Sinkholes have plagued Lebanon County, Pennsylvania for decades due to the region’s underlying Karst limestone formations.
The unpredictability of limestone depths and presence of voids make foundation work in limestone extremely challenging. For the past 50 years, ongoing sinkholes in the Palmyra Borough have prompted quick responses from PennDOT to make repairs to minimize road closure time and cost.
In June 2019, a series of sinkholes occurred in an area previously repaired with a geosynthetic reinforced safety grid below the roadway. Although the grid performed as intended, preventing a sudden roadway failure, PennDOT stakeholders needed a more robust solution.
On July 2, 2019, PennDOT District 8-0 engaged Gannett Fleming to help remediate a large sinkhole opening along SR 422 at the east end of downtown Palmyra. In less than five days, the rapid response team assessed site conditions, and determined that the safest course of action for the traveling public was the innovative approach of bridging the sinkhole-prone area with a 2-ft-thick, 280-ft-long x 38-ft-wide concrete roadway slab supported with 84 steel micropiles in a 10-ft x 15-ft grid.
Varying in length because of the variability of limestone depths and the presence of voids, these micropiles were drilled into competent bedrock as deep as 180 ft. To ensure safety in the event of future sinkholes, redundancy was achieved by designing the slab so that one pile could be missing at any location with the slab spanning that distance. Micropile installation, with depths ranging from 17 ft to 180 ft, was completed in six weeks. The entire project, from design to opening SR 422 to traffic, was completed in just over four months.
Of the project success, Chris Drda, P.E., District 8-0 Assistant District Executive for Design, said, “Gannett Fleming clearly demonstrated their technical knowledge and project delivery capabilities. Starting on the July 4th weekend, to the timely completion, they consistently delivered.”