Rendering illustrates concrete slab supported by 84 micropiles.

SR 422 Emergency Sinkhole Repair

When a large sinkhole closed SR 422 in Palmyra, Pennsylvania, causing hardship for the local economy and delays for 16,000 daily motorists, the Pennsylvania Department of Transportation (PennDOT), District 8-0 responded by quickly initiating an emergency remediation project. Working through the July 4th holiday, PennDOT and Gannett Fleming engineers developed an innovative solution to support a rebuilt roadway over the sinkhole-prone limestone soil.

The design and construction of a 280-foot-long, 2-foot-deep slab supported by 84 micropiles enabled PennDOT to safely reopen SR 422 just 18 weeks after its closure. Because of the team’s prompt action, travel time for commuters quickly returned to normal, and local businesses were able to reopen. The innovative solution was designed to enable the monitoring of subsequent ground movements, facilitating quick action to address future concerns.

What We Did

Varying in length because of the variability of limestone depths and the presence of voids, the micropiles were drilled into competent bedrock as deep as 180 feet. To protect against 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. A 3D GIS model of the proposed site activities communicated anticipated drilling depth and provided micropile status during construction.

Inspection ports in the slab allow PennDOT to monitor the effect of future belowground movement on the foundation system. Cameras lowered into these ports every six months capture potential problems, enabling quick responses to any voids under the slab. A time-lapse camera documented site activity, while biweekly drone photography created panoramic aerial photos of the project site, enhancing public outreach. The innovative solution provides a design that can be monitored, minimizes disruptions to travel, and provides long-term protection against future closures.

Key Features

  • Slab supports roadway over sinkhole-prone karst limestone formations.
  • Micropiles drilled into competent bedrock securely support roadway slab.
  • 3D GIS model effectively communicated anticipated drilling depths for secure micropile installation.
  • Inspection ports enable long-term monitoring of future belowground movements.

Awards & Recognition

  • Grand Conceptor Award, 2021, American Council of Engineering Companies (ACEC) Pennsylvania, Engineering Excellence Awards.
  • #10 Road, 2020, Roads & Bridges, Top 10 Roads.


  • Long-term solution to highway closures caused by persistent sinkholes.
  • Safely restored travel for 16,000 motorists every day.
  • Minimal disruption to area traffic and local businesses.


Pennsylvania Department of Transportation, District 8-0


Palmyra, Pa.


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