Liquefaction is associated with the loss of mean effective stress and increase of the pore water pressure in saturated granular materials due to their contractive tendency under cyclic shear loading. The loss of mean effective stress is linked to loss of grain contacts, bringing the granular material to a "semifluidized state" and leading to development and accumulation of large cyclic shear strains. Constitutive modeling of the cyclic stress-strain response in earthquake-induced liquefaction and post-liquefaction is complex and yet very important for stress-deformation and performance-based analysis of sand deposits. A new state internal variable named strain liquefaction factor is introduced that evolves at low mean effective stresses, and its constitutive role is to reduce the plastic shear stiffness and dilatancy while maintaining the same plastic volumetric strain rate in the semifluidized state. This new constitutive ingredient is added to an existing critical state compatible, bounding surface plasticity reference model, that is well established for constitutive modeling of cyclic response of sands in the pre-liquefaction state. The roles of the key components of the proposed formulation are examined in a series of sensitivity analyses. Their combined effects in improving the performance of the reference model are examined by simulating undrained cyclic simple shear tests on Ottawa sand, with focus on reproducing the increasing shear strain amplitude as well as its saturation in the post-liquefaction response.
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.
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.
Numerical simulations of LEAP-UCD-2017 were performed to validate the numerical modeling approach and provide insight to capabilities and limitations of the adopted constitutive model. This chapter focuses on using an extended version of the SANISAND constitutive model implemented in FLAC 3D program at UBC. The constitutive model was calibrated based on the available laboratory element tests on Ottawa F65 sand. It was then used for simulation of the centrifuge tests on a mildly sloping liquefiable ground of the same soil subjected to dynamic loading. The study covered the Types B and C simulations and the sensitivity analyses. Type B simulations were successful in capturing some aspects of measurements from the experiments. A simplified approach for changing the soil permeability was adopted in Type C simulations, and the improved simulation results were again compared with those measured in the experiments. In the numerical sensitivity analyses, the model appeared to provide reasonable trends for simulation of different sample densities, and ground motion intensities and frequency contents.
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