No one knows whether winter will bring pounding storms like those seen last season, but California water officials say Oroville Dam is ready for whatever comes.

The nation’s tallest dam became one of the fastest construction projects in modern state history after the dam’s spillway partially collapsed in February amid heavy rain and caused the evacuation of thousands. Hundreds of workers have been laboring around the clock for five months to rebuild the chute before wet winter weather returns.

This week, as the roar of bulldozers and the whirl of cranes reach a crescendo, crews will pour the final concrete for the half-mile-long spillway — just in time to meet the state’s Nov. 1 deadline for having the 770-foot-high dam ready to release water, should the lake behind it fill up.

“If things go well, we could be done on the 31st,” said Jeff Petersen, project director for Kiewit Corp., the firm hired by the state Department of Water Resources to fix the dam at record pace. “Everybody knew it was a tight schedule. We just used construction fundamentals to know mathematically it can work. We just broke it down to pieces and stuck to those details.”

While state water officials are resting easier now that the rebuilt spillway is in place, the work has not come without concerns. Costs have nearly doubled since construction began, to at least $500 million, and dam operators will face limits on how much water they can store in the lake until final repairs are done next year, meaning potentially less supply temporarily.

Critics of the state also question whether the spillway failure is just the first of many problems that may surface at the nearly 50-year-old dam or, for that matter, at other spots in California’s aging water infrastructure.

Lake Oroville, about 75 miles north of Sacramento, is the largest of nearly three dozen state-run reservoirs that supply water across California. It’s the second-biggest reservoir in the state after the federally managed Shasta Lake. The dam also provides flood control on the Feather River.

“We’re very confident in the reconstruction effort, and the dam remains safe and sound,” said Erin Mellon, spokeswoman for the Department of Water Resources.

Problems at the dam emerged Feb. 7 when a giant hole opened on the spillway as dam operators released large amounts of water from the lake. When managers closed the chute to take stock of the tear, an emergency spillway — essentially an unpaved hillside — was thrust into action. It quickly began to erode.

Fears that the hill would give way and reservoir water would gush uncontrollably downstream prompted authorities to evacuate 180,000 people. Many rushed from their homes with only what they were wearing, as highways clogged and gas stations ran out of fuel. Dam managers ultimately averted disaster by putting the damaged main spillway back into use, at least long enough to lower the lake to a safe level.

A panel of independent investigators has since determined that years of water seepage weakened the spillway. The group’s findings, which are yet to be finalized, suggest that better inspections could have identified the problem before the near-catastrophe.

The remodeled main spillway is designed to be much stronger.

That chute, which is as wide as a 15-lane freeway, is now 7½ feet thick on average, compared with 2½ feet in the original 1960s construction. The steel pillars that anchor the structure extend 15 to 25 feet into the bedrock, instead of the previous 5 feet. And drainage beneath the spillway has been modernized.

“It’s an amazing change in technology since the first one was built,” Petersen said. “It’s going to outlive all of us.”

Kiewit began the project in earnest May 20, after the late seasonal rains ended and the state drew up designs and put the job out to bid — a process that can normally take years. The company rushed more than 500 pieces of heavy machinery to the dam, recruited about 600 engineers, construction workers and tradesman, and built two concrete plants at the lake.

Much of the work over the ensuing five months deviated from the original plan as state officials learned more about the spillway damage and adjusted accordingly.

Petersen said securing the new chute was a much bigger task than expected. Crews had to dig deeper to get a foothold on the underlying rock and use more concrete to stabilize the structure, adding tremendously to the cost. The original estimate was $275 million.

“You’re spending this money to rebuild it, you might as well do it right,” Petersen said.

State officials hope federal emergency funds will cover 75 percent of the expense. The balance will be paid by the local water agencies that buy water from the state, including the Bay Area’s Santa Clara Valley Water District and Alameda County Water District.

Kiewit also has been working to stabilize the emergency spillway. Crews built a huge, 1,450-foot retaining wall on the barren slope next to the main chute over the summer, and next year they’ll pave the upper part of the hill. State officials, though, hope never to use this outlet again.

Next year, workers also will rebuild the top 730 feet of the main spillway, a section they didn’t have time to tackle over the summer.

Because of the remaining work, dam operators say water releases down the main spillway will be limited to 100,000 cubic feet per second this winter. The spillway’s capacity will be 270,000 cubic feet per second when the job is done next year. To maintain the lower outflow for now, dam operators plan to keep water levels 200 feet below the top of the 700-foot-deep lake.

State officials say the diminished storage at the reservoir will have a negligible effect on the state’s total water supply.

“It’s hard to say how allocations will shake out this year because weather is unpredictable,” said Mellon, with the Department of Water Resources. “However, the largest reservoirs in the state are at 120 percent of average for this time of year, so we’re certainly feeling much better.”

The problems at Oroville Dam have prompted state and federal water officials to enhance reviews of dozens of other spillways. The average age of California dams is 70 years.

At Oroville, an independent group of engineers has been working alongside the Department of Water Resources to make sure the rebuilt spillway and other parts of the dam are safe.

Still, some are skeptical. Robert Bea, a retired engineering professor at UC Berkeley who has studied the reservoir, wants more testing of the headworks, where the spillway gates open and close. He also thinks the state should examine the potential for water seepage behind the dam.

“Having gone through the surprise evacuation of 180,000 people, after telling them the dam was safe, I think we need to be careful,” Bea said.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander

$500 million

Cost to rebuild spillway.

7½ feet

New thickness of chute, compared with 2½ feet of original.

15 to 25 feet

Extension of steel pillars into bedrock, instead of the previous 5 feet.

270,000 cubic

feet per second

Spillway’s new capacity.