PSI - Issue 72

G. Gusev et al. / Procedia Structural Integrity 72 (2025) 464–469

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nonuniform contact of the attached masses, they experience a slight rocking. Hence, we will focus more closely on the response parameters at the lower natural frequency, which is less prone to abnormal behavior.

Fig. 4. Evolution of Fourier spectra of actuator impact response: (a) x axis; (b) y axis.

Figure 5 shows the parameters identified from processing the vibration signals (indicated by a dark circle for each measurement): the natural frequencies (in the lower frequency range) and the corresponding damping ratios. The plot also shows the air temperature (green curve) and the temperature in the pile at a depth of 0.2 m (red curve).

Fig. 5. (a) Evolution of first natural frequency and temperature; (b) Evolution of damping ratio corresponding first natural frequency.

As can be seen in Fig. 5, as the soil freezes, the lowest natural frequency quickly shifts from 17 to 25 Hz in about one day (November 26, 2024). When short-term air temperature increases occur, the soil temperature also rises, and the natural frequencies decrease. During these brief thaws, the soil does not completely thaw, and the frequencies do not revert to their initial values. Fig. 5 also shows how the dissipative properties of the system change over time. The damping ratio of the system in unfrozen and frozen states (under presumably dry soil conditions) ranges approximately from 0.005 to 0.012. As the winter progresses and the depth of the freeze increases, there is a gradual decrease in the system’s dissipation. During brief thaw periods in the topsoil, there is a sudden increase in the damping ratio to values of 0.02 – 0.03, which is higher than it was before freezing. This occurs because, during thawing, the surface soil becomes water-saturated while the layer beneath remains frozen. 5. Conclusion Using a digital accelerometer, a series of field experiments was performed to investigate the dynamic response of the “pile–soil” system to both natural background excitations and actuator-induced excitations under natural soil