Two lanes of M Street SW, between Delaware Avenue and First Street, lay cracked, sunken or broken open yesterday because of a water line rupture that has been attributed to tunneling for Metro's Green Line.

Repair work closed the two lanes yesterday and today, limiting traffic to one lane in each direction along that section of M Street.

Metro officials expected to reopen the two lanes by tomorrow's morning rush hour.

The four-inch water pipe, running about four feet beneath M Street just east of Delaware Avenue, apparently ruptured Sunday night as Metro's tunneling machine was boring through the earth about 50 feet below M Street, according to authorities and witnesses.

Metro construction officials theorized that the tunneling machine, moving west along the route between the future Navy Yard and Waterfront stations, hit a pocket of sand or soft earth, causing the street above to crack and sink about two to three feet, said transit agency spokeswoman Beverly Silverberg.

The earth movement probably broke the water pipe, causing water to spew out of the cracks and onto the street, where it was noticed about 8 p.m. Sunday, Silverberg said.

The pipe was one of two lines providing water to Anthony Bowen Elementary School, the only building affected by the break, said Tara Hamilton, a spokeswoman for the District's Department of Public Works.

The leak caused no injuries or noticeable water damage to Metro's underground construction area, Silverberg said.

Metro's director of construction, John F. McElhenny, characterized the break yesterday as "a minor problem."

The contractor on the project broke open part of M Street to find the water leak and begin repairs.

The cost of the repairs will be covered by Metro's self-insurance program, Silverberg said.