The New NY Bridge will stand on more than 1,000 pipe piles – enormous tubes of steel that provide the foundation to support the bridge. These piles are installed deep into the riverbed primarily using crane-suspended vibratory hammers, which tends to be among the louder operations on the project. While using noise shrouds reduces sound levels in the air, Tappan Zee Constructors, LLC (TZC) also developed additional techniques that specifically reduce underwater sound waves.
Sound waves cause pressure as they pass through water, and can confuse and even harm marine wildlife if left unimpeded. As these pressure waves pass by, they can rapidly compress and expand the air in a fish’s swim bladder, causing potential damage to organs. Underwater sound pressure waves can also affect a fish’s hearing, causing blood vessels to burst and in some cases lead to death. As a result, the New York State Department of Environmental Conservation and the National Marine Fisheries Service require that measures be taken to reduce these negative effects on Hudson River fish.
TZC’s solution was to place a curtain of air bubbles around each pile as they are placed in the riverbed.
Bubble curtains are created using aluminum rings which slide over pilings like rings on a shower curtain rod. Pressurized air is then pumped through these rings, creating a continuous cloud of bubbles around the pile. These bubbles absorb the energy of the sound-pressure waves created when the pile is struck. The pressure waves subsequently change shape and have a lower decibel (sound) level. In addition to softening the impact of the underwater pressure waves, the air bubbles are thought to deter fish from swimming near the construction area.
Since bubble curtains are a relatively new technology, a series of tests was required to ensure maximum efficiency. The tides and river currents in the Hudson River near the Tappan Zee Bridge are strong enough to potentially disrupt the bubble curtain if sufficient air flow is not used. By stringing the aluminum rings closely along the pile length under water, engineers are able to maintain a thick and consistent froth of bubbles to quiet underwater noise and protect marine life.
As part of the project’s environmental performance commitments, the bubble curtain system is required to reduce underwater noise by 10 decibels. In October, the New York State Thruway Authority proudly reported to state and federal environmental officials that the bubble curtains had surpassed those requirements. Thanks to the efforts of the project team, no sturgeon have been harmed as a result of construction activities on the New NY Bridge.
How do bubble curtains work?
Bubble curtains are created around steel piles by pushing pressurized air through small openings in a series of aluminum rings. The result is a constant stream of bubbles that significantly dampen the sound-pressure waves to protect fish and other marine life.
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