Dynamite blasting during construction at a new Fairfax County elementary school hurled large boulders into the side of the building yesterday, doing damage that may delay its scheduled Sept. 4 opening, officials said.

No one was injured in the blast, which caused an estimated $200,000 damage to Poplar Tree Elementary School in Chantilly, a fire department spokesman said. Fire marshals have revoked the blasting permit of the company, Capital Drilling Inc., he said.

Firefighters responded to an 11 a.m. call to a "structure collapse" at the school in the 13400 block of Melville Lane, according to Lt. Michael Reilly. Damaged were part of the building's air conditioning system and a wall of the school's boiler room, he said.

The company doing the blasting was trying to create a hole large enough for an 8,000-gallon petroleum tank about 70 feet from the building when the incident occurred, officials said. A spokesman for the company could not be reached for comment yesterday.

"See, when the blast went off, it threw boulders," said Scott Sanford, superintendent at the construction site. The boulders "just came in on a straight line to the building," he said. "Nobody was in the area, thank God."

Gene Kelly, project manager for the county school system, said it would take about a week to determine whether the school will open on time. Because the damage was "fairly extensive," some walls will have to be torn down to determine the extent of the structural damage and equipment will have to be replaced, he said.

Kelly said workers had been having difficulty digging the hole for the tank because of an extremely dense rock, called weathered silt stone, about six feet below the surface. The blasting company was hired to remove the same rock from a baseball field at the school, he said.

While blasting the hole, the company took safety precautions, including covering the dynamite with dirt, but "it all just shot out on the angle," Kelly said.

Reilly said the company's permit was revoked because the blast exceeded noise limits and earth movement restrictions. He said a magistrate declined to charge the company with a misdemeanor violation in the incident.