MONTPELIER, Vt. — Demolition at the flooded former state office complex in Waterbury is shifting into high gear with most interior work done and one building already gone as part of a $125 million project that will restore part of the office and construct a new building above the flood line.

So far, from Main Street, there is little to see of the work being done behind the fence that surrounds the project. Much of the construction is happening inside the buildings that are set to be demolished, said Michael Stevens, the special project administrator for Vermont Building and General Services.

About 125 workers are on the site, and the demolition of the building that once contained the state laboratory is due to start next week. The bidding process for the construction is underway, and some work is expected to begin in February with full-bore construction slated for the spring, Stevens said.

“There is a buzz on this campus. We are making progress. The work has started, and it feels energized,” Stevens said.

When Tropical Storm Irene flooded the Winooski River on Aug. 28, 2011, the river crept into the complex, forcing the evacuation of the Vermont State Hospital and requiring more than 1,000 state workers to find new offices. The only major building still in use from before the storm is the Public Safety Building.

The local economy took a hit after the workers disappeared overnight. Finding permanent locations for the displaced offices and paying for it was one of Vermont’s biggest challenges in recovering from the storm.

In August, coinciding with the second anniversary of the storm, state and federal officials finalized the plans for the complex.

The total cost of damage and repairs at the Waterbury complex is estimated at about $225 million. That figure includes the cost of building a new Vermont State Hospital, now underway in Berlin, and paying for the temporary offices used by Waterbury workers displaced by Irene.

The revived Waterbury complex will house about 900 employees of the Agency of Human Services.

The new office building is built about 6 inches above the estimated 500-year flood level and a full 3 feet above the level during Irene, Stevens said.

“If Irene occurs again, we won’t be back in the complex the next day,” Stevens said. “We’re going to have to clean up the roadways, clean up the site, but we sure won’t be out of this complex for years. We’ll be out of this complex for a week or two.”