86th Street cavern of the Second Avenue Subway. (Charles Eckert)

86th Street cavern of the Second Avenue Subway. (Charles Eckert) Credit: 86th Street cavern of the Second Avenue Subway. (Charles Eckert)

The first phase of the Second Avenue subway is more than half finished as of this month, while sandhogs 100-feet below the street continue blasting out the 86th Street station cavern.

Blasting for the 72nd Street station finished in February, but rock-crumbling explosives will be detonated inside the 86th Street station cavern through August, with possibly more blasting in ancillary areas.

“One of the things that people are not going to be able to see as quite often as they did before is the blasting,” said Michael Horodniceanu, head of capital construction at the MTA.

The end of blasting at 86th Street means the end of the muck houses that handle the debris.

“The muck bins and truck loading station will be dismantled by this fall,” Horodniceanu said, adding that residents will see an improvement in their quality of life near the 86th Street station. The muck house enclosure, meanwhile, will remain on the street until the summer of 2014.

On a press tour of construction below street level Thursday, construction crews in the expansive cavern were preparing areas for blasting by drilling holes into large mounds of 350 million-year-old bedrock called “Manhattan schist,” then stuffing them with sticks of wired explosives.

“We are blasting down to the lowest level,” Horodniceanu said.

The 86th Street blasting is part of a nearly $310 million cavern mining and lining contract that is 59% complete and is slated for completion in December 2014. Meanwhile, starting in April 2014, crews will add the station’s finishing touches, such as tracks, the third rail, and communications, power and signal cables. That is expected to be completed by May 2016.

“The finishes take time,” Horodniceanu said.

The 72nd Street station, meanwhile, is 86% done, with finishes to the station expected to start in September and end in November 2015.

The capital construction chief said the initial $4.5 billion phase of the Second Avenue subway — two tunnels between 92nd and 63rd streets totaling more than 15,000 feet — is on schedule to be completed by December 2016, according to Horodniceanu. So far, two contracts have been completed and eight are still active; the final contract for the first phase was awarded in June.

“We are on budget, and we expect not to go a penny over,” he said.

The next phases of the Second Avenue subway will cover 125th Street to 96th Street, then south from 63rd Street to Houston Street, with the final leg stretching down to Hanover Square in lower Manhattan.

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