The City of Charleston was installing new sewer to relieve the strain being put on its' existing 100 year old system.  The solution was a 100’ (30.48 m) deep tunnel that would carry the wastewater to the treatment plant.  In engineering the new system, the designers had to get the street level laterals connected to the main tunnel trunk line, nearly 70’ (21.33 m) deeper than the existing pipe lines.  The designer employed the use of vortex drop shafts in order to get the wastewater down to the main line.  The challenge facing the designers of these vortex structures was the amount of hydrogen sulfide that would be created.  The original design called for Portland cement concrete structures and a protective spray-on coating to be applied after installation.  The coating was designed to protect the concrete from the acid that would be generated for the hydrogen sulfide gas in the sewer.  The vortex structures have an inside diameter of 120” (3048 mm).  The joints between the riser sections had to withstand a 50 psi (3.44 bar) pressure test.  In order to accomplish this, the risers sections were constructed with a double o-ring AWWA joint design with an air test port between the two o-rings. The reinforcement was designed around the potential seismic loads that could occur in the Charleston region.

The decision to use precast polymer concrete structures resulted in a faster installation time.  The structures were shipped to the jobsite and staged next to the access shaft.  Once lowered into place, the structures were ready for service.  The use of the polymer structures eliminated the extra step of having to come back and coat them with a protective material.