The tunnel boring machine (TBM) contractor experienced obstructions, including steel wire, while tunneling beneath the Cedar River. The 3-foot diameter TBM eventually made it across the river, but became inoperable at the west river bank, approximately 30 feet from the Renton airport landing strip. To complete the project, the contractor needed to recover and repair the damaged TBM, and then continue tunneling west from this stopping point.

The TBM was stuck at a depth of about 25 feet below ground surface so the only way to recover the machine was an unplanned rescue shaft. However, there were space restrictions because the stopping point for the TBM was in between the Cedar River and the airstrip. In addition, severe limitations regarding equipment heights, operating times, etc. were imposed by the Renton airport. Soils above and below the TBM consisted of wet, loose sand and silt and dewatering would be very difficult and expensive so close to the Cedar River.

Frozen soil shoring was used to create a stable, dry rescue shaft to allow the contractor to remove the TBM. The soil below the TBM/steel casing was frozen by installing a number of angled freeze pipes that extended from the ground surface to points beneath the casing. After the TBM was removed, a 14-foot diameter pre-cast manhole was placed inside the shaft. The repaired TBM was then launched from this manhole to successfully complete the project.

With our specially engineered system, the shoring wall was frozen in a record time of 2 weeks. Ground freezing stabilized the loose sand and silt and created a strong, water-tight shoring system that allowed the contractor to safely remove the TBM and place the new manhole with no dewatering.