Roads & Bridges – March 2011
Maine DOT uses boat-mounted survey equipment to inspect bridges
The Maine Department of Transportation (MEDOT) has one of the most robust underwater bridge inspection programs in the country. In order to comply with state and federal inspection requirements, a 12-person professional dive team conducts bridge inspections on every bridge that relies on underwater structural support. Of the state’s 3,000-plus bridges, more than 500 require underwater inspections.
But strong currents and restricted visibility often create difficult and even dangerous conditions for the divers. As a result, MEDOT turned to a high-tech alternative: hydrography. Substructure Inc., Portsmouth, N.H., conducted high-resolution multibeam surveys of three bridges that span the Kennebec River in Bath and Richmond, Maine. Initial “baseline” surveys were completed in June 2009, and follow-up “current-conditions” surveys were conducted in July 2010 to obtain comparative hydographic survey and scour data.
Before each survey, Substructure established a real-time kinematic (RTK) differential global positioning system (DGPS) base station over a known control point to ensure high resolution, accuracy and consistency. From there, the company’s custom-made vessel, the Orion, conducted the surveys using a host of high-tech equipment, including an R2Sonic 2024 and Reson 8125 multibeam echo sounders. In addition, Orion was equipped with an Applanix 320 POSMV vessel motion re7ference and navigation unit, Odom Digibar speed of sound profiler and Hypack/Hysweep hydrographic data acquisition and processing software.
During the actual surveys, the multibeam transducer was rotated outward 30° so data could be acquired almost up to the water surface. This let Substructure maximize data coverage around the structural supports. The data collected from the surveys is useful for depicting overall bathymetry around the site and also shows a general outline of the bridge supports.
Additionally, Substructure was able to create numerous high-resolution point-cloud datasets of all of the valid multibeam soundings around each structural support.
While the acoustic surveys were not meant to replace periodic “hands-on” inspections, the new method provided a quick, easy and safe way to monitor erosion and depositional changes over time.