Browsing by Author "Tiznado, Juan Carlos"

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    Assessing soil liquefaction due to large-magnitude subduction earthquakes
    (2025) Santiago, Yrene; Ledezma, Christian; Tiznado, Juan Carlos
    Infrastructure failure due to soil liquefaction has been repeatedly observed in past megathrust earthquakes, causing significant material and structural functionality losses. In most seismic regions, soil liquefaction potential is assessed using updated versions of the cyclic-stress-based simplified procedure initially proposed by Seed and Idriss in 1971. However, the application of these procedures to large-magnitude (Mw > 7.5) subduction earthquakes has shown discrepancies between forward predictions and field observations, particularly regarding liquefaction triggering and manifestation. This paper proposes an alternative model to assess soil liquefaction due to large-magnitude subduction earthquakes based on excess pore water pressure ratios and shear deformations. The triggering criteria are based on the peak values of excess pore pressure ratio and shear strain anticipated within the critical, potentially liquefiable soil layer. The model considers liquefiable layer thickness and relative density, along with input motion's Cumulative Absolute Velocity (CAV), as the main predictors of soil liquefaction. To this end, a numerical model was first developed and validated against results from a dynamic centrifuge test simulating free-field conditions. The calibrated numerical model was then used to perform a numerical parametric study to identify the trends and key predictors of liquefaction in layered soil deposits subjected to large-magnitude subduction earthquakes. Finally, a simplified probabilistic procedure, validated against available case histories, was developed to estimate the probabilities of full, marginal, and no liquefaction occurrence within each critical layer.
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    Influence of pulse-like motions and extreme environmental loads on the seismic foundation response of offshore wind turbines on layered liquefiable soils
    (2024) Hwang, Yu-Wei; Tiznado, Juan Carlos
    Monopile foundations are known as the most common foundation solution for offshore wind turbines (OWTs). However, the state of practice for designing monopile foundations in high seismicity areas is still limited. In particular, the impact of soil liquefaction on the seismic soil-foundation-OWT interaction is not yet well understood. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite-element analyses performed in the OpenSees numerical platform were used to evaluate the seismic performance of a series of hypothetical 5 MW OWTs on monopile foundations in layered, liquefiable sites. A suite of earthquake recordings with and without strong velocity pulses (i.e., near fault, pulse-like and ordinary motions, respectively) was used to investigate the impact of ground motion characteristics on the seismic response of the OWT system. Also, the influence of soilstructure interaction and earthquake shaking coupled with extreme environmental loading (i.e., wind and wave loads) on the seismic performance of soil-OWT systems was evaluated. The numerical results showed pile movements induced by extreme climate loading led to a bias in permanent settlement accumulation across the foundation area and accumulation of soil deformations in the proximity of the pile. Ground motion velocity pulses increased the cyclic stress demand in soil and, therefore, the potential for the occurrence of soil liquefaction. A subsequent, limited numerical sensitivity study showed that the foundation rotations of the OWT system were influenced by ground motion characteristics such as polarity and velocity pulses, and the presence of the wind and wave loads. The cumulative absolute velocity (CAV) was identified as the optimum ground motion intensity measure for permanent foundation settlement and tilt as well as for peak transient foundation tilt of the OWT system under extreme environmental loadings. The net outcome of these factors determined the magnitude and orientation of the foundation rotations at the end of shaking. This study highlights the importance of considering the effects of extreme loadings and pulse-like motions in the design and performance of OWT systems.
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    Seismic performance of mat-founded building clusters on liquefiable soils treated with ground densification
    (2023) Hwang, Yu-Wei; Dashti, Shideh; Tiznado, Juan Carlos
    Current guidelines for evaluating the performance of ground densification as a liquefaction countermeasure near buildings are based on free-field conditions or, at best, consider one structure experiencing soil-structure interaction (SSI) in isolation. However, in urban areas, where structures are constructed in close vicinity of each other, structure-soil-structure interaction in liquefiable deposits near two (SSSI2) or multiple (>= 3) buildings in a cluster (SSSI3+) has been shown as consequential on key engineering demand parameters (EDPs), particu-larly differential settlement. Furthermore, the potential tradeoffs associated with ground improvement in urban settings, considering SSSI2 and SSSI3+, are currently not well understood or defined. In this paper, three-dimensional (3D), fully-coupled, nonlinear, dynamic finite element analyses are first validated with centrifuge models of SSI and SSSI2, including ground densification. These models are subsequently used to explore the influence of building arrangement (two adjacent structures and four structures in a square block) and spacing on key EDPs for mitigated structures undergoing SSSI2 and SSSI3+ compared to that under isolated SSI. For the conditions evaluated, it is shown that both SSSI2 and SSSI3+ could reduce the average settlement of mitigated structures compared to SSI at building spacings (S) > 0.5Wfnd (where Wfnd is the foundation width), particularly in larger clusters experiencing SSSI3+. On the other hand, both SSSI2 and SSSI3+ amplified the permanent tilt of the mitigated structures compared to SSI at S < 0.5Wfnd. The impact of these interactions on tilt reduced at larger spacings. A limited, subsequent numerical sensitivity study showed that pulse-like input motions together with the stress and flow-path bias introduced by SSSI2 and SSSI3+ can increase the uneven accumulation of soil strains below the mitigated structures compared to cases experiencing SSI or the same building clusters subject to non -pulse-like motions. This led to a greater amplification in tilt of mitigated structures experiencing SSSI2 and SSSI3+ at shorter spacings under the selected pulse-like motions. Overall, the results point to the importance of considering the impact of building cluster arrangement, spacing, soil and structural properties, and ground motion characteristics in the design of ground improvement in urban settings.
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    Two-dimensional nonlinear dynamic analysis of a liquefaction case history considering spatial variability and long-duration megathrust earthquakes
    (2024) Saldana-Sotelo, Hector; Montalva, Gonzalo; Escribano, Daniella; Nunez-Jara, Sebastian; Tiznado, Juan Carlos
    We analyzed the unusual liquefaction response of the Los Presidentes site in Concepci ' on, Chile, during the 2010 Mw 8.8 Maule earthquake, using a series of twodimensional nonlinear dynamic analyses (NDAs) on a well characterized subsurface. This site, characterized by interbedded clean and silty sands followed by a dense sand unit, included four identical towers that experienced different settlements, with maximum values reaching 40 cm for the most damaged tower to zero for the less affected one. We used SPT, CPTu, and Vs tests to characterize the soil's spatial variability. The NDAs used the PM4Sand and PM4Silt soil constitutive models implemented in OpenSees. The results suggest that the shear-induced deformation mechanism controls the dynamic response. The computed settlements matched post-earthquake LiDAR and field measurements, while liquefaction vulnerability indexes underestimated observed manifestations. Our results show that liquefactioninduced settlements due to megathrust earthquakes greatly differ from those induced by crustal ground motions.