Hey — want to dig a hole? How about a tunnel 3,900 feet long? That's the job facing a public-private consortium that's working to improve traffic flow in the Port of Miami, an economic powerhouse that helps drive the city's economy, but stresses the quality of life near the heavily used docks. At the core of the ambitious project is a huge, self-propelled Tunnel Boring Machine (TBM) nicknamed Harriet.

Coastal Connection reported on the giant rig's arrival in Miami last fall ("Port of Miami Tunnel Project Gears Up to Go," October 10, 2011). Now, the earth-chewing monster has completed half its task, tunneling one of the two tubes designed to route two-way truck traffic away from downtown streets that now connect the highway to the docks. The Miami Herald had that story at the end of August (" Halfway done: Port tunnel drilling reaches a milestone," by Arianna Prothero). Now, technicians are tearing the machine apart and reassembling it to face back the way it came, primed to dig the second shaft. Excavation starts again in six weeks, reports the Herald (" PortMiami tunnel's next leg to begin in October," by Alfonso Chardy). "We're on time, we're on schedule and we're on budget," Chris Hodgkins, vice president of Miami Access Tunnel, the multinational consortium building the tunnel, told the Herald.

Here's a YouTube amateur video of the boring machine's rotating disk breaking through the concrete shoring at the end of its first dig path ("Port of Miami Tunnel TBM Break-Through," by rtoddy55).

Members of the crowd on hand to watch the giant digging machine emerge from its hole took snapshots and hand-held video as operators inside the huge rig narrated a play-by-play account.

And here's a long and detailed animation (taken from a subway project in Europe) that explains how the tunnel boring machine technology works ("TBM/Tunneling Video," by MuniCentralSubwaySF). As the grinding face removes rocks, hydraulic pistons drive the grinder into the earth, while a screw conveyor and belts remove the spoil. Section by section, concrete and steel tunnel reinforcing walls are added to the growing tunnel — each new section of wall serving as a base for the pistons to push against as the machine bores further along its path.

The new tunnel will have obvious benefits: preventing congestion and noise in a downtown area that the city wants to revitalize, and improving the port's economic efficiency. But there's also a cost — and along with the tunnel, Miami is digging itself a financial hole. To fund the project, the city floated a big, short-term loan — and it has a huge balloon payment coming due. The Herald has that story (" $45 million PortMiami tunnel dig payment threatens Miami's finances," by Kathleen McGrory).

"The tunnel loan payment is but the latest crisis-du-jour for Miami, which is already trying to plug a budget hole, replace key finance managers, negotiate with restive federal securities regulators and placate nervous bond raters," the Herald reports. "Some City Hall observers and former employees say it's another sign Miami is hurtling toward a financial meltdown."

But Miami Commissioner Marc Sarnoff, who is chairman of the special district board that is managing redevelopment in the blighted port area, says there is still ample time to refinance the $45 million loan, complete a city bond issue, or take the debt to the private equity market. He told the Herald, "We can get this done in time."

Meanwhile, the tunnel itself should be all dug by springtime — and project managers expect traffic to flow by mid-2014.