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.     

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.     

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.     

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.     

Influence of Nonplastic Fines on Shear Wave Velocity-Based Assessment of Liquefaction

Many false positives (no liquefaction detected when the normalized shear wave velocity-cyclic stress ratio (Vs1-CSR) combination indicated that it should have been) are observed in the database used in the simplified liquefaction assessment procedure based on shear wave velocity. Two possible reasons for false positives are the presence of a thick surface layer of nonliquefiable soil and the effects of fines on cyclic shear resistance (CRR) and Vs1. About 67% of the false positives that could not have been caused by an overlying thick surface layer are associated with silty sands with less than 35% fines. The effects of fines on the liquefaction resistance of silty sands and on the shear wave velocity are analyzed. Theoretical CRRfield?versus?Vs1 curves for silty sands containing 0 to 15% nonplastic fines are established. They show that the theoretical CRR-Vs1 correlations for silty sands with 5 to 15% nonplastic fines are all located to the far left of the semi-empirical curves that separate liquefaction from no-liquefaction zones in the simplified liquefaction potential assessment procedures. The results suggest the currently used shear wave velocity-based liquefaction potential curves may be overly conservative when applied to sands containing nonplastic fines.     

Discriminant Model for Evaluating Soil Liquefaction Potential Using Cone Penetration Test Data

Statistical analysis using a discriminant model is applied to 399 cone penetration test (CPT) data sets of both liquefaction and nonliquefaction cases, including 174 sets from the Chi-Chi earthquake in Taiwan and 225 sets of synthesized data. The discriminant model employed is a multivariate statistical method. In situ testing results of cone tip resistance qc and sleeve friction ratio Rf are adopted as the major parameters for analyses. A model for evaluating liquefaction potential using CPT-qc data is also established in this study, which allows calculated results to be compared with the empirical curves.     

CPT-Based Evaluation of Liquefaction and Lateral Spreading in Centrifuge

Two series of centrifuge model tests were conducted using Nevada sand. Four saturated models placed in a mildly inclined laminar box and simulating a 6-m-thick deposit were shaken inducing liquefaction effects and lateral spreading. The sand was deposited at a relative density, Dr = 45 or 75%; two of the 45% models were subjected to overconsolidation or preshaking. The second series involved in-flight measurements of static cone tip penetration resistance, qc, simulating the standard cone penetration test (CPT) 36-mm cone. Values of qc increased with Dr, overconsolidation, and preshaking. A normalized resistance, qc1N, was assigned to each of the four liquefaction/lateral spreading models. Increases in Dr, overconsolidation, and preshaking decreased liquefaction and ground deformation, but relative density alone captured these effects rather poorly. Conversely, qc1N predicted extremely well the liquefaction and lateral spreading response of the four models, confirming Seed’s hypothesis to explain the success of penetration-based seismic liquefaction charts. The depth to liquefaction measured in the four centrifuge models is consistent with the field CPT liquefaction chart.     

Updated Liquefaction Resistance Correction Factors for Aged Sands

Data from over 30 sites in 5 countries are analyzed to develop updated factors for correcting liquefaction resistance for aged sand deposits. Results of cyclic laboratory tests on relatively undisturbed and reconstituted specimens suggest an increase in the correction factors of 0.12 per log cycle of time and an average reference age of 2 days for the reconstitute specimens. Laboratory and field test results combined with cyclic resistance ratio (CRR) charts suggest an increase in the correction factors of 0.13 per log cycle of time and an average reference age of 23 years. A reference age of 23 years seems appropriate for the commonly used CRR charts derived from field liquefaction and no liquefaction case history data. Because age of natural deposits is often difficult to accurately determine, a relationship between measured to estimated shear-wave velocity ratio (MEVR) and liquefaction resistance correction factor is also derived directly from the compiled data. This new MEVR-liquefaction resistance correction factor relationship is not as sensitive to MEVR as in the relationship derived indirectly in a previous paper.     

CPT-Based Probabilistic Soil Characterization and Classification

Due to lack of soil sampling during conventional cone penetration testing, it is necessary to characterize and classify soils based on tip and sleeve friction values as well as pore pressure induced during and after penetration. Currently available semiempirical methods exhibit a significant variability in the estimation of soil type. Within the confines of this paper it is attempted to present a new probabilistic cone penetration test (CPT)-based soil characterization and classification methodology, which addresses the uncertainties intrinsic to the problem. For this purpose, a database composed of normalized corrected cone tip resistance (qt,1,net), normalized friction ratio (FR), fines content (FC), liquid limit (LL), plasticity index (PI), and soil type based on the unified soil classification system was complied. Soil classification was performed by laboratory testing of the standard penetration test disturbed samples retrieved from the boreholes within mostly 2?m of each CPT hole. The resulting database was probabilistically assessed through Bayesian updating methodology allowing full and consistent representation of relevant uncertainties, including (1) model imperfection; (2) statistical uncertainty; and (3) inherent variability. As a conclusion, different sets of FC, LL, PI, and A-line boundary curves along with a new CPT-based, simplified soil classification scheme are proposed in the qt,1,net and FR domain. Probabilistic uses of the proposed models are illustrated through a set of illustrative examples.     

Nondestructive Sample Quality Assessment of a Soft Clay Using Shear Wave Velocity

While improvements in equipment and sampling methods have enabled collection of better quality samples of soft clays for more reliable engineering design and performance prediction, current sample quality assessment methods typically require destructive laboratory testing performed long after samples are taken. This paper describes a nondestructive technique for sample quality assessment of soft clays using shear wave velocity. A portable bender element device was used to measure shear wave velocity (Vvh) in the field immediately following collection of Sherbrooke block, tube, and split spoon samples of Boston blue clay. Vvh values were compared to in situ values from seismic piezocone (VSCPTU) tests. The ratio Vvh/VSCPTU was compared with results from a conventional, laboratory-based assessment method. Results indicate a consistent correlation between laboratory-based methods and the Vvh/VSCPTU ratio, which ranges from Vvh/VSCPTU = 0.77 for the block samples to 0.28 for split spoon samples. The portable bender element device and nondestructive assessment technique offer the potential for field quality assessment and allow for real time adjustments to sampling techniques and/or more effective selection of samples for laboratory testing.     

Probabilistic Models for Cyclic Straining of Saturated Clean Sands

A maximum likelihood framework for the probabilistic assessment of postcyclic straining of saturated clean sands is described. Databases consisting of cyclic laboratory test results including maximum shear and postcyclic volumetric strains in conjunction with relative density, number of stress (strain) cycles, and “index” test results were used for the development of probabilistically based postcyclic strain correlations. For this purpose, in addition to the compilation of existing data from literature, a series of stress-controlled cyclic triaxial and simple shear tests were performed on laboratory-constituted saturated clean sand specimens. The variabilities in testing conditions (i.e., type of test, consolidation procedure, confining pressure, rate of loading, etc.) were corrected through a series of correction schemes, the effectiveness of which were later confirmed by the discriminant analyses results. Volumetric and shear strain boundary curves were developed in the cyclic stress ratio versus N1,60,CS or qc,1 domain. In addition to being based on significantly extended and higher quality databases, contrary to the existing judgmentally derived deterministic ones, proposed correlations have formal probabilistic bases, and so provide insight regarding uncertainty of strain predictions or probability of exceeding a target strain value. Probabilistic uses of the proposed correlations were illustrated by three sets of examples. A companion paper applied and calibrated the proposed volumetric strain correlation to semiempirically evaluate postearthquake settlement of level, free-field sites. For the calibration, case history soil profiles, composed of a broad range of sand types and depositional characteristics, shaken by a number of earthquakes, were used. Superior predictions of field settlements by this laboratory data-based cyclic strain assessment approach were concluded to be strongly mutually supportive.     

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance (qt) and penetration pore-water pressure at the shoulder (u2). Parametric studies for normalized cone tip resistance (Q = qcnet/σv0′) and normalized excess pressures (Δu2/σv0′) as a function of overconsolidation ratio (OCR = σvy′/σv0′) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter (Bq = Δu2/qcnet). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δu2/σv0′ space is superior to Q–Bq space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and Δu2/σv0′ be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling.     

Correlation between Cyclic Resistance Ratios of Intact and Reconstituted Offshore Saturated Sands and Silts with the Same Shear Wave Velocity

The cyclic liquefaction resistance of intact medium dense specimens of sands and silts obtained from offshore platform sites was compared to that of specimens reconstituted to the same values of shear wave velocity. The shear wave velocity was measured using a new system that is comprised of torsional piezoelectric ceramic ring transducers mounted in a triaxial cell, a multiwave measuring device, and special watertight connectors. The relationship between cyclic resistance ratio and the number of cycles to liquefaction Nf of intact and reconstituted specimens was compared at the same values of consolidation pressure and shear wave velocity. There was good agreement between cyclic resistance ratios of intact and reconstituted specimens with similar values of shear wave velocity if liquefaction is defined as ? 6% peak-to-peak axial strain. The results of this study support the hypothesis that the cyclic liquefaction resistance of reconstituted specimens may be restored to in situ conditions when their shear wave velocity is restored to in situ values.     

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.     

Estimation of Load Capacity of Pipe Piles in Sand Based on Cone Penetration Test Results

Pipe piles can be classified as either closed- or open-ended piles. In the present paper, the load capacity of both closed- and open-ended piles is related to cone penetration resistance qc through an experimental program using calibration chamber model pile load tests and field pile load tests. A total of 36 calibration chamber pile load tests and two full-scale field pile load tests were analyzed. All the test piles were instrumented for separate measurement of each component of pile load capacity. Based on the test results, the normalized base resistance qb/qc was obtained as a function of the relative density DR for closed-ended piles, and of both the relative density DR and the incremental filling ratio (IFR) for open-ended piles. A relationship between the IFR and the relative density DR is proposed as a function of the pile diameter and driving depth. The relationship between IFR and DR allows the estimation of IFR and thus of the pile load capacity of open-ended piles at the design stage, before pile driving operations.     

Evaluation of Remolded Shear Strength and Sensitivity of Soft Clay Using Full-Flow Penetrometers

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites.     

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.     

Computation of Cavity Expansion Pressure and Penetration Resistance in Sands

A cavity expansion-based theory for calculation of cone penetration resistance qc in sand is presented. The theory includes a completely new analysis to obtain cone resistance from cavity limit pressure. In order to more clearly link the proposed theory with the classical cavity expansion theories, which were based on linear elastic, perfectly plastic soil response, linear equivalent values of Young's modulus, Poisson’s ratio and friction and dilatancy angles are given in charts as a function of relative density, stress state, and critical-state friction angle. These linear-equivalent values may be used in the classical theories to obtain very good estimates of cavity pressure. A much simpler way to estimate qc—based on direct reading from charts in terms of relative density, stress state, and critical-state friction angle—is also proposed. Finally, a single equation obtained by regression of qc on relative density and stress state for a range of values of critical-state friction angle is also proposed. Examples illustrate the different ways of calculating cone resistance and interpreting cone penetration test results.     

Dynamic Properties of Highly Organic Soils from Montezuma Slough and Clifton Court

The nonlinear dynamic properties of highly organic soils from two levee sites in the Sacramento-San Joaquin Delta in California are described. Cyclic triaxial, resonant column and torsional shear tests were performed on thin-walled tube samples obtained from beneath levee crests, beneath adjacent berms, and in the free field such that the in situ vertical effective stresses (σvo′) ranged from about 16?to?67?kPa. These highly organic soils had considerably different organic characteristics from those used in previous studies of dynamic properties. The tested samples had organic contents of 14 to 61%, initial water contents (wo) of 88 to 496%, shear wave velocities (Vs) of 20?to?130?m/s, and organic components that ranged from highly fibrous to highly decomposed and amorphous. Secant shear modulus (G), normalized secant shear modulus (G/Gmax), and equivalent damping ratio (ξ) versus cyclic shear strain amplitude (γc) relations are presented, and their dependence on variables such as consolidation stress, organic content, prior loading history, testing device, and loading frequency are illustrated. Findings are compared to previously published results.     

Nonlinear Dynamic Properties of a Fibrous Organic Soil

The nonlinear dynamic properties of a fibrous peaty organic soil beneath a levee in the Sacramento–San Joaquin Delta in California are described herein. Thin-walled tube samples were obtained from four locations between the levee crest and the free field such that the in situ vertical effective stresses (σvo′) ranged from about 12 kPa in the free field to about 135 kPa beneath the levee crest. The peaty organic soil was very soft and highly compressible in the free field with initial water contents (wo) of 236–588% and shear wave velocities (Vs) of typically 22–27 m/s, and moderately firm beneath the levee crest with wo of 152–240% and Vs of typically 88–129 m/s. Stress–strain measurements in a cyclic triaxial device showed that the normalized secant shear modulus (G/Gmax) and equivalent damping ratio (ξ) versus cyclic shear strain amplitude (γc) relations were dependent on the consolidation stress (σvc′). Tests involving prior overstraining followed by reconsolidation showed that the effects of sample disturbance were likely small. Stress history, creep, and loading frequency effects were also examined. Tests on reconstituted specimens provided supplementary data on the functional relation between maximum shear modulus (Gmax) and consolidation stress conditions. Summary relations are provided for G/Gmax and ξ versus γc and for Gmax versus σvc′.