Workers yesterday began installing a new retaining system at the construction site of the $100 million City Center office building at 14th and H streets NW, eight weeks after a cave-in swallowed parts of nearby streets, buried equipment and crippled several downtown blocks for days.

Previously, the only work at the site had been the removal of debris in the hole, replacement of parts of the retaining system that were not buried by the collapse and the placement of new steel pilings to prevent further damage to adjacent streets.

Completion of the new retaining system is the first major step toward removing tons of dirt used to stabilize the site and resuming construction of the building.

Despite the progress at the site, officials of the federal Occupational Safety and Health Administration, which is investigating the Nov. 19 collapse, say the effort to determine the cause could take months.

"This is a pretty complicated matter," said Gary R. Griess, regional director for OSHA's Baltimore office. "We are still gathering information."

"Right now, I think it's wide open as to what could have caused the failure," said Vincent Ford, chief of D.C. building inspectors.

"There is some talk that it may have been water seeping in in the upper portions of the excavations. Or that the shoring kicked out at the bottom because of water problems there," Ford said. "The thing is that while we may be able to speculate . . . we may never come up with the truth."

Work resumed after the D.C. Department of Consumer and Regulatory Affairs approved modifications late Thursday to the original construction plans for the five underground levels of the structure.

Arthur J. Schultz, a spokesman for the developer, Rubloff Real Estate and Capital Inc., said workers began yesterday to drill for new tiebacks, long reinforced steel cables that extend deep into the earth and help secure the pilings. Besides the tiebacks, the retaining system consists of extensive pilings and steel girders.

Once the new retaining system is in place, workers will begin removing the 1,000 truckloads of dirt dumped into the hole immediately after the collapse to shore up the streets. "As the fill dirt is removed, they will also be taking out the debris, which will be handed over to OSHA for inspection and removal to their site for further tests," Schultz said.

Schultz said the prime contractor, Schal Mid-Atlantic Inc., predicts that most of the construction equipment that toppled into the hole will be salvageable, including a 38-ton mobile crane, a backhoe, a track excavator (similar to a front-end loader) and a grade-all machine, used for exacting excavation.

As the dirt and debris are removed, workers will resume pouring concrete for the lower levels. After those levels are completed, the area around the building will be filled in and damage to streets, waterlines, electrical conduits and other utilities will be repaired.

No estimates of the final cost of the collapse are available, Schultz said, because the total will depend on how long it takes to restore the construction site and surrounding area. "We believe it will be a few million dollars," Schultz said.

Who pays for the damage apparently will depend on what caused the cave-in. Schultz said that both Rubloff, the developer, and Schal, the contractor, have insurance that would cover the costs.

Most of the permanent damage was to streets and utilities. Steve Arabia, a spokesman for Potomac Electric Power Co., said it cost "in six figures" to restore electrical service cut by the cave-in. Still to be replaced are large underground electrical conduits along the north side of H Street NW, he said.

Tara Hamilton, spokeswoman for the District's Department of Public Works, said the developer has paid $93,000 to replace two traffic signals, a traffic control box and four street lights. Still to be repaired, she said, are the streets, curbs, gutters and a water main. Another water main will have to be replaced, and two sewer lines will have to be rebuilt, Hamilton said.