Fortified by more than 1,000 truckloads of borrowed dirt, the area around the massive construction site cave-in at 14th and H streets NW was stabilized yesterday, and work began immediately to try to keep today's expected showers from causing new damage.

Giant sheets of plastic were spread over the fresh dirt used to shore up 14th and H streets along the excavation site, and workers started building an asphalt berm to prevent street runoff from flowing into the pit. Workers also sealed the crack along H street to keep rain from flooding into the pit under the street.

Power was restored to the area around the cave-in site, but utility company officials said service was only one-third the normal level because of damage to 13,000-volt feeder lines.

Authorities said the area around the cave-in, at the site of the new 12-story City Center office building on the northwest corner of 14th and H streets, will be closed through the weekend, but may reopen for Monday's morning rush hour.

"The emergency is over," said Arthur J. Schultz, spokesman for building developer Rubloff Real Estate and Capital Inc. "Friday the reconstruction begins . . . . We think that in six weeks the site will be back to the same status as it was before the cave-in."

The first cave-in occurred about 8:30 p.m. Monday when the shoring along the eastern edge of the 52-foot deep mammoth hole pulled loose, sending a wall of dirt crashing into the construction pit and eventually triggering the collapse of portions of the sidewalk along 14th and H streets and an alley that had separated the site from the adjacent United Press International Building.

A secondary collapse about 1:40 p.m. Tuesday swallowed up the sidewalk and part of the curb lane along a 100-foot portion of H street. No injuries or deaths were reported in either collapse.

The building's contractors and developers and city and federal investigators still do not know what caused the collapse, and officials said the exact reason for the cave-in might never be determined.

Bonnie Rampersaud, a spokeswoman for the D.C. Department of Consumer and Regulatory Affairs, said yesterday that Schal Mid-Atlantic Inc., the prime contractor, had obtained all the required city permits, a total of 11, and regular and routine site inspections before the cave-in had not detected any problems.

Before the cave-in, the excavation site had been secured by three shoring methods in different areas of the pit, according to Schultz. Along 14th Street, where the collapse began, the walls of earth were held back by H-shaped vertical beams that were secured to the walls by long reinforcing rods that passed under the street. Between the H-beams, which extend five to 10 feet beneath the floor of the pit, the earth was held back by three-inch-thick horizontal beams and horizontal steel cables.

Other methods of reinforcing the H-beams were used in the middle and western sections of the excavation pit, Schultz said, because the long rods would have interfered with other structures, including the nearby Metro subway tunnel. Those areas were reinforced with steel bracing girders placed on the floor of the pit.

Throughout yesterday, consumer and regulatory affairs inspectors and officials of the federal Occupational Safety and Health Administration remained at the site overseeing the shoring-up. Rampersaud said city inspectors are monitoring the site every hour and have not detected any shifts since the second collapse on Tuesday.

Around the cave-in site, the return to normalcy began early yesterday when city building inspectors decided that the UPI Building was stable, and allowed tenants to return to the structure. The southeast corner of the UPI Building's foundation had been exposed by the cave-in.

UPI began operating its worldwide news service Monday night from such far-flung bureaus as Los Angeles and its business office in Vienna, Va. Last night, UPI said virtually all of its services had been restored, including telephone lines.

At midday yesterday, a hand-lettered sign at the entrance on a UPI Building restaurant proclaimed, "Mingles Is Open," and Tina's Coffee Shop was doing a brisk business serving construction workers and curiosity seekers.

All the buildings in the area also had water and electrical service, though the Potomac Electric Power Co. was still operating on what spokeswoman Nancy Moses said was on the order of a "biblical miracle."

"We got by through the night on one dangling, overloaded feeder," in an area usually served by six 13,000-volt feeders, Moses explained.

A second feeder line was put in service early yesterday, and Pepco workers toiled around the clock to repair a third feeder line. "This is not the fast-lane kind of work. This is more like delicate surgery," Moses said.

For the second day, Pepco asked customers in the area to curtail electricity usage as much as possible. "We are thankful for Thanksgiving in more ways than one," Moses said. She said the power company hoped to have all cables repaired by the weekend.

The parade of dump trucks that began at 7 a.m. Tuesday continued unabated until about 2 p.m. yesterday. The dirt was poured into the pit to create a 45 degree-angle fortification where the shoring collapsed.

The dirt, most of which was borrowed from the farm of one of the dump truck company's owners and two nearby farms, will be returned after new shoring is installed in the damaged areas, Schultz said.

Work will begin tomorrow to remove girders and debris not buried by the fill dirt so the materials can be examined by OSHA and independent engineers.

OSHA and consumer affairs officials said that all work at the site will be closely monitored. "This will probably be the safest building in the District by the time it's finished," quipped Schultz.