East Side Access project progress as of Dec. 3, 2013,...

East Side Access project progress as of Dec. 3, 2013, along Sunnyside Yard and Harold Interlocking in Queens. Credit: MTA Capital Construction / Rehema Trimiew

The MTA says it has filled in all the sinkholes discovered at a critical construction site for the nearly $11 billion East Side Access project, but can't rule out new holes forming in the future.

In April, the Metropolitan Transportation Authority announced it had found five surface depressions at the Harold Interlocking -- a large train junction in Long Island City, Queens, where much of the construction work is going on for East Side Access, which has been plagued by years of cost overruns and delays. The agency launched an investigation that included hiring outside geotechnical engineers to inspect the sinkholes.

MTA spokesman Aaron Donovan said Thursday that the MTA has wrapped up the investigation after finding 11 sinkholes in total. Eight measured between 1 and 3 feet deep, but three larger ones were from 7 to 9 feet deep. All had diameters of 1 to 3 feet.

Investigators determined that the holes were caused by a combination of loose soil conditions at the Harold site and heavy rains this past spring. On two particularly rainy days -- March 29 and April 30 -- the downpours pushed down the soft dirt into empty underground pockets, causing sinkholes to form on the surface, Donovan said.

"It naturally wants to go down and fill in the underground open space," Donovan said of the soil. "The real villain behind this whole thing is Mother Nature."

Although crews had to temporarily suspend some work near the sites of the sinkholes, Donovan said there was "not a measurable impact" on the budget or the timeline of East Side Access. Five contractor crews working at about a dozen other sites were able to continue at those locations while the sinkholes were remedied. Also, Donovan said, East Side Access has contingency funding built in to cover such "real-world problems."

Donovan said workers filled in all the identified holes, using soil similar to that displaced. While Donovan said the MTA is "confident" that the holes won't recur at the same locations, he said it's possible new sinkholes will form if rainfall again pushes down on the soft ground at Harold, which includes earth excavated when the East River tunnels were built a century ago.

East Side Access, referred to by the MTA as the largest public works project going on in the United States, aims to link the Long Island Rail Road to Grand Central Terminal via newly bored tunnels.

Setbacks have increased both the timeline and the cost of the so-called "megaproject," once pegged for completion by 2009 at a price tag of $4.3 billion.

The Federal Transit Administration now says it won't be finished before 2023 and projects the cost at nearly $11 billion.

Latest video