Harrisburg, Pa. — The American Council of Engineering Companies of Pennsylvania (ACEC/PA) recognized the Digital Photogrammetry for Rock Cut Slope Design project and the Nesbitt Dam Rehabilitation project in its 2014 Diamond Awards for Engineering Excellence competition on Feb. 24. The digital photogrammetry entry received the Honor Award in the Surveying and Mapping Technology category; the Nesbitt Dam entry received the Honor Award in the Water Resources category. The Honor Award is the highest honor bestowed upon a project in each category.
Gannett Fleming introduced the use of digital photogrammetry, a new advancement in precision geologic mapping, to rock cut slope design in the Appalachian Mountains on a Pennsylvania Department of Transportation (PennDOT) project to modify an existing cut carrying U.S. Highway 15 through Bald Eagle Mountain, near South Williamsport. Digital photogrammetry was used to safely measure the orientations of fractures and bedding exposed in the existing cut, which was experiencing occasional rockfalls.
Roadway safety improvements undertaken by PennDOT Engineering District 3-0 at this location included removal of an existing rockfall fence; modification of an existing rock cut to flatten the slope; and installation of a rockfall catchment zone to prevent falling rock travel hazards and reduce on-going maintenance issues.
The digital photogrammetry deployed by Gannett Fleming geologists and geotechnical engineers safely and rapidly evaluated site conditions so that analysis and design could proceed more quickly. At the completion of the project, roadway sight lines were improved; highway rock fall hazards were mitigated; and maintenance costs were reduced.
The more than $27 million upgrade of Nesbitt Dam addressed complex safety deficiencies related to spillway capacity, seepage, and a long history of hillside instability. Because it is a primary source of water supply for 75,000 residents, the rehabilitation had to be completed with a nearly full reservoir, requiring the spillway to be in operation throughout the project.
The challenging dam geology with the left masonry portion of the dam perched on a vertical rock ledge and the right earth embankment founded on soil also led to many design and construction concerns. Despite all its challenges, the project was completed 15 months ahead of schedule, and not only meets the Pennsylvania Department of Environmental Protection standards but, more importantly, provides the safe operation of this essential water supply dam into the next century.