Metro will restore full subway service to its once-flooded Blue Line today at 2 p.m., in time for the evening rush hour.

By that time, Metro general manager Theodore Lutz said yesterday, there will be an "extra margin" of protection for the subway tunnels from the waters of the Washington Channel, which rolled through three barriers last Thursday and flooded two stations.

Since that time, seven stations on the Blue Line have been closed and subway service on the Blue Line has been limited to 11 stations - from National Airport to Federal Triangle. Stations serving the Southwest government office buildings, the Capitol and Capitol Hill have been closed since Thursday.

During that time, special free shuttle buses have run between the closed subway stations. That service will continue today until full subway operations are resumed at 2 'oclock.

Other special bus service that has been added during the subway problem also will be discontinued, Metro officials said. Extra buses will be held in reserve to augment paid service on the lines serving Pennsylvania Avenue SE, the main transportation corridor that lost subway service.

Two things have happened. Lutz said after a briefing from key officials, that permit the restoration of service:

1. A bulkhead, a gigantic 40,000-pound steel plate, has been successfully placed over the underwater entrance to the flooded tunnels. That has permitted contractors to pump water out of the tunnels.

2. With the water gone, contractors are building two 42-inch thick reinforced concrete plugs in the two tunnels that were flooded. Forms and reinforcing steel were being placed yesterday and pouring of the concrete was scheduled for last night.

The concrete is expected to set in about 12 hours, or sometime in the late morning or early afternoon today.

Even without the plugs, Lutz said, he and his consultants are convinced "we have a safe situation." The hardened concrete plugs provide the "extra margin" he talked about.

"Our shuttle bus service has been fairly effective serving the morning fush hour," Lutz said, "but less effective in the evening. That's another reason we decided to wait until midday" to restore full subway service.

Loss of the seven subway stations has cost Metro in excess of 40,000 riders a day, according to early estimates. Public transit ridership - in fact traffic in general - is normally very low in the last week before Labor Day because of vacations.

The two stations that received severe flooding - L'Enfant Plaza and Federal Center SW - have been cleaned. Tracks have been checked and electronic and electrical circuits restored. Metro has run several trains through these sections without passengers to keep rust from forming on the rails and interfering with good electrical circuitry.

With the new bulkhead and plugs in place, contractors and Metro can then safely begin cleaning up the mess in the Washington Channel and, at the same time, determine exactly what happened.

What is known is that channel water breached two barriers - called cofferdams - that were built into the channel to keep water out of construction work on a new Metro tunnel.

Then, water also got into the tunnels by breaching one of the two existing bulkheads that were designed to hold out the water by themselves. With one bulkhead gone, both tunnels filled becuase of connections between the two.

Water rolled to the L'Enfant Plaza station and overflowed onto the Blue Line tracks there.

What is not known is why the cofferdams and the bulkhead failed. The bulkheads were placed by Traylor Brothers, the contractor for the completed tunnels.

The cofferdams, both of interlocking pile-driven steel, were placed by a joint venture of Perini, Horn, Morrison Knudsen.

All three barriers are under water, making inspection and determination of cause difficult.

Thousands of tons of fill were poured into the cofferdam enclosure on both sides of the second barrier after the breach Thursday night. That will all have to be removed and the water pumped out before inspection of failed barries and continuing of construction are possible.

The process will take months, said Roy T. Dodge, Metro's assistant general manager for construction and design. Water must be drawn down carefully to make sure the Washington Channel water does not collapse the barriers. With the bulkheads and two plugs in place, that work can proceed safely, Dodge said.