric tensor that could describe their load-bearing characteristics in response to the external applied stress. The norm and principal direction of the fabric tensor both exhibited drastically different evolution patterns under varying loading conditions. This can provide further insights into the underlying failure mechanism of granular soils.

Undrained Seismic Compression of Unsaturated Sand

Unsaturated soils in engineered geostructures like embankments or retaining walls may experience seismic compression during earthquakes due to particle rearrangement associated with large-strain cyclic shearing. Although previous studies have investigated volume changes during drained cyclic shearing of unsaturated soils, undrained cyclic shearing presents a more complex situation. During undrained cyclic shearing, changes in total volume, matric suction, degree of saturation, effective stress, shear modulus, and damping ratio may occur that have complex coupling effects that affect seismic compression. To better understand these coupling effects, this study performed a series of undrained cyclic simple shear tests on unsaturated sand specimens. Contractile volumetric strain after 200 cycles in these tests was found to vary nonlinearly with degree of saturation. The largest volumetric contraction occurred at a degree of saturation of 0.4 and coincided with the largest decrease in mean effective stress. The sand specimens followed a wetting-path scanning curve during shearing, with small changes in matric suction. The decrease in mean effective stress during cyclic shearing for all specimens followed the same linear relation with the accumulation of volumetric strain.