Department of Civil and Environmental Engineering, UCLA

A complete model of a soil-foundation-structure system for use in response history analysis requires modification of input motions relative to those in the free-field to account for kinematic interaction effects, foundation springs and dashpots to represent foundation-soil impedance, and a structural model. The recently completed ATC-83 project developed consistent guidelines for evaluation of kinematic interaction effects and foundation impedance for realistic conditions. We implement those procedures in seismic response history analyses for two instrumented buildings in California, one a 13-story concrete-moment frame building with two levels of basement and the other a 10-story concrete shear wall core building without embedment. We develop three-dimensional baseline models (MB) of the building and foundation systems (including SSI components) that are calibrated to reproduce observed responses from recorded earthquakes. SSI components considered in the MB model include horizontal and vertical springs and dashpots that represent the horizontal translation and rotational impedance, kinematic ground motion variations from embedment and base slab averaging, and ground motion variations over the embedment depth of basements. We then remove selected components of the MB models one at a time to evaluate their impact on engineering demand parameters (EDPs) such as inter-story drifts, story shear distributions, and floor accelerations. We find that a “bathtub” model that retains all features of the MB approach except for depth-variable motions provides for generally good above-ground superstructure responses, but biased demand assessments in subterranean levels. Other common approaches using a fixed-based representation can produce poor results.

Site factors are used to modify ground motions from a reference rock site condition to reflect the influence of geologic conditions at the site of interest. Site factors typically have a small-strain (linear) site amplification that captures impedance and resonance effects coupled with nonlinear components. Site factors in current NEHRP Provisions are empirically-derived at relatively small ground motion levels and feature simulation-based nonlinearity. We show that NEHRP factors have discrepancies with respect to the site terms in the Next Generation Attenuation (NGA) ground motion prediction equations, both in the linear site amplification (especially for Classes B, C, D, and E) and the degree of nonlinearity (Classes C and D). The misfits are towards larger linear site factors and stronger nonlinearity in theNEHRP factors. The differences in linear site factors result largely from theirnormalization to a reference average shear wave velocity in the upper 30 m of about 1050 m/s, whereas the reference velocity for current application is 760 m/s. We show that the levels of nonlinearity in the NEHRP factors are generally stronger than recent simulation-based models as well as empirically-based models.

We review site parameters used in ground motion prediction equations (GMPEs) for various tectonic regimes and describe procedures for estimation of site parameters in the absence of site-specific data. Most modern GMPEs take as the principal site parameter the average shear wave velocity in the upper 30 m of the site (V_s30) either directly or as the basis for site classification into categories. Three GMPEs developed for active regions also use basin depth parameters. We review estimation procedures for V_s30 that utilize surface geology, terrain-based site categories, ground slope, or combinations of these. We analyze the relative efficacy of those procedures using a profile data set from California assembled in a recent NGA project. The results indicate that no single procedure is most effective and that prediction dispersion is lower for young sediments than for stiff soils or rock.

          The Salton Sea is the largest lake in California and is an endorheic, shallow, hypersaline lake. The surface water elevation of the Sea is currently 238 feet below sea level, and has been maintained by agricultural return flows from Imperial Valley farming, and two rivers- the New River and Alamo River- which originate in Mexicali, Mexico. The current salinity is at 74 ppt and is expected to increase due to the Quantification Settlement Agreement that was signed in 2003, stipulating the transfer of 500,000 acre-ft of Colorado River water to urban areas until 2075. This results in less flow to the Salton Sea and the declining water level has exposed 220 square miles of dried up playa, creating dust storms that have become the highest risk factor for asthma and cardiovascular diseases to the population around the Sea. Massive fish and bird kills began in the 1980s and continue to occur periodically. The Sea that was once the main Pacific flyway is now named as “IBA in Danger” by BirdLife International.

            In this study, the Delft3D numerical modeling suite- FLOW, WAVE and WAQ- was utilized to investigate transport and cycling of nutrients under the influence of wind-induced sediment resuspension activity. The three-dimensional hydrodynamic and water quality combined model was applied to simulate mitigation scenarios to assess long-term effects on salinity and water quality of 1) emerged islands, 2) seawater import/export, and 3) seawater import/export in addition to treating tributary rivers to remove nutrients treatment.

              Overall, this study supports the findings from previous studies and showed that sediment resuspension is an important factor that influences orthophosphate concentration in the water column, and that emerged islands have long term potential on enhancing burial activity for pollutants removal in the Salton Sea. Furthermore, the seawater import/export mitigation scenario showed promising results of reducing salinity level from 46 ppt to 38-39 ppt in two years. The three-dimensional hydrodynamic/water quality model developed in this work is the latest numerical model tailored to the Salton Sea’s system, and has the potential to improve understanding of biogeochemical processes of chemical substances that lead to detrimental effects, and facilitate future restoration plans for the Salton Sea. 

Global sea levels are expected to increase up to five meters by the year 2150 (Fox-Kemper

et al., 2021). Ocean water level fluctuations will cause an increase in coastal

groundwater in addition to extreme water levels from storm surge and spring tides. These

fluctuations pose multiple interrelated coastal hazards, including beach erosion and infrastructure

damage. Currently, there has been minimal attention paid to the effects of sea level rise on groundwater,

specifically how rising groundwater levels interact with other hazards, such as earthquake-induced

soil liquefaction. In collaboration with the California Geologic Survey, the United States

Geologic Survey, and the California Seismic Safety Commission, we attempt to

quantify the effects of sea level rise on earthquake-induced liquefaction.

We propose to use a probabilistic liquefaction hazard assessment

(PLHA) (Kramer and Mayfield, 2007) to obtain annual rates of nonexceedance of factor of safety of liquefaction.

This methodology will be used to analyze the effects of sea level rise on earthquake-induced soil liquefaction in

coastal California, which is an area of land which houses over 25 million people and has important transportation and economic implications.

PLHA is an extension of probabilistic seismic hazard analysis (PSHA). It conceptually accounts for the rate of occurrence of all earthquake events that might shake a site of interest. This includes measures of ground motion intensity resulting from each earthquake, the groundwater conditions at the time of the

earthquake, and the likelihood of liquefaction given a particular shaking intensity and groundwater

level. We propose to integrate the liquefaction uncertainty into the hazard calculation, which is a more direct end-to-end approach. The new method integrates the PLHA directly with the PSHA without requiring a deaggregation. The liquefaction hazard is assessed probabilistically for all the rupture scenarios within any fault source model being used. With currently available computing power, running the probabilistic liquefaction hazard inside the hazard integral can be done efficiently (seconds), requiring only slightly more computation time than a PSHA. There is an added benefit that we can deaggregate the liquefaction hazard, which is not possible when the calculations are performed outside the hazard integral.

Using traditional stress-based liquefaction evaluation methods, practitioners are unable to determine the

return period associated with the computed factor of safety. This new methodology allows us to choose the factor of safety against liquefaction for a desired return period. The Uniform California Earthquake Rupture Forecast (Field and Cornell, 2003) Fault Source model is used for both the PSHA and PLHA. Preliminary results

 have been calculated and calibrated for Cardiff State Beach in Southern California. Specifically, preliminary results of a 1-meter of sea level rise scenario using a calibrated groundwater model shows over a 100-year reduction in return period. Initial results suggest that for sites where more events contribute to the hazard the deterministic factor of safety and integrated PLHA factor of safety will differ.  Additional research includes applications of additional ground motion models (2D and 3D over longer time periods) and use of more ground motion and PLHA triggering models.

When the direct shear test is performed in accordance with ASTM guidelines, the measured shear stresses at failure estimate drained strength parameters.  We investigate the possibility of estimating undrained strength using direct shear testing at variable shear displacement rates on specimens composed of various combinations of kaolinite and bentonite. Even at fast displacement rates, constant volume conditions are not achieved in the direct shear device because of changes in specimen height that are large relative to allowable ASTM thresholds for constant volume simple shear testing. However, undrained strengths established by constant volume simple shear testing at slow strain rates are well approximated by direct shear tests conducted at fast shear displacement rates (time to failure < t₅₀/8, where t₅₀=time to 50% consolidation in a conventional oedometer test). Because of the simplicity of direct shear testing, such estimates of undrained strength may be useful in engineering practice when access to a simple shear device is limited. Nevertheless, fast direct shear tests have shortcomings, including lack of control of rate effects, and constant volume testing is recommended for critical projects.

Global sea levels are expected to increase up to five meters by the year 2150. Higher marine water levels as well as ocean fluctuations will also raise unconfined coastal groundwater levels. These fluctuations pose multiple interrelated coastal hazards. Currently, there has been minimal attention paid to the effects of sea level rise on groundwater, specifically how rising groundwater levels interact with hazards such as earthquake-induced soil liquefaction. We propose to use a probabilistic liquefaction hazard assessment (PLHA) to obtain annual rates of nonexceedance of factor of safety of liquefaction. Our approach integrates the liquefaction uncertainty into the hazard calculation by performing a PLHA for every event in the PSHA. Existing codes for performing probabilistic liquefaction hazard analysis (PLHA) are decoupled as first a probabilistic seismic hazard analysis (PSHA) is required to obtain a hazard curve. Drawbacks of this method are it only accounts for the influence of magnitude on liquefaction because it uses a small number of magnitudes and selection of the magnitudes requires judgment. We adopt the UCERF3 source model, which uses over a million events. For each event, ground motion at the site is a random variable that is used to compute cyclic stress ratio (CSR). Cyclic resistance ratio (CRR) is also a random variable obtained from a liquefaction triggering model. Because CSR and CRR are log normally distributed, we can use a closed-form solution to obtain a distribution for factor of safety (FoS). The rate of the event is then multiplied by the cumulative distribution function (CDF) of FoS and the rate-weighted CDF’s are summed over all events to compute the liquefaction hazard curve. Using efficient vectorized operations, the PLHA runs within seconds for a single site despite the large number of calculations. The code is used to compute the PLHA for an example soil profile at Cardiff State Beach in Southern California for current groundwater conditions and for a 1-meter sea level rise scenario.

A complete model of a soil-foundation-structure system for use in response history analysis requires modification of input motions relative to those in the free-field to account for kinematic interaction effects, foundation springs and dashpots to represent foundation-soil impedance, and a structural model. The recently completed ATC-83 project developed consistent guidelines for evaluation of kinematic interaction effects and foundation impedance for realistic conditions. We implement those procedures in seismic response history analyses for two instrumented buildings in California, one a 13-story concrete-moment frame building with two levels of basement and the other a 10-story concrete shear wall core building without embedment. We develop three-dimensional baseline models (MB) of the building and foundation systems (including SSI components) that are calibrated to reproduce observed responses from recorded earthquakes. SSI components considered in the MB model include horizontal and vertical springs and dashpots that represent the horizontal translation and rotational impedance, kinematic ground motion variations from embedment and base slab averaging, and ground motion variations over the embedment depth of basements. We then remove selected components of the MB models one at a time to evaluate their impact on engineering demand parameters (EDPs) such as inter-story drifts, story shear distributions, and floor accelerations. We find that a “bathtub” model that retains all features of the MB approach except for depth-variable motions provides for generally good above-ground superstructure responses, but biased demand assessments in subterranean levels. Other common approaches using a fixed-based representation can produce poor results.

The present work details the development of carbon fiber-reinforced epoxy membranes with excellent rejection of small-molecule dyes. It is a proof-of-concept for a more sustainable membrane design incorporating carbon fibers, and their recycling and reuse. 4,4'-methylenebis(cyclohexylamine) (MBCHA) polymerized with either bisphenol-A-diglycidyl ether (BADGE) or tetraphenolethane tetraglycidylether (EPON Resin 1031) in polyethylene glycol (PEG) were used to make monolithic membranes reinforced by nonwoven carbon fibers. Membrane pore sizes were tuned by adjusting the molecular weight of the PEG used in the initial polymerization. Membranes made of BADGE-MBCHA showed rejection of Rose Bengal approaching 100%, while tuning the pore sizes substantially increased the rejection of Methylene Blue from ~65% to nearly 100%. The membrane with the best permselectivity was made of EPON-MBCHA polymerized in PEG 300. It has an average DI flux of 4.48 LMH/bar and an average rejection of 99.6% and 99.8% for Rose Bengal and Methylene Blue dyes, respectively. Degradation in 1.1 M sodium hypochlorite enabled the retrieval of the carbon fiber from the epoxy matrix, suggesting that the monolithic membranes could be recycled to retrieve high-value products rather than downcycled for incineration or used as a lower selectivity membrane. The mechanism for epoxy degradation is hypothesized to be part chemical and part physical due to intense swelling stress leading to erosion that leaves behind undamaged carbon fibers. The retrieved fibers were successfully used to make another membrane exhibiting similar performance to those made with pristine fibers.

We describe the data model that we intend to use in a publicly available site profile database under development for the United States. The initial implementation of the database contains data from California. Currently, our prototype data model consists of JavaScript Object Notation (JSON) format files for storing metadata and data. For a site to be included in the database, the minimum metadata requirements are geodetic coordinates and elevation values, and the minimum data requirement is a shear-wave velocity profile. The JSON files are structured in a hierarchal manner to store metadata and data using a nested structure consisting of location, velocity profiles, dispersion curve data (for surface-wave methods), geotechnical data, and horizontal-to-vertical spectral ratios. The database schema at the current stage of the project, and as we continue to develop the data model we will consider including other relevant data, as well as evaluate other file formats to increase the efficiency of data storage and querying. In the current data model, location information includes site geodetic values (latitude, longitude, and elevation) and various site descriptors related to surface geology, geomorphic terrain category, slope gradient at various resolutions, and a geotechnical site category. Velocity data include the geophysical method(s) used to obtain the shear-wave velocity profile, type of data recorded, modeled primary- and shear-wave velocity as a function of depth, modeled profile maximum depth, and the calculated VS30 value. In the case of surface-wave based data, dispersion curve data can be recorded in data structure as phase velocity versus either wavelength or frequency. Geotechnical data includes boring logs penetration resistance, cone penetration test sounding logs, and laboratory index test results. Horizontal-to-vertical spectral ratio plots are given as a function of frequency.