Eagleview Dam, Chester County, Pennsylvania

Overview: In lieu of a new bridge, the Hankin Group planned to build a more economically sound box culvert where Wharton Boulevard crosses Shamona Creek in the Eagleview development of Chester County, Pennsylvania. Extensive approach embankments, however, subjected the project to the regulatory requirements of a new dam— particularly because more than 12 ft of water would be detained in the event of a 100-year flood. According to the Pennsylvania Department of Environmental Protection (PADEP), this roadway and culvert combination would function as a flood detention dam. While adhering to PADEP regulatory requirements, GeoStructures  provided a geotechnical investigation to accommodate the construction of the box culvert. Construction testing and inspection services were also performed.

The completed structure has a maximum height of 20 ft and comprises a 17-ft wide by 10-ft high concrete box culvert with 10 ft of soil cover and 250-ft approach fills at each end. Design and permitting required dam safety engineering and the development of typical details and sections for 2:1 slopes on the upstream and downstream sides of the embankments, which is steeper than most earth dams. GeoStructures’ engineers specified an earth dam with a clay core and completed the following services:

• Bearing capacity evaluation of native soils to support the structure
• Field exploration, laboratory classification, and permeability testing of borrow areas for earth fill
• Sliding stability at the base and slope stability along the 2:1 side slopes (during the rapid drawdown case for 24-hour detention of floodwaters)
• Design of a clay core to prevent steady state seepage
• Design of a filter diaphragm section around the outlet of the box culvert
• Determination of erosion countermeasures (to mitigate contraction flow scour plus erosion of the stream bed at the inlet and outlet of the culvert)
• Development of subgrade requirements and benching sections for embankment construction

Additionally, our engineers provided construction testing and inspection services. Challenges included assisting the contractor with stream diversion, dewatering for embankment fill placement, and subgrade stabilization below the culvert through the use of a mud mat.