Undrained Cyclic Torsional Shear Research Articles

Experimental study on the influence of cyclic loading frequency on liquefaction characteristics of saturated sand

A series of undrained cyclic torsional shear tests was conducted to investigate the effect of cyclic loading frequency on the liquefaction characteristics of saturated sand using a hollow cylinder apparatus. The test results show that the dilative and contractive tendencies of various saturated sands are not only related to the physical properties of the sand, but are also affected by loading frequency. Under low-frequency loading, the saturated sand has a dilative behaviour, excess pore water pressure fluctuates after initial liquefaction and the soil maintains the ability to resist liquefaction to some extent after the initial liquefaction. The liquefaction mode in terms of the stress–strain relationship generally performs as the ‘cyclic mobility’. However, under high-frequency loading, the saturated sand has a contractive behaviour, and excess pore water pressure generally remains stable after the initial liquefaction. The liquefaction mode in terms of stress–strain relationship generally exhibits as ‘cyclic instability’. The deformation caused by low-frequency loading is significantly larger compared with that caused by high-frequency loading. At higher loading frequencies, the phase transformation stress ratio increases with the increase of loading frequency, and gradually approaches the failure stress ratio.

Effects of seepage flow on liquefaction resistance of uniform sand and gap-graded soil under undrained cyclic torsional shear

Internal erosion is the transportation of soil particles from within or beneath geotechnical structures, caused by seepage flow, that impacts the subsequent mechanical and hydraulic behaviour of the soil. However, it is difficult to predict the liquefaction resistance of eroded soil due to several factors related to the soil fabric. The present study investigates the impact of seepage flow on the undrained cyclic behaviour of two types of soil: uniform sand and gap-graded soil with a fines content of 20%, using a novel erosion hollow cylindrical torsion shear apparatus. From the results for the uniform sand, the soil fabric formed by moist tamping (MT) leads to higher liquefaction resistance than that formed by air-pluviation (AP). However, after applying seepage flow, the liquefaction resistance of the eroded MT specimens becomes even lower than that of the non-eroded AP specimen. Therefore, the liquefaction resistance of soil is expected to decrease due to the rearrangement of the initially stable coarse particles during seepage flow. On the other hand, the liquefaction resistance of the gap-graded soil tends to increase after the removal of fines as the number of stable contacts between the coarse particles is increased. Under these test conditions, the latter effect is found to be greater for the given gradation, leading to a slight increase in the liquefaction resistance of the tested gap-graded soil after internal erosion. Furthermore, the intergranular void ratio and small-strain shear modulus are seen to be well correlated with the liquefaction resistance of the tested soil.

Effects of consolidation conditions on the dynamic shear modulus of saturated coral sand over a wide strain range

The utilization of coral sand as a construction material for ports and other infrastructure systems in coral reefs is prevalent, wherein the soil elements within slopes and embankments typically experience an anisotropic stress state. It is imperative to accurately characterize the degradation behavior of the stratigraphic material's stiffness across a wide range of strains. In this study, the maximum and strain-dependent shear modulus (G0 and G) characteristics of saturated coral sand under isotropic and anisotropic consolidations were investigated by performing multi-stage stress-controlled undrained cyclic torsional shear tests with cyclic hollow cylinder shear apparatus. The experimental results indicated that G0 and G generally rose with increasing initial effective consolidation pressure but decreased with increasing inclination of initial major principal stress from the vertical (αc). In contrast, the effect of consolidation stress ratio (kc) on G0 and G was nonmonotonic, and that of kc and αc on the strain-dependent shear modulus reduction (G/G0) curve was slight. By defining anisotropic consolidation indexes δ0 and δr used for quantifying the effect of consolidation-induced anisotropy on the G0 and the G/G0, respectively. A unified formula of G0 obtained by multiplying Hardin's model by δ0, as well as a unique formula of G/G0 obtained by correcting γr in the Davidenkov skeleton curve expression as a linear function of δr were established under various consolidation conditions, respectively. The proposed G formula over a wide strain range is well validated by the experimental data for siliceous sandy soils from the literature.

Effect of cyclic loading frequency on cyclic behaviour of saturated sand

In practical engineering, sand fill or sediments subjected to cyclic loadings at varying frequencies due to earthquakes, traffic, and waves may cause complex soil responses. To investigate the effect of loading frequency on the cyclic behaviour of saturated sand, a series of undrained cyclic torsional shear tests covering a wide range of loading frequencies were conducted on Fujian sand. The excess pore water pressure (EPWP) in saturated sand can be divided into two components: residual and transient. Special focus was on the effect of loading frequency on the development of residual EPWP, transient EPWP, and shear strain amplitude. The key finding demonstrates that the loading frequency has a considerable effect on the association between the double amplitude of transient EPWP and shear strain amplitude. Based on the test data, a threshold strain is proposed that can be used to predict the onset of shear dilatancy in saturated sand. The liquefaction strength in the cyclic torsional shear tests was also investigated and was found to be significantly influenced by the loading frequency. The cyclic resistance ratio (CRR15) of saturated sand shows an increasing trend with the increase in loading frequency.

Experimental study on cyclic deformation properties of saturated Fujian sand at different loading frequencies

In this study, a series of undrained cyclic torsional shear tests were conducted to investigate the effect of cyclic loading frequency f on the pre-liquefaction (shearing contractive (SC) period and initial shearing dilative (ISD) period) and post-liquefaction (late shearing dilative (LSD) period) deformation properties of saturated Fujian sand. The secant shear modulus G and damping ratio λ in the entire cyclic loading process, and the unloading tangent shear modulus GL1 and flow deformation tangent shear modulus GL2 in the ISD and LSD periods were adopted to quantitatively characterize the evolution of hysteresis loop with an increase in shear strain amplitude γa. The test results show that the effect of f on G of saturated Fujian sand in the SC period is not apparent. However, all the G-γa, GL1-γa, and GL2-γa curves in the ISD and LSD periods showed a downward trend with an increase in f. This study also proposes a modified method for calculating λ to compensate for the analytical error caused by the non-closure of hysteresis loop. Compared with the classical curves that mainly applied in geotechnical engineering, the λ first increases and then decreases with the increase of γa. Furthermore, the λ evaluated by the modified method is approximately 10%–15% more than the λ evaluated by the traditional method when the λ reaches its peak value.

Experimental study on liquefaction characteristics of saturated marine coral sand based on dissipated energy evaluation

The liquefaction of saturated marine coral sand is of a great issue for the safety of ocean engineering. A series of undrained cyclic torsional shear tests on saturated coral sand are carried out to further explore the liquefaction characteristics of coral sand. Based on the energy method, the effects of relative density (D r), effective mean principal stress (), and cyclic stress ratio (CSR) on undrained responses of saturated coral sand are investigated. Two failure criteria are discussed to evaluate the dissipated energy required to cause initial liquefaction (W l). The test results show that the W l is significantly affected by D r and The generation patterns of excess pore water pressure (EPWP) and single amplitude torsional strain amplitude (γ SA) along with cumulative dissipated energy (W c) are described, and the unified generation model of both u e and γ SA along with W c are established. The liquefaction resistance curves for various D r and are strongly related to W l. Analyzing the cumulative dissipated energy required to trigger liquefaction, a modified stress level index termed CSR e is proposed to normalize the liquefaction resistance curves of saturated coral sand under different D r and The test data in literature also verify the applicability of CSR e.

Comparisons of cyclic response and reconsolidation volumetric strain of saturated coral sand and siliceous sand

Reported here are undrained cyclic torsional shear tests and reconsolidation tests carried out to compare the characteristics of the undrained cyclic response and reconsolidation volumetric strain (ε v) of saturated coral sand and siliceous sand (Nanjing fine sand and Ottawa sand). An important finding is that the cyclic resistance ratio of saturated coral sand increases with increasing relative density (D r) and decreases with increasing consolidation confining pressure () under isotropic consolidation. For given D r and the liquefaction resistance of saturated coral sand is higher than that of saturated siliceous sand. There is good correlation between ε v and the excess pore water pressure (u e) of sand before liquefaction. For a given consolidation stress with increasing maximum shear strain (γ max), ε v of saturated coral sand tends to stabilize at a value defined as the ultimate reconsolidation volumetric strain, which decreases with increasing D r. Also, ε v of saturated siliceous sand is less than that of saturated coral sand, and with increasing and D r, the measured ε v approaches the theoretical post-liquefaction maximum volumetric strain. Prediction models for ε v of saturated coral sand are developed based on the liquefaction level and γ max, respectively.

Evaluation of post-liquefaction volumetric strain of reconstituted samples based on soil compressibility

In this study, we performed undrained cyclic–torsional–shear tests on clean and silty sands to examine post-liquefaction volumetric strain. Accordingly, we proposed a novel density index—ultimate volumetric strain—to evaluate the density of clean sand and silty soils; the proposed density index is based on soil compressibility. We discussed the test results with respect to relative density, skeleton void ratio, and ultimate volumetric strain. Consequently, we confirmed that ultimate volumetric strain showed good correlation with post-liquefaction volumetric strain irrespective of soil type, whereas values for relative density and skeleton void ratio were discordant. Further, we observed that, compared to maximum shear strain, accumulated shear strain was a better index for representing the effect of loading history. From these results therefore, we proposed a chart to evaluate post-liquefaction volumetric strain based on ultimate volumetric strain and accumulated shear strain.

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...

Numerical analysis of LEAP centrifuge tests on sloping liquefiable ground: Influence of dilatancy and post-liquefaction shear deformation

The Liquefaction Experiments and Analysis Projects (LEAP) provides an international collaboration platform to assess key aspects related to seismic induced soil liquefaction. Within this scope, the current study investigates the influence of dilatancy and post-liquefaction shear strain on the seismic response of mildly sloping liquefiable ground, based on simulations of LEAP 2017–2019 centrifuge tests. A unified plasticity model for large post-liquefaction deformation, which has been implemented in the OpenSees finite element framework, is used in the numerical simulations. The model parameters are calibrated rigorously against element tests performed for Ottawa F65 sand used in LEAP 2017–2019, especially for liquefaction resistance and post-liquefaction shear strain observed in undrained cyclic hollow cylinder torsional shear tests. The calibrated model is used to simulate a series of centrifuge shaking table tests on sloping ground constructed at different soil densities. Good agreement between simulation and test results is achieved, validating the constitutive model and numerical simulation method. In depth analysis on the influence of dilatancy and post-liquefaction shear strain on the seismic response of the slope model is then conducted. The results show that these two factors can significantly affect the seismic acceleration, excess pore water pressure, and lateral displacement response of sloping ground, and therefore must be appropriately reflected in seismic liquefaction analysis oriented constitutive models.

Local deformation behaviour of saturated silica sand during undrained cyclic torsional shear tests using image analysis

A series of undrained cyclic torsional shear tests was conducted to investigate the development of local deformations of silica sand specimens directly using an image-based technique and a transparent membrane. The results for saturated sand specimens with varying densities showed that the vertical slippage of sand particles relative to the membrane was initiated at the liquefaction stage. Additionally, the quantity of vertical slippage of dense specimens was lower than that of loose specimens tested under the same conditions. Moreover, measured on the surface of the specimen, the large accumulated movement during cyclic shearing significantly increased the potential for inducing vertical slippage. A comparison of local deformation among the different tests revealed that the threshold of shear strain to generate non-uniform local deformations decreased with an increase in the relative density of the specimen. A lower liquefaction resistance layer, which formed at the upper part of the specimen after initial liquefaction, would accelerate the concentration of local shear strain; this was especially distinct when the specimen was completely liquefied.

Effect of stress direction on the cyclic resistance of sand

In total, 55 undrained cyclic torsional shear tests were conducted at a constant mean confining stress, σ0m, constant intermediate principal stress ratios, b = (σ2 − σ3)/(σ1 − σ3), and principal stress directions, α. The cyclic resistance of loose, isotropically consolidated specimens exhibited a dip in strength, with the lowest values for α being 45°, an angle at which α was oriented such that the potential shear band directions dovetailed with the plane having the weakest strength (i.e. horizontal plane). For a given α, the cyclic resistance decreased to the lowest values for b = 0·5, at which the value of the principal stresses was different in the three major directions. For dense specimens, the direction of the major principal stress had no significant influence on the cyclic resistance. For loose, anisotropically consolidated specimens, higher cyclic resistances were observed at the point of approaching the direction of the major principal stress during the consolidation and loading stages.

Influence of principal stress rotation of unequal tensile and compressive stress amplitudes on characteristics of soft clay

Soil behavior can reflect the characteristics of principal stress rotation under dynamic wave and traffic loads. Unequal amplitudes of tensile and compressive stresses applied to soils have complex effects on foundation soils in comparison with the pure principal stress rotation path. A series of undrained cyclic hollow torsional shear tests were performed on typical remolded soft clay from the Ilexi area of Nanjing, China. The main control parameters were the tensile and compressive stress amplitude ratio (α) and the cyclic dynamic stress ratio (η). It was found that the critical η tended to remain constant at 0.13, when the value of the compressive stress amplitude was higher than the tensile stress amplitude. However, the influence of the tensile stress was limited by the dynamic stress level when α= 1. For obvious structural change in the soil, the corresponding numbers of cyclic vibration cycles were found to be independent of α at low stress levels and were only related to η. Finally, a new method for evaluating the failure of remolded soft clay was presented. It considers the influence of the tensile and compressive stresses which caused by complex paths of the principal stress rotation. This criterion can distinguish stable, critical, and destructive states based on the pore-water-pressure-strain coupling curve while also providing a range of failure strain and vibration cycles. These results provide the theoretical support for systematic studies of principal stress rotation using constitu tive models.

On the influence of initial static shear on large deformation behavior of very loose Toyoura sand in undrained cyclic torsional shear tests

On the influence of initial static shear on large deformation behavior of very loose Toyoura sand in undrained cyclic torsional shear tests

A stress–strain description of saturated sand under undrained cyclic torsional shear loading

A constitutive model to describe the cyclic undrained behavior of saturated sand is presented. The increments in volumetric strain during undrained loading, which are equal to zero, are assumed to consist of increments due to dilatancy and increments due to consolidation/swelling. This assumption enables the proposed model to evaluate increments in volumetric strain due to dilatancy as mirror images of increments in volumetric strain due to consolidation/swelling, thus simulating the generation of excess pore water pressure (i.e., reduction in mean effective principal stress) during undrained cyclic shear loading. Based on the results of drained tests, the increments in volumetric strain due to consolidation/swelling are evaluated by assuming that the quasi-elastic bulk modulus can be expressed as a unique function of the mean effective principal stress. On the other hand, in evaluating the increments in volumetric strain due to dilatancy, a normalized stress–plastic shear strain relationship is employed in combination with a novel empirical stress–dilatancy relationship derived for torsional shear loading. The proposed stress–dilatancy relationship accounts for the effects of over-consolidation during cyclic loading. Numerical simulations show that the proposed model can satisfactorily simulate the generation of excess pore water pressure and the stress–strain relationship of saturated Toyoura sand specimens subjected to undrained cyclic torsional shear loading. It is found that the liquefaction resistance of loose Toyoura sand specimens can be accurately predicted by the model, while the liquefaction resistance of dense Toyoura sand specimens may be slightly underestimated. (i.e., the liquefaction potential is higher). Yet, the model predictions are conservative.

Relationship between limiting shear strain and reduction of shear moduli due to liquefaction in large strain torsional shear tests

With the spread of performance-based design concepts in geotechnical earthquake engineering, conducting a practical and accurate analysis for estimating the liquefaction-induced ground deformation has become important. However, there is a difficulty in setting relevant parameters of liquefied soil that would be employed in the analysis because experimental investigations on large deformation behaviour of liquefied soil are still limited. Therefore, in order to investigate the liquefaction-induced ground deformation characteristics, a series of undrained cyclic torsional shear tests was performed by using a modified torsional shear apparatus that is capable of achieving double amplitude shear strain up to about 100%. The tested materials were saturated Toyoura sand, two kinds of in-situ frozen samples having different geological ages and their reconstituted samples. The in-situ samples were retrieved from Pleistocene deposits. In all the undrained cyclic torsional shear tests, cyclic mobility was observed and the double amplitude shear strain approached 100%, irrespective of the different initial conditions of the specimens. A limiting value of double amplitude shear strain to cause strain localization, which would be linked to the maximum possible liquefaction-induced ground deformation, was evaluated based on the change in the deviator stress during the undrained cyclic torsional loading. The limiting value was found to increase with decrease in initial values of small strain shear moduli which were evaluated by dynamic measurement. In addition, we measured tangent shear moduli at the limiting state as well as after strain localization, and evaluated a reduction ratio of shear moduli due to liquefaction, which would be employed in the ALID framework. These characteristics measured by such large strain liquefaction tests would be useful in estimating the maximum liquefaction-induced ground deformation.

Strain localization characteristics of loose saturated Toyoura sand in undrained cyclic torsional shear tests with initial static shear

Strain localization, or the formation of shear bands, is a key aspect in understanding soil failure mechanisms. While efforts have been made in terms of measuring the shear band properties and the stress–strain behavior within shear bands, there are still uncertainties regarding when shear bands initiate and their influence on the development of large ground deformation. In this paper, the limiting value of shear strain, at which strain localization appears during undrained cyclic torsional shear tests with initial static shear, performed on loose Toyoura sand specimens (Dr=44–48%) up to a single amplitude of shear strain exceeding 50%, was evaluated. Non-uniform specimen deformation was observed at strain levels larger than 20%. However, the onset of strain localization could not be defined on the basis of visual observations. Therefore, the limiting values for half of the double amplitude (γDA/2) and single amplitude (γSA) shear strain, to initiate strain localization, were determined from test results based on changes in the deviator stress response and strain accumulation properties as well as changes in the strain-softening behavior during cyclic shear. It was found that γSA is a more appropriate parameter than γDA/2. Irrespective of the static shear stress level, the limiting strain value for γSA was evaluated to be in the range of 23–28% for liquefied loose Toyoura sand specimens (i.e., stress reversal and intermediate tests). Alternatively, the limiting strain value could not be properly defined when liquefaction did not occur (i.e., non-reversal stress tests), although various methods were employed.

Effects of initial static shear on liquefaction and large deformation properties of loose saturated Toyoura sand in undrained cyclic torsional shear tests

This study focused on the role which static shear plays on the large deformation behavior of loose saturated sand during undrained cyclic loading. A series of undrained cyclic torsional shear tests was performed on saturated Toyoura sand specimens up to single amplitude shear strain exceeding 50%. Three types of cyclic loading patterns, i.e., stress reversal, intermediate and non-reversal, were employed by varying the initial static shear level and the cyclic shear stress amplitude. The observed types of failure could be distinguished into liquefaction (cyclic and rapid flow) and residual deformation by comparing both monotonic and cyclic undrained behavior. It was found that the presence of initial static shear does not always lead to an increase in the resistance to liquefaction or strain accumulation; they could either increase or decrease with an increasing initial static shear level depending on the type of loading pattern and failure behavior. In addition, according to the failure behavior which the specimens exhibited, three modes of development of large residual deformation were observed.

Simulations of cyclic shearing of sand at low effective stress

This paper presents a very simple soil model of reduced dimensionality, that, using only two adjustable parameters, captures all the essential features of a set of six undrained cyclic torsional shear tests carried out on Toyoura sand by Oka and co-workers. The same values for all the model parameters (all but two of which are obtained from published data) are used for all six tests and very good agreement between the experimental test results and the simulations is achieved. Simulations of drained tests are also presented. The model uses a parameter that stores the recent strain history of the soil and that the authors show can be related to Houlsby's anisotropy parameter. The relationship between the volume and shear strain increments contains two terms. The first term simulates continual compaction while the second can predict additional compaction or dilatancy, depending on the direction of the shear strain increment. These two terms cancel each other out during continuous deformation at the critical state, but reinforce each other on reversal to predict the rapid densification observed experimentally.