- ENGINEERING INFO
- CASE STUDIES
The City of Raytown, Missouri, a suburb of Kansas City with a population of approximately 30,000 residents, maintains 157 miles of sanitary sewer pipe and over 4,000 manholes. The recently completed White Oak Sanitary Sewer Replacement Project (West Line A) consisted of over 5,000 LF of sewer pipe replacement in various sizes. The project was necessary to provide additional capacity to the existing conveyance system along 83rd Street, from Raytown Road to Spring Valley Road. This area had a history of sewer backups caused by an undersized system. The majority of the new pipe was installed via open-cut and consisted of bell-and-spigot PVC pipe. However, two sections of line involved terrain and obstacles where open-cut installation was not an option.
One of these sections of the alignment involved a creek crossing. Based on the need to limit environmental impact, the creek crossing was designed using trenchless installation methods. Further complicating the design, the proposed gravity line was to be installed through a rock formation present in the creek area. Bid documents provided contractors the option to insert bell-and-spigot PVC pipe inside a steel casing using jack & bore installation, or install 30-inch Fusible PVC® pipe via horizontal directional drill (HDD). The installing contractor selected HDD with Fusible PVC® pipe due to significant cost savings versus the jack & bore option.
The 30-inch HDD installation was challenging due to a number of factors:
1. On-grade HDDs require careful attention to maintain proper grade at all points during the drilling process. A minor variation in slope during drilling can result in grade variations that are unacceptable for gravity systems.
2. Site access was limited by geographic constraints, with the drill rig and fused pipe string both located on one side of the installation. As a result, the pilot hole and reaming passes were performed with the drill rig set up on the same side as the pipe fusing operations. Pipe pull-in was accomplished using a cable and excavator that was moved into place on the opposite side of the creek following completion of the reaming process.
3. The initial vertical alignment would have resulted in pipe being installed in a mixed face zone (part soil and part rock). This alignment would have made it extremely difficult to meet the tolerances required for an on-grade HDD. The driller requested that the line be vertically lowered to allow the entire bore path to be drilled in the rock zone. The overall sanitary sewer profile accommodated this adjustment and the line was drilled-in at the lower elevation within the required tolerances.
|Pipeline Details and Project Summary|
|Project:||White Oak Sanitary Sewer Project|
|Length and Size:||220 LF of 30” DR25 FPVC® pipe 520 LF of 14” DR21 FPVC® pipe|
|Owner:||City of Raytown|
|Contractor:||Linaweaver Construction, Inc.|
|HDD Contractor:||Diversified Boring|
The second section requiring trenchless installation consisted of approximately 520 LF of 15-inch sanitary sewer that was installed via HDD because it ran parallel to an existing drainage ditch. By utilizing HDD installation, the line was installed without damaging the existing concrete-lined channel. This section was installed in better soils, which eliminated some of the difficulties experienced on the 30-inch installation. However, there was not enough room to fuse the entire 520 LF pipe string prior to installation, so an intermediate fusion was required during pull-in. A 14-inch DR21 Fusible PVC® pipe was used for this section, because the 14-inch ductile iron pipe (DIPS) size was the same outside diameter as 15-inch sewer pipe, allowing for standard sewer connections to be utilized.
Diversified Boring was selected by general contractor, Linaweaver Construction, to perform the bores based on its experience with on-grade HDDs. Chris Flood, owner of Diversified Boring, stated: “We have worked with Fusible PVC® pipe in the past and like the material. Not only does it pull without stretching, it maintains its shape and dimensionality in the borehole better than other plastic pipe materials. This is critical when boring on-grade, as localized deflections can lead to sags in the line.”