Flat Rock Dam Fish Passageway,
Montgomery County, Pennsylvania

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Overview: Built in the early 1800s as part of the Schuylkill Navigation System, the Flat Rock Dam is a gravity dam located along a section of the Schuylkill River in the Lower Merion Township of Montgomery County, Pennsylvania. To assist in fish migration, Pennsylvania’s Department of General Services (DGS) planned to install a fish ladder on the dam. GeoStructures provided a geotechnical investigation to characterize the subsurface of the site.

The completed 200-ft, S-shaped structure along the west bank of the Schuylkill River involves 21 upward sloping chambers that allow fish to climb 15 ft and reach the 10-ft deep impounded waters on the upstream side of the dam.

The geotechnical exploration provided by GeoStructures involved 4 core borings on the rip rap of the west bank using a special skid rig. The rig was precariously cantilevered over the rapids on the downstream side at two locations. Expertise provided by our engineers on this project included:

  • Specialized drilling in a precarious setting over the river
  • Detailed rock characterization for compressive strength, hardness, weathering, fracturing, and discontinuities
  • Quantifying rock cut for attraction chamber and valve pit
  • Sliding/uplift stability for tanks of the passageway
  • Bearing capacity of alluvial soils below the higher chambers
  • Assessment of rock in terms of scour and countermeasures
  • Retaining wall design
  • Cut slope design/stability for soil cuts
  • Dewatering criteria

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