< Sioux City, Iowa Replaces Floyd River Sewer Siphon with New Fusible PVC<sup>®</sup> Carrier Pipes and Casings

Sioux City, Iowa Replaces Floyd River Sewer Siphon with New Fusible PVC® Carrier Pipes and Casings

The Floyd River intersects downtown Sioux City, Iowa and presents a significant obstacle in conveying wastewater to its treatment plant. Sioux City uses an inverted sewer siphon to transport sewer flows across the river, but planned improvements to Interstate 29 through the city required placement of a new bridge abutment in the location of the existing sewer siphon. As a result, the existing siphon needed to be abandoned and a new siphon was constructed approximately 1,100 feet to the north of the current crossing.

The City retained the design firm HR Green, Inc. to begin design of the new siphon. Based on the sewer flows that needed to be transported across the river and the velocities required to make the siphon operate properly, HR Green determined the new dual barrel siphon would require 24-inch and 30-inch diameter carrier pipes. Regulatory requirements for the river and railroad crossings added an extra level of containment security to the carrier pipes by requiring that they be installed inside 36-inch diameter casings.

In order to install the new sewer siphon, HR Green and the City evaluated various construction options before ultimately deciding to specify a horizontal directional drill (HDD) installation. Micro-tunneling under the river was considered, but was deemed to be significantly more expensive and time consuming than HDD installation at the depths required for the project. Timely completion of the project was critical due to the tightly phased schedule associated with the I-29 improvements project.

Once HDD was selected as the installation method, the next task was to evaluate various pipe material options for the siphon. Steel, ductile iron, HDPE, and Fusible PVC® pipe were all evaluated for the sewer siphon pipe material. Bend radius limits and corrosion concerns eliminated steel and ductile iron pipe as viable options. While HDPE pipe met the bend radius requirements and was corrosion resistant, the soil loading requirements for the project depths necessitated a DR9 HDPE pipe to provide sufficient long-term buckling resistance. In order to meet the minimum inside diameter required for the casings, the project would have required 42-inch DR9 HDPE pipe, which was not readily available commercially. Finally, the project was located in an industrial area and there were concerns regarding the presence of hydrocarbons and other contaminants that might permeate and eventually weaken the HDPE casing pipe.

After each pipe material was evaluated, Fusible PVC pipe was selected for the sewer siphon carrier piping and casings. Pete Merten of HR Green noted, “The decision to use Fusible PVC® pipe was based on the pipe’s strength characteristics, the elimination of corrosion concerns, and the pipe’s superior resistance to hydrocarbon permeation.” In addition to crossing under the Floyd River, there were railroad tracks on the west side of the river that needed to be crossed as well. HR Green worked with the owner of the rail spurs to approve an HDD installation under the tracks using the American Railway Engineering and Maintenance-of-Way Association pipe crossing requirements. It was determined that, due to the 50-foot depth of the borehole beneath the tracks, there would be enough soil bridging to prevent any settlement.

The final design of the project required directionally drilling two parallel 36-inch casings under the Floyd River and the adjacent railroad spur, resulting in a surface-to-surface drill length of approximately 1,140 feet. The project was competitively bid, with the contract awarded to Minger Construction of Chanhassen, Minnesota. Minger subcontracted with Gabe’s Construction of Sheboygan, Wisconsin to perform the HDD work.

Gabe’s used several reaming passes to open up 52-inch boreholes for the 36-inch casings. Due to the elevation change between the surface of the drill and the bottom of the boreholes, it was decided to pull the casing and carrier piping simultaneously into the borehole. Based on this plan, the carrier piping was pre-loaded into the 36-inch casings. The 30-inch carrier within the 36-inch casing was the first siphon line to be installed and the combined pipe bundle pulled in smoothly. Gabe’s ballasted the siphon line with water during installation to reduce the amount of friction in the borehole and minimize pull force. The second siphon line containing the 24-inch carrier and the 36-inch casing pipe was also successfully installed adjacent to the first line.

Once Gabe’s had both of the new siphon casings and carrier lines installed, the annular space between casing and carrier piping was grouted and the lines were successfully pressure tested. Minger then constructed the siphon structures on either side of the river and tied in the carrier lines.

The successful relocation of the inverted sewer siphon will allow the City to abandon its existing sewer siphon and clear the right-of-way needed for the Iowa Department of Transportation to continue moving forward with improvements to I-29.

Pipeline Details and Project Summary
Project: I-29 Interceptor Floyd River Siphon
Location: Sioux City, IA
Installation HDD
Owner: City of Sioux City
Engineer: HR Green, Inc.

Floyd River to be Crossed with Siphon

Drill Rig Setup on East Side of River

Excavators Begin to Elevate Pipe for Insertion

Pipe Inserted into Borehole