The state Transportation Department said Tuesday the sinking by four-tenths of an inch is within safety limits and had been anticipated during construction. The department says the viaduct was strengthened and monitors were installed to measure how much the viaduct moves.
Officials say the viaduct remains safe for everyday use, but it could collapse during an earthquake. That's why it's being replaced by the new tunnel to carry Highway 99 traffic.
However, tunneling has been stalled since early December because of a mechanical problem with the machine called Bertha. The department expects to know later this week how long repairs will take and when tunneling may resume.
Earlier estimates said it could be months before the tunneling machine is fixed.
State transportation officials insist the tunnel is still the "most expedient way" to replace the Alaskan Way Viaduct.