Workers at PortMiami have disassembled parts of the giant tunnel boring machine and are slowly reassembling them to begin digging the second leg of the under-the-bay project next month.

The tunnel consists of twin tubes that will carry a two-lane highway each way to convey vehicles between the port and the MacArthur Causeway. The eastbound tube from the MacArthur Causeway median at Watson Island to the port was completed July 31. Boring of the westbound tunnel is scheduled to begin Oct. 19.

The $1 billion tunnel is one of three major transportation projects now being built in South Florida. The other two are the $1.7 billion transit hub known as the Miami Intermodal Center just east of Miami International Airport and the $1.8 billion reconstruction of Interstate 595 in Broward.

As workers prepare to bore the tunnel’s second leg, the Florida Department of Environmental Protection has approved a permit for the builder to transport more rock and dirt to Virginia Key.

According to the permit, an additional 311,000 cubic yards of material to be extracted from the westbound excavation can be deposited at a site in Virginia Key where the firm building the tunnel was previously authorized to dump 287,000 cubic yards of spoil.

Using a combination of large truck trailer beds and rail tracks, workers are gradually pulling disassembled parts out of the completed eastbound tunnel segment, and are positioning them at the site where the machine will start digging the westbound tube. It will take the mammoth machine, named Harriet, about six months to complete the job.

“We are on time, we’re on schedule and we’re on budget”, said Chris Hodgkins, vice president of Miami Access Tunnel, the multinational consortium building the tunnel.

The tunnel is intended to divert cargo trucks from congested downtown Miami streets, giving them a direct connection to the port from Interstate 95 and State Road 836. The tunnel will link the port to the MacArthur Causeway, which is a continuation of the I-395 expressway that links up with 836 and I-95. Currently, tractor-trailers headed to the port clog downtown streets because that’s their only route to the port.

The eastbound tube, begun Nov. 11, will contain a two-lane road linking the MacArthur Causeway from Watson Island to the port under the cruise ship terminal. The westbound tube will have another two-lane road from the port to Watson Island, where the entrance portal will be located across from Jungle Island.

Once completed in 2014, the project will include not only the tunnel but also a widened MacArthur Causeway bridge and improvements to the port’s road network. The tunnel is part of a major port overhaul that also includes dredging of a deeper cargo harbor and an upgraded rail line to move container cargo north to other U.S. cities.

In the operation to turn around the tunnel boring machine, workers have already positioned the heavy circular cutter head at the precise spot where it will begin slicing rock and dirt. Now, they attaching to the cutter head five gantries that make up the rest of the digging machine.

Each gantry contains parts of the system including a command booth, a small rest lounge and eating area for personnel and a room where workers can take refuge in case of emergency. Two of the gantries were still inside the completed eastbound tube last Wednesday, but three others were outside.

The process is slow because the parts are massive. The $45 million, German-built tunnel-boring machine is 43 feet wide and 428.5 feet long. The cutter head, the heaviest piece, weighs 1,500 tons, Hodgkins said.

A photographer and a reporter from El Nuevo Herald toured the turn-around site and the newly finished eastbound tube Wednesday.

The completed tube, just under a mile long, looks like it is almost ready to use.

Oval concrete panels line the walls after being installed by the tunnel-boring machine as it pushed its way to the port.

The tunnel gently slopes downward at a 5-degree incline from the MacArthur Causeway — to a depth of 120 feet below the surface. Then it curves upward to the port.

“The eastbound tunnel has been completely bored,” said Hodgkins as he led the reporter and photographer on a walk through the tunnel.

As the tunnel dips downward, it goes under the MacArthur Causeway and then under Government Cut, the channel to the cruise ship terminal.

“The curve takes us underneath the MacArthur Causeway and then we go about another 100 feet and we’re underneath Government Cut,” said Hodgkins.

What’s still missing is the two-lane road deck, which will be built later. Right now, workers drive inside the tunnel in vehicles that resemble large golf carts.