The Healdsburg Memorial Bridge will be closed to vehicular traffic Wednesday and Thursday for an exploratory drilling project.

The test bore from the deck of the bridge into the bed of the Russian River, near the existing center pier, is part of initial work for a major rehabilitation of the span scheduled for next year.

The work will begin at 8 a.m. today and continue overnight until 6 p.m. Thursday. Although the bridge will be closed to motorized vehicles, it will remain open for bicycles and pedestrians.

The drilling is intended to ascertain soil characteristics that will determine the design of a new centerpiece and foundation for the 91-year-old Russian River crossing.

That major work, scheduled for mid-2013, is expected to require closure of the bridge to all traffic for 18 to 24 months.

While vehicles can use the nearby Highway 101 bridge as an alternate, city officials are trying to figure out how to re-route bicycles.

Possibilities include using the railroad bridge upstream, or having a shuttle service of some type.

The City Council is set to discuss the subject at its Aug. 20 meeting.