THEW BLOG

Utilizing Laser Scanning Technology for Existing Conditions Surveying and Mapping

The Saint Lawrence Seaway Development Corporation (SLSDC) was planning to reconstruct a failing tainter valve at the Snell Lock located in Massena, New York. The SLSDC determined that the valve was not closing completely and may have been out of alignment. To confirm the alignment and determine the best course of action, the SLSDC retained Thew Associates to perform an existing conditions survey of the valve trunnions, guide bushings, culvert walls, bull nose plate, chimney, and strut arm (topside).

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Thew Associates utilized a Riegl VZ-400i terrestrial laser scanner to perform the existing conditions survey of the tainter valve. The topside strut arm was referenced to the valve trunnions, guide bushings, bull nose plate, and culvert walls. A complete three-dimensional model of the tainter valve components to include the surrounding chimney and culvert was created using the terrestrial laser scanner.

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The terrestrial laser scanner and survey crew were lowered into the chimney and culvert by crane and man basket. Scanning in the culvert shaft was performed on a tripod at ten-foot intervals along the centerline of the culvert. The crew and scanner were then lifted into the chimney to begin scanning the vertical portion of the project. Anchor bolts mounted on the face of the chimney wall at 10-foot intervals, beginning 5 feet from the bottom of the shaft and ending 1-foot from the top of the shaft were required to mount the scanner on a pre-fabricated mount. The crew mounted the scanner and were subsequently lifted out of the chimney before remotely triggering the VZ-400i allowing the scanner to measure the existing conditions at each vertical increment.

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The data was post processed and a 3D model was prepared. The required measurements were extracted from the 3D model and expressed on 2D plan section view mapping. SLSDC rebuilt the components in the horizontal shaft using the information from the existing conditions survey and successfully installed a new tainter valve that is now operational.

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UNDERWATER STRUCTURAL ASSESSMENT WITH MULTI-BEAM SONAR

Thew Associates was retained in January of 2019 by an engineering firm providing design services for the stabilization of a concrete flood wall on the northerly bank of Rondout Creek in Rosendale, New York.  The engineer required bathymetric surveying and mapping of the Creek in the vicinity of the flood wall to facilitate their design. Thew Associates collected bathymetric data utilizing a Norbit iWBMS multi-beam echo sounder mounted on a remotely controlled EchoBoat. In addition to the bathymetry, the multi-beam sonar utilized for the survey was able to confirm areas of significant scouring and undermining of the flood wall footings.

The 1.8-meter EchoBoat USV was utilized due to restricted access (steep banks), swift currents, and variable water depths.  The vessel was secured with a tether held by personnel on a bridge adjacent to the upstream end of the survey area.  The Norbit iWBMS integrated multi-beam sonar system is compact and lightweight enough to be easily used with the USV.  A simplified graphical user interface that communicates with the shore station and operator through a remote connection allows the user to navigate the USV and observe the data being collected.  Additionally, the system has the ability to transmit the data in real-time to the office so the Project Manager can also view the data being collected.

Multi-beam sonar was chosen over vertical single-beam sonar because of the ability to provide complete coverage of the Creek bottom and the ability to collect bathymetric data under the flood wall to map the extent of the undermining caused by scour beneath the flood wall footings.

During the survey, raw position and motion data was logged on the survey vessel, while GPS/GNSS observations were logged by a local base station set up over a project control point.  This data was combined and post-processed to provide very accurate positioning of each sounding, even in areas where the GPS/GNSS signals are interrupted, such as near the bridge or immediately adjacent to the flood wall.  The data was all initially post-processed and validated utilizing Hypack hydrographic surveying processing software.  The bathymetric data was subsequently exported as an XYZ file to create a representative bathymetric surface, as well as an LAS point cloud file of all soundings.  The LAS file was imported into TopoDOT to extract detailed breakline data for the flood wall, footings, extents of undermining, and adjacent concrete apron.  The breakline and surface area was brought into an Autodesk Civil 3D environment to prepare the final bathymetric mapping, including plan and cross-sectional views to illustrate the existing conditions and for use in the remedial design.

Figure1
Figure 1: Visible undermining of the concrete footing to the left of the image. The vertical section of the flood wall is seen to the extreme left of the image.

Multi-beam sonar can be used to inspect the integrity of many underwater components including:

  • Bridge piers/abutments
  • Piles
  • Retaining walls (submerged installed sheet pile retaining wall on Buffalo River project)
  • Dam structures
  • Submerged intakes/outfalls
  • Abandoned-in-place infrastructure (submerged roads/bridges, buildings, walls)
  • Breakwater armor (stone, blocks: dolos, antifer, tetrapod, etc)

Figure2
Figure 2: The flat horizontal green line is the exposed portion of the top of the concrete footing.  The blue represents the Creek bottom.

Figure3
Figure 3: The vertical flood and concrete footing is readily visible with the adjacent Creek bottom.  Scour was not occurring in this area.

Land

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Canton

6431 US Hwy 11
Canton, NY 13617

Phone: 315-386-2776 
Fax: 315-386-1012

Utica

9478 River Road
Marcy, NY 13403

Phone: 315-733-7278
Fax: 315-797-1957

Saranac Lake

135 Broadway
Saranac Lake, NY 12983

Phone: 315-386-2776

Syracuse

135 Old Cove Rd, Suite 20
Liverpool, NY 13039

Phone: 315-244-2151

Thew Associates Land Surveyors
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