Four columns of the Alaskan Way Viaduct have sunk another three-eighths of an inch so far this year, while an emergency project to strengthen...

Four columns of the Alaskan Way Viaduct have sunk another three-eighths of an inch so far this year, while an emergency project to strengthen the underpinnings is getting finished in the nick of time.

The critical area where the sinking is occurring is near the state ferry terminal. It was measured during a maintenance shutdown of the elevated highway last weekend. At that location, the Viaduct has now sunk 5 ½ inches since the Nisqually earthquake seven years ago.

Crews have been building deeper, wider steel-and-concrete foundations surrounding the weakest columns, and hope to finish the work next month.

Vibrations caused by the recent foundation construction played a role in the most recent sinking, but it was expected, said Harvey Coffman, bridge preservation engineer for the state Department of Transportation.

The elevated highway’s central portion, built on weak fill soil, was built in 1953, followed by the stretch south of South Dearborn Street in 1959. Coffman says he expects it to settle another three-eighths of an inch near Yesler Way, before its weight rests securely on the new foundations.

Gov. Christine Gregoire has said the one-mile waterfront section will be removed or replaced starting in 2012. Other stretches of the Viaduct would be refurbished sooner.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com