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  1. Free, publicly-accessible full text available February 1, 2025
  2. Free, publicly-accessible full text available December 1, 2024
  3. Lemnitzer, A. ; Stuedlein, A.W. (Ed.)
    This study presents a laboratory investigation of the monotonic, cyclic, and post-cyclic responses of a lightly overconsolidated, low plasticity silt deposit conducted to support the geotechnical design of a proposed bridge replacement crossing the Willamette River in Corvallis, OR. The design seismic hazard corresponded to the 975-year return period with the Cascadia Subduction Zone contributing the greatest portion of the hazard. The response of the intact, natural specimens was compared to that of specimens reconstituted from the same material for comparison of the effect of soil fabric. Constant-volume cyclic stress controlled direct simple shear tests (CDSS) conducted on the low plasticity silt deposit showed cyclic mobility type behavior and increases in cyclic resistance with OCR. The exponent of the power relationship between cyclic resistance ratio (CRR) and the number of cycles, N, was shown to be smaller than that commonly assumed within the simplified method for cyclic softening of fine-grained plastic soil. Despite higher density, the reconstituted specimens exhibited approximately 16% lower cyclic resistance than their undisturbed counterparts, indicating the importance of soil fabric on the cyclic resistance of natural silt soils. The post-cyclic volumetric strain of the silt deposit was found to be independent of OCR and increased with the maximum excess pore pressure ratio generated during the cyclic tests. 
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    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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