An official website of the United States government
Calibrations of the PM4Silt constitutive model are presented for two low-plasticity fine-grained soils that exhibit significantly different cyclic loading be-haviors. The PM4Silt model is a stress-ratio controlled, critical state compatible, bounding surface plasticity model that was recently developed for representing low-plasticity silts and clays in geotechnical earthquake engineering applications. The low-plasticity clayey silt and silty clay examined herein were reconstituted mixtures of silica silt and kaolin with plasticity indices (PIs) of 6 and 20. Un-drained monotonic and undrained cyclic direct simple shear (DSS) tests were per-formed on normally consolidated, slurry deposited specimens. Calibration of the PM4Silt model was based on the monotonic and cyclic DSS test data, plus em-pirical relationships for strain-dependent secant shear moduli and equivalent damping ratios. The calibration process and performance of the PM4Silt constitu-tive model are described for each soil. The results illustrate that PM4Silt is capa-ble of reasonably approximating a range of monotonic and cyclic loading behav-iors important to many earthquake engineering applications and is relatively easy to calibrate.
more »
« less
PM4Silt (Version 1): A silt plasticity model for earthquake engineering applications
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 shear 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.
more »
« less
- Award ID(s):
- 1635398
- PAR ID:
- 10079691
- Date Published:
- Journal Name:
- Report No. UCD/CGM-18/01, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, CA, 108 pp.
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT This study investigates linkages between volume change, pore fluid drainage, shear wave velocity, and temperature of soft clays using a thermal triaxial cell equipped with bender elements, a measurement approach that has not been explored widely in past thermo-mechanical studies. Two kaolinite specimens were consolidated mechanically to a normally consolidated state and then subjected to drained and undrained heating-cooling cycles, respectively. After cooling, the specimens were subjected to further mechanical consolidation to evaluate changes in apparent preconsolidation stress. Both specimens showed net contractive thermal strains after a heating-cooling cycle and overconsolidated behavior during mechanical compression immediately after cooling. The shear wave velocity increased during drained heating, but negligible changes were observed during drained cooling, indicating permanent hardening because of thermal consolidation during the heating-cooling cycle. The shear wave velocity decreased during undrained heating because of a reduction in effective stress associated with thermal pressurization of the pore fluid but subsequently increased when drainage was permitted at elevated temperature. The shear wave velocity increased slightly during undrained cooling but decreased when drainage was permitted at room temperature. Net increases in small-strain shear modulus of 17 and 11 % after heating-cooling cycles under drained and undrained (with drainage after reaching stable temperatures) conditions, respectively, provide further evidence to the potential of thermal soil improvement of normally consolidated clays. Transient changes in shear modulus also highlight the importance of considering drainage conditions and corresponding changes in effective stress state during heating-cooling cycles.more » « less
-
Calibration and validation of constitutive models and numerical modeling techniques used in analysis of soil liquefaction and its effects are often based on extensive comparisons with the results of element tests and centrifuge experiments. While good quality experimental data are available to understand and quantify the stress-strain-strength response of liquefiable soils in monotonic and cyclic drained/undrained element (triaxial and direct simple shear) tests, the results of these experiments are often less repeatable when the soil approaches liquefaction state and relatively large deviatoric strains suddenly develop within a few cycles of loading. The main source of these less repeatable patterns of soil behavior appears to be instability rather than the attainment of a state of material failure. The goal of this paper is to investigate the role of instability on the stress-strain response of liquefiable soils by using a critical state sand plasticity model that is enriched with an internal length scale representing the potential shear bands that may develop during monotonic or cyclic loading conditions. Through a series of numerical simulations, it is shown that the global stress-strain response measured in the element tests is a good approximation of the soil constitutive response before an unstable condition such as shear banding or liquefaction develops in the soil specimen.more » « less
-
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 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) 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).more » « less
-
null (Ed.)This paper presents results from an experimental program that measured the thixotropic behavior of several remolded soft clays with varying plasticity. Measurements were conducted at various curing times ranging from 1.0 hour to 64 days. The undrained shear strength s/sub u/ was measured using the fall cone while companion specimens were also prepared and mounted in a bender element jig for measurement of temporal changes in shear wave velocity V/sub vh/. The bender element specimens allowed for much greater frequency of repeated data collection since the measurement was nondestructive and only one specimen was needed per clay for the full curing period. Furthermore, the combination of measurements provided data at each curing period for the rigidity index I/sub R/ taken as the ratio of the small-strain shear modulus G/sub max/ to s/sub u/. Values of s/sub u/ and V/sub vh/ increased significantly with time for all the clays, especially at a higher rate for the first 2 to 4 days. The full time-series dataset of each clay enabled the evolution of I/sub R/ during thixotropic hardening, which was found to be nearly constant over time for each of the studied clays. These data shed light on analyzing the behavior of offshore infrastructure and in situ tools that remold soft clays during installation such as pipelines, embedment anchors, and full-flow penetrometers.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
