Hundreds of people worked through the day yesterday to shore up a massive construction site cave-in at 14th and H streets NW, pouring in tons of dirt to keep adjacent streets from collapsing further.

A parade of dump trucks and the deft maneuvering of front-end loader operators working close to the cave-in edge attracted crowds of gawkers who were just as surprised as the experts when a 100-foot stretch of H street tumbled into the giant hole about 1:40 p.m.

The secondary cave-in also plunged the area into an electrical emergency.

By nightfall, only one 13,000-volt Potomac Electric Power Co. feeder line was supplying a six-square-block area normally served by six lines.

Metro subway service was operating normally through the nearby McPherson Square station, but above ground, several blocks were closed to traffic, snarling holiday week commuters throughout the downtown area.

Parts of 14th and H streets NW are expected to stay closed at least until Saturday. A half-dozen nearby buildings were closed and water service in the area was shut off to lessen stress on exposed water mains.

Experts said it would take two to three days to pack in enough dirt to stabilize the cave-in, which devoured an adjacent alley and parts of 14th and H streets.

The gaping hole, five stories deep, resembled an urban landfill strewn with twisted steel girders, a half-buried construction trailer, chunks of pavement, remnants of a large purple sign and exposed water mains and electrical circuits.

Arthur Schultz, a spokesman for Chicago-based Rubloff Real Estate and Capital Inc., developers of the site, said officials had no immediate estimate on the cost of the shoring-up operation.

He said dump trucks from several companies were being used to haul in dirt from excavation sites in the region. The dumping operations continued through last night.

Schultz said that Rubloff and Schal Mid-Atlantic Inc., the primary contractor, estimated the cave-in would delay completion of the planned 12-story City Center office building by about a month.

The initial cave-in, which occurred about 8:30 p.m. Monday on the northwest corner of 14th and H streets NW, is believed to be the largest in the city's history. It set off a spectacular light show as power lines snapped and building equipment plummeted into the hole.

The alley between the construction site and the United Press International building at 1400 I St. NW was ripped away, exposing a part of the building's foundation. City officials ordered the structure closed until possible damage can be determined.

D.C. firefighters had to wait until yesterday morning before they could inspect several trailers that plummeted into the 52-foot-deep excavation site.

No bodies or survivors were found in the trailers, but fire officials said they could not rule out the possibility that someone walking along the alley or sidewalk might have been buried in the collapse.

Nearly 100 D.C. firefighters rushed to the cave-in site Monday night because of initial fears that construction workers might be trapped, according to D.C. Fire Department spokesman Ted Holmes.

Rescue workers used a thermal imager, a sensor that locates sources of heat, to try to determine if there were people in the huge hole.

Two sources of heat were located even though construction officials said all their personnel were accounted for.

Just before dawn, two firefighters were lowered into the area to search where the heat sources had been detected. They found nothing.

Phil Buffa, one of the two firefighters lowered into the area, inspected the construction trailer.

"I did a quick search and turned over a few tables, but I didn't see anyone," Buffa said.

Dozens of D.C. and federal safety officials were at the cave-in site yesterday, but they said it would be weeks or months before they could determine what triggered the collapse.

"We would be very reluctant to speculate what caused the cave-in," said Alan McMillan, deputy assistant labor secretary for occupational health and safety.

A geotechnical engineer who helps test soil and design foundations for major buildings in the Washington region said the earth in the downtown area is generally fairly strong, but some areas contain layers of highly plastic clays that "can exert some swelling pressures if exposed to water."

The engineer, who asked not to be identified, said water pressure behind a normally stable wall sometimes can trigger events such as Monday night's collapse.

James Moran, director of occupational safety and health for the Laborers Health and Safety Fund, a joint union-management group, also suggested that water might have contributed to the collapse.

"It makes the soil a lot heavier and it moves much more easily and places a great additional load on a protective barrier system," Moran said.

Charles Culver, director of construction and engineering for the Occupational Safety and Health Administration, said yesterday that the collapse apparently started on the 14th Street, or eastern, end of the excavation.

H-shaped steel pilings that hold back railroad-tie-size shorings were ripped from their moorings and propelled about 30 feet inward.

He said "something moved and those supports buckled," and all of the dirt they were holding back poured into the open pit.

The collapse of the 14th Street side of the shoring then pulled away the shoring holding up the alley and shoring near the corner of H Street NW.

Three construction trailers that had been hoisted above the sidewalk on the 14th Street side crashed into the open hole, along with a small truck, two portable toilets, parts of the wooden sidewalk protection, a mobile crane, three city light posts, a traffic signal, fire hydrants, several trees, two manholes and a Pepco transformer.

Construction equipment on the floor of the excavation was engulfed.

Another large construction trailer was left hanging above the H Street side of the site.

When the alley next to the UPI building collapsed, telephone cables carrying 2,700 lines were cut, according to C&P Telephone Co. spokesman Michel Daley.

Pepco spokeswoman Nancy Moses said one of six 13,000-volt feeder lines that serve a six-block area around the cave-in site was destroyed during the Monday night incident and another was damaged.

The second collapse yesterday afternoon took two more of the cables out of services. Later another line was downed and Pepco was operating primarily on one feeder line.

"We are holding onto things by a thread," Moses said last night. "Our lines are practically holding together the street," she added.

She said that when two lines were working, both of them were "literally steaming" because of the large demand.

"We have been calling our major customers in the area and asking them to cut back dramatically, if not entirely, on their usage," Moses said.

Pepco officials said last night they hoped to have three of the feeder lines in service by this morning.

C&P officials said they did not know when they would be able to repair the phone lines, but that they might have to string temporary lines through windows to get phone service restored.

Staff writer Carlos Sanchez contributed to this report.