This involves building dozens of rectangular-shaped concrete forms, 40 feet long and driving them through the ground with powerful hydraulic rams, 18 inches per thrust. The dirt pushed into the advancing tunnel box is excavated as the forms push forward.

Soil mixing and building tunnels from prefabricated tubes are fairly well established technologies in the United States, far more so than highway-sized tunnel jacking.

To combine all three in one place and to such an enormous extent, however, is virtually unprecedented. "We're not breaking new ground, but we're taking technology that is proven and doubling it," said Michael T. Bertoulin, a senior project consultant.

Although the new design largely resolves the crises the project faced in mid-1994, it has not quieted Gillette's concerns about work outside its front door. Anthony M. Termine, a senior Gillette official monitoring the project, said the redesign would still have "severe negative impacts."

The Gillette factory depends on channel water for its massive cooling system. Earlier this year, with the project footing the bill, it completed a 70-million-gallon-a-day water intake just east of the tunnel route. But Gillette is concerned that the cross-channel tunnel, which would be just 12 feet below mean sea level, could dam up water flow in the channel and prevent the tides from flushing out sewage-tainted runoff that spews from the Roxbury sewer outfall near the Broadway Bridge.

Gillette also opposes a plan to seize some of its land as the for a new Dorchester Avenue bridge.

Officials are confident the tunnel will not hinder Gillette's cooling system, Bertoulin said, but "we're still working with them" to negotiate design and land-taking changes.

Postal Service spokesman Robert Cannon said the agency is "comfortable but cautious" about the artery's ability to ensure full access for the 3,100 vehicles that enter the postal site daily. "It's critical for us that no matter what the project does, no matter how they work in, through or around us, that both sides of the building stay open 24 hours a day, seven days a week," he said. "If we're closed for one day, you'd shut down 9 million pieces of mail."

Here is a closer look at some of the key technologies the artery-tunnel plans to use:

Tunnel jacking is most commonly used to install sewer and utility pipes, typically 30 or 48 inches in diameter. The largest sections of highway tunnel to be jacked outside South Station are 28 feet high and 65 feet wide -- the girth of a comfortable two-story house.

Most if not all of the tunnel jacking done in this country under highways or railroad tracks has involved culverts or utility lines.; project officials said they did not know of any similar highway construction by tunnel jacking. But at least 30 such projects have been done in rail-dependent England and Western Europe, where closing down busy train lines for road work is considered unthinkable. The artery-tunnel project is in fact turning to English engineers for advice.

The process begins with excavation of a pit adjacent to the tracks. The first concrete tunnel section is lowered by crane into the pit and outfitted with a cowl-shaped steel shield that acts as a leading, cutting edge.

In the pit, a hydraulic machine rams the section forward, about 18 inches at a pop, while the dirt and muck forced into the frame are dug out or hauled off by conveyor belt. A water-clay mixture called bentonite slurry is sprayed around the edges of the concrete sections to act as a crude lubricant.

Once the first 40-foot section has been forced ahead, the jacking machine is retracted, and the second section dropped into place. Then both sections are pushed forward. The process is repeated for the length of the tunnel.

Engineers can curve the tunnel horizontally or vertically as they go, by increasing pressure on one side or the other of the jacked sections.

Prefabricated tunnel tubes would seem like a tough technique to use in the channel. Usually tubes are built elsewhere and floated in, as were the 12 steel tubes of the Williams Tunnel. Those were built in Maryland and hauled by tugboat to Boston Harbor.

But Fort Point Channel is blocked by four low bridges of old and new Northern avenues, Congress Street and Summer Street. Removing them to let tubes in could cause traffic chaos.

To get around that problem, project officials are building next to Gillette a "casting basin," essentially a gigantic dry dock where the tubes will be made. The tunnel sections will look like huge, open-ended concrete warehouses.

As they are completed, two at a time, the floodgates will be opened to the casting basin. Huge winches will tow the boxes out, and concrete will be pumped into the bottom of the box both to sink it and to create the vehicle driving surface.

Where the highway passes over the Red Line, each tube will be supported by up to 48 caissons, huge watertight sleeves rammed down to bedrock and filled with concrete to form a pillar.

Structurally, Bertoulin said, "the tunnel is really like a covered bridge under water." Because any damage to the aging subway tunnels could send catastrophic floods into the Red Line, the tunnel tubes will straddle it, with their weight -- as much as 180 million pounds per tube -- fully borne by the caissons.

Once the six tubes are done, the casting basin will be filled in and walled to form the next stretch of I-90 into South Boston.

Soil mixing, or adding cement to soil, will be used where the tunnel is not tubed or jacked. The soil is so weak that its compressive strength -- the amount of weight it can bear -- ranges from just 50 pounds per square inch to effectively zero.

A good concrete sidewalk, by comparison, has a compressive strength of 3,000 to 4,000 p.s.i. In the channel area, Bertoulin said, the soil mixing will boost the soil's strength to 400 to 1,000 p.s.i. -- strong enough to support highways and retaining walls, but not so strong that excavation equipment cannot dig through it.

However, the process is arduously slow. Soil mixing is typically done by truck-mounted rigs that look like three long egg beaters. To drill and mix three 30-foot-wide shafts down 100 feet can easily take an hour, and in the channel area 700,000 cubic yards of soil -- enough to bury all of Boston Common six feet under -- must be bolstered.

Even working around the clock, the job will take three years. It will cost $195 million and require 500,000 tons of cement.