Rehabilitation or Replacement? That Is the Question
Prepared for the 2014 NC American Water Works Association & Water Environment Association (AWWA-WEA) Annual Conference by Chris L. Windley PE, Ben R. Latino, Jr., PE, and Greg Anderson, PE
When designing to accommodate additional future flows, is it more economical to rehabilitate the existing sewer along with installing a new parallel line, or to replace the existing sewer with a single, larger diameter gravity sewer? And how does a municipality go about making that decision?
The City of Raleigh Public Utilities Department (CORPUD) plans to rehabilitate and parallel or replace approximately 16,000 linear feet of 48-inch and 54-inch gravity sewer in the lower portion of the Walnut Creek sewer basin. In order to make an educated and informed decision regarding rehabilitation versus replacement, the CORPUD initiated an evaluation of multiple rehabilitation technologies and methods for large diameter gravity sewer.
CCTV and pole camera investigations had already been performed prior to start of the project, and indicated that the majority of the existing gravity sewer main and manhole conditions were rated as 4 or 5 under the Pipeline Assessment and Certification Program (PACP) and Manhole Assessment and Certification Program (MACP) rating systems developed by the National Association of Sewer Service Companies (NASSCO). The results of the investigations made it apparent that any rehabilitation technology considered in the preliminary study would need to provide structural integrity independent of the existing host pipe.
Four minimum criteria were utilized for evaluating a dozen rehabilitation technologies. Analysis determined that three of the 12 technologies met the minimum criteria. These included segmented sliplining, geopolymer centrifugal spray-in-place pipe, and cured-in-place pipe (CIPP). Each method was thoroughly researched and ranked for its ability to meet seven additional requirements, and these rankings indicated that CIPP offered a slight advantage over the other two methods for this particular project and client.
Upon conclusion of the rehabilitation evaluations, the project team evaluated four design alternatives:
- A single, replacement interceptor located in the same alignment.
- A single, parallel interceptor replacement sized to carry the entire future flow.
- Rehabilitation of the existing interceptor and construction of a parallel interceptor.
- A combination of Alternatives 2 and 3 to address areas with potential constructability issues.
Additional analysis indicated that Alternative 2 was the most viable and economical for the CORPUD. This was primarily due to the fact that the parallel line sizes were only one to two typical pipe diameters smaller than the single replacement line, and the cost to install the parallel lines in addition to rehabbing the existing lines outweighed the cost of a single replacement line.
The presentation will discuss the overall project need, rehabilitation technologies and methods evaluated, flow projections and pipe sizing, alternative analyses, and final recommendations for the project.