Evaluation of Cyclic Softening in Silts and Clays

Procedures are presented for evaluating the potential for cyclic softening (i.e., onset of significant strains or strength loss) in saturated silts and clays during earthquakes. The recommended procedures are applicable for fine-grained soils with sufficient plasticity that they would be characterized as behaving more fundamentally like clays in undrained monotonic or cyclic loading. The procedures are presented in a form that is similar to that used in semiempirical liquefaction procedures. Expressions are developed for a static shear stress correction factor and a magnitude scaling factor. Guidelines and empirical relations are presented for determining cyclic resistance ratios based on different approaches to characterizing fine-grained soil deposits. The potential consequences of cyclic softening, and the major variables affecting such consequences, are discussed. Application of these procedures is demonstrated through the analysis of the Carrefour Shopping Center case history from the 1999 Kocaeli earthquake. The proposed procedures, in conjunction with associated liquefaction susceptibility criteria, provide an improved means for distinguishing between the conditions that do and those that do not lead to ground deformations in fine-grained soil deposits during earthquakes.     

Subsurface Characterization at Ground Failure Sites in Adapazari, Turkey

Ground failure in Adapazari, Turkey during the 1999 Kocaeli earthquake was severe. Hundreds of structures settled, slid, tilted, and collapsed due in part to liquefaction and ground softening. Ground failure was more severe adjacent to and under buildings. The soils that led to severe building damage were generally low plasticity silts. In this paper, the results of a comprehensive investigation of the soils of Adapazari, which included cone penetration test (CPT) profiles followed by borings with standard penetration tests (SPTs) and soil index tests, are presented. The effects of subsurface conditions on the occurrence of ground failure and its resulting effect on building performance are explored through representative case histories. CPT- and SPT-based liquefaction triggering procedures adequately identified soils that liquefied if the clay-size criterion of the Chinese criteria was disregarded. The CPT was able to identify thin seams of loose liquefiable silt, and the SPT (with retrieved samples) allowed for reliable evaluation of the liquefaction susceptibility of fine-grained soils. A well-documented database of in situ and index testing is now available for incorporating in future CPT- and SPT-based liquefaction triggering correlations.     

Cyclic Behavior of Fine-Grained Soils at Different pH Values

The effects of pH on the liquefaction susceptibility of fine-grained soils were examined by performing undrained cyclic ring-shear tests on artificial mixtures and a natural soil under different pH conditions. Solutions of diluted sulphuric acid (H2SO4) and dissolved sodium hydroxide (NaOH) were used to create acidic and alkaline environments, respectively, while distilled water was used as a reference liquid. Low plasticity kaolin and illite-sand mixtures and a medium plasticity bentonite-sand mixture were selected to investigate the influence of plasticity and clay mineralogy on the pH-dependent response of soil to cyclic loading. The results showed that the effects of pH were more pronounced for the medium plasticity mixture, and depended greatly on the mineralogy of clay fraction. For example, in an acidic medium, the kaolin-sand mixture became slightly more resistant to liquefaction while the illite-sand mixture became more susceptible to liquefaction. The bentonite-sand mixture was observed to be the most sensitive to changes in pH environment. While resistant to liquefaction in distilled water, it rapidly liquefied in acidic and alkaline mediums. Cyclic behavior of a medium plasticity soil, which was collected from an earthquake-induced landslide, was also affected by changes in pH. Although being overall resistant to liquefaction regardless of pH, it decreased its cyclic strength in both acidic and alkaline environments. Based on the available literature and the obtained results, an attempt was made to explain the influence of pH on the undrained cyclic behavior of fine-grained soils.     

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.     

Zero-Displacement Lateral Spreads, 1999 Kocaeli, Turkey, Earthquake

Three potential lateral spreads exhibited negligible displacements during the 1999 Kocaeli, Turkey Earthquake (Mw = 7.5) even though they were located within 7?km of the fault rupture. These spreads are analyzed to verify and augment current procedures for predicting liquefaction resistance and lateral spread displacement. The sites include ?ark Canal and Cumhuriyet Avenue in Adapazari, underlain by fine-grained sediment, and Degirmendere Nose adjacent to Izmit Bay, a steeply sloping area underlain by moderately dense silty sand. The plasticity index and moisture content criteria of Bray and Sancio set forth in 2006 indicate that much of the fine-grained sediment is liquefiable. Even though liquefaction likely occurred, lateral spreading did not occur due either to the dilative nature of fine-grained, sandlike sediments or the inherent strength of claylike sediments. Corrected blow counts, (N1)60, in moderately dense sand at Degirmendere Nose range from 15 to 25 blows/30?cm, indicating that liquefaction should have occurred but that the silty sand was too dense and dilative to deform. This finding is consistent with the MLR procedure of Youd et al. set forth in 2002 that identifies liquefiable sands with (N1)60 greater than 15 blows/30?cm as resistant to lateral spread during earthquakes with M<8.     

Liquefaction Susceptibility Criteria for Silts and Clays

New liquefaction susceptibility criteria for saturated silts and clays are presented that are based on the mechanics of their stress-strain behavior and which provide improved guidance for selecting engineering procedures for estimating potential strains and strength loss during seismic loading. Monotonic and cyclic undrained loading test data for silts and clays show that they transition, over a fairly narrow range of plasticity indices (PI), from soils that behave more fundamentally like sands (sand-like behavior) to soils that behave more fundamentally like clays (clay-like behavior), with the distinction having a direct correspondence to the type of engineering procedures that are best suited to evaluating their seismic behavior. It is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine-grained soils exhibiting sand-like behavior, whereas the term cyclic softening failure be used to describe similar phenomena in fine-grained soils exhibiting clay-like behavior. For practical purposes, clay-like behavior can be expected for fine-grained soils that have PI ≥ 7, although a slightly lower transition point for soils with a CL-ML classification (perhaps PI ≥ 5 or 6) would be equally consistent with the available data. Issues related to the practical application of these criteria are discussed.     

Cyclic Softening of Low-Plasticity Clay and Its Effect on Seismic Foundation Performance

During the 1999 Chi-Chi Earthquake (Mw = 7.6), significant incidents of ground failure occurred in Wufeng, Taiwan, which experienced peak accelerations ～ 0.7?g. This paper describes the results of field investigations and analyses of a small region within Wufeng along an E–W trending line 350?m long. The east end of the line has single-story structures for which there was no evidence of ground failure. The west end of the line had three to six-story reinforced concrete structures that underwent differential settlement and foundation bearing failures. No ground failure was observed in the free field. Surficial soils consist of low-plasticity silty clays that extend to 8–12?m depth in the damaged area (west side), and 3–10?m depth in the undamaged area (east side). A significant fraction of the foundation soils at the site are liquefaction susceptible based on several recently proposed criteria, but the site performance cannot be explained by analysis in existing liquefaction frameworks. Accordingly, an alternative approach is used that accounts for the clayey nature of the foundation soils. Field and laboratory tests are used to evaluate the monotonic and cyclic shear resistance of the soil, which is compared to the cyclic demand placed on the soil by ground response and soil–structure interaction. Results of the analysis indicate a potential for cyclic softening and associated strength loss in foundation soils below the six-story buildings, which contributes to bearing capacity failures at the edges of the foundation. Similar analyses indicate high factors of safety in foundation soils below one-story buildings as well in the free field, which is consistent with the observed field performance.     

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.     

Liquefaction Resistance of Soils from Shear-Wave Velocity

A simplified procedure using shear-wave velocity measurements for evaluating the liquefaction resistance of soils is presented. The procedure was developed in cooperation with industry, researchers, and practitioners and evolved from workshops in 1996 and 1998. It follows the general format of the Seed-Idriss simplified procedure based on standard penetration test blow count and was developed using case history data from 26 earthquakes and >70 measurement sites in soils ranging from fine sand to sandy gravel with cobbles to profiles including silty clay layers. Liquefaction resistance curves were established by applying a modified relationship between the shear-wave velocity and cyclic stress ratio for the constant average cyclic shear strain suggested by R. Dobry. These curves correctly predicted moderate to high liquefaction potential for >95% of the liquefaction case histories and are shown to be consistent with the standard penetration test based curves in sandy soils. A case study is provided to illustrate application of the procedure. Additional data are needed, particularly from denser soil deposits shaken by stronger ground motions, to further validate the simplified procedure.     

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.     

Evaluation of Flow Liquefaction and Liquefied Strength Using the Cone Penetration Test

Flow liquefaction is a major design issue for large soil structures such as mine tailings impoundments and earth dams. If a soil is strain softening in undrained shear and, hence, susceptible to flow liquefaction, an estimate of the resulting liquefied shear strength is required for stability analyses. Many procedures have been published for estimating the residual or liquefied shear strength of cohesionless soils. This paper presents cone penetration test-based relationships to evaluate the susceptibility to strength loss and liquefied shear strength for a wide range of soils. Case-history analyses by a number of investigators are reviewed and used with some additional case histories. Extrapolations beyond the case-history data are guided by laboratory studies and theory.     

Accounting for Soil Aging When Assessing Liquefaction Potential

It has been recognized that liquefaction resistance of sand increases with age due to processes such as cementation at particle contacts and increasing frictional resistance resulting from particle rearrangement and interlocking. As such, the currently available empirical correlations derived from liquefaction of young Holocene sand deposits, and used to determine liquefaction resistance of sand deposits from in situ soil indices [standard penetration test (SPT), cone penetration test (CPT), shear wave velocity test (Vs)], are not applicable for old sand deposits. To overcome this limitation, a methodology was developed to account for the effect of aging on the liquefaction resistance of old sand deposits. The methodology is based upon the currently existing empirical boundary curves for Holocene age soils and utilizes correction factors presented in the literature that comprise the effect of aging on the in situ soil indices as well as on the field cyclic strength (CRR). This paper describes how to combine currently recorded SPT, CPT, and Vs values with corresponding CRR values derived for aged soil deposits to generate new empirical boundary curves for aged soils. The method is illustrated using existing geotechnical data from four sites in the South Carolina Coastal Plain (SCCP) where sand boils associated with prehistoric earthquakes have been found. These sites involve sand deposits that are 200,000?to?450,000?years in age. This work shows that accounting for aging of soils in the SCCP yields less conservative results regarding the current liquefaction potential than when age is not considered. The modified boundary curves indicate that old sand deposits are more resistant to liquefaction than indicated by the existing empirical curves and can be used to evaluate the liquefaction potential at a specific site directly from the current in situ properties of the soil.     

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.     

Influence of Amorphous Clay-Size Materials on Soil Plasticity and Shrink-Swell Behavior

The influence of amorphous clay-size materials on geotechnical engineering properties is recognized only for soils developed from volcanic ash under extremely wet, alumina-rich soil environments (called Andisols). The objective of this study was to quantify the amorphous clay-size materials in less weathered volcanic soils that are rich in silica, and to determine the influence of the amorphous materials on plasticity and shrink-swell behavior of these soils. Soil and weathered rock samples were taken from a slow-moving landslide site in Honolulu. Quantification of amorphous and crystalline clay content was performed with x-ray diffraction and the Rietveld method. Atterberg limits and shrink-swell potential of the soil samples were determined. The results showed that clay-size fraction in both soil and weathered rock samples were predominantly amorphous (55–74% in soil and 48–63% in weathered rock). Smectite and halloysite were the primary crystalline clay minerals, constituting about 15–30% of the clay fraction in soils. Atterberg limits of the soil ranged from 65 to 135 for liquid limit, from 30 to 40 for plastic limit, and 9 to 25 for shrinkage limit. Volumetric free swell ranged from 2 to 21%. The plasticity and shrink-swell potential increased with increasing the content of amorphous clay-size materials in the soil. Air drying and oven drying did not significantly change the plasticity. The study concluded that silica-rich amorphous materials dominate the clay mineralogy of the soils studied, resulting in the plasticity and shrink-swell behavior similar to that of smectite-rich soils and distinct from that of Andisols.     

Evaluation of Liquefaction Using Disturbed State and Energy Approaches

The disturbed state concept (DSC) and the dissipated energy approach can provide simplified, fundamental, and mechanistic methods for the identification of the initiation and growth of liquefaction in saturated soils under cyclic and earthquake loading. Both approaches are developed and used for the analysis of liquefaction in the soil deposits at Port Island, Kobe, Japan, during the Hyogo-ken Nanbu earthquake. They are also used to analyze liquefaction of two sands during laboratory cyclic tests using torsional and multiaxial devices. It is shown that the DSC and energy criteria can lead to improved understanding of the mechanism of liquefaction, and to rational and simplified procedures compared to those based on empirical and index properties. Furthermore, the DSC possesses certain advantages over the energy approaches, particularly in terms of its implementation in computer (finite-element) programs for dynamic and liquefaction analysis.     

Dynamic Response of Compacted CG, DM, and CG-DM Blends

The cyclic behavior of 9.5 mm (3/8 in.) minus curbside-collected crushed glass (CG) blended with dredged material (DM), classified as an organic silt by the Unified Soil Classification System, was evaluated using a cyclic triaxial testing program. Tests were performed on 100% CG and 100% DM specimens, and 20/80, 40/60, 60/40, and 80/20 CG-DM blends (dry CG content is reported first). The specimens were compacted to a dry unit weight equivalent to 95% of the maximum dry density based on ASTM D1557. For each material, a minimum of three specimens was tested at cyclic stress ratios of 0.20, 0.35, and 0.45. The DM used in this study exhibited significant plasticity, which would be expected to display cyclic softening behavior according to liquefaction susceptibility criteria proposed by Boulanger and Idriss in 2006. However, the high density of the material resulted in transitional behavior between cyclic mobility and cyclic softening. These findings suggest that as long as the CG, DM, and CG-DM blends are compacted, they should not be susceptible to strength loss or large strain under cyclic loading.     

Correlation between Cyclic Resistance and Shear-Wave Velocity for Providence Silts

As an alternative to a field-based liquefaction resistance approach, cyclic triaxial tests with bender elements were used to develop a new correlation between cyclic resistance ratio (CRR) and overburden stress-corrected shear-wave velocity (VS1) for two nonplastic silts obtained from Providence, Rhode Island. Samples of natural nonplastic silt were recovered by block sampling and from geotechnical borings/split-spoon sampling. The data show that the correlation is independent of the soils’ stress history as well as the method used to prepare the silt for cyclic testing. The laboratory results indicate that using the existing field-based CRR-VS1 correlations will significantly overestimate the cyclic resistance of the Providence silts. The strong dependency of the CRR-VS1 curves on soil type also suggests the necessity of developing silt-specific liquefaction resistance curves from laboratory cyclic tests performed on reconstituted samples.     

Laboratory Investigation on Assessing Liquefaction Resistance of Sandy Soils by Shear Wave Velocity

Shear wave velocity (Vs) offers engineers a promising alternative tool to evaluate liquefaction resistance of sandy soils, and the lack of sufficient in-situ databases makes controlled laboratory study very important. In this study, semitheoretical considerations were first given based on review of previous liquefaction studies, which predicted a possible relationship between laboratory cyclic resistance ratio (CRRtx) and Vs normalized with respect to the minimum void ratio, confining stress and exponent n of Hardin equation. Undrained cyclic triaxial tests were then performed on three reconstituted sands with Vs measured by bender elements, which verified this soil-type-dependent relationship. Further investigation on similar laboratory studies resulted in a database of 291 sets of data from 34 types of sandy soils, based on which the correlation between liquefaction resistance and Vs was established statistically and further converted to equivalent field conditions with well-defined parameters, revealing that CRR will vary proportionally with (Vs1)4. Detailed comparisons with Vs-based site-specific investigations show that the present lower-bound CRR–Vs1 curve is a reliable prediction especially for sites with higher CSR or Vs1. The framework of liquefaction assessment based on the present laboratory study is proposed for engineering practice.     

Model Tests on Discontinuities in Subsurface Barrier Installed by the Vibrating Beam Method

In the present note, the susceptibility to necking of a subsurface barrier installed by the vibrating beam method in fine-grained soils is investigated by physical model tests. Two soils are investigated, a clayey soil and a silty soil. The model soil is prepared by moist tamping. The recipe of slurry in the model tests is widely used in the foundation engineering industry. The dynamic loading exerted by the vibration of the adjacent panel is simulated by a shake table. The test results show that soil plasticity and water content are the major influence factors on the susceptibility to necking. The plasticity index can be used as an indicator for the susceptibility to necking of subsurface barrier installed by the vibrating beam method in fine-grained soils. Other influence factors on necking are also investigated and their implications for practice are discussed.     

Liquefaction, Cyclic Mobility, and Failure of Silt

It is known that the mechanical properties of low-plasticity silt are similar to those of sand, and yet silts are frequently used as coastal reclamation materials in many cities and industrial areas and will thus be susceptible to liquefaction. Samples of a low-plasticity silt have been tested under monotonic and cyclic loading under isotropic and anisotropic stress conditions to characterize liquefaction, cyclic failure, and to develop an empirical model describing its cyclic strength. A sedimentation technique produced samples that had the highest susceptibility to liquefaction. Contractive behavior of monotonically loaded samples was triggered when the stress path reached an initial phase transformation (IPT) in both compression and extension tests. The samples became dilative after reaching a phase transformation (PT) point. The cyclic shear behavior of the silt samples prepared using the sedimentation method and consolidated at various initial sustained deviator stress ratios was examined in terms of two different failure criteria: a double amplitude axial strain εa,DA = 5% for reversal conditions; or axial plastic strain εa,P = 5% for nonreversal. For isotropically consolidated samples the initial phase transformation determined from undrained monotonic extension tests was the boundary between stable and contractive behavior. For anisotropically consolidated samples failure was defined by a bounding surface formed by undrained monotonic compression tests. An empirical model was developed relating the number of cycles to failure under conditions of both liquefaction and cyclic mobility to the initial anisotropic sustained deviator stress and cyclic deviator stress ratio.