Project Scope
From April 3 to 10, 2006 NMGI performed an underwater evaluation of the 10 bridge piers on Liberty Street and Independence Street bridges. For the purpose of this inspection the Bascule piers were inspected using underwater acoustic imaging and the smaller or minor piers were inspected using surface supplied air diving techniques. bridge had two bascule piers and three minor piers.

Inspection Techniques Used
Two types of underwater inspection techniques were used for this inspection. The first was surface supplied air diving and the second underwater acoustic imaging. The commercial dive inspection was conducted in accordance with current FHWA guidelines for underwater inspection and in accordance with all OSHA and USCG diving regulations. The inspection included a level I structural inspection. The level I inspection is a general visual inspection of 100% of the units and does not involve cleaning of structural elements. The level I structural inspection was to identify and quantify damage, cracking, settlement, and deterioration of the concrete piers.

The Level I scour inspections included a visual commercial dive inspection and acoustic imaging to determine the extent of any scour present. The two inspection techniques were to identify voids if present under the footings, surrounding foundations as well as the general condition of the riverbed. This information was to be utilized to perform a level I scour analysis and assessment to determine the appropriate repairs if needed.

The dive team was composed of a four-man team utilizing one engineer as the dive inspector, two dive technicians as tender/standby diver and a dive supervisor. The divers utilized surface supply diving techniques to support the underwater inspections. An underwater color video camera was mounted to the divers dive hat to document all areas of inspection for future review. However, due to the large suspended sediment load in the river, the camera had poor visibility of the piers underwater. The diver was able to preform a tactile inspection and this data is included in the report.

Finally, a visual inspection was conducted from the waterline to 10 feet above the waterline to identify and quantify damage, cracking, settlement, and deterioration of the concrete piers.

Underwater Imaging
The underwater imaging of the bascule piers was conducted using the MS 1000 sector scan. Three types of images were captured using the MS 1000. The first is the plan view image, second is the vertical view image and third is the profiling view image. For the plan view three distances were used to obtain these images. These distances were 60 feet, 90 feet and 150 feet. The purpose of the different ranges was to obtain full coverage of the river bottom, identify debris around or on the piers and classifying the impact of the local scour events on the piers. The vertical imaging allowed us to capture the side faces of the pier and any associated defects. Profiling allowed us to capture how the pier, footings and river bed are connected, i.e., degradation or aggregation. For profiling, the beam path travels out from the pier for about 45 feet in front of the pier face. Using all these images, we are able to create a mosaic of the piers allowing for a complete image underwater.