Hundreds of houses and an elementary school in western Fairfax County were evacuated yesterday after a bulldozer ripped a hole in a large underground gasoline pipeline, shooting a geyser of unleaded fuel 100 feet in the air and leaving the ground drenched with the highly flammable liquid.

Police and fire officials cordoned off a four-square-mile area around the site in Centreville, spraying 10,000 gallons of foam over the gas and laying down pontoons to keep the liquid from running into streams and storm drains. Officials said it appeared that none of the gasoline had contaminated the county's water supply.

Last night, police and fire officials escorted about 50 residents of the Singleton's Grove subdivision into their homes, where they were given five minutes to pack some of their belongings. Some were told they might not be able to return for several days. One resident collapsed on one of the buses and was taken to Fair Oaks Hospital, but no other injuries were reported.

Cars, streets and town houses in the subdivision, parts of which are still under construction, were covered with white foam. Helicopters hovered overhead. Some houses apparently were drenched with gasoline, which lay in puddles every where. Leaves on trees looked charred. The air, reeking with noxious fumes, was lit with floodlights.

"It looks like a war zone," remarked Fairfax County Fire & Rescue Department Lt. Mike Reilly.

Citing the potential of fire and explosion from the gasoline and vapors, emergency workers evacuated about 700 children and staff members from the Union Mill Elementary School and residents from more than 300 homes in the Sunset Ridge and Singleton's Grove subdivisions. All electrical power was shut off. About 150 construction workers also were evacuated.

"It's only a pure miracle that we didn't have ignition," said Fairfax County Fire Chief Warren E. Isman, who called the spill "one of the most major incidents we've had . . . in years."

If there had been a spark, Isman said, "we would have lost every town house in that subdivision."

Officials said it was unclear how much gasoline was spilled in the incident, which occurred about 9:55 a.m. in the 6100 block of Singleton Way, south of Rte. 29 (Lee Highway) and east of Rte. 28 (Centreville Road).

Noel Griese, a spokesman for Colonial Pipe Line Co. of Atlanta, which owns the pipeline, said about 2,100 gallons of super unleaded gasoline escaped after a 4-inch-by-4-inch hole was punched in the line, which is 32 inches in diameter and stretches about 300 miles from Greensboro, N.C., to Dorsey Junction, Md., outside of Baltimore.

Isman, however, said that "tens of thousands" of gallons may have gushed out.

Officials said the incident occurred when a bulldozer that was leveling ground for a sidewalk punched a hole in the pipe, spraying gas for about 10 minutes.

Keith Carpenter, 37, a worker for F.E. Gregory & Sons Inc. who was operating the bulldozer at the time, said the machine "hooked it {the pipe} somehow and all I saw was gasoline {spraying} into the air."

"It was just enough to scare you to death," he said.

Barry Hice, 21, another Gregory & Sons worker, said he was sweeping the curb about 10 feet away when a tooth on the bulldozer's shovel punctured the pipe and he heard a spraying sound.

"I thought it was water at first," Hice said. "Once I found out it was gas I took off like a jet. I told my friend behind me, 'Don't light up any cigarettes. Let's get . . . out of here!' "

About 400 firefighters -- a third of the county's force -- and 22 county police officers converged on the scene and were assisted by units from Dulles International Airport, Arlington, Alexandria and the Bethesda-Chevy Chase Rescue Squad, officials said.

There were no estimates last night on how long the cleanup would take. Firefighters were using absorbent materials to soak up some of the gasoline and officials said that much of the contaminated dirt would have to be hauled away.

There were no damage estimates.

Investigators were taking measurements to determine if the pipe was deep enough to meet federal regulations, which stipulate that gasoline pipes be 30 to 36 inches below grade, officials said.

An official of the U.S. Department of Transportation's Office of Pipeline Safety, which regulates underground gasoline lines, said "The fire department tells us that the pipe was at least 36 inches deep . . . . The pipe was buried in accordance with regulations, as best as we can understand."

Griese, the Colonial spokesman, said construction workers notified the company that they had punctured the pipe abut the same time that workers in the company's Atlanta computer control room noticed that the line was losing pressure. The Atlanta workers immediately activiated a computer sequence, lasting about 30 seconds, that shut the line down in stages so pressure would not build up in other sections of the pipe and cause another rupture.

At the time of the incident, the pipeline was operating at 67.5 percent of capacity, pumping 567,000 gallons of gasoline per hour under 190 pounds per square inch of pressure, according to Griese. He said officials estimate that it will take two days to repair the pipe.

Evelyn Hall of Ohio was visiting her brother's family in the Singleton's Grove subdivision when "I heard a lot of commotion" and looked out the window. "It was gushing, gushing wide," she said. "It looked like dirty water."

Most evacuees from the two housing subdivisions were sent to Centreville Elementary School, where Red Cross workers served them coffee, doughnuts and soft drinks and showed videotapes while classes continued undisturbed.

Union Mill Elementary School officials said last night that no decision had been made whether to reopen the school today. They said if the school remained closed, the students should report to Clifton Elementary.

Singleton's Grove developer, U.S. Homes, reserved a block of rooms at a hotel for displaced residents.

About 7:45 p.m., police escorted two busloads of residents into the subdivision. Assistant Fire Chief Richard Steinberg told one group they were about to enter "a very dangerous atmosphere."

"This area is saturated with gasoline. It could easily be a week before you can return to your homes. We just don't know," he said.

After warning residents that anyone who did not follow instructions would be arrested, officers escorted them to their homes to gather clothes, medicines and other belongings.

One resident, Edward Roy, 55, collapsed and was taken to Fair Oaks Hospital, where he was treated for chest pains and released last night.

Residents raced through their homes, filling suitcases and laundry baskets, grabbing their pets, stuffing anything they could carry under their arms.

"I thought my house was going to ignite," said Leslie Kinkead, 33. "It's sinking in how bad this is."

Staff writers D'Vera Cohn, Pierre Thomas and Douglas Stevenson contributed to this report.