Snaking its way between the winding Juniata River and the steep slope of Shade Mountain, the Lewistown Narrows, a section of Rte. 22/322 in Pennsylvania, has historically been a notorious stretch of road. As a vital artery between the Harrisburg/Hershey and Lewistown/State College regions, the passage has been used to traverse central Pennsylvania for centuries, and today the rural highway is typically used by 20,000 motorists a day.
The narrow, winding two-lane highway was generally bottlenecked and had gained national notoriety for being cited as one of the nation’s “most dangerous highways” by Reader’s Digest. To reverse the roadway’s reputation and better the quality of life for surrounding communities, the Pennsylvania Department of Transportation (PennDOT) committed to a nearly 10-mile massive reconstruction. The $142 million enterprise would include the second-largest construction project ever awarded by PennDOT at the time and the longest mechanically stabilized wall in the U.S. The Narrows would prove to be one of the more complex projects undertaken by PennDOT, but once completed would offer safe passage for thousands of daily motorists as well as amenities for public recreation and education.
PennDOT partnered with The EADS Group Inc., headquartered in Altoona, Pa., to be the prime design consultant responsible for roadway and structure design, project management and agreement administration of the design contract. Subconsultants on the project team included Erdman Anthony and Associates Inc.; GTS Technologies Inc.; Heberling Associates; and Transportation Resource Group Inc.
The first domino in a long line of challenges faced by the project design team was the proximity of the Juniata River to the roadway, coupled with concerns for the historically significant remnants of the Pennsylvania Canal system. Though the Juniata River is an important environmental concern, the greater challenge for the design team was to preserve the unique historical areas of the Pennsylvania Canal system running between the existing road and the river. The already-constricted passage quickly became even narrower as a report on historical structures classified several sections along the proposed construction zone as “pristine” areas that could not be affected by the reconstruction. As an answer to the challenge, the design team chose a bifurcated alignment for 3.2 miles of the roadway. This configuration pushed the westbound lanes approximately 20 ft above the eastbound lanes and into the unstable slope of Shade Mountain.
Though the alignment relieved much of the concern for the canal, PennDOT agreed to create a canal park to mitigate any impacts the reconstruction may have on the area. Included in the park were plans for a new visitor area complete with a recreation/picnic area, a restored section of the Pennsylvania Canal lift lock system, a restored towpath spillway, interpretive signs and trail markers to guide visitors along the towpath for a 1.5-mile journey along the Juniata River. The project also includes a fish-and-boat access area located at the end of towpath trail. The fish-and-boat access was provided to mitigate the impacts to river access along the entire length of the Narrows Project.
With the river and canal safe, the team focused on stabilizing the roadway. The design of the highway was complex in that it required minimizing excavation into the slope and providing stability of the highway. To support the bifurcated alignment, two mechanically stabilized earth (MSE) median walls were required between the eastbound and westbound lanes. The longest of the MSE walls is 12,100 ft (or 2.3 miles) long and according to the designer and supplier of the wall, Reinforced Earth Inc., are the largest of its kind in the U.S. and Canada and the second largest in the entire world.
In addition to the two median walls, nine river walls were included in the design to retain the roadway and prevent encroachment on the canal and the river. The longest of these walls is 1,330 ft long. Other structures include three box culverts and two bridges. All in all, the Narrows section of the project included 16 structures.
Made for the Shade
Though the bifurcated alignment alleviated concerns for the Juniata River and Pennsylvania Canal, the design pushed two lanes of traffic into the extremely steep Shade Mountain, which introduced new loads to the unstable talus, a naturally occurring loose buildup of rock covering large portions of the mountainside.
The next challenge faced by the design team involved finding the best way to stabilize the mountainside. To begin, the team conducted geotechnical investigations and analyses. Over 40,000 linear ft of core borings were performed in addition to seismic and ground-penetrating investigations to determine subsurface conditions. Months were spent analyzing the global stability of Shade Mountain and its talus rock by both federal and state government, as well as several independent consultants. After examining several reinforcement options, subconsultants Erdman Anthony and Associates Inc. concluded the most viable was to use independent piles to strengthen and stabilize the slopes.
To implement the independent-piles alternative, consideration was given to three types of piles: caissons, steel beam H-piles and micropiles. Although caissons and steel beam H-piles are more rigid and might intuitively appear to provide better stability, a careful study of soil-structure interaction indicated that micropiles provided a solution that was both safe and the most cost-effective. Put simply, although micropiles are made of narrow-diameter pipe, they carry loads deeper below the surface, where the soil is stronger, than either caissons or steel beam H-piles. In addition, when considering the large amount of rock and boulders in the overburden, micropile installation proves to be more productive. Maximum production rates for H-piles were estimated at two piles per day, while eight micropiles, including concrete backfill, could be installed per day. With specialized equipment, actual production rates for micropiles reached 16 per day.
Prior to the Lewistown Narrows project, very little design criteria or reference material was available that could be applied to the problem of slope failure for this unique project. As a result, Erdman Anthony and GTS Technologies Inc. developed the analysis, design methodology and ultimately the design criteria for the project’s slope stability system, which was approved by PennDOT prior to implementation. Through geotechnical and structural analysis, the team developed criteria that called for micropiles to be spaced at a maximum of 12 times their diameter or at 6 ft, whichever was smaller. This spacing was sufficient to inhibit the “flow” of soil down the failure surface.
The final slope stability system for the Lewistown Narrows included more than 15,000 linear ft of retaining walls reinforced with 8,800 micropiles made out of 7-in. pipe. The micropiles averaged 26 ft in length, which meant that the total quantity of pipe required exceeded 43.5 miles. The significance of the slope stability system to this project is reflected in its cost, which represents 18% of the overall project cost. The use of micropiles helped to keep this cost from being even higher and provided a solid foundation for the construction of a safe highway facility.
In addition to the structural design of the project, the team faced several challenges in maintaining manageable traffic flow through the construction zone. Because there are few alternative routes to Rte. 22/322 through the Lewistown Narrows and official detours would have been about 30 to 60 miles long, it was extremely important to keep the roadway open throughout construction. Additionally, construction was scheduled to continue year-round to meet extremely ambitious schedules. Traffic was maintained one lane in each direction and construction traffic interruptions were limited to those that were absolutely necessary. Because of the magnitude of the project, delays were inevitable. However, motorists were continually updated on the status of the project through the PennDOT website, highway signage and highway advisory radio. Also, local media was often invited for tours of the construction site to help the public better understand the changing traffic patterns, especially the many that may not have traveled the highway on a daily basis. The project team also included in the reconstruction plans an incident management plan, discussed with local emergency response teams, to provide for situations which may have occurred in the congested construction zone.
Once the design challenges had been tackled, the Lewistown Narrows highway was bid in three sections for construction—starting in December 2002 with the eastern interchange at Arch Rock ($12.6 million) and followed by the interchange at the western end with the existing Lewistown Bypass in December 2002 (approximately $18 million). The final section constructed the 6.6-mile Narrows starting in November 2003 ($108 million). All three sections were scheduled to be completed by the end of 2008, but were finished one full year ahead of schedule. The project was opened to traffic on Dec. 14, 2007.
The once-treacherous roadway has now been transformed through the innovative design work of an integrated project team. The previously bottlenecked two-lane roadway is now a modern, four-lane, limited-access highway, which links all the communities along a narrow, 100-mile-long corridor. Not only has the reconstruction eliminated conditions that have been a bane to drivers for decades, it has given back to the community by adding access to recreational and historically educational areas. Though most drivers may not realize, the most impressive aspects of the new highway are either buried beneath the slope or appear to be “just a wall.” With design challenges overcome, all—from vacationers to truckers—can agree the Lewistown Narrows is a much improved, much safer passage.