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Section 8: Underwater Inspections

Underwater Inspections are performed every sixty months or more frequently if conditions warrant. Perform an Underwater Inspection on structures where the submerged portions of the structure have a history of water depths of at least four feet year round or where the submerged elements are in less than four feet of water, but wading would be unsafe due to channel bottom conditions, high current or localized scour.

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Underwater Inspection Methods

There are currently three methods used to conduct Underwater Inspections. These are:

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  • Wading -- The most basic of the three methods, wading requires only a probing rod and wading boots to be effective. This is performed during Routine Inspections.
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  • Scuba diving -- A method that allows a more detailed examination of substructure conditions below the waterline. The diver has freedom of movement and may carry a variety of small tools with which to probe or measure.
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  • Surface Supplied Air Diving -- Involves the use of sophisticated diving equipment and a surface supplied air system. This inspection method is well suited when adverse conditions will be encountered, such as high water velocity, pollution, and unusual depth or duration requirements.

The choice of which method to employ depends largely on accessibility and the required inspection detail.

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Levels of Underwater Inspection

Standard levels of inspection originated in the U.S. Navy. Three levels have been established as the result of the process through time.

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  • Level I -- A simple visual or tactile (by feel) inspection, without the aid of tools or measuring devices. It is usually employed to gain an overview of the structure and will precede or verify the need for a more detailed Level II or III inspection.
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  • Level II -- A detailed inspection which involves physically cleaning or removing growth from portions of the structure. In this way, hidden damage may be detected and assessed for severity. This level is usually performed on at least a portion of a structure, supplementing a Level I.
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  • Level III -- A highly detailed inspection of a structure which is warranted if extensive repair or replacement is being considered. This level requires extensive cleaning, detailed measurements, and testing techniques that may be destructive or non-destructive in nature.
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Underwater Structural Elements

The elements of a bridge structure that may be located below the water line are abutments, bents, piers, and protection systems. Bents are distinguished from piers in that they carry the loads directly to the foundation rather than using a footing.

Abutments normally do not require an Underwater Inspection, but in rare instances may be continuously submerged. Although usually founded on piles or drilled shafts, abutments occasionally rest on spread footings. Scour is almost always the primary consideration when an underwater abutment inspection is being conducted. Local scour is often detectable during diving inspections, although sediment will eventually refill a scour hole between the events that cause the scour. More general scour, or channel degradation, will usually be undetectable to the diver and must be determined from known channel cross sections or historical data.

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Underwater Inspection Devices

Divers may use several types of sounding or sensing devices in underwater investigations. The most common device is the black-and-white fathometer. It uses sound waves reflected from the channel bottom and records the depths continuously. It provides an inexpensive, effective means of recording channel depths, but does not detect a refilled scour hole. Another device is the color fathometer. It uses different colors to record different densities and in this way often detects scour refill. Other devices include ground penetrating radar, which works well for shallow water but has limited usefulness in murky water, and fixed instrumentation, which is reliable but requires periodic monitoring and resetting to be effective.

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Underwater Structural Materials

Piers and bents, if located in a navigable waterway area, are often subject to material defects, collision damage and scour. Concrete is the most common type of material encountered in Underwater Inspections, followed by timber, steel, and masonry. Common defects in concrete substructures include cracking, spalling, laitance, and honeycombing. Minor or even moderate damage to concrete can be tolerated if it does not endanger the reinforcing. Corrosion of the reinforcing can lead to serious difficulties.

Timber has frequently been used for piles, especially in fenders or protection systems. The most common type of damage to timber members is from biological organisms, such as fungus, insects, and marine borers. In order to control infestations, timber is usually treated to poison the wood to block a food source for organisms. In time the treatment may leach out of the wood or the treatment layer may be penetrated. Pay particular attention to the area of the waterline and the vicinity of connectors where this type of damage may occur.

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