Characterization of silty soil thin layering and groundwater conditions for liquefaction assessment

Abstract

Tools for characterizing thin layering and groundwater table conditions are evaluated at silty soil sites being assessed for liquefaction. Thin interlayered stratigraphy and groundwater table fluctuation are two potential causes for inconsistencies observed during the Canterbury earthquake sequence, wherein liquefaction did not manifest at several silty soil sites, despite simplified liquefaction assessment procedures indicating severe manifestations would be expected. Site investigations should capture these features to allow for improved assessment of liquefaction potential at silty soil sites. Cone penetration tests (CPTs), mini-CPTs, and sonic borings do not adequately capture thin layering. However, detailed logging of high-quality samples captures the actual in situ layering that may help explain the limitations of simplified liquefaction assessment procedures at these sites, revealing the need to understand underlying limitations in current site investigation techniques. Piezometers, sonic borings, high-quality sampling, crosshole testing, and regional groundwater maps are evaluated to assess their ability to capture complex groundwater conditions. Multiple groundwater measurement methods are typically required to characterize groundwater fluctuations. An approach to using enhanced site characterization tools is recommended for liquefaction assessments at silty soil sites with thin layering and groundwater fluctuations.

Effective Stress Analysis of Liquefaction Sites and Evaluation of Sediment Ejecta Potential

“…The thin-layer effect may significantly influence the CPT tip resistance measurement of a soil layer thinner than 0.5 m. However, most of the critical layers at sites studied in this research have a thickness more than 1.0 m, making the thin-layer effect less significant. Beyzaei et al [2020] used the smaller CPT cone size to study the effect and found that the relative influence is small. We have performed analysis using parameters derived from inverse-CPT measurement (i.e., The EPI can distinguish sites with and without ejecta manifestation, as shown in Figure 5.5.…”

PEER Workshop on Liquefaction Susceptibility

“…Better integration of qualitative geologic information about the soils at a site and the quantitative information from in situ and laboratory engineering tests is essential for quantifying and minimizing the uncertainties associated with site characterization (Beyzaei et al 2020). At the site scale, one potential way to do this is to use proxies for depositional environments.…”

Dynamic, in situ, nonlinear-inelastic response and post-cyclic strength of a plastic silt deposit

“…This observation is corroborated by the recent centrifuge studies on reconstituted sand reported by Adamidis and Madabhushi (2018) and Ni et al (2020), as well as the observations of the Sand Array reported by . Since drainage is unavoidable during earthquakes (Beyzaei et al 2018(Beyzaei et al , 2019 and owing to the frequency content of the ground motions, the in-situ tests reported herein produce realistic soil responses to seismic shaking.…”

“…Geotechnical earthquake engineering practice regarding the seismic response of silt deposits generally center on assessments of their cyclic and post-cyclic responses. Such assessments may consider the plasticity index, PI, fines content, FC, overconsolidation ratio, OCR, effective confining or vertical stress, ' v0 , static shear stress, and soil fabric (Sanin and Wijewickreme 2006;Soysa 2015;Dahl et al 2010Dahl et al , 2014Beyzaei et al 2019;Wijewickreme et al 2019;Jana and Stuedlein 2020). Critical dynamic soil properties useful for calibrating site response and constitutive models include the threshold shear strain to trigger nonlinearity,  te , threshold shear strain to trigger nonlinear-inelasticity and generate residual excess pore pressure,  tp , and the relationship between shear strain amplitude and residual excess pore pressure ratio, r u,r , defined as ratio of residual excess pore pressure, u e,r , and ' v0 (e.g., Hashash et al 2010;Markham et al 2016).…”

“…Some amount of disturbance is inevitable and is generally more severe for silts of lower plasticity, the consequence of which is significantly lower cyclic resistance relative to insitu conditions (Kurtulus and Stokoe 2008;Dahl et al 2010;Wijewickreme et al 2019). Whereas significant progress has been made in understanding the elemental cyclic response of silt in the D r a f t laboratory, researchers have identified differences between the observed in-situ penetration testbased Yost et al 2019) and laboratory-predicted response of natural silts (Beyzaei et al 2018(Beyzaei et al , 2019. The complexity of the in-situ dynamic response arises from excess pore pressure diffusion, soil variability, interlayering, and multi-directional seismic shaking (Dobry and Abdoun 2015;Adamidis and Madabhushi 2018, Beyzaei et al 2018, 2019Ni et al 2020;, which are difficult to simulate in laboratory settings.…”