Jefferson Barracks Tunnel Receives 2024 Tunnel Achievement Award

Tunnel Business Magazine has awarded their Tunnel Achievement Award to Jefferson Barracks Tunnel in Saint Louis, Missouri. The Award recognizes successfully completed projects that demonstrate innovation and teamwork and provide benefits to the community.

Brierley Associates became involved in 2019 when excavation of the Jefferson Barracks CSO tunnel came to a halt because the main beam TBM advanced into an apparent karst feature and encountered flowing sand, gravel and mud. This 11-ft diameter stormwater conveyance tunnel was designed to be entirely within good quality, competent limestone bedrock, below known karst zones. Concurrent with a grouting program implemented by the contractor, Affholder (formerly SAK Construction), Brierley performed a geotechnical investigation between the stalled TBM and a proposed rescue shaft location.

Based on the investigation, Brierley designed a 200-ft deep by 40-ft diameter recovery shaft approximately 200-ft ahead of the stalled TBM. Affholder excavated the recovery shaft to the tunnel elevation and conventionally excavated the recovery tunnel to a point approximately 75-ft ahead of the machine, where additional karst voids were encountered. At this location, a temporary 8-ft thick reinforced concrete bulkhead designed by Brierley was installed to allow probe-hole drilling and high-pressure grouting from inside the recovery tunnel. Once the initial grouting through the temporary bulkhead was complete, a double row pipe canopy was installed through the bulkhead.

Affholder was prepared to advance sequentially with a partial heading if necessary. However, based on the probing and grout takes at the bulkhead, it was decided to remove the bulkhead and continue carefully with full face drill-and-blast excavation. Numerous karst features voids, typically four to eight feet across, were encountered along this final reach of the excavation.

In July 2021, Affholder and Brierley Associates celebrated a reverse hole-through when the Jefferson Barracks recovery tunnel was successfully excavated to the face of the stalled TBM. The machine was advanced to the recovery shaft and removed for refurbishment. A replacement 13.5ft machine was relaunched out of the same shaft. The new TBM holed through in December of 2022, with no major additional karst zones being encountered.

Ray Scherrer, Division Inspector for the MSD, cited teamwork and communications as keys to the successful completion of tunneling. “It took a great deal of coordination of the entire project team to not only deal with the technical issues, but third parties as well,” he said. “Even something as seemingly straightforward as building the recovery shaft was challenging due to coordination between parties including the Missouri Historical Society, Great Rivers Greenway, Union & Pacific railroad, and others. Every time we put a shovel in the ground we were facing a delay to get approvals.”

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