Published: Feb. 18, 2019
  • Full Project Title: Liquification of Silty Sands 
  • 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 behaviour of this material during dynamic events is contradictory, and has presented no useful conclusions on how to account for this material.  Utilizing my experience with this material gained during my PhD, I intend to focus on how we can 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:

    • Develop 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.