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SR 99 update: Contractor begins backfilling TBM access pit

Seattle Tunnel Partners (STP) has started backfilling the access pit on the Alaskan Way Viaduct replacement project in preparation for tunnelling to resume.

The 36.5m (120ft) deep pit was built to access and repair the tunnel boring machine (TBM) after it stalled in December 2013.

STP is now filling the access pit with sand and soil following several weeks of preliminary TBM tests, including rotation of the cutterhead in the open air at the bottom of the pit.

During the backfilling process, STP will incrementally turn off the dewatering wells that crews have been using to control groundwater.

Washington State Department of Transport says it will continue to monitor the movement of the ground, structures, utilities and the viaduct during this process.

Tunnelling is expected to resume on 23 December.

TBM repairs

The TBM – named Bertha – is currently sitting on a concrete cradle at the bottom of the access pit, approximately 6m (20ft) from the wall it will tunnel through when tunnelling resumes. This phase of the repair effort includes:

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 13 December, STP began filling the pit with sand, which will rise up to 1.2m (4ft) above the top of the TBM. The excavation chamber behind the cutterhead will also be filled with sand. As sand flows better than other soils and quickly allows water to move through it, this will help crews achieve the required density.

Step 4
The remaining 15.5m (51ft) 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 tunnelling.

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