Non-Linear Ground Motion Amplification Functions for Fine Grained and Highly Organic Soils
Non-linear (NL) ground motion amplification functions have been developed for fine grained and highly organic soils using ground response analysis for two profiles in the Kushiro area of Hokkaido, Japan. The NL ground motion amplification functions will support a broader research effort to develop fragility functions for levees resting on organic soils in Hokkaido. The ground motion amplification functions were developed using one-dimensional (1D) NL ground response simulations in DEEPSOIL (Hashash et al. 2016) based on a suite of earthquake ground motions compiled by Baker (Baker et al. 2011). Corrections to the small-strain Darendeli (2001) modulus reduction curve for fine grained soil recommended by Yee et al. (2013) were used to realistically represent the large-strain portion of backbone curves by asymptotically approaching the shear strength at large strains. The GQ/H material model (Groholski, 2016) in DEEPSOIL was utilized to preserve the input undrained shear strength while still providing the ability to represent small-strain stiffness nonlinearity. The resulting ground motion amplification values, defined as the spectral acceleration of the surface motion divided by the spectral acceleration of the input outcrop motion, was regressed using the functional form for NL site ground motion amplification by Stewart et al. (2014).
Ground motion amplification functions for organic soils in the Sacramento / San Joaquin Delta were previously developed by Kishida, et al. (2006 and 2009a) using 1D equivalent-linear (EL) ground response analysis (GRA) that were subsequently regressed to derive ground motion amplification functions. These simulations utilized linear and nonlinear regression models for dynamic properties of highly organic soils through ongoing research on this topic that was ultimately concluded by Kishida et al. (2009b). However, it is not clear whether these ground motion amplification functions are applicable to the Hokkaido system due to differences in the Peat layers in Hokkaido compared with the Delta, nor is it clear the extent to which the equivalent linear assumption influenced the ground motion amplification functions. This thesis also presents a brief comparison between the NL ground motion amplification functions developed herein and the EL ground motion amplification functions for highly organic soils presented by Kishida et al. (2009a).