Gravity Sewer Installation

Project Summary

Robinson Construction was selected to construct a 72 in. gravity sanitary sewer. The new sewer line runs parallel to an existing sewer and includes approximately 5100 ft of 72 in. PCCP and 500 ft of 48 in. PCCP. The project includes an inverted siphon under the Wolf River and four concrete structures. The siphon consists of two concrete structures, one on each side of the river, that divert the flow from the 72 in. PCCP into three 32 in. HDPE pipes installed under the river bottom. The other two structures are constructed around the existing sewer line and will channel the flow from the existing sewer into the new sewer. The 72 in. PCCP was installed by the open-cut method at depths to 28 ft. A dewatering system was used to lower the water table. The inverted siphon was constructed by first installing a cofferdam in the Wolf River. The cofferdam was designed as two separate phases and was constructed by driving 30 ft sheet piles into the clay layer below the river bottom, cutting off the flow of water into the structure. The cofferdam was then excavated, the bottom 2 ft of the encasement was poured, the prefabricated HDPE pipes were anchored to the slab, and finally, the pipes were fully encased in concrete. The sheet piles remained in place to protect the new siphon lines. The completion of the project involved bypassing the 42 in. existing sewer so that the flow could be diverted into the new line through one of the new concrete structures.