< The City of Modesto, California Upgrades 1970-Era Sewer Line by Installing Two New Sewer Siphons under the Tuolumne River

The City of Modesto, California Upgrades 1970-Era Sewer Line by Installing Two New Sewer Siphons under the Tuolumne River

In 1970, the City of Modesto, California installed two new concrete sewer lines under the Tuolumne River. Prior to the development of trenchless installation methods, river crossings were typically completed using open trench construction. The contractor would first build a temporary dam to divert river flow and then trench the river bottom to install the new pipe. Today, due to cost and permitting issues, river crossings are typically completed using trenchless installation methods, most commonly horizontal directional drilling (HDD).

For the last several years the City of Modesto, O’Dell Engineering (lead engineer), and Bennett Trenchless (HDD design engineer) had been working together to design, permit and manage local logistics to replace the existing sewer lines. The project design required HDD installation of two 700-foot, 18-inch sewer siphons at a depth of 40 feet below the bottom of the Tuolumne River. The two sewer lines were designed to traverse the river between the site of the former Modesto Tallow Plant and the Dryden Park Golf Course.

During design phase, Bennett Trenchless recommended two pipe material options for the general contractor to complete the HDD portion of the project. Fusible PVC® pipe and HDPE were specified because both pipe materials can be thermally fused together to create a fusion joint that is equal to the nominal strength of the surrounding pipe barrel. The fused pipe string is then able to sustain an axial load during HDD pull back.

Although both pipe materials were deemed suitable for the installation, they have very different material properties, most notably significantly different strength characteristics. Based on the required hydraulic demand and calculated installation stresses, Bennett Trenchless specified 18-inch DR21 Fusible PVC® pipe and 22-inch DR9 HDPE pipe. Because PVC pipe has a hydrostatic design basis (HDB) of 4,000 psi compared to an HDB of 1,600 psi for HDPE pipe, the 18-inch DIPS Fusible PVC® pipe was able to provide equal pressure capacity and a slightly larger ID than the 22-inch HDPE pipe.

The smaller OD of the Fusible PVC® pipe reduced the overall bore volume of the borehole by 20%, in turn lowering the total installed cost compared to the HDPE pipe option at bid time.

Mozingo Construction of Oakdale, California, the general contractor for the project, selected The HDD Company, Inc. of Cameron Park, California to perform the HDD installation of the new 18-inch sewer lines. The two lines were installed in January and February of 2015, and placed into service shortly thereafter.

NOTE: Recommended bore diameter for pipe diameters less than 24-inch is typically calculated by multiplying the pipe OD by 1.5 and rounding up to the nearest inch. Bore diameters for pipe diameters 24-inch and larger are typically calculated by adding 12" to the pipe OD and rounding up to the nearest inch.

Pipeline Details and Project Summary
Project: Shackelford Sewer Main
Location: Modesto, CA
Pipe Size: 1,400 LF of 18” DR21 Fusible PVC® pipe
Installation: Horizontal Directional Drill
Owner: City of Modesto
Engineer: General Engineer: O’Dell Engineering HDD Design Engineer: Bennett Trenchless
Contractor: General Contractor: Mozingo Construction HDD Subcontractor: The HDD Company

Jeff Graham, Project Manager for Mozingo Construction, commented: “Mozingo Construction chose the fused PVC option because it was the most cost-effective in this application. The Underground Solutions crew performed extremely well on the fusing operation, and the directional drilling by HDD and subsequent pipe pulling operation went flawlessly. The City of Modesto was extremely satisfied with the whole process. Mozingo Construction will definitely look at Fusible PVC® pipe in all future projects.”

Rick Evans of The HDD Company remarked: “We’ve completed several projects and currently have three projects contracted where Fusible PVC® pipe will be installed. From initial design through construction, the Shackelford project is just one more example of our previous experience with Fusible PVC® pipe. The team of O’Dell Engineering, Bennett Trenchless, Mozingo Construction, The HDD Company, and Underground Solutions did an excellent job of working together to make the project a success.”