Pilot Tube Method of Guided Boring over Soil Stabilization in the Great Lakes Basin

This paper will present a project in Novi, MI where 300 feet of 10-inch sanitary sewer was installed over a Peat and Marl Zone using a trenchless stabilization process and the pilot tube method of guided boring (PTM). The study is a comparison between the originally designed 'open excavation' project and the value engineered project using PTM and grout injection for the Grand River Road project (adjacent to two busy thoroughfares and a Cadillac Dealership). In 1996, the City of Novi, permitted development of a 10-acre parcel which included a sanitary sewer line that was added to their existing collection system. Located in Oakland County, this installation was originally constructed using PVC Truss Pipe at cover depths ranging from 10 to 12 feet. Below the pipe, the soil consistency was layers of peat and marl. In 2006, 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 / soil structure as well as the peat / 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 peat/marl zone within the sewer elevation (on the west end) and below that, was a 5-foot layer peat zone above a 7-foot layer of marl. Moving to the east, the marl zone was just 2 foot below the sewer invert. The initial bid design required over-excavation of the peat and marl, the addition of a geotextile wrap, and 1-3 inches of backfill with crushed stone. This foundation improvement was required for pipeline support using conventional open-trench installation methods. This paper outlines the soil stabilization and installation method of PTM used to replace the sanitary sewer. The project was originally specified conventional open-excavation and the Contractor value engineered the project using grout injection and then used Pilot Tube Method of Guided Boring install the sanitary pipe.