Trenchless Soil Stabilization of Marl Prior to Using Pilot Tube Method to Install New Sanitary Sewer

In 1996, the City of Novi, Michigan, permitted the development of a 10-acre parcel which included a sanitary sewer line that had been part of their system since 1986. Located in Oakland County, this installation was originally constructed using 10” PVC SDR 35 pipe at cover depths ranging from 10 to 12 ft. Below the pipe, the soil consistency was predominantly marl and peat. In 2005, the site was redeveloped with an additional 10-foot of fill added to the area above the sanitary sewer line. This surcharge load affected the flexible pipe and soil structure, as well as the peat and marl foundation below the pipe zone; the result was a failed pipeline. A new sanitary sewer line was proposed, designed, and bid as a conventional open trench excavation with over-excavation of the marl zone. Soil borings indicated the existence of a marl zone within the west end of the sewer elevation with a seven-foot layer marl zone over a five-foot layer of peat below. Moving to the east, the marl zone shifted to 2-ft below the sewer invert. The initial bid design required a foundation improvement of over-excavation of the marl, the addition of a geotextile wrap, and two to 7 ft of crushed stone in gradations of one to 3-in. This foundation improvement was required for pipeline support using conventional open-trench installation methods. The contractor value-engineered the project by identifying an alternative soil stabilization and installation method that was reviewed and approved by the engineer and the city. This paper outlines the soil stabilization and installation method used to replace the sanitary sewer. A cost comparison will be provided between the originally specified, conventional method and the trenchless soil stabilization with pilot tube method of guided boring (PTM) installation.