Cracks are growing on the Alaskan Way Viaduct, which will need a few brief closures for inspections and repairs near Seneca Street.

Some cracks are new and others have increased since they were found last year. The problem is unrelated to construction of a four-lane tunnel, because that work is happening a half-mile south, says Tom Baker, chief bridge engineer for the Washington State Department of Transportation (WSDOT).

Instead, the most likely culprit is the 2001 Nisqually earthquake, which probably caused the column foundations to shift in ways that aren’t entirely understood, he said. Baker mentioned that technical records for the 1953-vintage structure are incomplete.

But the WSDOT has noted the viaduct has sunk more than 3 inches near Seneca Street since the quake, including continued sagging last year.

The cracking demonstrates how the state is on a race against time to keep the fragile viaduct usable until the replacement Highway 99 tunnel is finished. That milestone will be delayed until mid-2016 or later because of damage to the giant boring machine, nicknamed Bertha.

The fact the viaduct is gradually sagging supports Baker’s theory, said Neil Hawkins, of Clyde Hill, professor emeritus of civil engineering at the University of Illinois and a leading expert on concrete. In the very weak fill soil near Seattle’s waterfront, it’s entirely possible that foundations could shift for 20 to 30 years after a quake, he said.

“If they had a settlement like that as recently as two or three years ago, the cracks could be related to the earthquake,” Hawkins said.

The widest crack found during a March 1 quarterly inspection is around 1/16 of an inch, and the longest crack is about 8 feet, state engineers said. Some marked and labeled with chalk are visible from the ground.

To get a better look, crews will remove ivy from the columns and girders, then close the viaduct March 22. Repair work might require three overnight closures, said Dave McCormick, assistant regional administrator for WSDOT.

Workers will inject epoxy into the cracks, he said. Similar repairs using epoxy were made to the first batch of Highway 520 floating-bridge pontoons where the end walls and bottoms cracked in 2012.

New cracks on the viaduct pose no immediate safety risk to drivers, said Keith Metcalf, assistant chief engineer for WSDOT.

Hawkins agreed, saying the cracking released quake-related stresses in the concrete, so those are probably borne by reinforcing steel in the viaduct.

Of course, the structure remains vulnerable to a major earthquake. According to a state disaster video published in 2009, a temblor slightly worse than the Nisqually quake, and closer to Seattle, would cause a full collapse within minutes.

Some citizens, including engineer Victor Gray and architect Art Skolnik, urged the state to retrofit the old structure, but were rebuffed. State-funded studies said it would take hundreds of millions of dollars to meet modern seismic standards. Gov. Chris Gregoire in early 2009 announced that a four-lane tunnel tube was her choice to replace the viaduct, and lawmakers approved.

The four-lane, $2 billion tunnel was supposed to be done by 2015 but has been delayed by damage to the main bearing of the giant tunnel-boring machine Bertha. WSDOT says traffic will stay on the old viaduct until the tunnel is done — which it now seems will be in mid- to late 2016.

Baker said the cracks discovered March 1 illustrate the importance of replacing the 61-year-old viaduct.

Besides repairs, load limits might be considered, the WSDOT said, though engineers suspect heavy vehicles are not a major cause of cracking.

But Hawkins said that even legal loads, especially if there are uneven weights over the axles, could contribute to cracks on such an old structure. “Cumulatively, that would have an effect,” he said.

Down near Yesler Way, the viaduct has sagged about 5½ inches since the earthquake. The area settled an additional four-tenths of an inch this winter due to tunnel-related work — groundwater loss, and vibrations from the construction of a block of concrete where Bertha will stop for maintenance near the viaduct.

This spring a temporary 120-foot-deep pit must be dug to repair the stranded tunnel drill. The pit excavation requires careful monitoring and protection of the viaduct columns, to be followed by a viaduct closure late this year while Bertha passes beneath.

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