Year: 2019

Participants:  April Bowman, Shideh Dashti, Joelle Westcott, Erin Nebel, Rob Drevno

Summary: Liquefaction, or the generation of pore water pressure between soil grains due to seismic excitation, reduces the shear strength of a granular skeleton and is a significant cause of damage in earthquakes. To prevent such damage, designers use one-dimensional, free-field settlement as a basis to predict displacement, and the input variables are based on the density of the sand and earthquake-induced input stress. It has been shown through centrifuge modelling that these simplifications neglect an array of mechanisms that contribute to settlement. Most experimental investigations into the liquefaction phenomenon have utilized clean sands. Yet research into the 1989 Loma Prieta Californian and the 1999 Izmit, Turkey earthquakes have established that the make-up of the soil heavily influenced the outcome. In both cases, and more, the granular material in question was a silty-sand.

Silty-sand is an extremely complex material. Made up of sand and fine-grained, non-plastic or plastic, granular mediums, it has been shown through element testing that even static prediction of the anticipated strengths and behaviors is challenging. The limited, available literature on the behavior of this material during dynamic events is contradictory, and has presented no useful conclusions on how to account for this material. Utilizing experience with this material gained during her PhD, April intends to focus on how to use silty-sands in the centrifuge to produce accountable results to predict the influence of this material on liquefaction; hopefully leading to incorporation into performance-based design.

The specific goals include: Developing a method for placement and saturation of silty-sands in a centrifuge model. Several studies have shown that soil behaviour is highly dependent on sample preparation and no reliable placement method for silty-sands has yet been established in centrifuge modelling.

Incorporate new dynamic instrumentation into the centrifuge models, being developed by the Technion, Israel, which can measure the dynamic stress-state of the model. 

Understand how the liquefaction behaviour changes in the free field and beneath a single degree of freedom structure as non-plastic fines content is increased in a single homogenous soil layer.   

Examine the effect of layering and interlayering.

As silty-sand behaviour is better understood, complexity to the centrifuge model can be added such as the applicability of existing mitigation measures and the impact on multiple structures.

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Full Project Title: Seismic Response of Embankments on Liquefiable Soils Improved with Compacted Granular Columns

Year: 2018

Participants: Juan Carlos Tiznado Aitken, Shideh Dashti, Hiral Gandhi, Lauren Strand, Madison Philips

Summary: The main goal of this project is to investigate how the use of compacted granular columns (CGC) may help improve the seismic performance of embankments on potentially liquefiable soils. First, the separated effects of densification, reinforcement, and enhanced drainage mechanisms provided by CGC's will be evaluated through several reduced-scale centrifuge model tests perfomed at CIEST. Then, non-linear finite element models will be calibrated against the experimental data in order to perform a parametric study considering different soil conditions and design scenarios. Finally, the insights from the previous phases will be used to evaluate the validity and limitations of current engineering design procedures for CGC’s. This project is intended to enhance our understanding of the seismic behavior of geo-structures affected by liquefaction and lateral spreading as well as to collaborate in bringing engineering practice towards a performance-based design philosophy.

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Year: 2018

Participants: Daniel Green, David Manlove, Jordan Thoning, Nicholas Gentz, Steven Karl, Zachary Trahey, Dr. Shalom Ruben, Dr. Brad Wham, Hiral Gandhi

Primary Investigator: UC Boulder Mechanical Engineering Senior Design in conjunction with Design Center Colorado and Sandia National Labs

Summary: The Sandia Senior Design Team were tasked to design a switch that activates at 9g, as part of a cubesat subsystem. As part of the test phase of their device, the team used the 15-g centrifuge at CIEST to replicate a cubesat launch profile and observed their switch activate through video stream.

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Summary: This Faculty Early Career Development (CAREER) grant will create a new approach for evaluating the behavior of clusters of buildings on liquefiable ground during earthquakes and pave the way toward designing mitigation measures that improve building performance at a system level. Earthquake-induced soil liquefaction can cause substantial damage to urban areas where multiple buildings and infrastructure systems are clustered. Previous studies have shown that buildings located in close proximity to one another can interact in earthquakes affecting ground motions, settlement patterns, and building damage potential. The parameters that control the seismic performance of building clusters are poorly understood. As a result, mitigation measures that are currently designed perform poorly, particularly when the performance of a building is evaluated in the context of its surroundings. This award supports a systematic study of the impact of adjacent buildings on the effectiveness of liquefaction remediation techniques. In doing so, this award contributes to the resilience of cities globally. 

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Full Project Title: NEESR: Seismic Response of Shallow Underground Structures in Dense Urban Environments

Summary: Shallow underground structures used for public transportation are a key component of sustainable cities. In dense urban environments, underground structures are often built near tall buildings. Although such buildings have the potential to alter ground motions in their vicinity and transmit significant forces to adjacent underground structures, the impact of these forces remains uncertain. This project will study the seismic response of temporary and permanent cut-and-cover box structures near mid- to high-rise buildings using a combination of centrifuge testing and numerical simulations. The data from centrifuge tests will serve two purposes: first, to understand seismic soil-structure-underground structure-interaction (SSUSI), and second, to calibrate and improve numerical models. Nonlinear numerical simulations of centrifuge tests will help assess and improve the capabilities of existing numerical tools in capturing SSUSI and key loading mechanisms. Parametric studies using calibrated models will be conducted to make design recommendations. Data from this project will be archived and made available to the public through the NEES Project Warehouse/data repository.

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Summary: The Western Transportation Institue has conducted a series of modelling tests of deep patch repairing method used in Highway in our 400 g-ton centrifuge. The purpose of this project is to evaluate and improve current FHWA Office of Federal Lands Highway (FLH) and US Forest Service (USFS) deep patch design and construction methods.

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Full Project Title: Physical Modeling of Buried Explosion in Soils by Geotechnical Centrifuge

Summary: Funded by the Multidisciplinary University Research Initiatives (MURI) program by Office of Naval Research (ONR). Buried explosion is a complex phenomenon, involving high strain-rate soil dynamics, fluid dynamics, fractures, shocks and multi-scale physics. A comprehensive experimental program using geotechnical centrifuge modeling was conducted in conjunction with computational modeling using finite element, boundary element and discrete element and meshless methods to achieve a new level of scientific understanding of the complex multi-phase phenomena. To address the experimental gaps in the current literature, an extensive series of centrifuge tests were conducted to examine the high-rate dynamic soil behavior under explosive loads with parametric variations of the charge size and shape, burial depth, soil type, water content, and g-level with both in-flight and post-detonation measurements. A novel integration of a high-speed imaging system into the centrifuge domain was developed to capture the blast response and allowed detailed characterization of the transient, multiphasic soil blast mechanics including early soil disaggregation and ejecta, gas–particle interactions, shock propagation and soil dome evolution. The blast-induced changes in the in-soil stress, acceleration, as well as the above-ground pressure and acoustic intensity were measured by an extensive suite of miniaturized sensors. Precise crater dimensions and geometry were captured by laser profilometry. The advanced centrifuge scaled modeling studio can be used for detailed parametric experimental investigations as well as physical benchmarks for validation and calibration of computational simulations for soil blast, cratering and many other applications.

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