In Situ, Nonlinear Soil Response Applying an Active Source

Soil sites have a profound effect on ground motion during earthquakes due to their low wave speeds, layered structure, and nonlinear constitutive behavior. Measurements of nonlinear soil response under natural conditions are critical to understanding soil behavior during earthquakes. Currently, quantitative measurements of nonlinear soil response are derived from laboratory experiments on small samples. In this paper, we extend laboratory methods for measuring nonlinear soil response to field-scale. We observe the in situ, nonlinear response of a natural soil formation using measurements obtained immediately adjacent to a large vibrator truck. The source generates a steady-state wavefield in the soil formation at a range of discrete source frequencies and source amplitudes. Accelerometers within the source provide an estimate of the source output to the soil, and an array of 4 accelerometers adjacent to the source record the wavefield at 1.5 m spacing. We develop a homodyne analysis to extract the steady-state amplitude at each discrete source frequency and amplitude without contamination from source harmonics. Steady-state amplitude ratios are computed between the receivers and the source, and between adjacent receiver pairs within the array. Both sets of amplitude ratios show dramatic decreases in peak frequency as the source amplitude is increased. These peak frequency shifts are qualitatively similar to the nonlinear soil response observed for laboratory samples under resonance conditions. Amplitude ratios between adjacent receiver pairs suggest the nonlinear soil response persists across the receiver array and is not limited to the source-soil contact region. The magnitudes of the observed peak shifts appear to depend on their frequency, a proxy for depth, which is consistent with the confining pressure dependence of soil nonlinearity observed in laboratory experiments. Future work will include measurements of steady-state phase velocities across the array to better understand the nature of nonlinear wave propagation within natural soil formations (wavelengths, polarization, etc.). Forward modeling to match these field-scale observations of nonlinear soil response may lead to nonlinear site characterization tools and ultimately better predictions of local, nonlinear site response.