29 research outputs found
LEAP-2017 Simulation Exercise: Calibration of Constitutive Models and Simulation of the Element Tests
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)
LEAP-2017: Comparison of the Type-B Numerical Simulations with Centrifuge Test Results
This paper presents comparisons of 11 sets of Type-B numerical simulations with the results of a selected set of centrifuge tests conducted in the LEAP-2017 project. Time histories of accelerations, excess pore water pressures, and lateral displacement of the ground surface are compared to the results of nine centrifuge tests. A number of numerical simulations showed trends similar to those observed in the experiments. While achieving a close match to all measured responses (accelerations, pore pressures, and displacements) is quite challenging, the numerical simulations show promising capabilities that can be further improved with the availability of additional high-quality experimental results
Design of Piles in Liquefied Soils for Combined Inertial and Kinematic Demands
The design of piles penetrating liquefiable soils poses a number of challenges, including 1) accounting for the effects of liquefaction on lateral soil springs (p-y), 2) estimating liquefaction-induced kinematic demands (lateral spreading displacements) including pile-pinning effects, and 3) combining the effects of inertial demands from superstructure and kinematic demands from liquefied ground. This paper illustrates two approaches to address the above questions: A) a simplified decoupled approach adopting equivalent static analysis (ESA), and B) a detailed, coupled approach adopting nonlinear dynamic analyses (NDA). These approaches are presented through example projects showing the limitations and the potential conservatism associated with the simplified decoupled ESA approach, the benefits of using more rigorous coupled NDA approach, and recommendations on how to modify ESA methods, when used, to approximate outputs from advanced NDA for slope instability-induced demands on piles. These modifications include using pile-restrained, rather than free-field, soil displacements in the ESA, to reduce conservatism associated with these methods
Recommended from our members
Development and Implementation of Semiempirical Framework for Modeling Postliquefaction Shear Deformation Accumulation in Sands
A framework for the estimation of coseismic deformations in the postliquefaction regime is developed based on an extensive database of available cyclic undrained stress-controlled tests on clean sand samples without static shear bias, covering a wide range of relative densities. Based on fundamental experimental observations, a compliance rate is defined as the postliquefaction shear strain rate per cycle over the shear stress amplitude. Semiempirical relationships of the compliance rate as a function of relative density are developed to provide guidance for estimating postliquefaction shear strains. The proposed framework provides a basis for the calibration of advanced constitutive models capable of capturing postliquefaction strain accumulation. A calibration methodology is proposed using both existing liquefaction resistance curves and the newly developed semiempirical relationships for estimating postliquefaction shear strain accumulation. The validity of the proposed methodology is demonstrated by numerical simulations, using the PM4Sand model, of two well-documented centrifuge tests focusing on liquefaction-induced demands on engineering structures
Bioactive Metabolites of the Stem Bark of <i>Strychnos aff. darienensis</i> and Evaluation of Their Antioxidant and UV Protection Activity in Human Skin Cell Cultures
The genus Strychnos (Loganiaceae) is well-known as a rich source of various bioactive metabolites. In continuation of our phytochemical studies on plants from Amazonia, we examined Strychnos aff. darienensis, collected in Peru. This species has been traditionally used in South America and is still presently used as a drug by the Yanesha tribe in Peru. Phytochemical investigation of this plant led to the isolation and structure elucidation by ΝuclearΜagnetic Resonance and High Resolution Mass Spectroscopy of 14 compounds that belong to the categories of phenolic acids [p-hydroxybenzoic acid (1) and vanillic acid (2)], flavonoids [luteolin, (3),3-O-methyl quercetin (4), strychnobiflavone (5), minaxin (6) and 3’,4’,7-trihydroxy-flavone (7)], lignans [syringaresinol-β-D-glucoside (8), balanophonin (9) and ficusal (10)] and alkaloids [venoterpine (11), 11-methoxyhenningsamine (12), diaboline (13) and 11-methoxy diaboline (14)]. The isolated flavonoids—a class known for its anti-aging activities—were further evaluated for their biological activities on normal human skin fibroblasts. Among them, only (6), and to a lesser extent (7), exhibited cytotoxicity at 100 µg/ml. All five flavonoids suppressed intracellularreactive oxygen species (ROS) levels, either basal or following stimulation with hydrogen peroxide or both. Moreover, luteolin and strychnobiflavone protected skin fibroblasts against ultraviolet (UV)-irradiation-induced cell death. The isolated flavonoids could prove useful bioactive ingredients in the cosmetic industry