On the low-pitched banks of the Anacostia River, workers have surveyed and cleared a flat stretch of ground to start digging subway tunnels for Metro's most severely delayed route: the Green Line.

The construction -- under way on the east side of the river, midway between the South Capitol (Douglass Memorial) and 11th Street bridges -- marks the first work on the Green Line's southeastern branch since a federal judge lifted a court order that stalled the project for three years.

"The Green Line is moving," said Metro General Manager Carmen E. Turner, "and we are as anxious as anyone to see that line built."

Metro's first job will be to excavate tunnels beneath the river. Eventually these concrete-lined tubes will connect a proposed Anacostia station, to be built near I-295 and Howard Road SE, with a planned stop at the Washington Navy Yard on the west side of the river. The stations are expected to open in 1990.

Behind a green wooden fence about 100 yards from the east bank, workers have posted stakes to mark the starting point. Soon they will begin excavating a 62-foot shaft. Then they will lower a huge tunneling machine into the shaft to bore the holes under the river.

The twin 2,500-foot tunnels are to end at another shaft near the west bank. There workers already are clearing a site by tearing down a large brick warehouse, once used as a gun foundry, at the Navy Yard Annex. To demolish the building, Metro had to get approval from historic preservation agencies.

The protracted delays in constructing the Green Line, the only unopened line in the Metro system, have long caused concern among local and federal officials because the route is designed to serve low-income neigbhorhoods where many residents depend heavily on public transportation.

Construction on the Green Line's southeastern branch was held up largely because of a court battle among Prince George's County residents and businesses over the route's proposed terminus. Some groups advocated a terminus near Branch Avenue, while others pressed for a site near Rosecroft Raceway.

The fight ended last year when Metro's board of directors reversed an earlier vote and endorsed the Branch Avenue plan. U.S. District Court Judge Norman P. Ramsey cleared the way for construction to start by rescinding a March 1982 order that had blocked the project.

Plans for other parts of the Green Line have been delayed because of neighborhood opposition and shortages of funds. Last week, Metro officials revised their proposals for a section of the route through Northwest and Northeast Washington, reviving a conflict that has dragged on since the 1970s.

The work on the Anacostia River tunnels began in recent weeks without fanfare, announcement or ground-breaking ceremonies. The omission was attributed partly to a desire by Metro and other local officials to avoid reviving another controversy that had enveloped the project.

This dispute centered on allegations last year that a partner in the construction group submitting the lowest bid for the tunneling job had ties to firms doing business in South Africa, a volatile issue at a time of widespread protests over South Africa's policies of racial discrimination.

The Metro board eventually awarded the $25.6 million contract after concluding that Franki-Denys Inc., the minority partner, had "very remote" ties with South African businesses. The firm is partly controlled by a Belgian company with subsidiaries in South Africa. Harrison Western Corp., the major partner, was not the focus of any allegation.

Metro officials say they will belatedly hold ground-breaking ceremonies when work begins later this year on the Anacostia station. This will be a larger project, officials said, and the station will be closer to Anacostia neighborhoods.

The tunneling job that now is getting started will rely on some highly sophisticated machinery and a bit of tricky engineering.

"We're worried about a material called T5, which runs easily -- sort of a sandy material," said Walter A. Mergelsberg, a veteran Metro construction manager who is overseeing work on the Green Line's southeastern branch.

T5 is an engineers' designation for a layer of earth composed of relatively fine sand mixed with gravel and boulders. The boulders may pose a key problem, officials said. It is possible that some may be too large to be swallowed by the tunneling machine. Workers would have to smash the boulders apart before pushing ahead.

Aside from this troublesome rock layer, the tunnels will mainly be carved from clay formations, about 50 feet beneath the river bottom. A laser-guided tunneling machine, built by the Japan-based Hitachi Zosen Corp., a shipbuilding company unrelated to the giant Hitachi Ltd., is to arrive here in the fall to start excavation.

The nearly 24-foot-long machine will employ techniques so novel that they have never before been used in the eastern United States. Nevertheless, some officials have found a degree of simplicity in its complex design.

"Its simplicity is amazing. All it is is a damn auger affixed to a bulkhead," said Phil Bell, a seasoned tunneler who is general superintendent of the construction outfit.

The cylindrical machine, nearly 19 feet in diameter, will gnaw through the earth at a pace of four inches a minute. Its rotating face will be dotted with 129 steel bits to cut through rock and clay. The loosened earth will be gobbled up through slits in its face and gulped down by a giant screw.

The muck, as excavated rock is known, will be hauled away through the rear of the machine and out of the tunnel on conveyors or small railroad cars. As the machine presses ahead, the walls of the tunnel will be lined with concrete. The two parallel tunnels are expected to be completed by late 1987.

Another mammoth machine already is at work at the Navy Yard Annex. Outfitted with massive jaw-like shears, the 13-ton contraption is ripping down the vacant 602-foot warehouse.

"They call it 'Pac-Man,' " said Bell. "He has a snack and 10 minutes later the building is gone. He could eat your house in five minutes."

Officials said it would take three months to raze the warehouse, built in 1914 and described as a typical example of early 20th century foundry architecture. Once used to cast steel parts and forgings for guns, the building recently was the storage site for presidential inaugural parade floats.

Under a 1981 agreement approved by the federal Advisory Council on Historic Preservation and the D.C. historic preservation office, Metro was given permission to demolish the building in exchange for ensuring that a "permanent record," including photographs and architectural drawings, would be preserved.

Work also is expected to begin soon on other parts of the Green Line's southeastern branch. In May, construction is to start on a three-level parking garage at the Anacostia station. Work on the station is scheduled to follow about a month later.

The Waterfront station at Fourth and M streets SW was largely completed when work was halted by the court order. Recent inspections have found the structure sound. Its concrete walls need to be sandblasted to remove dirt, and water must be pumped out of electrical conduits, said Daniel J. Wallace, a consultant.

In September, officials plan to start digging tunnels from Waterfront to the Navy Yard. Construction of the Navy Yard station is to begin early next year, along with excavation of tunnels to link the station with the tubes beneath the Anacostia River.