Dec. 14 project update: Seattle Tunnel Partners begins backfilling the access pit

Posted on Dec 14 2015 9:35 AM

Seattle Tunnel Partners continues to prepare the SR 99 tunneling machine for mining. Crews have spent several weeks conducting a series of preliminary tests – including rotation of the cutterhead in the open air at the bottom of the 120-foot-deep pit that was built to access and repair the machine. Over the weekend, STP began filling in the access pit with sand and soil – the final step before they resume mining.

The following graphics illustrate this phase of STP’s repair effort.

Current conditions

Bertha currently sits on a concrete cradle at the bottom of the access pit, approximately 20 feet from the wall she will tunnel through when mining resumes.

Step 1

STP has placed gravel under and beside the machine to fill the gap between the machine and cradle.

Step 2

Next, STP added a low strength concrete-sand mixture to the portion of the cradle in front of the machine to assure good contact between the rings and the cradle. This provides a firm surface for the machine to mine through.

Step 3

On Sunday, STP began filling the pit with sand, which will rise up to 4 feet above the top of the tunneling machine. The excavation chamber behind the cutterhead will also be filled with sand. Sand flows better than other soils and quickly allows water to move through it, which will help crews achieve the required density.

Step 4

The remaining 51 feet to the ground surface will be filled with compacted soil originally excavated from the access pit. Once the pit has been backfilled, STP will commence final testing and prepare to resume mining.

Controlling groundwater

During the backfilling process, STP will incrementally turn off the dewatering wells that crews have been using to control groundwater. We will continue to monitor movement of the ground, structures, utilities and the viaduct. Additional information about our ground monitoring program can be found here.

STP's most recent schedule shows the machine will be ready to resume mining in the pit on Dec. 23. The state cannot verify the contractor's schedule.