Section 9: Underwater Inspections

Overview

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, localized scour, or lack of freeboard. With the transition from the Coding Guide to the Specification for the National Bridge Inventory (SNBI) in accordance with 23 CFR 650.311, the collection and use of certain SNBI data is required to establish inspection intervals and to update and implement the reduced inspection interval policies by June 6, 2024. Prior to SNBI data collection TxDOT will use equivalent criteria per the Coding Guide to determine the reduced interval inspection criteria, provided later in this section.

Underwater Inspection Methods

The methods used to conduct Underwater Inspections are:
  • 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.
  • 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.

Levels of Underwater Inspection

Standard levels of underwater inspection originated in the U.S. Navy. Three levels have been established as the result of the process through time.
  • Level I -- A simple visual or tactile (by feel) inspection, without the aid of tools or measuring devices. It is used to gain an overview of the underwater elements of the structure.
  • Level II -- A detailed inspection which involves physically cleaning or removing growth from portions of the structure to assess hidden defects. This level of effort usually supplements a Level I inspection.
  • 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 nondestructive in nature.

Underwater Structural Elements

The elements of a bridge structure that may be located below the waterline 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 done. Local scour is often detectable during diving inspections, although sediment will sometimes refill a scour hole between the events that cause scour. More general scour, or channel degradation, will usually be undetectable to the diver and must be determined from known channel cross-sections and historical data.

Scour 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 often detects scour infill. 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.

Underwater Structural Materials

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. Timber is usually treated to control infestations. 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.

Reduced Inspection Interval

The following outlines TxDOT policy and requirements for risk-based inspection intervals outlined in 23 Code of Federal Regulations (CFR) 650.311 using Method 1 inspection interval determination for changing the interval of Underwater Inspections. The level of inspection for underwater inspections with decreased inspection intervals will be the same as that required for those scheduled on a 60-month interval.
Bridges meeting the criteria below require reduced interval inspection of not greater than 24 months per Method 1 provisions as amended below:
  • Underwater Inspection Condition (Item B.C.15) ≤ 4 [Substructure Condition Rating (Item 60) ≤ 4]
  • Channel Condition (Item B.C.09) ≤ 4 [Channel and Channel Protection Condition Rating (Item 61 ≤ 4]
  • Channel Protection Condition (B.C.10) ≤ 4 [Channel and Channel Protection Condition Rating (Item 61) ≤ 4]
  • Scour Condition Rating (Item B.C.11) ≤ 3 [Scour Critical Bridges (Item 113) ≤ 3]
The inspection interval may be assigned as either 12 or 24 months based on the severity and recent progression of the deficiency. The basis for assigning either a 12-month or a 24- month interval must be clearly documented in the inspection report.
Reducing a condition rating to a 3 or lower for Pier or Abutment Protection (NBI Item 111) due to a reduction in protection provided by the protection feature will be cause to perform special underwater inspections on an interval of 12 or 24 months. The interval assigned is based on the severity of the damage to the protection feature. The special underwater inspection will be limited to the pier or abutment protection and the substructure components adjacent to the damaged feature. The level of inspection for substructure components inspected will be the same as that for routine underwater inspections.

Extended Inspection Interval

Inspection intervals up to 72 months are allowed provided all the following criteria are met:
  • Underwater Inspection Condition (Item B.C.15) ≥ 6 [Substructure Condition Rating (Item 60) ≥ 6 or Culvert Condition Rating (Item 62) ≥ 6]
  • Channel Condition (Item B.C.09) ≥ 6 [Channel and Channel Protection Condition Rating (Item 61) ≥ 6]
  • Channel Protection Condition (Item B.C.10) ≥ 6 [Channel and Channel Protection Condition Rating (Item 61) ≥ 6]
  • Scour Vulnerability (Item B.AP.03) = A or B [Scour Critical Bridges (Item 113) = 5 or 8]
  • Scour Condition Rating (Item B.C.11) ≥ 6

Bridge Inspection Interval Tolerance

The requirements and acceptable tolerances for inspection intervals for UW inspections is the same as that for Routine Inspections. See Section 4 for Routine Inspection Interval Tolerance requirements not repeated here.