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1. Dynamic, In-situ, Nonlinear-Inelastic Response and Post-Cyclic Strength of a Plastic Silt Deposit

   https://doi.org/10.1139/cgj-2020-0652  

   Jana, Amalesh; Stuedlein, Armin W. (April 2021, Canadian Geotechnical Journal)

   null (Ed.)

   This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicate that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high frequency P-waves are negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranging from 0.002 to 0.003% and 0.008 to 0.012%, respectively, where the latter corresponded to ~30% of Gmax. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in-situ compared to that of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in-situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens. 

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2. Comparison of Seismic Compression Procedure Predictions: Case History from Japan

   Green, R.A. and (September 2020, Proc. 17th World Conference on Earthquake Engineering)

   null (Ed.)

   Seismic compression is the accrual of contractive volumetric strain in unsaturated or partially saturated sandy soils during earthquake shaking and has caused significant distress to overlying and nearby structures, to include the 2007, Mw6.6 Niigata-ken Chuetsu-oki, Japan earthquake. Of specific interest to this study is the seismic compression that occurred during this event at the Kashiwazaki-Kariwa Nuclear Power Plant (KKNPP) site. What makes this case history of particular value is that the motions at the site were recorded by a free-field downhole array (Service Hall Array, SHA) and the magnitude of the seismic compression was accurately determined from the settlement of soil around a vertical pipe housing one of the array seismographs. The seismic compression at the site was ~10-20 cm. The profile at the site was well characterized by in-situ tests and laboratory tests performed on samples from the site, which allows seismic compression models to be calibrated. The study presented herein compares the predictions of the simplified and non-simplified forms of the expanded Byrne model. The predictions are in good accord with field observations, but the slight under-prediction by the non-simplified model may relate to estimated soil properties, assumed orientation of the ground motions and accounting for multidirectional shaking, and/or the numerical site response analyses used to compute the variation of the shear strains during shaking at depth in the soil profile. 

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3. Smooth Crustal Velocity Models Cause a Depletion of High-Frequency Ground Motions on Soil in 2D Dynamic Rupture Simulations

   https://doi.org/10.1785/0120200311  

   Huang, Yihe (June 2021, Bulletin of the Seismological Society of America)

   ABSTRACT A depletion of high-frequency ground motions on soil sites has been observed in recent large earthquakes and is often attributed to a nonlinear soil response. Here, I show that the reduced amplitudes of high-frequency horizontal-to-vertical spectral ratios (HVSRs) on soil can also be caused by a smooth crustal velocity model with low shear-wave velocities underneath soil sites. I calculate near-fault ground motions using both 2D dynamic rupture simulations and point-source models for both rock and soil sites. The 1D velocity models used in the simulations are derived from empirical relationships between seismic wave velocities and depths in northern California. The simulations for soil sites feature lower shear-wave velocities and thus larger Poisson’s ratios at shallow depths than those for rock sites. The lower shear-wave velocities cause slower shallow rupture and smaller shallow slip, but both soil and rock simulations have similar rupture speeds and slip for the rest of the fault. However, the simulated near-fault ground motions on soil and rock sites have distinct features. Compared to ground motions on rock, horizontal ground acceleration on soil is only amplified at low frequencies, whereas vertical ground acceleration is deamplified for the whole frequency range. Thus, the HVSRs on soil exhibit a depletion of high-frequency energy. The comparison between smooth and layered velocity models demonstrates that the smoothness of the velocity model plays a critical role in the contrasting behaviors of HVSRs on soil and rock for different rupture styles and velocity profiles. The results reveal the significant role of shallow crustal velocity structure in the generation of high-frequency ground motions on soil sites. 

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4. Assessing the Significance of Dynamic Soil-Structure Interaction Using Large-Amplitude Mobile Shakers

   Farrag, Sharef; Gucunski, Nenad; Cox, Brady; Menq, Farnyuh; Moon, Franklin; DeVitis, John (April 2019, 2019 Geo Congress, ASCE)

   To accurately describe the dynamic characteristics of bridges, it is important in some instances to take into consideration the flexibility and damping of the soil-foundation system. The ability to evaluate those properties in the field can serve as both a check for the design assumptions, and as assistance in the design of bridges with similar superstructure/substructure loading and soil conditions in the future. The goal of the presented study is to demonstrate the use of large-amplitude shaking as an effective tool in measuring actual response/behavior of bridges, and developing better understanding of the dynamic response of bridge systems. For that purpose, a large-amplitude shaking of a bridge in Hamilton Township, New Jersey, was carried out. The T-Rex, a mobile shaker from the Natural Hazards Engineering Research Infrastructure (NHERI) experimental facility at the University of Texas, Austin was employed to shake the bridge. A large number of sensors, geophones and accelerometers, were installed at various locations on the bridge deck, pier cap, and on the adjacent ground to capture the dynamic response of the bridge system. Furthermore, the results from field testing were used to calibrate a 3D finite element model of the bridge. The model was used to conduct a comparative analysis of the bridge response for the assumption of the bridge with fixed foundation conditions, and the bridge with the consideration of dynamic soil-structure interaction (DSSI) effects. The comparison with the field testing results demonstrate that the fixed foundation assumption model does not fully capture the behavior of the bridge, as opposed to the model with DSSI considerations. 

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5. In-situ and Laboratory Cyclic Response of an Alluvial Plastic Silt Deposit

   Dadashiserej, A.; Jana, A.; Stuedlein, A.W.; Evans, T.M.; Zhang, B.; Xu, Z.; Stokoe II, K.H.; Cox, B.R. (May 2022, Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney 2021)

   Current best practices for the assessment of the cyclic response of plastic silts are centered on the careful sampling and cyclic testing of natural, intact specimens. Side-by-side evaluation of in-situ and laboratory element test responses are severely limited, despite the need to establish similarities and differences in their characteristics. In this paper, a coordinated laboratory and field-testing campaign that was undertaken to compare the strain-controlled cyclic response of a plastic silt deposit at the Port of Longview, Longview, WA is described. Following a discussion of the subsurface conditions at one of several test panels, the responses of laboratory test specimens to resonant column and cyclic torsional shear testing, and constant-volume, strain-controlled cyclic direct simple shear testing are described in terms of shear modulus nonlinearity and degradation, and excess pore pressure generation with shear strain. Several months earlier, the in-situ cyclic response of the same deposit was investigated by applying a range of shear strain amplitudes using a large mobile shaker. The in-situ response is presented and compared to the laboratory test results, highlighting similarities and differences arising from differences in mechanical (e.g., constant-volume shearing; strain rate-effects) and hydraulic (e.g., local drainage) boundary conditions and the spatial variability of natural soil deposits. 

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