Analysis of the attenuation characteristics of environmental vibrations with different frequencies is of great benefit for preventing many vibration-related problems. This study carried out an in situ experimental study on the attenuation characteristics of a series of single frequency ground vibrations caused by harmonic excitations. An electromagnetic vibration excitation system was adopted to generate harmonic excitations at frequencies varying from 10 Hz to 80 Hz with steps 5 Hz. 5 low frequency, 1 component, 991B vibration sensors fastened on the ground surface were used for the measurement of the vertical velocity time histories of ground vibrations at 5 test points with different distances from the vibration source. The vibration level in terms of the vertical peak particle velocity of single frequency ground vibrations in general decayed monotonically but nonlinearly with distance, however, the attenuations of 15 Hz, 45 Hz, 65 Hz and 75 Hz ground vibrations were oscillatory due to vibration interference on the ground surface. The attenuation of single frequency ground vibrations was more rapid in the zone close to the vibration source than that in the zone far away. This study demonstrates that comparison of the attenuation characteristics of high and low frequency ground vibrations should take into account the difference in the amplitudes of the corresponding ground vibrations.
Thickening the foundation slab and improving the subgrade soil using geo-techniques are effective measures for controlling unwanted vibrations at high-tech facilities. In this study, the vibration-reduction performance of 1-m-thick concrete slabs with natural and cement-improved subgrades was investigated based on in situ frequency sweep tests. One 1-m-thick concrete slab rested on 1-m-thick compacted sandy gravel backfill atop an undisturbed subgrade was constructed on the north side of the experimental site, and another identical concrete slab rested on 1-m-thick compacted sandy gravel backfill atop a cement-improved subgrade was constructed on the south side. The vibration-reduction effect was evaluated by comparing the free-field ground vibrations and surface vibrations of the two slabs at three pairs of evaluation locations. In terms of peak velocity, the 1-m-thick concrete slab with the natural subgrade exhibited a slight vibration amplification effect at low frequencies and a significant reduction effect at middle and high frequencies; the 1-m-thick concrete slab with the cement-improved subgrade exhibited a continuous vibration reduction action at all frequencies. In terms of RMS velocity, the vibration-reduction performance of the 1-m-thick concrete slab with the cement-improved subgrade was better than that with the natural subgrade. The results demonstrated that the vibration-reduction effect of the thick concrete slab was significant and could be increased by improving the subgrade using the cement grouting method.
Knowledge of the attenuation characteristics of truck-induced ground vibrations has gained importance for micro-vibration mitigation of high-tech facilities. This study conducted an in-situ experimental study on the attenuation characteristics of truck-induced ground vibrations. The ground vibrations were generated by the passage of a truck on a low-cost road with different weights and speeds. Eight low frequency, one component, 941B vibration sensors were fastened on the ground surface to record the vertical velocity time histories of ground vibrations at eight measurement points with different distances. The vibration level was evaluated in terms of the vertical maximum displacement obtained by integrating the measured velocity time histories. The vibration level represented by the vertical maximum displacement decayed monotonically but nonlinearly with distance from the road. Ground vibrations at the measurement points that were close to the road decayed more rapidly than that at the measurement points far away. In general, the maximum displacement of ground vibrations induced by the 30-ton truck at a measurement point was smaller than that of ground vibrations induced by the 18-ton truck with the same speed.
Reduction of road traffic-induced vibrations has gained importance with rapid development of high-tech industry and nanotechnology. This study focuses on the in situ vibration measurement and transmissibility-based vibration prediction for the foundation slab design of a high-tech lab subjected to truck-induced vibrations. The truck-induced vibrations come from a proposed road 30 m away from the high-tech lab. The allowable vertical vibration velocity for the foundation slab of the high-tech lab was 60 μm/s in the frequency range of 5–50 Hz. The truck-induced ground vibrations in the proximity of an existing road with the same design as the proposed road were taken as the vibration source response used in the foundation design. The ground vibration transmissibility from the proposed road area to the high-tech lab area was determined by conducting frequency sweep tests in the free field. Based on the vibration source response and the ground vibration transmissibility, two antivibration foundation prototypes with different thicknesses were constructed at the site. The vibration transmissibility from the subgrade soil to the surfaces of the two foundation prototypes was obtained by measuring the ground vibrations at the high-tech lab area and the surface vibrations of the two foundation prototypes. The vertical vibration velocities of the two foundation prototypes were predicted based on the measured transmissibility and the vibration source response. The final thickness of the antivibration foundation was determined by comparing the predicted vibration velocities with the allowable vibration velocity. After construction of the high-tech lab and the road, vibration tests were conducted to assess the performance of the actual antivibration foundation. The results showed that the actual antivibration foundation was able to reduce the vibration levels at the high-tech lab to acceptable levels.
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