Modal identification of a centrifuge soil model using subspace state space method

• Modal parameters of a layered soil deposit are identified using the subspace state space system identification method.
• Identified natural frequencies are validated against transfer function spectra and the theoretical amplification functions.
• Normal mode shapes are extracted from the identified complex valued mode shapes and compared with the analytical models.
• Acceleration-time histories and Fourier spectra are predicted for three events.
• Reasonable predictions are made for the small and moderate shaking events.

In this paper, modal parameters of a layered soil system comprising of a soft clay layer overlying a dense sand layer are identified from accelerometer recordings in a centrifuge test. For the first time, the subspace state space system identification (4SID) method was employed to identify the natural frequencies, damping ratios, and complex valued mode shapes while considering the non-proportional damping in a soil system. A brief review of system identification concepts needed for application of the 4SID techniques to structural modal identification is provided in the paper. The identified natural frequencies were validated against those estimated by transfer function spectra. The computed normal mode shapes were compared with closed-form solutions obtained from the one-dimensional shear wave propagation equation. The identified modal parameters were then employed to synthesize state space prediction models which were subsequently used to simulate the soil response to three successive base motions. The identified models captured acceleration time-histories and corresponding Fourier spectra reasonably well in the small and moderate shaking events. In the stronger third shaking event, the model performed well at greater soil depths, but was less accurate near the surface where nonlinearities dominated.