A sonar scan was conducted in February 2014, when the water was frozen over, so multiple holes were cut in the ice sheet to deploy the sonar to the scan location. Using a bucket boom truck, the equipment and operator were lowered to the waterline from the bridge deck above.
Underwater portions of bridge structures traditionally have been inspected visually by certified divers. However, visual inspection quality is contingent upon water clarity and is subject to river or ocean current, environmental influences, and safety concerns.
In 2012, the Minnesota Department of Transportation contracted to perform routine bridge underwater inspections to document underwater structure conditions. Typical underwater inspections include visual and tactile inspection for scour, cracked or deteriorated concrete, exposed reinforcement, and other detrimental conditions. Inclusive of hands-on diver inspection are probings and soundings of the subsurface profile. The inspection report for the Minnesota structure indicated corrective maintenance to the substructure should be programmed. MnDOT received the report, reviewed the recommendations, and began planning the necessary repairs based on the diver’s findings. The dive team inspection reported that there was an area scoured out underneath the bridge pier support structure that had escalated significantly since the last inspection cycle (see Figure 1).
Because of the location of the Saint Anthony Falls and complex underlying geologic conditions, MnDOT hired a consultant with underwater repair expertise and extensive experience working with infrastructure near Saint Anthony Falls.
MnDOT decided to begin repair plans based on the inspection report findings and recommendations. Because of the location of the Saint Anthony Falls and complex underlying geologic conditions, MnDOT hired a consultant with underwater repair expertise and extensive experience working with infrastructure near Saint Anthony Falls. The consultant started work in mid-November 2013, but early onset of frigid winter temperatures and ice buildup around the area of interest caused contractor inspection teams to turn down further underwater exploration.
The project seemed stalled: To produce a low-risk repair plan there was a need for additional information on the substructure condition, and the harsh conditions combined with the dangerous location had deterred conventional underwater investigation techniques.
Teledyne BlueView’s 3D mechanical scanning sonar does not need positioning, heading, pitch, and roll sensors to collect quality data.
MnDOT contacted Teledyne BlueView in January 2014 for feedback on how its BV5000 3D mechanical scanning sonar could be used to provide the information required to move the project forward. Teledyne BlueView, part of Teledyne’s Marine Acoustic Imaging Group, enables companies to use cutting-edge acoustic instruments to inspect dams and highway and railroad bridges that span water. These structures require routine inspection to verify the integrity of support structures. Teledyne BlueView works with civil engineering dive and inspection teams around the world to help enhance the deliverables to their customers using both 2D and 3D imaging sonar. BlueView’s 2D and 3D systems help prioritize engineering decisions for inspection, repair, maintenance, and verification of third-party contractor-completed work to specifications, in addition to enhancing diver safety.
Figure 1: Region noted in Circle 2 indicated an undercut area and concrete deterioration with a footing cavity varying in penetration
MnDOT recognized that BlueView’s sonar equipment offered the ability to deploy and perform in the harsh conditions with low risk to personnel. Teledyne Blueview not only confirmed it was possible, but also offered to demonstrate how the BV5000 could create detailed 3D point cloud data for these situations that surpasses the level of detail an underwater inspector might be able to provide.
“The ability to deploy with ease and very little safety risk to state employees or contracted personnel is an incredible combination,” said Paul Pilarski, principal engineer, Metro Region, Bridge Construction and Maintenance Section, MnDOT. “BlueView’s sonar capability to capture 3D point cloud data is a valuable tool for use by a bridge owner. The sonar brings both an accurate depiction of underwater structure and is also measureable in a CADD environment. At a minimum, it is a strong assessment tool for a bridge owner when there may be uncertainty whether there really is an issue. This sort of assessment would otherwise require contracting with underwater divers, which takes time, resources, and which may ultimately not have been needed.”
Teledyne BlueView’s 3D mechanical scanning sonar does not need positioning, heading, pitch, and roll sensors to collect quality data, so the BV5000 system is easy to setup and operate. If positioning sensors are available, the data can be input at the time of the scan to adjust the point cloud for any offsets.
The sonar was placed on the bottom in different locations around the nose of the MnDOT bridge support to image the scoured-out areas. The project was conducted in February 2014, when the water was frozen over, so multiple holes were cut in the ice sheet to deploy the sonar to the scan location. Using a bucket boom truck, or “snooper,” the equipment and operator were lowered to the waterline from the bridge deck above. The snooper was used because, despite frigid temperatures, the river ice is unpredictable and continually flows over St. Anthony Falls, less than 50 feet away from the scan location.
Detailed 3D point cloud data provides an accurate depiction of underwater structures and is also measureable in a CADD environment.
Lowering the BV5000 to the river bottom gives the sonar a unique perspective on any scour and vertical structure. This low perspective gives the user the capability to measure the height, width, and depth of the scour. In many instances, the generated point cloud can be used with software to calculate the volume of material removed or missing.
After the area of interest is scanned, the individual scan locations are merged using point cloud processing software. This complete mosaicked image of the area provides the information needed to move to the next step in repair. The level of detail provided gives construction and repair contractors the capability to make more accurate bids on the repair project by taking out most of the guess work that comes from a traditional inspection report.
“Contracting with inspection, construction, and repair companies is a long process, and with this equipment we could go to suspected deterioration or scour locations ourselves to do a quick assessment,” Pilarski said. “After a repair project, we could use 3D scanning sonar to inspect completion of the repairs.”
“We have been working with Teledyne BlueView using the BV5000 for different applications, and underwater structure inspection is a large, growing market for this equipment,” said Matt Meyer, general manager from OARS, which provides offshore and inshore data analysis as well as field support and training for acoustic instruments. “The BV5000 is great to use in environments where divers see hazards such as ice, high current, and nearby falls as we have at this work site.”
Information provided by Teledyne BlueView and MnDOT.