New Neponset Valley Relief Sewer

The New Neponset Valley Sewer System is comprised of 8.4 miles of gravity and force main relief sewer up to 48 inches in diameter. It includes approximately 17,000 feet of 48- inch-diameter force main, a new 50- million gallons per day pumping station, and approximately 10,000 feet of 48-inch diameter gravity sewer. GEI was responsible for all geotechnical and environmental aspects for design and construction of the project.

Work included feasibility analyses of pipeline installations along the alignment, as well as evaluation of alternate construction methods for a 48-inch-diameter, 2,000-foot long section in the river valley floodplain.

Environmental investigations indicated that the aquifer was contaminated below the proposed depth of sewer excavation. GEI evaluated alternate trenchless construction techniques, such as microtunneling, ground freezing, and pipe jacking, to minimize or eliminate dewatering and handling of contaminated water. Recommendations were provided by GEI for five pipe jacking crossings at major highways or railroads, a siphon crossing of the Neponset River, a pumping station foundation, controlled blasting removal of bedrock, dewatering of silts and fine sands, and sewer bedding and backfill. GEI also provided geotechnical recommendations for protection of an existing unreinforced, 48-inch sewer located immediately adjacent to the proposed sewer over the entire length of the new sewer.

During final design and preparation of contract documents, GEI developed detailed contract specifications for microtunneling and slurry face or pressure balance technology to install a 2,000-feet, 72-inch diameter casing pipe for the 48-inch-diameter force main crossing through the river flood plain. GEI gave detailed requirements for jacking and receiving pits, intermediate jacking stations between the receiving pits, and dewatering.

GEI reviewed contractor submittals, and provided consultation and monitoring during several construction activities including effects of bedrock blasting on existing unreinforced 48-inch sewer, effects of TBM advancement immediately under a 10-inch diameter, high pressure (700 psi) steel pipe gas transmission main.