Nonlinear Seismic Effects in Soils: Numerical Simulation and Studyстатья
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Дата последнего поиска статьи во внешних источниках: 26 декабря 2017 г.
Аннотация:To study the effects of nonlinearity in the seismic response of soils, a
numerical simulation of the propagation of vertically incident seismic waves in horizontal
soil layers were performed. Shear noiselike and monochromatic seismic
waves of various intensities were used as input signals. The behavior of soils was
described by a nonlinear hysteretic model. To extract and study nonlinear components
in the ground response, the nonlinear system identification method and analysis
of higher-order spectra of oscillations on the surface were applied. Even for weak
input signals, the response of the simulated soils contained a noticeable nonlinear
component. An increase in the intensity of input signals led to increasing distortions
of propagating signals, due to the generation and growth of combination-frequency
harmonics. The results show that odd types of nonlinearity are most typical for soils,
such as cubic and fifth-order nonlinearities, causing generation of the third and fifth
higher harmonics of main frequencies of input signals. Nonlinearities of even types,
such as quadratic, fourth-order, and sixth-order, concerned with asymmetry, or skewness,
of oscillations (i.e., quasi-static deformations of the surface) are usually weak,
except some special cases, in which a stress-strain relationship of a soil can be represented
by functions with noticeable even components. A weak nonlinearity results
in an increase in high-frequency components, due to the generation of higher harmonics.
In cases of strong nonlinearity, in which a decrease in amplification and in
shear moduli become noticeable, changes in spectra of propagating signals achieve
their maximum. As a result, input signals with arbitrary spectra are transformed into
output signals with spectra of the type of E(f ) ^ f -k, where k depends on the properties
of the medium.