PA 309 Expressway, a 10-mile stretch of State Route 309, is a major commuter route from Philadelphia to the northern suburbs, and carries approximately 63,000 vehicles daily. Constructed in the 1950s, the expressway was designed with the short acceleration and deceleration lanes, limited site distances, and lack of shoulders characteristic of many highways of that era. To improve safety, reduce congestion, and improve mobility, Pennsylvania Department of Transportation (PennDOT) has initiated a $250M overhaul of the entire freeway.
Freeway reconstruction began on a two-mile section north of the Fort Washington/PA Turnpike interchange. Scope of the work included excavation of the existing northbound and southbound slopes to widen the highway for new access and exit lanes; construction of noise barriers and Mechanically Stabilized Earth (MSE) walls; construction of new bridge abutments; and modifications to the existing drainage. General Contractor Allan A Myers LP, Worcester, PA, invited bids for temporary support of the cuts created for roadbed widening.
Design-Build Soil Nail Solution
The proposed northbound and southbound cuts ranged in depth from 20 to 40 feet, with an overall wall length of 1,880 feet. The geotechnical investigation had revealed slope soils consisting of approximately 16 feet of silty sand overlaying an eight foot-thick stratum of highly weathered sandstone. Beneath this, medium-grained, moderately weathered sandstone extended to 40 feet below grade. After evaluating the site conditions and soil profile, Moretrench successfully submitted a design-build soil nail wall system that would provide effective earth support while significantly reducing the overall construction schedule. Work was scheduled to be accomplished in two separate phases over a two-year period, involving more than 100,000 square feet of soil nailed wall.
The first 1,880-foot phase of soil nail wall construction was along the northbound lanes. Several factors added to challenge of working along an active highway:
- Limited room for equipment and material staging
- Existing houses within 20 feet of the top of the proposed wall
- Maintaining open access to roads and driveways during the work
- Temporary drainage pipe and existing utilities behind the wall
During the work, sacrificial test nails were installed to verify bond values. Up to six levels of production nails were installed, depending on the depth of cut. In the upper stratum, Moretrench elected to reinforce the face of the cut with wire mesh and a four-inch thick shotcrete facing before drilling and grouting the soil nails in place. In the rock below, the wire mesh was eliminated and the shotcrete was reinforced with steel fibers.
For each level, a lift of five to six vertical feet was exca- vated, extending for a horizontal length that could be stabilized during the workday. Nails were typically installed at 15 degrees from horizontal, with lengths varying from 12 to 30 feet. In the vicinity of the drainage pipe and utilities, adjustments were made to the layout to avoid these obstructions while ensuring that the overall integrity of the wall was not compromised.
Although harder rock than anticipated extended the lift excavation time, the soil nail system was completed on schedule and within budget.