Small-strain Shear Wave Velocity Research Articles

SWprocess: a workflow for developing robust estimates of surface wave dispersion uncertainty

Non-invasive surface wave methods are increasingly being used as the primary technique for estimating a site’s small-strain shear wave velocity (Vs). Yet, in comparison to invasive methods, non-invasive surface wave methods suffer from highly variable standards of practice, with each company/group/analyst estimating surface wave dispersion data, quantifying its uncertainty (or ignoring it in many cases), and performing inversions to obtain Vs profiles in their own unique manner. In response, this work presents a well-documented, production-tested, and easy-to-adopt workflow for developing estimates of experimental surface wave dispersion data with robust measures of uncertainty. This is a key step required for propagating dispersion uncertainty forward into the estimates of Vs derived from inversion. The paper focuses on the two most common applications of surface wave testing: the first, where only active-source testing has been performed, and the second, where both active-source and passive-wavefield testing has been performed. In both cases, clear guidance is provided on the steps to transform experimentally acquired waveforms into estimates of the site’s surface wave dispersion data and quantify its uncertainty. In particular, changes to surface wave data acquisition and processing are shown to affect the resulting experimental dispersion data, thereby highlighting their importance when quantifying uncertainty. In addition, this work is accompanied by an open-source Python package, swprocess, and associated Jupyter workflows to enable the reader to easily adopt the recommendations presented herein. It is hoped that these recommendations will lead to further discussions about developing standards of practice for surface wave data acquisition, processing, and inversion.

Influence of source-to-site distance and diagenesis on liquefaction triggering of 200,000-year-old beach sand

Liquefaction triggering of the outcropping 200,000-year-old Ten Mile Hill beds sand facies (Qts) near Charleston, South Carolina is characterized in this paper. The characterization includes evaluating the relative susceptibility to liquefaction of Qts assuming a uniform seismic load and using seismic cone penetration testing at 46 sites, the ratio of measured small-strain shear wave velocity to estimated small-strain shear wave velocity (MEVR), and the liquefaction potential index (LPI). The computed LPI values indicate a slight decrease in the liquefaction susceptibility of Qts moving away (5 to 30 km) from the likely source of the 1886 Charleston earthquake (Mw ~ 7). Also characterized is the liquefaction potential of Qts during the Charleston earthquake and a range of other earthquake loadings. The results for the range of other earthquake loadings are summarized in terms of liquefaction probability curves, which provide a method for mapping liquefaction potential in Qts. A back-calculated, lower-bound diagenesis correction factor of 1.11 for areas of Qts where liquefaction surface manifestations were not observed agrees well with the published data.

Assessment of shear strength and compressibility characteristics of a newly developed lunar highland soil simulant (LSS-ISAC-1) for Chandrayaan lander and rover missions

The engineering properties (shear strength and compressibility) of lunar soils influence the design of landers, rovers, and their mobility behavior on the lunar surface. Assessing such properties is important for any lunar-based studies, and it is usually being determined by using suitable lunar soil simulants. This paper explains the shear strength parameters, stress-strain relationships, and volume change behavior of the newly developed lunar highland soil simulant (LSS-ISAC-1), apart from particle size distribution, which have a profound influence on the engineering behavior. In addition, to the above, other engineering properties such as critical state of the angle of internal friction, dilatancy angle, Young's modulus, Poisson's ratio, internal erodibility, small-strain shear wave velocity, and shear modulus have also been evaluated and presented. Mohr-Coulomb and friction-dilatancy theories are considered as a base for the analysis with the use of statistical predictive equations. The relative density and confining pressures are used as functions in the predictive equation to assess the behavior of the LSS-ISAC-1. The comparison of properties with the actual lunar highland soil samples of the Apollo 16 mission reveals a reasonable similarity of LSS-ISAC-1 with the lunar highland soils.

State of the art in the assessment of aging effects on soil liquefaction

A review of the current state of the art in the assessment of aging effects on soil liquefaction is presented in this paper. The review includes a summary of several field case histories indicating greater resistance to liquefaction in aged soils than in young uncemented soils during earthquakes, a discussion of the mechanisms that increase liquefaction resistance with time, an evaluation of proposed methods for quantifying the influence of aging processes (or diagenesis) on liquefaction resistance (KDR), and an evaluation of proposed predictor variables for KDR. The published literature indicates physical diagenetic processes tend to dominate the nature of interactions at the grain-to-grain contacts of sand deposits in the absence of sufficient cementing agents, while chemical processes tend to dominate where sufficient cementing agents are present. Variables proposed for predicting KDR include: the time since deposition or last critical disturbance; the ratio of measured to estimated small-strain shear wave velocity (MEVR); the ratio of small-strain shear modulus to cone tip resistance (Gmax/qc); the adjusted Gmax/qc (KG); and the ratio of measured to estimated adjusted Gmax/qc (MEKG). It is shown that MEVR, Gmax/qc, KG and MEKG are all ratios of measured to estimated or reference shear wave velocity. MEVR and Gmax/qc are slightly more robust predictors (i.e., regression equations with higher coefficient of regression and lower root mean square error) of KDR than KG and MEKG. Time is the least robust predictor of KDR.

Dynamic characterization of fine-grained soils in Central Italy by laboratory testing

The investigation of soil response to dynamic loads is necessary to predict site-specific seismic hazard. This paper presents the results of cyclic and dynamic laboratory tests carried out after the 2016–2017 Central Italy Earthquake sequence, within the framework of the seismic microzonation studies of the most damaged municipalities in the area. The database consists of 79 samples investigated by means of dynamic resonant column tests, cyclic torsional shear tests or cyclic direct simple shear tests. Results are firstly analysed in terms of field and laboratory values of small-strain shear wave velocity, highlighting the influence of the sample disturbance and of the mean effective consolidation pressure. The cyclic threshold shear strains as a function of plasticity index are then compared with findings from the published literature and the outliers are analysed. Subsequently, the dynamic soil behaviour is investigated with reference to the small-strain damping ratio. Differences between results from different tests are analysed in the light of the loading frequencies. Finally, the database is used to develop a predictive model for soil nonlinear curves according to plasticity index, mean effective confining stress, and loading frequency. The model represents a useful tool to predict the nonlinear stress–strain behaviour of Central Italy soils, necessary to perform site-specific ground response analyses.

Assessment of the cyclic strain approach for evaluating liquefaction triggering

The cyclic strain approach was proposed in the 1980s as a potential alternative to the stress-based simplified liquefaction evaluation procedure. However, despite its fundamental basis and many positive attributes, it has not been embraced by practice. One reason for this may be the need to perform cyclic laboratory tests on undisturbed/reconstituted samples to develop a relationship among excess pore water pressure, cyclic strain amplitude, and number of applied strain cycles. Herein an alternative implementation of the strain-based procedure is proposed that circumvents this requirement, using a strain-based pore pressure generation model in lieu of laboratory test data. To assess the efficacy of the alternative implementation, several hundred small strain shear wave velocity (Vs) and Standard Penetration Test (SPT) field liquefaction case histories are evaluated. The results are compared with both field observations and with predictions from the stress-based procedures. It was found that the stress-based approach yielded considerably more accurate predictions compared to the cyclic strain approach. One likely reason for this is the strain-based procedure's inherent and potentially fatal limitation of ignoring the decrease in soil stiffness due to excess pore pressure when representing the earthquake loading in terms of shear strain amplitude and number of equivalent cycles.

Shear wave velocities of Mississippi embayment soils from low frequency surface wave measurements

Deep unconsolidated sediments in the Mississippi embayment will influence ground motions from earthquakes in the New Madrid seismic zone. Shear wave velocity profiles of these sediments are important input parameters for modeling wave propagation and site response in this region. Low-frequency, active-source surface wave velocity measurements were performed to develop small-strain shear wave velocity ( V S ) profiles at eleven deep soil sites in the Mississippi embayment, from north of New Madrid, Missouri to Memphis, Tennessee. A servo-hydraulic, low-frequency source was used to excite surface wave energy to wavelengths of 600 m, resulting in V S profiles to depths of over 200 m. The average V S profile calculated from the eleven sites is in good agreement with common reference V S profiles that have been used in seismic hazard studies of this region. The variability in V S profiles is shown to be associated with changes in formation depth and thickness from site-to-site. Using lithologic information at each site, average formation velocities were developed and compared to previous studies. We found average V S values of about 193 m/s for alluvial deposits, 400 m/s for the Upper Claiborne formations, and 685 m/s for the Memphis Sand formation.

Influence of Soil Suction on Small-Strain Stiffness of Compacted Residual Subgrade Soil

The small-strain stiffness of compacted residual lateritic subgrade soil was determined. A bender element apparatus and the filter paper method were integrated to develop a systematic testing procedure to investigate the influence of matric suction on small-strain stiffness, including small-strain shear wave velocity and small-strain shear modulus, with different wetting paths. Soil specimens were compacted at three moisture contents (optimum moisture content, dry of optimum, and wet of optimum), then wetted to various moisture contents by using an environmental simulation apparatus developed to simulate in-service moisture conditions. Test findings indicate that specimen moisture contents increase gradually and finally attain equilibrium in the wetting process. Suction decreases with increasing moisture content and degree of saturation; consequently, small-strain shear wave velocity and small-strain shear modulus decrease. The specimens compacted at dry of optimum have a much lower small-strain shear wave velocity and small-strain shear modulus than specimens compacted at wet of optimum. Nevertheless, the small-strain shear modulus exhibits higher linearity with matric suction than with moisture content. Results indicate that matric suction is a key parameter for evaluating the dynamic properties of compacted soil.

Dynamic Periods and Building Damage at Charleston, South Carolina during the 1886 Earthquake

Fundamental dynamic periods of Quaternary deposits beneath the peninsula of Charleston, South Carolina, are characterized spatially using an updated isopach map of Quaternary thickness, characteristic small-strain shear wave velocity information, a 1:24,000 geologic map, and a simple approximating equation. The updated isopach map is developed from subsurface information from 266 investigation sites. Estimates of fundamental periods for the Quaternary sediments primarily range between 0.3 and 0.7 s. These periods are lower end estimates of actual ground periods, based on a comparison with modeled response-spectra ratios. Estimates of fundamental periods range from 0.1 to 0.4 s for over 95% of the buildings present in 1886. Thus, the overlap between the range of building periods and the range in periods corresponding to high spectral ratios is not great. This finding agrees with the observation of Marciano and Elton that damage was independent (or only slightly dependent) of building height.

Comparing liquefaction evaluation methods using penetration- VS relationships

Three methods that follow the general format of the Seed-Idriss simplified procedure for evaluating liquefaction resistance of soils are compared in this paper. They are compared by constructing relationships between penetration resistance and small-strain shear–wave velocity ( V S) implied from cyclic resistance ratio (CRR) curves for the three methods, and by plotting penetration- V S data pairs. The penetration- V S data pairs are from 43 Holocene-age sand layers in California, South Carolina, Canada, and Japan. It is shown that the V S-based CRR curve is more conservative than CRR curves based on the Standard Penetration Test (SPT) and Cone Penetration Test (CPT), for the compiled Holocene data. This result agrees with the findings of a recent probability study where the SPT-, CPT-, and V S-based CRR curves were characterized as curves with average probability of liquefaction of 31, 50, and 26%, respectively. New SPT- and CPT-based CRR equations are proposed that provide more consistent assessments of liquefaction potential for the Holocene sand layers considered.