SITUATION
This project involved the rehabilitation of 8,806 feet of a 60-inch and 66-inch combined sewer which was more than 65-years-old, utilizing cured-in-place pipe (CIPP) and geopolymer technologies. The project also included television inspection and cleaning, bypass pumping of 45 MGD, pipe rehabilitation, manhole and structure rehabilitation, and miscellaneous work.
CHALLENGE
Situated in the hot, humid low country of South Carolina, and positioned on a low gradient accepting flows from many long force mains, the reinforced concrete pipe (RCP) interceptor had severe corrosion and deterioration in the crown of the pipe from the long-term presence and effects of hydrogen sulfide (H2S) gases. Additional design considerations and project challenges were
:
• bypass pumping of 45 MGD of sewer flow
• pipe alignment near I-26 and two sets of railroad tracks
• two locations in the combined sewers where storm drain pipelines crossed through the top half of the RCP
SOLVED
NCSD and the engineering firm they hired to design the project selected CIPP as the rehabilitation technology for the interceptor. Prior to installing CIPP liners the flow in the interceptor must be completely bypassed, to facilitate cleaning and television inspection of the pipeline segments, and then the CIPP installation. Two critical factors faced SAK: 1) the interceptor being rehabilitated was the main feeder line into NCSD’s Felix C. Davis WWTP, which required designing and constructing a bypass capable of handling the line’s peak flow of 45 MGD to the treatment plant, and 2) the alignment of the interceptor being rehabilitated was in very close proximity I-26 and 2 sets of live railroad tracks. SAK’s construction manager overcame these two issues by obtaining required permits from the state and federal DOT’s, and both Norfolk Southern and CSX railroads, to not only build and operate the bypass, but to also complete the CIPP installations in these tight work areas. The bypass pumping system consisted of six 18-inch pumps (five primary and one standby), with each pump utilizing a 24-inch diameter HDPE discharge pipeline running parallel to the railroads. There were two locations where SAK had 36-inch diameter casings install using the jack and bore method, so they could feed the bypass pipelines through the casings and cross underneath both sets of railroad tracks on the route to the treatment plant. The cooperation of these stakeholders, the client and SAK working together allowed for the successful rehabilitation of these sections of the combined interceptor.
SAK enjoys a very good working relationship with this client and engineer, having successfully completed two prior interceptor rehabilitations for NCSD. The third challenge identified above was a condition they asked for input on during the design of this project, because of SAK’s vast experience with multiple pipe rehabilitation technologies. The presence of storm drains crossing through the interceptor in the top half of the pipe made rehabilitating these sections with CIPP risky, unless two expensive point repair excavations were made to relocate the storm drains. SAK recommended the client have the engineer design the rehabilitation of the two segments utilizing geopolymer lining. Geopolymer is a spray applied lining for large diameter pipes which can address irregular shapes and pipe deformities such as the presence of these storm drains, and still deliver a fully structural rehabilitation. NCSD and the engineer agreed this was a better approach than expensive excavations in the tight work area, so 204 feet of the 60-inch, and 359 feet of the 66-inch combined was rehabilitated with a fully structural geopolymer lining.
