Liquefaction Resistance of Sandy Soils under Partially Drained Condition

In order to simulate the effect of drainage on soils adjacent to gravel drains that are installed as countermeasure against liquefaction, several series of cyclic triaxial tests were performed on saturated sands under partially drained conditions. The condition of partial drainage under cyclic loading was simulated in the laboratory using triaxial testing equipment installed with a drainage control valve to precisely regulate the volume of water being drained from test specimens. Effects of both drainage conditions and loading frequencies on cyclic response were incorporated through the coefficient of drainage effect, α*. Experimental results showed that for sand exhibiting strain softening, the partially drained response was controlled by the critical effective stress ratio while for sand showing strain hardening behavior, the controlling factor was the phase transformation stress ratio. Moreover, test results indicated that the minimum liquefaction resistance under partially drained conditions can be used as a parameter to describe the liquefaction resistance of sands improved by the gravel drain method. From these results, a simplified procedure for designing gravel drains based on the factor of safety (FL) concept was proposed.     

Use of Artificial Neural Networks in the Prediction of Liquefaction Resistance of Sands

A backpropagation artificial neural network (ANN) model has been developed to predict the liquefaction cyclic resistance ratio (CRR) of sands using data from several laboratory studies involving undrained cyclic triaxial and cyclic simple shear testing. The model was verified using data that was not used for training as well as a set of independent data available from laboratory cyclic shear tests on another soil. The observed agreement between the predictions and the measured CRR values indicate that the model is capable of effectively capturing the liquefaction resistance of a number of sands under varying initial conditions. The predicted CRR values are mostly sensitive to the variations in relative density thus confirming the ability of the model to mimic the dominant dependence of liquefaction susceptibility on soil density already known from field and experimental observations. Although it is common to use mechanics-based approaches to understand fundamental soil response, the results clearly demonstrate that non-mechanistic ANN modeling also has a strong potential in the prediction of complex phenomena such as liquefaction resistance.     

Characterization of Liquefaction Susceptibility of Sands by Means of Extreme Void Ratios and/or Void Ratio Range

The liquefaction susceptibility of various graded fine to medium saturated sands are evaluated by stress controlled cyclic triaxial laboratory tests. Cyclic triaxial tests are performed on reconstituted specimens having global relative density of 60%. In all cyclic triaxial tests, loading pattern is selected as a sinusoidal wave form with 1.0 Hz frequency and effective consolidation pressure is chosen as 100 kPa. Liquefaction resistance is defined as the required cyclic stress ratio causing initial liquefaction in 10 cycles during the cyclic triaxial test. The results are used to draw conclusions on the effect of the extreme void ratios and void ratio range on the liquefaction resistance of various graded sands.     

Pore Pressure Generation of Silty Sands due to Induced Cyclic Shear Strains

It is well established that the main mechanism for the occurrence of liquefaction under seismic loading conditions is the generation of excess pore water pressure. Most previous research efforts have focused on clean sands, yet sand deposits with fines are more commonly found in nature. Previous laboratory liquefaction studies on the effect of fines on liquefaction susceptibility have not yet reached a consensus. This research presents an investigation on the effect of fines content on excess pore water pressure generation in sands and silty sands. Multiple series of strain-controlled cyclic direct simple shear tests were performed to directly measure the excess pore water pressure generation of sands and silty sands at different strain levels. The soil specimens were tested under three different categories: (1) at a constant relative density; (2) at a constant sand skeleton void ratio; and (3) at a constant overall void ratio. The findings from this study were used to develop insight into the behavior of silty sands under undrained cyclic loading conditions. In general, beneficial effects of the fines were observed in the form of a decrease in excess pore water pressure and an increase in the threshold strain. However, pore water pressure appears to increase when enough fines are present to create a sand skeleton void ratio greater than the maximum void ratio of the clean sand.     

Liquefaction and Undrained Response Evaluation of Sands from Drained Formulation

A general approach has been established to assess the undrained stress-strain curve and effective stress path under monotonic loading from drained triaxial tests. An appropriate formulation of a drained and drained rebounded (i.e., overconsolidated) triaxial test response is developed that, in turn, allows the assessment of developing liquefaction and the undrained behavior of saturated sands. The formulation presented is based upon reported experimental drained test results that were obtained from different investigators using different testing techniques. This formulation is a function of the confining pressure and basic properties of the sand, such as relative density, uniformity coefficient, and particle shape (roundness), which can be obtained from visual inspection. The approach is verified by comparing predicted and reported (observed) undrained behavior. The developed formulas allow one to predict the potential of sand to liquefy, the type of liquefaction, the peak and residual strength values, as well as the whole undrained stress-strain curve and effective stress path. The simplicity of this approach makes it an attractive general method to characterize the undrained behavior of sands in a preliminary analysis with no need to run sophisticated experimental tests.     

Undrained Shear Strength of Granular Soils with Different Particle Gradations

A series of undrained tests were performed on granular soils consisting of sand and gravel with different particle gradations and different relative densities reconstituted in laboratory. Despite large differences in grading, only a small difference was observed in undrained cyclic shear strength or liquefaction strength defined as the cyclic stress causing 5% double amplitude axial strain for specimens having the same relative density. In a good contrast, undrained monotonic shear strength defined at larger strains after undrained cyclic loading was at least eight times larger for well-graded soils than poorly graded sand despite the same relative density. This indicates that devastating failures with large postliquefaction soil strain are less likely to develop in well-graded granular soils compared to poorly graded sands with the same relative density, although they are almost equally liquefiable. However, if gravelly particles of well-graded materials are crushable such as decomposed granite soils, undrained monotonic strengths are considerably small and almost identical to or lower than that of poorly graded sands.     

Monotonic and Cyclic Liquefaction of Very Loose Sands with High Silt Content

The results from an experimental study on sands with high nonplastic silt content are presented. Drained and undrained triaxial compression tests, undrained cyclic triaxial tests, and drained∕undrained instability tests were performed on specimens of loose Nevada sand with 40% silt content. The behavior was observed to be somewhat different from previously published tests with sands at lower silt content. The greater silt content appears to provide a more volumetrically contractive response throughout the entire stress-strain curve. However, some aspects of the response were similar to sands with lower silt content. Monotonic undrained tests indicated “reverse” behavior, i.e., static liquefaction occurred at low confining pressures and increasing dilatant volume-change tendency was observed with increasing confining pressure. Analyzing the results using the concepts of steady state resulted in a unique steady-state line only when undrained tests were sheared from the same isotropic compression line. When specimens of different initial densities were tested at the same initial confining pressures, the resulting steady-state points did not fall on the same steady-state line.     

Liquefaction Resistance of Clean and Nonplastic Silty Sands Based on Cone Penetration Resistance

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy in the assessment of the likelihood of liquefaction at a site affects the safety and economy of the design. In this paper, curves of cyclic resistance ratio (CRR) versus cone penetration test (CPT) stress-normalized cone resistance qc1 are developed from a combination of analysis and laboratory testing. The approach consists of two steps: (1) determination of the CRR as a function of relative density from cyclic triaxial tests performed on samples isotropically consolidated to 100 kPa; and (2) estimation of the stress-normalized cone resistance qc1 for the relative densities at which the soil liquefaction tests were performed. A well-tested penetration resistance analysis based on cavity expansion analysis was used to calculate qc1 for the various soil densities. A set of 64 cyclic triaxial tests were performed on specimens of Ottawa sand with nonplastic silt content in the range of 0–15% by weight, and relative densities from loose to dense for each gradation, to establish the relationship of the CRR to the soil state and fines content. The resulting (CRR)7.5-qc1 relationship for clean sand is consistent with widely accepted empirical relationships. The (CRR)7.5-qc1 relationships for the silty sands depend on the relative effect of silt content on the CRR and qc1. It is shown that the cone resistance increases at a higher rate with increasing silt content than does liquefaction resistance, shifting the (CRR)7.5-qc1 curves to the right. The (CRR)7.5-qc1 curves proposed for both clean and silty sands are consistent with field observations.     

Effects of Nonplastic Fines on the Liquefaction Resistance of Sands

A laboratory parametric study utilizing cyclic triaxial tests was performed to clarify the effects of nonplastic fines on the liquefaction susceptibility of sands. Studies previously published in the literature have reported what appear to be conflicting results as to the effects of silt content on the liquefaction susceptibility of sandy soils. The current study has shown that if the soil structure is composed of silt particles contained within a sand matrix, the resistance to liquefaction of the soil is controlled by the relative density of the soil and is independent of the silt content of the soil. For soils whose structure is composed of sand particles suspended within a silt matrix, the resistance to liquefaction is again controlled by the relative density of the soil, but is lower than for soils with sand-dominated matrices at similar relative densities. In this case, the resistance to liquefaction is essentially independent of the amount and type of sand. These findings suggest the need for further evaluation of the effects of nonplastic fines content upon penetration resistance, and the manner in which this relationship affects the simplified methods currently used in engineering practice to evaluate the liquefaction resistance of silty soils.     

Lateral Loaded Pile Response in Liquefiable Soil

This paper provides a new analysis procedure for assessing the lateral response of an isolated pile in saturated sands as liquefaction develops in response to dynamic loading such as that generated during earthquake shaking. This new procedure predicts the degradation in pile response and soil resistance due to the free-field excess porewater pressure generated by the earthquake, along with the near-field excess porewater pressure generated by lateral loading from the superstructure. The new procedure involves the integration of the developing (free- and near-field) porewater pressure in the strain wedge (SW) model analysis. The current SW model, developed to evaluate drained response (a nonlinear three-dimensional model) of a flexible pile in soil, has been extended in this paper to incorporate the undrained response of a laterally loaded pile in liquefied sand. This new procedure has the capability of predicting the response of a laterally loaded isolated pile and the associated modulus of subgrade reaction (i.e., the p–y curve) in a mobilized fashion as a result of developing liquefaction in the sand. Current design procedures assume slight or no resistance for the lateral movement of the pile in the liquefied soil which is a conservative practice. Alternatively, if liquefaction is assessed not to occur, some practitioners take no account of the increased free-field porewater pressure, and none consider the additional near-field porewater pressure due to inertial interaction loading from the superstructure; a practice that is unsafe in loose sands.     

Assessment of the Undrained Response of Sands under Limited and Complete Liquefaction

The technique presented deals with the assessment, based on drained test behavior and formulation, of the undrained postcyclic stress-strain behavior of sands under limited or complete (full) liquefaction and its associated strength. At present, there is no particular procedure that allows assessment of such undrained postcyclic behavior that could develop full (pore-water pressure ratio, ru = 1) or limited (ru<1) liquefaction. The prediction of the undrained postliquefaction (full or limited liquefaction) response presented here is based on basic properties of sand such as its relative density (Drc) [or (N1)60 blowcount], the effective angle of internal friction (φ), the roundness of the sand grains (ρ), and the drained axial strain at 50% stress level (ε50). The technique presented accounts for the excess pore-water pressure induced by cyclic loading (Δuc) and the postcyclic excess pore-water pressure generated under undrained monotonic loading (Δud).     

Use of = 0 as a Failure Criterion for Weakly Cemented Soils

There is considerable uncertainty in the determination of effective stress strength parameters of cemented soils from undrained triaxial tests. Large negative excess pore pressures are generated at relatively large strains (typically 4–5% for cemented silty sand) in isotropically consolidated undrained (CIU) tests, which results in gas coming out of solution during shear and significant variability in the measured peak deviator stress. In this study, different failure criteria for weakly cemented sands were evaluated based on the results of CIU and isotropically consolidated drained triaxial compression tests conducted on samples of artificially cemented sand. The use of = 0 as a failure criterion eliminates the variability between the undrained tests and also ensures that the mobilized failure strength is not based on the highly variable negative excess pore pressures. In addition, the resulting strains to failure are comparable to the strains to failure for the drained tests. Mohr-Coulomb strength parameters thus estimated from the undrained tests are generally lower than strength parameters obtained from drained tests, and the difference between the failure envelopes from undrained tests increases as the level of cementation increases. This divergence is attributed to differences in the stiffness of the cemented soil under the different loading conditions. The stiffness under undrained loading conditions decreases with increasing cementation due to an increase in the generation of positive excess pore pressure at low strains.     

Degree of Saturation and Liquefaction Resistances of Sand Improved with Sand Compaction Pile

Sand compaction pile (SCP) is a ground improvement technique extensively used to ameliorate liquefaction resistance of loose sand deposits. This paper discusses results of laboratory tests on high-quality undisturbed samples obtained by the in situ freezing method at six sites where foundation soils had been improved with SCP. Inspection of samples revealed that the improved ground was desaturated during the ground improvement. Degree of saturation (Sr) was lower than 77% for the sand piles and 91% for the improved sand layers, while Sr was approximately 100% for improved clayey and silty soils. A good correlation was found between Sr and 5% diameter of the soil; the larger 5% diameter of soils (D5), the lower the degree of saturation. It appeared that the variation of Sr with D5 for soils within a month after the ground improvement work was quite similar in trend to that after more than several years. Degree of saturation of soils after several years was noticeably, but not significantly, higher as compared with that shortly after ground improvement, indicating longevity of air bubbles injected in the improved soil. Undrained cyclic shear tests were also carried out on saturated and unsaturated specimens and effects of desaturation on undrained cyclic shear strength were studied. The test results were summarized in a form of liquefaction resistance with reference to normalized standard penetration test N-value.     

Verification of the Soil-Type Specific Correlation between Liquefaction Resistance and Shear-Wave Velocity of Sand by Dynamic Centrifuge Test

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy of the liquefaction potential assessment at a site affects the safety and economy of an engineering project. Although shear-wave velocity (Vs)-based methods have become prevailing, very few works have addressed the problem of the reliability of various relationships between liquefaction resistance (CRR) and Vs used in practices. In this paper, both cyclic triaxial and dynamic centrifuge model tests were performed on saturated Silica sand No. 8 with Vs measurements using bender elements to investigate the reliability of the CRR-Vs1 correlation previously proposed by the authors. The test results show that the semiempirical CRR-Vs1 curve derived from laboratory liquefaction test of Silica sand No. 8 can accurately classify the (CRR,Vs1) database produced by dynamic centrifuge test of the same sand, while other existing correlations based on various sandy soils will significantly under or overestimate the cyclic resistance of this sand. This study verifies that CRR-Vs1 curve for liquefaction assessment is strongly soil-type dependent, and it is necessary to develop site-specific liquefaction resistance curves from laboratory cyclic tests for engineering practices.     

Elastoplastic Model for the Long-Term Behavior Modeling of Unbound Granular Materials in Flexible Pavements

The constitutive modeling of cyclic plasticity of soils has made great progress, especially in the area of sands liquefaction modeling. Nowadays, the problem of rutting of flexible pavements linked to permanent deformations occurring in the unbound layers is taken into account only by empirical formulas. This paper presents an elastoplastic model with both isotropic and kinematic hardening. The yield surface, plastic potential, and isotropic hardening are based on a model for sands, which takes into account the influence of the initial void ratio and of the mean stress on the mechanical behavior. A kinematic hardening has been added in order to take into account the mechanical behavior of the material for large cycle numbers. A complete model is then developed, simulations are presented, and comparisons with repeated load triaxial tests carried out on a subgrade soil (clayey sand), have been made. These comparisons underline the capabilities of the model to take into account the monotonic, cyclic, and ratchetting behavior of unbound materials for roads.     

Design of Foundations on Sensitive Champlain Clay Subjected to Cyclic Loading

Sensitive clay subjected to cyclic loading may experience gradual loss of its shear strength, which may lead to liquefaction. Foundations built on this clay would suffer extensive settlement and significant loss of bearing capacity or perhaps catastrophic failure. This paper presents an experimental investigation on sensitive (Champlain) clay obtained from the city of Rigaud, Quebec (Canada). Consolidation tests, static and cyclic undrained and drained triaxial tests were performed on representative samples of this clay. The objective of this investigation was to examine the influence of the physical and mechanical parameters, which govern the shear strength of sensitive clay subjected to cyclic loading. Based on the results of the present investigation and those available in the literature, it can be reported herein that the undrained response is the most critical for these foundations; furthermore, the preconsolidation pressure is considered as an important parameter in establishing the shear strength of sensitive clay. A design procedure is developed to determine the safe zone for the undrained and drained responses, within which a combination of the cyclic deviator stress and the number of cycles for a given soil/loading/site conditions can achieve a quasielastic resilient state without reaching failure. The proposed design procedure is applicable to all regions around the world, where sensitive clays can be found. Furthermore, this procedure can be adopted to examine the conditions of existing foundations built on sensitive clay at any time during its lifespan.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

Liquefaction Testing of Stratified Silty Sands

The cyclic behavior of stratified silty sandy soils is at present poorly understood, yet these materials are commonly found in alluvial deposits and hydraulic fill, which have a history of liquefaction during earthquakes. The main objective of this research project was to compare the behavior of stratified and homogeneous silty sands during seismic liquefaction conditions for various silt contents and confining pressures. A comprehensive experimental program was undertaken in which a total of 150 stress-controlled undrained cyclic triaxial tests were performed. Two methods of sample preparation were used for each soil type. These methods included moist tamping (representing uniform soil conditions) and sedimentation (representing layered soil conditions). The silt contents ranged from 10 to 50%, and confining pressures in the range of 50 to 250 KPa were considered. The results indicated that the liquefaction resistances of layered and uniform soils are not significantly different, despite the fact that the soil fabric produced by the two methods of sample preparation is totally different. The findings of this study justify applying the laboratory test results to the field conditions for the range of variables studied.     

Pressure-Level Dependency and Densification Behavior of Sand Through Generalized Plasticity Model

Natural soil deposits and man-made earth structures exhibit complicated engineering behavior that is influenced by factors such as the stress level and drainage conditions. The stress conditions within a soil structure vary greatly, ranging from very low to very high values, due to the dead weight, loading and boundary conditions. Saturated sand deposits that exhibit drained conditions under static loading become undrained when subject to earthquake excitations. The Pastor–Zienkiewicz–Chan model has demonstrated considerable success in describing the inelastic behavior of soils under isotropic monotonic and cyclic loadings, including liquefaction and cyclic mobility. This study proposed modifications to the Pastor–Zienkiewicz–Chan model so that effects of stress level and densification behavior are simulated. The proposed model suggested that the angle of internal friction, elastic and plastic moduli are dependent on the pressure levels. Relevant modifications were made to incorporate a power term of mean effective stress on the loading plastic modulus so that a stress-level dependent volume change is obtained in combination with the stress-dilatancy relationship. To better simulate cyclic loading with reference to densification behavior, an exponential term of plastic volumetric strain is included for the unloading and reloading plastic moduli. A total of 11 parameters are needed for monotonic loading, whereas 15 parameters are needed in describing the cyclic behavior. The model simulations were compared with undrained and drained triaxial test results of several kinds of sand under dense and loose states. The predictive capability for monotonic and cyclic loading conditions was also demonstrated.     

How Reliable is the Plasticity Index for Estimating the Liquefaction Potential of Clayey Sands?

This paper examines the validity of the plasticity index (PI) as a criterion for estimating the liquefaction potential of clayey soils under cyclic loading. The results of undrained cyclic stress-controlled ring-shear tests on artificial mixtures of sand with different clays saturated with water indicated that an increase in PI decreased the soil potential to liquefy, and soil with PI>15 seemed to be nonliquefiable, a finding that is in agreement with the results of other researchers. However, in this study some deviations from this relation were found when a bentonite–sand mixture was treated with solutions of different ions, thus bringing into question the effectiveness of PI as a measure of the liquefaction potential of clayey soil having a certain pore water chemistry.