One of the principal water mains supplying Fairfax County was cracked open yesterday afternoon in a construction accident in the Lorton area, reducing water pressure to the houses of an estimated 100,000 people in the eastern part of the county, a water authority official said last night.

Repairs to the 30-inch main were still under way early today, and James Warfield, deputy director of administration for the county water authority, said he hoped the pipe would be back in service before 4 a.m.

Residents of the area served by the main -- the part of the county east of Telegraph Road -- were asked in broadcast appeals to restrict their water usage after a bulldozer cracked open the main about 2:30 p.m.

Warfield said that after the accident, water was fed from other sections of the county into the affected area. Some water also came from storage tanks, he said. Before the break could be isolated, water may have been cut off to a few houses, he said.

County fire officials said they sent a pumper truck to Mount Vernon Hospital to supplement the water supply there. In addition they said they sent extra equipment on all runs in the affected area.

Warfield said the broken pipe is one of three major mains that supply the county from the Occoquan River treatment plant. Repairs were made more difficult because the break occurred at a joint, requiring replacement of sections of pipe on either side of the fracture.