Soil behavior is inherently nonlinear, including the strain-dependent modulus and damping values measured in the resonant column apparatus. This paper describes the frequency response behavior in the resonant column test using a theory that is valid for all frequencies including those that differ from the resonant frequency, and for strain amplitudes ranging from near zero to those approaching failure. The theory is based on the ASTM D4015-15 Standard for devices having a fixed specimen base, with torque and rotation measured at the top of the specimen. The theory accounts for damping in the apparatus. Results show that the frequency response at all shear strain amplitudes is significantly different from that predicted by linear viscoelastic theory, especially at small shear strains where both shear modulus and damping are most highly dependent on shear strain amplitude. Proof of this dependency at low strains is provided by plotting shear modulus and damping to natural scales rather than logarithmic scales as is traditionally done.
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