Lewistown Narrows

Junitata and Mifflin Counties, PA

This project consisted of improvements to the 6.5-mile, two-lane section of U.S. Route 22 to enhance the safety and service of this route by widening the existing two-lane roadway into a four-lane, limited access facility to match the four-lane facilities at each end.

AEG was responsible for all geotechnical engineering and related work. The geotechnical challenge was to site a bifurcated roadway on a very narrow strip of existing roadway bounded by the Juniata River and an old Pennsylvania Canal to the south and by very steep boulder fields and unstable colluvial soil slopes to the north. At the time of construction, this project was the second-most expensive construction section and likely the most complex geotechnical project ever contracted by the Pennsylvania Department of Transportation.

Project challenges included a complex geology of unstable colluvial and talus slopes overlying a residual shear plane, along which sliding was likely. To understand the conditions, AEG performed over 800 borings along the entire alignment. Geophysical investigations were also used to supplement the boring program consisting of very low frequency survey, seismic refraction profiles, and ground-penetrating radar.
To satisfy the roadway stability issues, a system of discrete pin-pile elements was developed. AEG performed stability analyses considering 56 possible modes of failure at intervals of 200 to 300 feet along the roadway. The entire project included over 220,000 linear feet of pin-piles to maintain stability of the roadway and structures.

The project included the design of six bridges, nine cantilever retaining walls totaling over 8,500 feet in length, four MSE retaining walls totaling over 15,000 feet in length, and four culverts. AEG performed all investigations and provided recommendations for the associated roadway issues. Roadway issues included the design and installation of 344 rock anchors to stabilize a rock cut with unfavorable bedrock dip, design of rock cuts 145 feet in height, design of steepened embankment slopes supported by a rock buttress overlying the canal, remediation of rock fall hazards via fencing, grouting, bolting and nets, design of 11,500 feet of temporary geosynthetic reinforced soil slopes, and the stability monitoring of the construction with 28 piezometers, 35 inclinometers, and 28 strain gauges.