Ever notice the piles of dirt on Drum Island as you zoom over the Cooper River bridge from Mount Pleasant to Charleston?  Seem like they’ve been moving around for the last few months, but for no apparent reason?
Well, there’s a method to what’s happening as explained in a recent visit to the 160-acre island of dredge material, or spoil, that the bridge crosses.
The S.C. State Ports Authority is preparing to raise the walls of a dirt dike that surrounds the island so that it can dispose of silt removed from berths at Union and Columbus street terminals.  When the walls are around six feet taller around Drum Island, the SPA says it will be able to put dredge material on Drum Island for eight to 10 more years.
To get the island ready, a lot of dirt needs to be moved to facilitate the drying process.  On the surface, the dirt looks like a tan drought landscape pocked by deep surface cracks.  But if you dig down just one or two feet, the dry dirt will quickly change to liquid pluff mud that is unmanageable, explains SPA project engineer Ben Morgan.
“We cut ditches across the island to lower the water table,” he said.  “The purpose of the ditches is to dry out the silt.”  Water seeps from the spoil basin into ditches and drains into Town Creek and the Cooper River through four big outfall pipes.
An excavating machine deepens ditches only after it has spread its weight across a wooden mat of support.  The “mat” is a network of parallel wooden beams that look like extra-long railroad ties with metal hoops at the end.  When the excavator is done with one portion of a ditch, it picks up beams from one part of the mat and moves them further along the ditch so the machine can move forward and dig some more.
It’s a long process that takes patience.  Months pass as the silt basin dries and operators dig and re-dig ditches to allow water to drain.
Toward the end of summer, it should be time to start moving the dried silt.  First, a bulldozer — which can operate on the cracked dirt of the island because its weight is spread across its treads — will push piles between two drainage ditches to form a center “haul road,” says lead operator Jeremy McCarren, a contractor from Calabash, N.C.
Next, a tractor will drag a special scoop along the road to scrape up thousands of pounds of dried spoil, which then is moved to the exterior of the island to build the dike higher.  As the tractor takes away spoil, a bulldozer will push more dry spoil from the area between the ditches to the haul road to keep the process going.  After three or four months, the dike should be at the right height to be strong enough to add more spoil to the basin for several years.
Despite all of the big earth-moving equipment on Drum Island, the huge project is pretty simple in one sense because it relies on prepping the island’s basin for time and Mother Nature’s gravity.  When they’ve done their work, the machines kick into high gear to move the dirt to raise the wall.
Now, we know!
Andy Brack is publisher of StatehouseReport.com and CharlestonCurrents.com.  He can be reached at:  brack@statehousereport.com.

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