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1. PM4Silt (Version 1): A silt plasticity model for earthquake engineering applications

   Boulanger, Ross W. ; Ziotopoulou, Katerina ( January 2018 , Report No. UCD/CGM-18/01, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, CA, 108 pp.)

   The PM4Silt plasticity model for representing low-plasticity silts and clays in geotechnical earthquake engineering applications is presented herein. The PM4Silt model builds on the framework of the stress-ratio controlled, critical state compatible, bounding surface plasticity PM4Sand model (version 3) described in Boulanger and Ziotopoulou (2015) and Ziotopoulou and Boulanger (2016). Modifications to the model were developed and implemented to improve its ability to approximate undrained monotonic and cyclic loading responses of low-plasticity silts and clays, as opposed to those for purely nonplastic silts or sands. Emphasis was given to obtaining reasonable approximations of undrained monotonic shear strengths, undrained cyclic shearmore »strengths, and shear modulus reduction and hysteretic damping responses across a range of initial static shear stress and overburden stress conditions. The model does not include a cap, and therefore is not suited for simulating consolidation settlements or strength evolution with consolidation stress history. The model is cast in terms of the state parameter relative to a linear critical state line in void ratio versus logarithm of mean effective stress. The primary input parameters are the undrained shear strength ratio (or undrained shear strength), the shear modulus coefficient, the contraction rate parameter, and an optional post-strong-shaking shear strength reduction factor. All secondary input parameters are assigned default values based on a generalized calibration. Secondary parameters that are most likely to warrant adjustment based on site-specific laboratory test data include the shear modulus exponent, plastic modulus coefficient (adjusts modulus reduction with shear strain), bounding stress ratio parameters (affect peak friction angles and undrained stress paths), fabric related parameters (affect rate of shear strain accumulation at larger strains and shape of stress-strain hysteresis loops), maximum excess pore pressure ratio, initial void ratio, and compressibility index. The model is coded as a user defined material in a dynamic link library (DLL) for use with the commercial program FLAC 8.0 (Itasca 2016). The numerical implementation and DLL module are described. The behavior of the model is illustrated by simulations of element loading tests covering a range of conditions, including undrained monotonic and cyclic loading under a range of initial confining and shear stress conditions. The model is shown to provide reasonable approximations of behaviors important to many earthquake engineering applications and to be relatively easy to calibrate.« less
2. Dynamic, In-situ, Nonlinear-Inelastic Response and Post-Cyclic Strength of a Plastic Silt Deposit

   https://doi.org/10.1139/cgj-2020-0652  

   Jana, Amalesh ; Stuedlein, Armin W. ( April 2021 , Canadian Geotechnical Journal)

   This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicate that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high frequency P-waves are negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiatemore »soil nonlinearity and to trigger generation of residual EPP ranging from 0.002 to 0.003% and 0.008 to 0.012%, respectively, where the latter corresponded to ~30% of Gmax. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in-situ compared to that of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in-situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens.« less
3. Quality Assurance Criteria for Cyclic Direct Simple Shear Tests for Evaluating Liquefaction Triggering Characteristics of Cohesionless Soils

   https://doi.org/10.32075/17ECSMGE-2019-0470  

   Ulmer, K.J. ( September 2019 , Proceedings of the XVII ECSMGE-2019)

   Undrained or constant volume direct simple shear (CDSS) tests are commonly used to evaluate the liquefaction triggering characteristics of cohesionless soils. However, while the American Society for Testing of Materials (ASTM) has developed standards for monotonic direct simple shear testing, they have not developed a standard for CDSS. As a result, herein the authors review their test database and assign “grades” A-D to different aspects of the tests, e.g.: accumulated shear strain and imposed shear stress on the specimen during the consoli-dation phase, and maximum axial strain that occurs during the cyclic phase of constant volume CDSS testing. Additional gradesmore »are also assigned to the tests based on unusual behaviors in the stress paths. Acceptance criteria based on the cumulative test scores are then proposed for “high” quality tests. The slope of the relationship between cyclic stress ratio (CSR) and number of cycles to liquefaction (NL) is influenced by the exclusion of tests using the acceptance criteria, even though the excluded tests were of sufficient quality to have been included in most published studies.« less
4. LEAP-2017 Simulation Exercise: Calibration of Constitutive Models and Simulation of the Element Tests

   https://doi.org/10.1007/978-3-030-22818-7_9  

   Manzari, M.T. ( November 2019 , Proceedings of LEAP-UCD-2017 workshop)

   This paper presents a summary of the element test simulations (calibration simulations) submitted by 11 numerical simulation (prediction) teams that participated in the LEAP-2017 prediction exercise. A significant number of monotonic and cyclic triaxial (Vasko, An investigation into the behavior of Ottawa sand through monotonic and cyclic shear tests. Masters Thesis, The George Washington University, 2015; Vasko et al., LEAP-GWU-2015 Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraibymore »et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019) and direct simple shear tests (Bastidas, Ottawa F-65 Sand Characterization. PhD Dissertation, University of California, Davis, 2016) are available for Ottawa F-65 sand. The focus of this element test simulation exercise is to assess the performance of the constitutive models used by participating team in simulating the results of undrained stress-controlled cyclic triaxial tests on Ottawa F-65 sand for three different void ratios (El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019). The simulated stress paths, stress-strain responses, and liquefaction strength curves show that majority of the models used in this exercise are able to provide a reasonably good match to liquefaction strength curves for the highest void ratio (0.585) but the differences between the simulations and experiments become larger for the lower void ratios (0.542 and 0.515).« less
5. Particle Size Effects on the Strength and Fabric of Granular Media

   https://doi.org/10.1061/9780784482803.038  

   Kuei, Kevin C. ; DeJong, Jason T. ; Martinez, Alejandro ( February 2020 , GeoCongress 2020)

   The reliance on sand-based methods for the characterization and design of gravelly soils is necessitated in part by a limited understanding of how to account for the effects of particle size and distribution. In this paper, particle size effects on the strength and fabric of cohesionless, granular media are investigated using the discrete element method by testing specimens with particle size distributions (PSDs) of differing coefficient of uniformity (CU) and mean grain size (d50). Preliminary results show that when all particle shape characteristics are equal, increasing d50 yields equivalent stress-strain responses for the same relative density (DR) and CU. However,more »when increasing CU, specimens exhibit more dilative tendencies and greater shear strengths. Increasing CU results in more dispersive distributions of contact force, stress, and coordination number. The void ratios associated with minimum and maximum density, and constant volume conditions during shear also decrease with increasing CU.« less