In addition to mainline road widening and the creation of new acceleration lanes and shoulders, the ongoing total reconstruction of the original Pennsylvania Turnpike at Irwin, PA involved replacement of an existing single-span rigid frame bridge that passed over the mainline roadway with an adjacent two-span box beam bridge. Vertical cuts of up to 20 feet in height and approximately 180 feet in length were required along the existing northbound and southbound embankment to facilitate construction of the new bridge abutments. Originally, soldier beam and lagging walls had been proposed to provide the temporary excavation support. However, given the presence of overhead power lines this option was not viable. General Contractor Joseph B. Fay Company, Inc. of Tarentum, PA approached Moretrench to provide a soil nail alternative for this site.
Soil Nail Wall Design and Installation
The Pennsylvania Turnpike Commission’s subsurface investigation had revealed a profile within the zone of excavation of approximately 15 feet of silty clay overburden above 5 feet of limestone bedrock. Soil nailing is often accomplished by the iterative process of excavating a 5-to 6-feet lift extending for a horizontal length that can be stabilized during a work shift, installing the nails, placing reinforcing mesh and finally spraying a shotcrete facing. For this project, given the poor stand-up time anticipated for the overburden soils, Moretrench elected to place the mesh and the 4–inch thick shotcrete facing before installing the nails. In the rock below the overburden, the soil nailing was eliminated and reinforced shotcrete was used to cover portions of weathered rock.
The excavated faces were designed to slope back above each wall to minimize the retained area. The design also incorporated drainage strips behind the soil nail walls. During production work, sacrificial nails were installed to verify bond lengths. At each wall, three to four levels of nails, varying in length from 22 to 29 feet, were drilled and grouted in place at 15° from horizontal and on a 4.5 to 5-foot grid pattern.
Unanticipated Condition Overcome
When properly designed and installed, soil-nailed walls provide very effective and economic earth support. There are situations that occur where the soil condition as represented differ. Such was the case over a 40 feet by 10 feet area on this project. Investigation revealed that perched groundwater, attributed to heavy spring rains, was not able to be relieved because the original slope drainage system in that area had been compromised. Moretrench was proactive in the speedy installation of additional nails and shotcrete in the affected area, and the overall soil nailing work was completed within the scheduled timeframe.