East Bay / Calhoun Stormwater Pump Station, Charleston, South Carolina
 

With the award of the East Bay/Calhoun Stormwater Pump Station Project, Chandler Construction Services, Inc. was presented with the challenge of building a suspended concrete superstructure at an elevation of 130 feet above the invert of an existing 145-foot deep, 30-foot diameter shaft.  The historical, urban environment coupled with the demands of working alongside Charleston Harbor created a unique opportunity for Chandler to showcase our knowledge, capabilities and strong safety record.
 
The major component of the pump station wetwell is a concrete superstructure constructed in a 30-foot diameter shaft.  Scaffolding was erected from the bottom of the shaft to cast-in-place a suspended 14-inch thick concrete floor.


Three 325HP submersible propeller tube pumps rated at 40,000gpm capacity were installed to handle peak stormwater run-off.  The pumps, weighing  more than 11,000 pounds each, were suspended from the concrete superstructure.
 
Two 90HP 3,000gpm abrasive solids handling pumps were placed at the bottom of the 145-foot deep shaft for dewatering the tunnel and shafts.  Mounted to the shaft wall on 3-inch guide rails, the pumps can be lifted for repair.
 
The pump station building had to be elevated above the 100-year floodplain and constructed to compliment the surrounding historical area.  Brick masonry with features such as decorative columns, metal roofing and ornamental ironwork all added to the design.
 
To begin construction of the concrete outfall structure, a coffer dam was built into the Cooper River to shield water from the excavation.  The excavation was eleven feet below mean sea level.  The 25-foot x 25-foot by 19-foot deep outfall structure, set atop 60-foot deep "H" piles was designed with a 25-foot long weir wall to slow the energy and velocity of the water existing from the pump station and outfall lines.  By constructing the concrete structure below sea level and adding the weir wall, boats can be safely be secured in front of the outfall even when water is being expelled.
 
Before trenching could begin on the outfall lines, the entire Chandler crew needed to receive specialized training for working in and around hazardous materials.  The entire work area was a former shipyard and soils were contaminated with dangerous levels of heavy metals, hydrocarbons and PCB's.  543 tons of hazardous waste and 4,710 tons of contaminated material were excavated to install a 96-inch diameter fiberglass outfall line.  Dewatering effluent had to be tested and characterized before it could be disposed of.
 
The project was awarded a CAGC Pinnacle Award for its unique aspects, special value to the community and environment, special construction challenges, excellence in overall project management, and budget/schedule considerations.  The project engineer stated "Chandler performed outstandingly under the adverse conditions.  The project was completed with few additional costs and well within the contract schedule."