THE USE OF TRENCHLESS AND SEMI-TRENCHLESS TECHNOLOGIES ON THE NEMASKET INTERCEPTOR SEWER IMPROVEMENT PROJECT, MIDDLEBOROUGH, MASSACHUSETTS

Phase II of the Nemasket Interceptor Sewer Improvement Project is one of the largest pipe bursting projects, in diameter and total length, ever completed in the Northeast and possibly on the East Coast. Pipe bursting and other trenchless and semi-trenchless technologies were selected for both phases of the Nemasket Interceptor Sewer Improvement Project to minimize disruption to the significant amount of environmentally and historically sensitive areas, private property making up the project area, and “no dig” restrictions from the state highway department. Work on the project consisted of three distinct parts: (1) modeling and flow evaluation; (2) design, bidding and construction of Phase I improvements; and (3) design, bidding and construction of Phase II improvements.Constructed in 1950, the Nemasket Interceptor Sewer historically surcharged and, in some locations, overflowed during moderate storm events. In 1997, the town retained Weston & Sampson to determine the capacity of the Interceptor and evaluate various flow conditions through capacity analysis and modeling. The report on the capacity analysis and modeling recommended a two-phased improvement project. Phase I included the elimination of infiltration into the 15-inch Upper Nemasket Interceptor Sewer, and Phase II included elimination of sanitary sewer overflows from, and infiltration into, the existing 18-inch Lower Nemasket Interceptor Sewer.Design of Phase I of the project focused on rehabilitating the existing Upper Nemasket Interceptor Sewer through the use of trenchless technologies to minimize the disturbance to the wetland and buffer zones through which the Interceptor passes. This goal was accomplished as construction utilized 100% trenchless technologies to remove infiltration from the existing 15-inch Upper Nemasket Interceptor Sewer. In brief, construction of Phase I of the project included rehabilitation of 2,459 linear feet (LF) of sanitary sewer via cured-in-place pipe (CIPP) lining, and rehabilitation of 21 manholes via interior cementitious lining and exterior chemical grouting. Construction of Phase I was completed in February 1999.Design of Phase II of the project focused on expansion of the Lower Nemasket Interceptor Sewer. The hydraulic capacity analysis identified a shortfall of almost 50% during peak flows in the Lower Nemasket Interceptor Sewer (4.0 MGD vs. 2.8 MGD). To eliminate overflows and surcharging, the Lower Nemasket Interceptor Sewer would need to be increased from 18 inches to 24 inches in diameter. Unfortunately, the majority of the Lower Nemasket Interceptor Sewer lies in wetland and buffer zone areas and on private property where, although easements were taken during construction, a large number of encroachments now exist. For this reason, a variety of alternatives for providing additional capacity were evaluated; however, none of these alternatives were both feasible and cost-effective, and the final decision was to replace the existing Interceptor.Environmentally sensitive areas and easement encroachments on private property were not the only design challenges on the project. Additional challenges included, but were not limited to, a “no dig” restriction from the Massachusetts Highway Department (MassHighway), pipe buoyancy issues due to inadequate cover, historically significant areas, steep slopes, the need for an extensive by-pass pumping plan, and work in town recreational areas. In the end, final design of Phase II consisted of conventional excavation with multiple semi-trenchless technologies to minimize surface disturbance. In brief, it included approximately 3,500 LF of pipe bursting from 18-inch Vitrified Clay (VC) pipe to 24-inch High-Density Polyethylene (HDPE) pipe; excavation and replacement of approximately 6,500 LF of 18-inch VC pipe with 24-inch Polyvinyl Chloride (PVC) pipe; 100 LF of pipe ramming of 42-inch steel casing and installation of 24-inch PVC carrier pipe; and replacement of 50 sewer manholes. The project also required construction of a gravel access road with physical slope stabilization measures to provide access along steep slopes for construction and future operation and maintenance of the sewer. The project was publicly bid in May 2004. Construction began in August 2004 and is anticipated to be completed spring 2005.Pipe bursting and other innovative trenchless and semi-trenchless technologies provided the Town of Middleborough with valuable alternatives to conventional excavation for a project with significant design and construction hurdles.