BENEATH THE SEPTIMA P. CLARK PARKWAY — Friday afternoon approached an unseasonably warm 80 degrees in Charleston, but 140 feet underground, it was cool and still.

Inside the city's massive drainage tunnels, a boring machine was just 12 feet from completing the final shaft, a giant step in a nearly $200 million project that's so far stretched for a decade.

The tunnel walls still needed to be sealed with concrete, but for now, clad in wood panels and steel ribs, they proved an impressive site for a visit from media, Gov. Henry McMaster and members of the S.C. Floodwater Commission. They all traveled to the work site across from Burke High School for an up-close look.

"This is something that has to be done," McMaster told the gathered media after he emerged from underground in a cage linked to a crane. "The cost of not doing it is astronomical."

When completed, the tunnels, outfall and pump into the Ashley River will be able to move 360,000 gallons of water a minute off a sizable portion of the Charleston peninsula, including the Septima P. Clark Parkway, a key traffic artery known more commonly as the Crosstown among locals.

The underground tunnel, which will be 12 feet tall when finished, followed the same curve as that road above.

Construction workers spent two weeks hand mining at its subterranean intersection with the slightly smaller channel drilled under President Street to fit the final structure that will support the crossing.

John DiPonio, vice president of mining company Jay Dee Contractors, handed around a small clump of gray sediment to a group of reporters and flood commissioners who descended into the network after McMaster and Charleston Mayor John Tecklenburg had exited. It was an example of Cooper marl, the clay-like material that a cylindrical tunneling machine tore through at a rate of more than 50 feet a day. 

"It's excellent mining ground," DiPonio said. "The more challenging the ground conditions, the more expensive the tunnel potentially becomes." 

Marl dates back millions of years and was once the bed of a shallow estuary. It's such an ideal medium for the work because marl is both strong like sand and watertight like clay, said Michael Horton, the chief engineer with project administrator David & Floyd. 

Actually, it was not unlike a block of firm cheese, said Stephen O'Connell, a geologist for contractor Black and Veatch — relatively easy to dig a whole through but stiff enough to keep its shape. 

It's also deep enough underground that the tunnels likely wouldn't be disturbed during an earthquake from Charleston's nearby fault line, O'Connell said. 

The system must be durable, as it serves as a crucial piece of the city of Charleston's flood management in the coming years. Similar drainage projects are planned for other sections of the peninsula, and as city officials learned recently, they can easily get costly. 

The Crosstown project alone was recently revealed to be on track to overrun its original budget by $43 million and its timeline by four years. 

Much of that overrun will come due during the next phase of work, which will create the outfall that will actually release the water in the tunnels into the Ashley River. At the end of that phase, before the pumps are installed, gravity alone will be able to convey 200,000 gallons a minute into the Ashley at low tide. 

City Council has not awarded the contract to a construction firm for that portion of the project, and it's expected to address the matter Tuesday.

But without it, all of the work so far — more than 8,000 feet of new tunnels deep underground — will be for naught. 

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Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.

Chloe Johnson covers the coastal environment and climate change for the Post and Courier. She's always looking for a good excuse to hop on a boat.