OROVILLE — The cost of repairing the crippling damage to Oroville Dam’s spillways caused by last winter’s fierce storms has almost doubled, state water officials said Thursday.

Kiewit, the Nebraska-based construction firm that has the main contract to rebuild the main spillway and emergency spillway at Oroville, the nation’s tallest dam, estimated in its winning bid in April that the work would cost at least $275 million. But the price tag has now grown to at least $500 million, said Erin Mellon, a spokeswoman for the Department of Water Resources.

“When Kiewit put in the bid, only 30 percent of the project had been designed,” she said. “When you get on the construction site, there’s a lot more information that you glean.”

Jeff Petersen, project manager for Kiewit, said that once construction workers got on the site they discovered they had to dig much deeper to get down to bedrock than they had expected. That meant they are having to pour almost twice as much concrete this year — 870,000 cubic yards — as the company and the Department of Water Resources had planned.

Petersen said such discoveries are not uncommon on major dam projects.

“It’s like when you work on an old house or an old car,” he said.

Petersen said the project remains on schedule to hit a Nov. 1 deadline for repairs that will allow the main spillway to carry up to 100,000 cubic feet per second this winter. Seven hundred workers are rushing around the clock.

“I don’t want to jinx it,” he told reporters on a tour of the construction site, “but we’re five days ahead of schedule.”

On Thursday, the scene inside the main spillway — which is 3,000 feet long and 180 feet wide, twice the width of the Golden Gate Bridge — was abuzz.

Huge cranes swung steel beams and other materials overhead as fleets of giant dump trucks poured layer after layer of concrete, and bulldozers and mechanical rollers compacted it. Diesel engines roared from all directions, and the constant beeping of heavy equipment filled the air.

Additional work to finish the emergency spillway and cover the entire main spillway with reinforced concrete is scheduled to be completed by the end of 2018. About 41 percent of the main spillway will have concrete with modern rebar by Nov. 1.

Mellon said the state has asked the Federal Emergency Management Agency to pay 75 percent of the cost. The rest will come from contractors of the State Water Project, including the Santa Clara Valley Water District, the Alameda County Water District and the Metropolitan Water District of Southern California.

One other large expense of the February disaster is coming in lower than expected.

The original cost estimate for the emergency response — which included using fleets of helicopters to drop boulders, emergency concrete pouring and moving huge high-voltage power lines — was $274 million. In the end, however, it looks as if the actual cost will be between $140 million and $160 million, Mellon said.

Mellon said state officials are computing numbers now and more precise estimates will be ready by Nov. 1. “It’s an evolving project,” she said. “Most are.”

In early February, relentless winter storms dumped nearly 13 inches of rain in four days on the Sierra foothills. Waters raging over the dam tore an enormous hole in the spillway, leading to an unprecedented emergency that prompted the evacuation of 188,000 people from nearby towns.

After the hole developed, the Oroville spillway became an international story. State officials closed the gates on the main spillway, and the lake level rose to the top. Five days later, water poured over the emergency spillway, eroding it so badly it looked as if it would collapse.

Butte County Sheriff Kory Honea issued an emergency evacuation order with phrases like “This is NOT a drill” spread across social media, radio and TV. Chaos ensued.

Stores closed. Gas stations were overwhelmed. Panicked motorists drove on the shoulders of Highways 70 and 99.

The fear was that if the emergency spillway continued to erode, the top of the lake’s rim could give way, sending a wall of water from the 10-mile-long lake — California’s second largest reservoir — to the communities below.

State water officials made a gambit. They reopened the badly damaged main spillway to lower the lake. They knew the water — flowing at 750,000 gallons per second — would rip it apart, but they hoped the violently out-of-control flows would not start eroding away the face of the dam itself.

The plan worked. The lake level dropped. Water stopped flowing over the emergency spillway. The dam held. And two days later the evacuation order was lifted.

After the state put the repair job out to bid, Kiewit came in with the lowest price. The company, which also built Los Vaqueros Dam in Contra Costa County in the 1990s, went to work on two primary jobs: shoring up Oroville’s main spillway by filling in the chasms that had eroded and pouring new concrete — and building a new underground retaining wall at the hillside that functions as the dam’s emergency spillway, with layers of new concrete to reinforce its top.

An independent “forensics team” ordered by federal regulators to find out what went wrong at Oroville issued its preliminary findings in May, citing defects ranging from the dam’s construction in the 1960s to problems linked to poor maintenance and oversight by state and federal officials. Its final report is due this fall.

The new main spillway will be much stronger, Petersen said. Its concrete is between five and 13 feet thick in most places, with 612 concrete slabs each connected to 14 steel plates holding anchors drilled 20 feet or more into the bedrock.

After work began May 20, crews were in a race to stabilize the main spillway in five months.

“It’s not the biggest job we’ve ever worked on, but we usually have more time, compared to other jobs,” Petersen said. “We mobilized 500 pieces of equipment in 30 days.”

Construction workers have come from as far away as Alaska, Texas and Colorado, although 90 percent are Californians. In the summer, they worked in temperatures that exceeded 110 degrees inside the massive main spillway.

Several months ago, state officials decided to move the location of a huge underground concrete wall designed to prevent erosion on the main spillway from 350 feet away from the reservoir’s edge to 732 feet away. They had to do that to find solid bedrock. That decision doubled the amount of concrete armoring on the hillside, increasing costs.

Another driver of the higher-than-expected costs was the fact that crews filling two giant chasms in the spillway up to 80 feet deep had to excavate up to 20 feet deeper in some places than they had anticipated to find bedrock. That meant more concrete was needed to fill the holes.

“It’s like when you go to the dentist with a cavity and he says, ‘This has to come out too,'” Petersen said.