Impact driving steel piles through the water produces extremely high underwater sound levels that can harm aquatic wildlife. To address this problem, a series of research projects for the Washington State Department of Transportation has developed a double-walled pile to decrease the total amount of noise both in the water and reflecting from the sediment. This project conducted a full-scale test of the double-walled pile technology at Vashon Island, Washington.
Because of the critical state of many animal populations, the U.S. Fish and Wildlife Service and National Oceanic and Atmospheric Administration have established underwater noise thresholds to help in analysing disturbance and potential injury impacts to underwater fauna such as diving birds, whales and porpoises, sea lions, and various fish. Previous research has shown that without containing the noise, larger steel pipe piles cannot be installed with impact hammers without exceeding those underwater noise thresholds.
Research has shown that current pile driving noise attenuation techniques, such as bubble curtains and cofferdams, provide inconsistent noise reduction because of unconstrained Mach sound waves that propagate directly from the sediment surrounding the attenuation devices into the water. The double-walled pile concept was developed to comprehensively reduce both the pile driving noise that moves directly into the water and noise waves reflecting from the sediment.
The double-walled pile consists of two concentric steel pipe piles flexibly connected by a special driving shoe, which provides an air gap between the two tubes. Traditional equipment is used to drive the double-walled pile into the sediment, but it strikes only the inner pile. The air gap between the inner and outer pile and the flexible coupling then prevent the sound wave produced by the pile hammer from interacting with the water and the sediment. In one embodiment of the double-pile design the inner tube can even be removed and repeatedly reused.
This project performed a full-scale test of the double-walled pile technology at Vashon Island, Puget Sound, Washington. The tests involved a double-walled test pile, mandrel test pile, and control pile. Peak pressure measured during pile driving was less, in relation to the test pile, by more than 13 dB for the double and mandrel piles at a range of approximately 10 m. Unfortunately, unanticipated steel-to-steel contact with an installed template during the entire driving process decreased the sound attenuation performance of the double-walled piles. When the data were filtered to remove the effect of the steel-to-steel contact with the template, then estimated peak pressure decreased by over 17 dB for the double-walled pile and by 16 dB for mandrel pile in relation to the control pile. In addition, the noise generated by impact installation of the standard control pile exceeded all but one of the underwater noise thresholds for measuring impacts to sensitive species, whereas noise from the test piles largely fell below those thresholds.
Finally, by using WSDOT’s Geotechnical Design Manual Pile Driving Formula, the researchers estimated that the pile capacity of the new piles was comparable to that of a control pile of the same outer diameter.
WA-RD 861.1
See also: WA-RD 861.2
Authors:
Per Reinhall
Tim Dardis
UW Department of Mechanical Engineering
Julie Hampden
Marine Construction Technologies, PBC
Sponsor: WSDOT
WSDOT Technical Monitor: Jeri Bernstein
WSDOT Project Manager: Jon Peterson