Mattabassett District Outfall

The Mattabassett District's Water Pollution Control Facility processes wastewater from seven communities in Central Connecticut. The facility typically treats 15 to 20 million gallons per day, discharging clean water into the Connecticut River. In 2007, the District's new outfall - a 140 ft extension, with an additional 160 ft of 16 diffusers, at depths exceeding 20 ft - came online. 

According to Nick Tanionos, head of Specialty Diving Services, the pipeline project was initially designed for installation over bedrock. However, sections of the pipeline's footprint traversed coarse, fluid sand. As confirmed by a trial installation, the sandy river bottom could not support the weight of the 60 in. prestressed concrete cylinder pipe (PCCP). The challenge thus became the deployment of a scour-resistant bedding layer under continuously flowing water.

Engineers at Camp Dresser & McKee, Inc. (CDM) initially considered piles and various types of gabions; however, excavation and infill would have required a "massive" undertaking, according to Tanionos, adding, "So I suggested the Triton System to solve the problem." 

CDM's Paul Gilbert, P.E., Project Manager, stated that the Triton Marine Mattresses were readily available, capable of being filled and barged in a confined environment and, most importantly, able to provide a sound foundation for the support of the heavy piping. "Any significant settlement," he added, "would have reduced the efficiency of the large pipe diffusers."

The solution required sufficient mass and flexibility to properly conform to the site and resist movement, the stability to resist settlement, an adaptability to dynamic conditions and the capability for rapid installation. With each of these, the Triton Marine Mattress system excelled. With the strength of Tensar uniaxial (UX) geogrids, Triton Marine Mattresses resist degradation to provide rigorous scour protection in the most demanding climatic and underwater conditions. They conform readily to irregular or soft subgrades, steep slopes, banks and more. And with the use of readily available, natural fill materials, Triton Marine Mattresses can be significantly less expensive than conventional solutions such as riprap.