Dig it! Exploring Charleston’s new drainage tunnel
It took only a minute for a crane to lower the steel cage down through the concrete-lined caisson shaft some 140 feet below Market and Concord streets.
Market Street drainage tunnel
Total length: 3,955 feet
Current diameter: 11 feet, 3 inches
Finished diameter (when lined with concrete): 9 feet
Access shaft diameter: 20 feet, 3 inches
Market Street drainage basin size: 55.5 acres.
Drop shafts: Three (State, Anson and Church streets)
Total cost to date: $21.5 million
Total estimated cost: $26.5 million
Once it reached bottom, those who made the trip could see the first permanent solution to a Charleston drainage problem centuries in the making.
To the west and the north stretch more than 1,000 feet of freshly dug tunnels, which were dug through marl, a grayish clay.
Soon, these tunnels will be lined with concrete to handle the Market Area floodwaters that currently splash up against buildings and soak tourists’ feet.
City Engineer Laura Cabiness and Cliff Kassouf with Triad-Midwest Mole, the main contractor, provided the tour Wednesday to show off progress.
“The drainage work we’re doing in the city is unprecedented,” Cabiness said. “It’s also expensive.”
And it’s also ahead of schedule. The tunnels are completed, and they should be lined with concrete and tied to new drop shafts at State, Anson and Church streets by August 2014.
About two years after that, the rest of the Market Area’s drainage lines should tie into these new tunnels and shafts, work that also will involve rebuilding sidewalks and placing power lines underground.
Today, the tunnel is still a work in progress. Its bottom is lined with a narrow set of rail lines used to transport the tunnel-boring machine, the rail cars used to remove the marl and shuttle the workers.
A fabric tube runs along one side, pushing fresh, air-conditioned air into the space. A power line runs nearby. Across from that runs a red laser beam, which crews use to stay on track.
Kassouf emphasized the safety of the current work. Construction manager Stephen O’Connell with Black & Veatch said the biggest risk is no different than most other workplaces: “Slips, trips and falls.”
However, the project did dodge a bullet.
In February, construction crews discovered an abandoned water tunnel nearby, a tunnel possibly filled with dangerous amounts of water. If breached it would have put crews at risk of drowning.
To avoid that problem, the city redesigned the project to go down 140 feet instead of 80 feet.
The additional depth added about $2.7 million and six months to the $26.5 million project, but Kassouf said work is now about two months ahead of a revised schedule.
Kassouf said Charleston is the only Southern city he knows of that is building a new drainage network consisting of tunnels this deep underground. The city turned to this design because digging big trenches near the surface would have posed a huge disruption to traffic and neighboring property owners.
“It was really (Cabiness’) push to change everything into a deep-tunnel system,” Kassouf said.
The city drilled its first such tunnel 18 years ago, to build its first deep drainage line from Mary Street down to Marion Square and under Calhoun Street to a pump station next to the Charleston Maritime Center. That pump station also will handle the drainage from Market Street.
The city also is moving ahead with plans for a similar deep-tunnel system under the Septima P. Clark Parkway — the Crosstown — a project projected to finish around 2020.
When the Market Street work is completed in three years, the city will be able to pump 40,000 gallons of rainwater from here each minute.
And it will have solved a problem that goes back approximately two centuries, when the city began filling in Major Daniels Creek to create the area now known as Market Street.
Currently, the Market Area is drained with a 19th-century brick arch that measures less than 3 feet by 3 feet, and the rising sea level and accumulated sediment has rendered it largely ineffective.
Mayor Joe Riley noted that the deep-tunnel construction is largely hidden from view, but that doesn’t mean it’s insignificant.
“The complexity and cost to improve the drainage and respect the historic treasures of Charleston is a task that many mayors and councils have struggled with throughout Charleston’s history,” he said.
Reach Robert Behre at 937-5771.