Vibratory pile driving is commonly used to install small piles and/or may be used to initially drive a larger pile. Here, vibratory hammers sit on top of the pile, and a series of oscillating weights continuously transfer vertical vibrations into the pile at a specific frequency. These vertical vibrations cause the sediment surrounding the pile to liquefy, allowing the pile to penetrate the substrate. Vibratory hammers are available with several different vibration rates, ranging from about 1200-2400 vibrations per minute. The vibration rate chosen is influenced by soil conditions at the site. Vibratory hammers operate continuously.
Pile driving produces high sound pressure levels in both the surrounding air and underwater environment. Sound levels vary substantially, and the size of the hammer, diameter of the pile, as well as properties of the seafloor, influence the source level and frequency of the signals generated. During impact pile driving, sound from the hammer striking the pile radiates into the air and a pulse propagates down the length of the pile and into the substrate as well as the surrounding waters. The majority of energy in pile impact pulses is at frequencies below 500 Hz. Near source (within 10m of the pile driving activities) peak sound pressure levels range up to 220 underwater dB, or perhaps even higher.
Vibratory pile driving produces a continuous sound with peak pressures lower than those observed in pulses generated by impact pile driving. Sound signals generated by vibratory pile driving usually consist of a low fundamental frequency, from 20-40 Hz. Average, near source, peak sound pressure levels range from 165-185 underwater dB. Sound or vibrations generated by pile driving may also be transferred via the substrate and emerge at some distance from the source.Impact pile driving produces a loud, impulse sound that can propagate through the water and substrate. The underwater sound pressure levels caused by pile driving can be harmful to marine animals. The probability of impact are situational and vary with pile type, impact energy, exposure type, duration, site characteristics, and species’ auditory characteristics.
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