Sandy may have created air pockets under streets, buildings

Sandy may have created air pockets under streets, buildings
Holey ground: This image from ground-penetrating radar data shows an air pocket beneath a local residence. The smooth, gray portion shows solid ground of normal consistency, while the black and white stripes show where soil has been washed away by floodwaters, leaving voids reaching down as far as five feet which could turn into sinkholes and undermine building foundations.

Brooklynites in Sandy-ravaged neighborhoods are standing on air — literally.

Retreating floodwaters from the superstorm created underground air pockets just waiting to become sinkholes beneath streets, buildings and backyards across Southern Brooklyn, and most people aren’t aware of the danger under their feet, say local architects and companies specializing in subterranean scans.

“From Seagate to Manhattan Beach, a lot of these building’s foundations have been scoured,” said architect Walter Maffei, “and no one knows about it.”

According to Maffei, any structure that suffered severe flooding as a result of October’s hurricane is liable to have lost some of the sediment beneath its foundation as the water rapidly withdrew into the ocean, taking tons of soil with it.

“When you build a sand castle, everybody knows the water comes in relatively slowly, but when it goes out, it moves quickly and takes everything with it,” the architect explained. “A lot of scouring occurs at the foundation level, or below.”

Underground voids, or air pockets, can undermine the foundations of buildings and city infrastructure such as roadways. This means sink holes and structural stability is a big concern and potential hazard for people living in Red Hook and southern Brooklyn, according to Lou Neos, a technician at Ameriscan, which specializes in ground penetrating radar.

“It’s all over, wherever there was severe damage from Sandy,” said Neos, who has found voids underneath buildings in Sheepshead Bay and Brighton Beach. “If there’s another storm, people might lose their whole house because of a void.”

There are two methods for detecting subterranean voids. One is tearing up the floor, and probing the soil manually, while the other involves the use of ground-penetrating radar systems, a nondestructive method which uses electromagnetic radiation to detect the reflected signals from subsurface structures.

Either way, Maffei encourages anyone with property that sustained severe flooding to check for scouring before they begin any heavy construction or restoration work.

If property owners do detect voids, they can use a technique called super grouting, in which grout, or concrete is injected into the void underneath the building’s foundation.

If the voids go undetected, however, owners may start to see their walls crack, porches buckle, and homes tilt as the sands begin to settle, and the building’s foundation weakens.

“The foundations can settle unevenly. If that happens, walls can crack and houses may eventually begin to lean,” said Maffei. “It may not happen over night, but it will happen over a period of months or even years.”

Reach reporter Colin Mixson at [email protected] or by calling (718) 260-4514.

Crack up: These cracks in the foundations of a local home appeared after Sandy washed away soil from beneath the buliding, causing it to settle unevenly as the resulting air pockets collapsed.

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