PSI - Issue 72

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

467

Fig. 2. (a) Fourier spectrum of signal; (b) filtered signal around the natural frequency (  20 Hz) and its envelope; (c) filtered signal around the frequency (  100 Hz) and its envelope. 4. Experimental results During the experiment, background vibrations from natural sources and responses to impact excitation were recorded. Measurements were performed from late November to January during the period of seasonal soil freezing. A substantial change in the frequency spectrum over time was observed. Below are results starting from November 19, 2024, which was the last frozen-free week before a period of temperatures below 0 degrees Celsius. To illustrate the changes in frequency over time, spectrogram-like plots of the Fourier spectra have been generated. Fig. 3 shows the evolution of the spectra for the x and y components under microseismical vibrations. Each vertical stripe corresponds to the Fourier spectrum measured at a specific time. The color indicates the signal intensity at a given frequency. Normalization is performed according to the maximum value of each spectrum; the darkest shade corresponds to 100%. Due to the microseismic effects, the system response can be seen in the x and y accelerometer components (see Fig. 3 — darker shading) in the low-frequency region around 20 – 30 Hz. As time passes and the soil freezes, the peak frequency shifts from approximately 17 to 31 Hz (by 40 – 45%), which indicates a significantly increased stiffness of the pile – soil system.

Fig. 3. Evolution of Fourier spectra of microseismic vibration (a) x axis; (b) y axis.

Similar spectrograms were generated for the pile’s response to impact loads from the actuator. These spectra are shown in Fig. 4 for the x- and y- axis directions. As can be seen, the structure’s response occurs at several frequencies: the same lower frequency as in the microseismical case (about 20 – 30 Hz), as well as frequencies around 100 Hz. Note that starting from January 7, there was a change in how the energy was distributed among the frequencies in the response spectrum. In the second half of winter, the lower frequencies dominated the response. It can also be noted that in the higher part of the spectrum, two natural frequencies are visible. Possibly, due to a