Dynamic soil testing and assessment of the impact of vibrations from the RC/TS device using motion magnification technology

Abstract

This article presents the application of motion magnification technology for the qualitative and quantitative assessment of the distribution of horizontal displacement amplitudes along the height of a sample when vibrations at the resonant frequency are induced in it, and for identifying the potential influence of the apparatus’s own vibrations on the measurements taken. The experiments were carried out using a Wykeham Farrance 8,500 resonant column/torsional shear apparatus in a fixed–free configuration, with harmonic excitation of vibrations in a sand sample within the frequency range of 150–250 Hz. The optical measurements involved recording the movement of markers positioned along the height of the sample. Their movement was recorded using a high-speed camera (1,000 frames per second) under controlled lighting conditions. To visualize sub-pixel displacements, the modified Eulerian motion magnification method was used with multiband filtering corresponding to the frequency range of the excited vibrations. Using fast Fourier transform spectral analysis, the amplitudes of the displacements were determined and the resonance frequencies characterizing the motion of the markers were identified. The results showed monotonic changes in the amplitude of horizontal displacements along the height of the specimen, consistent with the theoretical distribution for a fixed–free system. At the same time, small but measurable vibrations of the structural elements of the testing apparatus were identified. The results indicate that the susceptibility of the apparatus to vibrations induced by internal systems applying dynamic loads to the test specimens may influence the interpretation of the results obtained.