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A numerical model based on the finite element framework was developed to predict the seismic response of saturated sand under free-field conditions. The finite element framework used a non-linear coupled hypoplastic model based on the u-p formulation to simulate the behaviour of the saturated sand. The u-p coupled constitutive model was implemented as a user-defined routine in commercial ABAQUS explicit 6.14. Results of centrifuge experiments simulating seismic site response of a layered saturated sand system were used to validate the numerical results. The centrifuge test consisted of a three-layered saturated sand system subjected to one-dimensional seismic shaking at the base. The test set-up was equipped with accelerometers, pore pressure transducers, and LVDTs at various levels. Most of the constitutive models used to date for predicting the seismic response of saturated sands have underestimated volumetric strains even after choosing material parameters subjected to rigorous calibration measures. The hypoplastic model with intergranular strains calibrated against monotonic triaxial test results was able to effectively capture the volumetric strains, reasons for which are discussed in this paper. The comparison of the numerical results to centrifuge test data illustrates the capabilities of the developed u-p hypoplastic formulation to perform pore fluid analysis of saturated sand in ABAQUS explicit, which inherently lacks this feature.

Following the M7.0 strike-slip earthquake near Kumamoto, Japan, in April of 2016, most geotechnical engineering experts believed that there would be significant soil liquefaction and liquefaction-induced infrastructure damage observed in the densely populated city of Kumamoto during the post-event engineering reconnaissance. This belief was driven by several factors including the young geologic environment, alluvially deposited soils, a predominance of loose sandy soils documented in publicly available boring logs throughout the region, and the high intensity ground motions observed from the earthquake. To the surprise of many of the researchers, soil liquefaction occurred both less frequently and less severely than expected. This paper summarizes findings from our field, laboratory, and simplified analytical studies common to engineering practice to assess the lower occurrence of liquefaction. Measured in situ SPT and CPT resistance values were evaluated with current liquefaction triggering procedures. Minimally disturbed samples were subjected to cyclic triaxial testing. Furthermore, an extensive literature review on Kumamoto volcanic soils was performed. Our findings suggest that current liquefaction triggering procedures over-predict liquefaction frequency and effects in alluvially deposited volcanic soils. Volcanic soils were found to possess properties of soil crushability, high fines content, moderate plasticity, and unanticipated organic constituents. Cyclic triaxial tests confirm the high liquefaction resistance of these soils. Moving forward, geotechnical engineers should holistically consider the soil’s mineralogy and geology before relying solely on simplified liquefaction triggering procedures when evaluating volcanic soils for liquefaction.

Letter to the Editor

Liquefaction or cyclic softening from earthquake shaking have caused significant damage of buildings with shallow foundations. In recent earthquakes, buildings have punched into, tilted excessively, and slid laterally on liquefied/softened ground. The state-of-the-practice still largely involves estimating building settlement using empirical procedures developed to calculate post-liquefaction, one-dimensional, consolidation settlement in the “free-field” away from buildings. Performance-based earthquake engineering requires improved procedures, because these free-field analyses cannot possibly capture shear-induced and localized volumetric-induced deformations in the soil underneath shallow foundations. Recent physical and numerical modeling has provided useful insights into this problem. Centrifuge tests revealed that much of the building movement occurs during earthquake strong shaking, and its rate is dependent on the shaking intensity rate. Additionally, shear strains due to shaking-induced ratcheting of the buildings into the softened soil and volumetric strains due to localized drainage in response to high transient hydraulic gradients are important effects that are not captured in current procedures. Nonlinear effective stress analyses can capture the soil and building responses reasonably well and provide valuable insights. Based on these studies, recommendations for estimating liquefaction-induced movements of buildings with shallow foundations are made.

Flooding is the most costly natural hazard event worldwide and can severely impact communities, both through economic losses and social disruption. To predict and reduce the flood risk facing a community, a reliable model is needed to estimate the cost of repairing flood-damaged buildings. In this paper, we describe the development and assessment of two models for predicting direct economic losses for single-family residential buildings, based on the experience of the 2013 Boulder, Colorado riverine floods. The first model is based on regression analyses on empirical data from over 3000 residential building damage inspections conducted by the Federal Emergency Management Agency (FEMA). This model enables a probabilistic assessment of loss (in terms of FEMA grants paid to homeowners for post-flood repairs) as a function of key building and flood hazard parameters, considering uncertainties in structural properties, building contents, and damage characteristics at a given flood depth. The second model is an assembly-based prediction of loss considering unit prices for damaged building components to predict mean repair costs borne by the homeowner, which is based on typical Boulder construction practices and local construction and material costs. Comparison of the two proposed models illustrates benefits that arise from each of the two approaches, while also serving to validate both models. These models can be used as predictive tools in the future, in Boulder and other US communities, due to adaptability of the model for other context, and similarities in home characteristics across the country. The assembly-based model quantifies the difference between the FEMA grants and true losses, providing a quantification of out-of-pocket homeowner expenses.

Liquefaction mitigation techniques are often used in the field to alleviate liquefaction hazard to the built environment. However, the current practice of designing mitigation techniques ignores the presence of buildings, and depends solely on satisfying the settlement criteria, and constructability. This design practice is due to lack of understanding of the influence of different mitigation strategies on the performance of soil-foundation-structure systems. In this dissertation, centrifuge experiments were designed and conducted to investigate soil-mitigation-foundation-structure systems, considering two potentially inelastic structures (3- and 9-story) placed on a layered liquefiable deposit (with and without a silt cap), with three different mitigation strategies: 1) enhanced drainage through prefabricated vertical drains (PVDs); 2) shear reinforcement using in-ground structural walls (SWs); and 3) enhanced drainage and damping, and shear reinforcement provided by an in-ground gravel-rubber panel wall system. The first set of test results show that PVDs and SWs reduced total foundation settlement compared to the unmitigated case. However, they amplified accelerations on the foundations, which could increase flexural deformations and P-Δ effects, with potentially adverse effects on foundation tilt (particularly for the taller, heavier, more deeply embedded, and weaker 9-story structure). The presence of soil interlayering (due to a silt cap) affected the overall response of unmitigated and PVD-mitigated structures, particularly impacting foundation tilt. Based on the insights gained from the tests with traditional mitigation techniques, we designed and tested a new mitigation strategy for shallow-founded structures: an in-ground gravel-rubber panel wall (GR) system. This system aims to reduce building settlements and tilts, while isolating the structure from the larger acceleration demands expected in mitigated ground. Test results showed that the GRs could be beneficial, roughly satisfying design objectives for the 3-story structure, but amplified tilt on the 9-story structure. Therefore, additional design considerations and shear reinforcement are required in the panel walls to improve total system response. The results presented in this dissertation point to the importance of considering the structure’s dynamic and geometric properties, force-deformation behavior, soil interlayering, and the possible increase in shaking intensity level due to different mitigation strategies, when designing traditional or innovative techniques to mitigate consequences of liquefaction.

Underground structures such as cut-and-cover box structures and retaining wall systems have performed relatively well during past seismic events. However, notable cases such as the failure of the Daikai Subway Station during the 1994 Kobe Earthquake show the importance of the seismic design of these types of structures. Braced excavations are a type of underground structure used to provide space for the construction of cut-and-cover tunnels, basements, foundations, and other permanent underground structures. Even though temporary, braced excavations need to be designed to withstand seismic loads. Current seismic design of underground structures assumes the case of isolation with no adjacent buildings. In reality, underground structures such as braced excavations and subway systems are located in densely populated downtown areas, with adjacent buildings. This research aims to provide insight into the seismic response of braced excavations near tall buildings. Data from three dynamic centrifuge tests were analyzed to evaluate soil-structure-underground structure-interaction (SSUSI) near a temporary braced excavation. Dry, medium dense, Nevada sand was used as the test soil. The first test studied the braced excavation in isolation with no adjacent buildings. The second and third tests studied the same excavation with an adjacent midrise and highrise building, respectively. Various methods of measuring small-strain soil properties in the far-field are explored in this thesis and compared. The seismic response of the braced excavation are presented during the three centrifuge tests in terms of racking displacements, dynamic lateral earth pressures, and bending moments along the excavation walls as well as axial forces on struts to evaluate the seismic impact of an adjacent tall building on the performance of a braced excavation. The conclusions and observations presented in this thesis are preliminary and based on experimental results alone. Final, generalized design recommendations cannot be drawn from these observations. Results from numerical simulations currently being performed by the University of Illinois-Urbana under the direction of Professor Hashash will be used in addition to these experimental results to provide design recommendations in the future.