Hearing Group Low-Frequency Cetaceans Mid-Frequency Cetaceans High-Frequency Cetaceans Phocid Pinnipeds Otariid Pinnipeds Impulsive Sound Source Peak = 219 dB SEL = 183 dBcum Peak = 230 dB SEL = 185 dB cum Peak = 202 dB SEL = 155 dBcum Peak = 218 dB SEL = 185 dBcum Peak = 232 dB SEL = 203 dBcum Non-impulsive Sound Source SEL = 199 dBcum SEL = 198 dBcum SEL = 173 dBcum SEL = 201 dBcum SEL = 219 dBcum Threshold limits established by NOAA Fisheries Service in July 2016 for impulsive (i.e., impact pile driving) and nonimpulsive sound sources (i.e., vibratory pile driving). These threshold limits indicate the onset of permanent changes in hearing (i.e., permanent threshold shift [PTS]) for different hearing groups of marine mammals (e.g., whales and dolphins [cetaceans] that hear high, medium and low frequencies, and sea lions or seals [phocid, otariid]). Today’s most popular solution, the bubble curtain, can be difficult to deploy and demonstrates highly variable performance for noise attenuation depending upon site and substrate conditions. Encapsulated bubble curtains perform more consistently, but require additional time and the right conditions for deployment . The AdBm curtain technology provides substantial noise attenuation, but, for the specific sound frequencies for which it is “tuned” to address, it may potentially benefit some species but not others depending upon their hearing frequencies. None of these technologies addresses the noise propagated into the water column and rebounded off the bottom sediments during pile installation. The industry needs a better way to mitigate marine pile driving noise. In this article, a promising new pile design is presented and discussed that blocks noise generation along the entire length of the pile, all the way to its terminal depth. This pile design has resulted in impressive test results for both noise attenuation and drivability. Schematic of double pile flexible watertight connection Details of the MCT Pile This new pile design, the MCT Pile, uses two conventional steel piles, one smaller pile sleeved inside of a larger pile, resulting in an air gap between the two piles. Into the outer pile, a fixed or reusable inner pipe is inserted for driving purposes, in essence creating a double-walled pile, which are attached together by a special driving proprietary shoe that encapsulates noise along the full length of the pile. With this design, only a minimal amount of noise escapes at the pile toe. To facilitate pile driving and welding if additional pile sections need to be affixed to create a longer pile, the driving hammer strikes the inner pipe, which is slightly longer than the outer pipe. Sound waves then travel along the inner pipe while the outer pipe and the air gap between the two piles mitigate noise transmission. After installation, using standard equipment, the inner pipe is either left in place or is removed and used as a reusable mandrel. When the inner pipe is left in place, the inner and outer piles are welded together to provide additional strength. The interstitial space can also be filled with concrete after installation to further strengthen the pile. Because of the double wall structure of the MCT Pile, the wall thicknesses of each pipe can be reduced, while still providing the same structural characteristics as a conventional pile, and the overall material required is only slightly higher. Schematic of the MCT pile 66 • DEEP FOUNDATIONS • MAR/APR 2017 Full-scale Testing at Two Sites In October 2014, a full-scale test of the double-walled pile was performed at a site within the Port of Tacoma in Puget Sound near Tacoma, Wash., with characteristics typical of those found where ferry terminals and other marine-oriented structures are constructed. During the test,
