Full-Scale Field Testing

To check the validity of our numerical simulations, we make comparisons to measurements taken at instrumented field sites. We completed our first field measurements in August 2014 at a Caltrans construction site in Goleta, CA, and anticipate visiting more sites by the conclusion of the project.

The field testing scheme consists of instrumenting the top of a pile foundation and the adjacent free field with accelerometers and then recording vibrations at the two locations simultaneously. A variety of vibrations sources are used, including "passive" sources such as the ambient noise caused by vehicle traffic, and "active" sources like a mechanical shaker or striking a steel plate or adjacent foundation with a sledge hammer (see picture at top of page). To minimize the effects of inertial soil-structure interaction resulting from excitation of the superstructure mass, we are specifically targeting sites that are under construction such that the pile foundations have been constructed but don't support a structure yet. This also conveniently provides easy access to the top of the foundation for attaching the accelerometer.

We compute transfer functions from the field data by computing Fourier spectra of the foundation motion and the corresponding free-field motion, then taking the ratio of the two in the frequency domain. An example is shown below:

This figure compares our analytical solutions for vertically propagating shear waves (bold red line) and Rayleigh waves (bold blue line) to measured transfer functions at the Goleta field site. The analytical Rayleigh wave solution seems to capture the reduction in FIM over the frequency range of engineering interest (about 1 to 30Hz) fairly well, but there is clearly a lot of scatter in the field measurements. We suspect that inertial effects from the foundation mass may be influencing the results, as well as the noise inherent in the instrumentation system. We plan to repeat similar experiments at other sites with more sensitive instruments in the future.

If you know of or are responsible for a site where we could access a deep foundation that doesn't support a structure yet, please contact us-- your support would be greatly appreciated!