The MBTA will close the Red Line north of Harvard Square on weekends from November through March to complete $80 million in repairs designed to keep trains from derailing, transit administrators said yesterday.
That means on a typical weekend, about 21,200 riders on Saturday and 14,200 on Sunday who board at Porter, Davis, and Alewife stations will have to clamber aboard substitute buses plying the crowded streets of Cambridge and Somerville or find another way to get around.
Starting Nov. 5, weekend service will stop to allow T crews to plug tunnel cracks and seal water leaks that have eroded the concrete track base and corroded power lines. Then crews will replace damaged track, supports, and electrical components.
Among the T’s estimated $4.5 billion in backlogged repair and replacement needs, this segment of the Red Line seized public attention in 2009 when it was singled out in an independent report on the MBTA ordered by Governor Deval Patrick. Report author David F. D’Alessandro, the former John Hancock chief executive, said the threat of derailment from deferred maintenance was so serious he would avoid riding the Red Line beyond Harvard.
The T put the project on its five-year capital plan and secured $4.3 million in federal stimulus funds to defray the cost.
The work is part of about $420 million being spent this year to maintain or replace vehicles and infrastructure in disrepair, a sum dwarfed by the system’s needs.
“Track conditions, switch conditions, power . . . if these things aren’t working well, people are going to be late, people are going to be delayed,’’ state Secretary of Transportation Richard A. Davey said in an interview yesterday after ribbon cutting for the rebuilt transit station at Ashmont, at the other end of the Red Line.
That event drew more than a dozen city, state, and federal officials and scores of onlookers, a gathering unimaginable at the completion of an underground track or unseen power project such as the one north of Harvard Square.
“This is the sort of investment that the state [needs] to make,’’ said Brian Kane, budget and policy analyst for the MBTA Advisory Board, which represents cities and towns served by the T. “It’s not glamorous; it’s not sexy; it’s not anything you cut a ribbon on. But it is the investment that allows the system that provides 1.2 million trips a day to function.’’
Riders at Davis Square said yesterday that they understood the need for the work but might use the T less frequently on weekends. They also expressed surprise that the MBTA had yet to post signs or announce the work, worrying that many would be caught off guard.
“We will only find out at the moment we’re actually taking the train,’’ said Jessica Martinez, 37, a Mattapan resident who commutes daily from Ashmont to Davis. ’’I don’t think they do enough to let passengers know what [they’re] planning.’’
Carmen Dicecca, 25, of Somerville said he would probably walk to Harvard to pick up the Red Line into Boston, rather than wait for the replacement buses, which the T will staff with 31 drivers, part of the project cost.
Tufts University freshman Kevin Kozikowski said he would simply wait for the end of the project before resuming his weekend rides downtown to visit friends at Emerson College. “Just because I don’t have to go into the city, at that point, I’d just rather not be inconvenienced,’’ he said.
Despite the 2009 report, T officials have stressed that there is no immediate danger of derailment on the Red Line, which is fully inspected twice a week. But the long-term potential for a derailment would increase without this project, said Michael A. Turcotte, the T’s assistant general manager for engineering and maintenance.
“There’s nothing unsafe here,’’ Turcotte said. “We’ve been monitoring this, and we’ve wanted to do this. But as you can see from the numbers, it’s a pretty hefty job.’’
The price and the complexity of the work contributed to its deferral. The specialized track along the Harvard-to-Alewife extension, which opened in 1985, rests on thousands of concrete slabs that float on absorbent rubber disks - resembling 13-inch-wide hockey pucks - that cushion noise and vibration for above-ground neighbors. Tunnel leaks have caused the “floating slabs’’ to crack and their metal track fasteners to corrode, posing a risk of rail movement and train derailment.
The T expects to work most weekends through the end of March, though trains will run during the Christmas and New Year’s weekends. Buses will otherwise provide substitute service, picking up and dropping off outside the closed stations.
T administrators said they intend to have buses departing more frequently than Red Line trains to minimize waits, but the buses will have to contend with road traffic, and riders will no longer have a one-seat ride downtown.
Meanwhile, beneath the surface, workers will chase and plug leaks; remove and repour nearly 100 concrete slabs, each of them 9 feet wide, 5 feet long, and 18 inches high; and lay 6,000 linear feet of new rails and 2,300 feet of electrified third rail.
The project cannot be managed solely within the 3 1/2-hour window early each morning when the Red Line tunnels are dormant, without disrupting service.
Materials must be hauled in and out each weekend on special trucks outfitted to drive on rails, with the nearest entry point at the mouth of the tunnel near Kendall Station, where the subway line emerges to cross the Charles River: 2.2 underground miles from Harvard and nearly 5 miles from Alewife.
Even on working weekends, the T must use a specialized concrete mix that can cure hard and fast enough for trains to resume Monday mornings, Turcotte said.
About half the cost goes to the slab, track, and power replacement on the portion of the line north of Harvard Square, and half covers leak chasing and filling throughout the Red Line, he said.
Globe correspondent Taylor M. Miles contributed to this story. Eric Moskowitz can be reached at firstname.lastname@example.org.