To study the propagation characteristics of Rayleigh waves and the isolation mechanism of a single-row of piles by isolation effects, in this paper we draw a two-dimensional contour map of ζ (normalized acceleration amplitude relative to a measure close at the vibration source) using a vibration test carried out on a sand foundation. In this experiment, we study, in addition to the free field and the single pile cases, settings with two and three piles. The result shows that the vibration caused by the point source in the free field excites Rayleigh waves in a radial direction along the surface of the foundation. Meanwhile, the vibrations of the points along the propagation path on the surface of the foundation are gradually weakened. There is a steady transition when the ζ drops to 0.6 and a placid decline when ζ decreases to less than 0.25. The vibration-shielded region, the strengthened region, and the strengthened strips will appear on the surface of the foundation. The vibration-shielded region is located behind the piles, and the region presents a trumpet-shaped area that takes the pile as the vertex. Increasing the quantity of piles contributes to increasing the vibration isolation effect, not only that involving the degree of isolation but also for the area of the shielded area. The vibration-strengthened regions include the diffraction regions at the pile corners on both sides of the single-row of piles and the scattering region at the gaps of the piles. In addition, the composite regions are located among the vibration source and the scattering and diffraction-strengthened regions. Increasing the number of piles has little influence on the scattering and diffraction-strengthened regions but can significantly enhance the vibrations of the composite regions. In general, the vibration-strengthened strips are connected with the scattering-strengthened regions. However, in the test of a single pile, the pile is connected to the diffraction-strengthened regions near its two anterior angles.

No abstract

To study the influence of annular trenches on a vibration isolation area, the depth, width, vibration source distance, and central angle of the trench are analysed as research variables, and a contour diagram of the amplitude reduction ratio is drawn based on an outdoor test of the trench. Taking an area with an amplitude reduction ratio less than 0.40 as the evaluation index of the effective vibration isolation area, the effects of the above geometric parameters on the vibration isolation area are analysed. Limited to the test conditions of this paper, the results show that the depth, vibration source distance, and deep width ratio are the important factors affecting the effective vibration isolation area; with the increase of the above parameters, the effective vibration isolation area increases significantly, but the area increase rate decreases gradually. The width has a relatively little effect on the effective vibration isolation area. When the ratio of depth to width is from 7.05 to 9.15, and the width reaches 0.23 times the Rayleigh wavelength, the annular trench can have a good effective vibration isolation area. When the central angle of the trench is less than 90°, a discontinuous effective vibration isolation area will form in the vibration isolation region. The selection of the central angle of the trench is related to the frequency. With the same trench size, the effective vibration isolation area decreases as frequency decreases. In addition, the effect of distance depth ratio on the effective vibration isolation area presents a fluctuation. When the ratio of distance to depth is from 1.21 to 2.05, a good effective vibration isolation area can be obtained and it is reasonable.