The use of Unmanned Aircraft Systems (UAS), commonly referred to as drones, has increased significantly over the last several years. This study explored the benefits of UAS technology when adopted by the South Carolina DOT (SCDOT), specifically for land surveying and bridge inspection. To evaluate the effectiveness of UAS in supporting bridge inspections, a test bridge was inspected twice using two different Bridge Inspection Engineers (BIE), on June 25, 2019, and on July 17, 2019. The test bridge, namely the Bates Bridge, was a concrete structure of 0.3 miles in length and had two lanes spanning over the Congaree River in southwest of Columbia, SC. When the test bridge was inspected using traditional methods, an Under Bridge Inspection Truck (UBIT), mobile crash attenuator trailer, traffic management signage and ten people were required.
To evaluate the effectiveness of using a UAS to support a bridge inspection team, two primary experiments were conducted on the same test bridge. The first experiment intended to evaluate the number of defects that the drone could identify given optimal conditions. For this experiment, the BIE that previously inspected the test bridge using a UBIT to re-inspect the bridge using the drone only. The second experiment was designed to evaluate the number of deficiencies that could be identified given real-world conditions when the deficiencies were unknown. For this experiment, a second BIE from another district who was unfamiliar with the test bridge was given the same drone inspection task. The unit costs used were from published state DOT average costs databases. To assess the savings a UAS could provide, it was assumed that even if a drone was deployed, a UBIT would still be needed, but for a reduced time period because the two experiments showed that not all inspections points could be seen with the UAS.
- The results showed that a significant advantage of drone deployment was the reduced need for UBIT, convenient documentation, keeping the BIEs away from traffic and the process being nearly invisible to the traveling public.
- The cost analysis revealed that the estimated cost to conduct the inspection performed for one of the test bridges using traditional methods was $5,242. The experiment also revealed an estimated cost savings of approximately $1,500 (a 27.5 percent cost reduction comparing traditional versus UAS) due to the reduced need for UBIT and traffic control.
- During the experiment, 91 percent of inspection points could be effectively observed with a drone equipped with a 30X zoom sensor and the study concluded that the use of UAS could improve more than 50 percent of bridge inspections.
- The study also highlighted few limitations of the UAS technology that at the present can only be met by in person inspections such as making a tactile contact with the structure and GPS signal blockages under bridges.
- Upon conclusion of the experiment, it was found that a drone inspection could not replace a comprehensive in-person inspection but would be beneficial for off-year and quick checks.
Figure 1. Traditional bridge inspection with UBIT (Source: SCDOT).
Figure 2. Bridge inspection using UAS (Source: SCDOT).