The City of Sergeant Bluff, Iowa, is located in western Iowa next to the Missouri River. The project was planned to eliminate an aging, costly lift station by installing a trunk line sanitary sewer. The City evaluated installation methods and estimated costs for conventional open-trench and trenchless options. Estimated costs for both options were presented to the City in 2009. The analysis showed open-trench at $225,000 less than the trenchless option. However, the City selected the trenchless method of pilot-tube microtunneling for the following reasons:
- Open-trench construction possible overruns,
- Open-trench would require entire street replacement,
- The project corridor was critical for school traffic,
- Existiing utilities would be better protected using trenchless technology, and
- Dewatering requirements at each shaft versus along the entire trench would better limit risk.
This project consists of the installation of 15- and 21-inch gravity sanitary sewer installed at depths up to 23 feet. The contractor, Minger Construction, selected 16-foot diameter shafts and a three-step installation process utilizing both powered reaming and cutter heads. The soil conditions encountered included silty clays, lean clays and poorly graded sands with blow counts ranging from two to 21. The pilot-tube microtunneling process, at the design slopes required for this project, is not a common technology used in the state of Iowa. The Iowa DNR required the City to file a construction permit variance to allow the technology to be used for this project.
The City of Sergeant Bluff, Minger Construction (contractor), Mission Clay (No-Dig Microtunneling pipe & Casing pipe) and Veenstra & Kimm, Inc (engineers for design work) worked together on this Microtunneling Project which is being considered for the North American Society for Trenchless Technology's (NASTT) award for Project of the Year.
The Lift Station Removal and Gravity Piping Project was a success!