Experiments on Nonlinear Effects in Ground-Surface Vibration

Summary

Results are presented of an experimental study of the nonlinear evolution and interaction of sinusoidal ground-surface vibration. The method is the same as used in acoustics: two vertical vibrators — sources of vertical harmonic excitation — were installed on the ground and driven separately and simultaneously, the resulting surface motion being picked up by an array of 8 vertical seismometers placed at various distances from the vibrators. The seismometer signals, representing the vertical component of the ground (particle) velocity, were recorded, then digitized and Fourier analysed. The novelty of the experiments in comparison with the similar work done in the USSR lies in the use of very small vibrators. This permitted generation of vibration with Mach numbers both larger and smaller than 10⁻⁵ — the strain level regarded in geomechanics as the boundary between linear and nonlinear soil behaviour.

In addition to harmonic generation and nonlinear interaction — effects observed earlier by other workers — the obtained data indicate the phenomena of self-modulation and Fermi-Pasta-Ulam recurrence in ground-surface vibration of sufficiently small amplitude.