By Mike Lindblom, December 5, 2014
The Alaskan Way Viaduct and nearby soil have sunk 1.2 inches this
fall alongside stranded tunnel machine Bertha, senior state engineers
said Friday afternoon.
The settling shows that the tunnel team is having trouble controlling
the soil, crucial to protecting downtown as the Highway 99 tunnel
project attempts to move ahead.
A few buildings in historic Pioneer Square have also settled to a lesser extent, the engineers said.
However, neither the viaduct nor the buildings are showing signs of
stress such as cracking or buckling, said Matt Preedy, deputy
administrator for the tunnel project. He said the viaduct remains safe
to drive, as the state collects more data.
Surveys by the state and contractor Seattle Tunnel Partners (STP)
confirmed the sinkage 1 1/2 weeks ago, but there hasn’t been further
settling this week, said Dave Sowers, a geotechnical engineer at the
state Department of Transportation.
This latest worry comes near the one-year anniversary of Bertha’s
stall, when the machine overheated and failed to advance Dec 6, 2013.
Contractors are digging a 120-foot deep vertical pit to reach, remove
and replace damaged bearing parts at the front of the machine’s
57.3-foot diameter cutter. This phase includes a sophisticated operation
to remove water from the soil that can disrupt nearby areas.
Dewatering may have caused the soil to sink, said Preedy, but there
could also be natural forces, or other construction, at work.
A state survey crew will measure the sides of the viaduct to confirm
ground-level and electronic data, and check for any continued slumping.