Liquefaction Potential Map of Charleston, South Carolina Based on the 1886 Earthquake

A liquefaction potential map of the peninsula of Charleston, S.C., is presented in this paper. Liquefaction potential is expressed in terms of the liquefaction potential index developed by Iwasaki et al. and calculated using 44 cone penetration test profiles. The cone profiles are supplemented with information from the 1:24,000 scale geologic map by Weems and Lemon, several first-hand accounts of liquefaction and ground deformation that occurred during the 1886 Charleston earthquake, and liquefaction probabilities determined by Elton and Hadj-Hamou based on standard penetration tests. Nearly all of the cases of liquefaction and ground deformation occurred in the Holocene to late Pleistocene beach deposits that flank the higher-ground sediments of the Wando Formation. To match the observed field behavior, a deposit resistance correction factor of 1.8 is applied to cyclic resistance ratios calculated for the 100,000-year-old Wando Formation. No corrections are needed for the younger deposits. In additional to 1886 field behavior, the deposit resistance corrections are supported by ratios of measured to predicted shear-wave velocity.     

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.     

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.     

Simplified Cone Penetration Test-based Method for Evaluating Liquefaction Resistance of Soils

This paper presents a new simplified method for assessing the liquefaction resistance of soils based on the cone penetration test (CPT). A relatively large database consisting of CPT measurements and field liquefaction performance observations of historical earthquakes is analyzed. This database is first used to train an artificial neural network for predicting the occurrence and nonoccurrence of liquefaction based on soil and seismic load parameters. The successfully trained and tested neural network is then used to generate a set of artificial data points that collectively define the liquefaction boundary surface, the limit state function. An empirical equation is further obtained by regression analysis to approximate the unknown limit state function. The empirical equation developed represents a deterministic method for assessing liquefaction resistance using the CPT. Based on this newly developed deterministic method, probabilistic analyses of the cases in the database are conducted using the Bayesian mapping function approach. The results of the probabilistic analyses, expressed as a mapping function, provide a simple means for probability-based evaluation of the liquefaction potential. The newly developed simplified method compares favorably to a widely used existing method.     

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.     

CPT-Based Probabilistic and Deterministic Assessment of In Situ Seismic Soil Liquefaction Potential

This paper presents a complete methodology for both probabilistic and deterministic assessment of seismic soil liquefaction triggering potential based on the cone penetration test (CPT). A comprehensive worldwide set of CPT-based liquefaction field case histories were compiled and back analyzed, and the data then used to develop probabilistic triggering correlations. Issues investigated in this study include improved normalization of CPT resistance measurements for the influence of effective overburden stress, and adjustment to CPT tip resistance for the potential influence of “thin” liquefiable layers. The effects of soil type and soil character (i.e., “fines” adjustment) for the new correlations are based on a combination of CPT tip and sleeve resistance. To quantify probability for performance-based engineering applications, Bayesian “regression” methods were used, and the uncertainties of all variables comprising both the seismic demand and the liquefaction resistance were estimated and included in the analysis. The resulting correlations were developed using a Bayesian framework and are presented in both probabilistic and deterministic formats. The results are compared to previous probabilistic and deterministic correlations.     

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.     

Index Properties-Based Criteria for Liquefaction Susceptibility of Clayey Soils: A Critical Assessment

The Cooper marl in Charleston, S.C., a deep layer of clayey soils approximately 5–21?m below the ground surface, is generally recognized as nonliquefiable material. Data from field cone penetration tests and laboratory tests of samples taken from the Cooper marl are used to investigate the adequacy of index properties-based criteria for assessing liquefaction susceptibility of clayey soils. In particular, the criterion based on soil behavior type index (Ic) and that based on Atterberg limits are examined. The results show that the Atterberg limits-based criterion adequately reflected the characteristics of the marl, whereas the Ic-based criterion erroneously identified the marl as being liquefiable. A possible reason for the deficiency of Ic and a modification to overcome this deficiency are presented.     

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.     

Standard Penetration Test-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Potential

This paper presents new correlations for assessment of the likelihood of initiation (or “triggering”) of soil liquefaction. These new correlations eliminate several sources of bias intrinsic to previous, similar correlations, and provide greatly reduced overall uncertainty and variance. Key elements in the development of these new correlations are (1) accumulation of a significantly expanded database of field performance case histories; (2) use of improved knowledge and understanding of factors affecting interpretation of standard penetration test data; (3) incorporation of improved understanding of factors affecting site-specific earthquake ground motions (including directivity effects, site-specific response, etc.); (4) use of improved methods for assessment of in situ cyclic shear stress ratio; (5) screening of field data case histories on a quality/uncertainty basis; and (6) use of high-order probabilistic tools (Bayesian updating). The resulting relationships not only provide greatly reduced uncertainty, they also help to resolve a number of corollary issues that have long been difficult and controversial including: (1) magnitude-correlated duration weighting factors, (2) adjustments for fines content, and (3) corrections for overburden stress.     

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.     

Correcting Liquefaction Resistance for Aged Sands Using Measured to Estimated Velocity Ratio

Factors for correcting liquefaction resistance for aged sands using ratios of measured to estimated shear-wave velocity (MEVR) are derived in this paper. Estimated values of shear-wave velocity (VS) are computed for 91 penetration resistance-VS data pairs using previously published relationships. Linear regression is performed on values of MEVR and corresponding average age. Age of the sand layer is taken as the time between VS measurements and initial deposition or last critical disturbance. It is found that MEVR increases by a factor of about 0.08 per log cycle of time, and time equals about 6?years on average when MEVR equals 1 for the recommended penetration resistance-VS relationships. The resulting regression equation is combined with the strength gain equation reported by Hayati et al. 2008 in “Proc., Geotechnical Earthquake Engineering and Soil Dynamics IV,” to produce a MEVR versus deposit resistance correction relationship. This new corrective relationship is applied to create liquefaction resistance curves based on VS, standard penetration test blow count, and cone tip resistance for sands of various ages (or MEVRs). Because age of natural soil deposits is usually difficult to accurately determine, MEVR appears to be a promising alternative.     

Liquefaction Hazard Mapping—Statistical and Spatial Characterization of Susceptible Units

In this paper, we present a three step method for characterizing geologic deposits for liquefaction potential using sample based liquefaction probability values. The steps include statistically characterizing the sample population, evaluating the spatial correlation of the population, and finally providing a local and/or global estimate of the distribution of high liquefaction probability values for the deposit. When spatial correlation is present, ordinary kriging can be used to evaluate spatial clustering of high liquefaction probability values within a geologic unit which in turn can be used in a regional liquefaction potential characterization. If spatial correlation is not present in the data, then a global estimate can be used to estimate the percentage of samples within the deposit which have a high liquefaction probability. By describing the liquefaction potential with a binomial distribution (high versus low), a global estimate can provide an estimate of the mean as well as uncertainty in the estimate. To demonstrate the method, we used a dense data set of subsurface borings to identify and characterize liquefiable deposits for hazard mapping in Cambridge, Mass.     

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.     

Influence of the Grain-Size Distribution Curve of Quartz Sand on the Small Strain Shear Modulus Gmax

The paper presents a study of the influence of grain-size distribution curve on the small strain shear modulus Gmax of quartz sand with subangular grain shape. The results of 163 resonant column tests on 25 different grain-size distribution curves are presented. It is demonstrated for a constant void ratio that while Gmax is not influenced by variations in the mean grain-size d50 in the investigated range, it significantly decreases with increasing coefficient of uniformity Cu = d60/d10 of the grain-size distribution curve. Well-known empirical formulas (e.g., Hardin’s equation with its commonly used constants) may strongly overestimate the stiffness of well-graded soils. Based on the RC test results, correlations of the constants of Hardin’s equation with Cu have been developed. The predictions using Hardin’s equation and these correlations are in good accordance with the test data. Correlations of the frequently used shear modulus coefficient K2,max with Cu and empirical equations formulated in terms of relative density, are also given in the paper. A comparison of the predictions by the proposed empirical formulas with Gmax-data from the literature and a micromechanical explanation of the experimental results are provided. Correction factors for an application of the laboratory data to in situ conditions are also discussed.     

Determination of Shear-Wave Velocities and Shear Moduli of Completely Decomposed Tuff

Based on theoretical derivations and considerations, five series of laboratory tests were planned to investigate and differentiate the degrees of inherent and stress-induced anisotropy, to study the effect of void ratio changes on shear-wave velocities and shear moduli, and to determine the relationship between shear-wave velocity and stress state on a completely decomposed tuff (CDT). Shear-wave velocities in three orthogonal horizontal and vertical planes [vs(hh), vs(hv), and vs(vh)] were measured in both vertically and horizontally cut block and Mazier specimens. Under isotropic stress conditions (K = 1.0), the degrees of inherent anisotropy [vs(hh)2/vs(hv)2 = Ghh/Ghv] were 1.48 and 1.36 for the block and Mazier specimens, respectively. At the anisotropic stress state (K = 0.4), the degrees of anisotropy of the block and Mazier specimens were 1.26 and 1.15, respectively, 15% reduction from the measured inherent anisotropy due to stress-induced effects. The measured higher shear-wave velocity in the horizontal plane of the CDT was confirmed by testing both vertically and horizontally cut specimens and the measured results reflect a stronger layering structure in the horizontal bedding plane of the natural material, in which K0 less than 1.0 is commonly assumed in designs. Under both isotropic and anisotropic stress states, the shear-wave velocities [vs(hh), vs(hv), and vs(vh)] of the block specimens are on average about 27% higher than those of the Mazier specimens.     

Large and Small Strain Properties of Sands Subjected to Local Void Increase

Local strain effects are proposed as a source of shear strength reduction in cemented particulate media, shales, and heterogeneous systems. Shear strength degradation through local straining may arise from particle dissolution, double-layer shrinkage in reactive clay phases, and volume change associated with thermal processes. This study focuses on mechanical property changes produced by local straining in particulate systems made of sand–salt mixtures. Local strains were induced by the dissolution of salt particles. Large-strain properties (angle of shear resistance and dilation rate) were measured using triaxial test methods. Small-strain properties (acoustic wave velocity and attenuation) were measured with a resonant column device and piezocrystals (bender elements). Experimental data showed that large-strain properties are sensitive to changes in aggregate volume; a reduction in the angle of shearing resistance up to 26% was observed for a 90% sand–10% salt mixture after salt dislocation. Acoustic wave velocity and attenuation values changed up to 25% during particle dissolution. Fine sand–salt specimens showed smaller changes in macroscopic parameters, compared to coarse-grained specimens. Changes at the microscale assessed using small-strain measurements are clearly reflected at the macroscale as a reduction in the angle of shearing resistance. Finally, it is shown that changes in macroscale parameters produced by internal volumetric strains can be estimated by considering the change in the void ratio and assuming a random distribution of internal strains. However, small-strain parameters cannot be evaluated using the same approach because the microstructure has a stronger effect on wave propagation parameters (velocity and attenuation) than the macroscopic parameters.     

Application of the Coupled Local Minimizers Method to the Optimization Problem in the Spectral Analysis of Surface Waves Method

The spectral analysis of surface waves (SASW) method aims to determine the small strain dynamic soil characteristics of shallow soil layers. The method involves an in situ experiment, the determination of an experimental dispersion curve, and the solution of an inverse problem, formulated as a nonlinear least squares problem. The latter is usually solved with a gradient-based local optimization method, which converges fast, but does not guarantee to find the global minimum of the objective function. The method of coupled local minimizers (CLM) combines the advantage of gradient-based local algorithms with the global approach of genetic algorithms. A cooperative search mechanism is set up by simultaneously performing a number of local optimization runs that are coupled by pairs of synchronization constraints. A synthetic example with two design variables (the shear wave velocity of two top layers of a layered half-space consisting of three layers on a half-space), demonstrates that the CLM method succeeds in finding the global minimum of an objective function with multiple minima and can successfully be used to solve the inverse problem in the SASW method. This is further illustrated by a complete inversion of the shear wave velocity profile accounting for seven design variables (the thickness and shear wave velocity of the three layers and the shear wave velocity of the underlying half-space). The inversion algorithm based on the CLM method is subsequently applied to invert the experimental dispersion curve derived from in situ data collected at a test site in Saluggia, Italy, consisting mainly of alluvial sediments. Up to a depth of about 25?m, the results show a reasonably good correspondence with crosshole test results.