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The majority of Lawrence, Kansas is built on the south side of the Kansas River. This includes downtown Lawrence, the University of Kansas, and the wastewater treatment plant that is located in close proximity to the Kansas River just east of downtown. There is, however, a significant portion of the city located north of the river (North Lawrence), including residential, commercial, and industrial properties that are a vital part of the community.
All of the wastewater from North Lawrence is sent to Pump Station PS04 where it is pumped under the Kansas River to the wastewater treatment plant. In January 2013, the City discovered that a 75-foot section of an existing 8-inch welded steel force main had become exposed on the north bank of the river. The force main was installed in 1956 and runs from PS04 to the wastewater treatment plant. The pipe was uncovered as a result of scour along the river bank that occurred during high flow periods along the river. Fortunately, the pump station had a redundant 18-inch force main to which all of the flows were diverted while a long-term solution was evaluated.
Professional Engineering Consultants (PEC) was hired by the City to conduct an evaluation of the overall system and recommend a long-term solution. Ultimately, the evaluation resulted in a recommendation to install two new force mains from PS04 to the wastewater treatment plant, with the existing 18-inch force main to remain as a backup and the exposed 8-inch force main to be abandoned in place.
|Pipeline Details and Project Summary|
|Project:||Redundant Force Main Project|
|Length:||2,000 LF of 12” DR18 FPVC® 2,000 LF of 14” DR18 FPVC®|
|Installation:||Horizontal Directional Drill (HDD)|
|Owner:||City of Lawrence, Kansas|
|Engineer:||Professional Engineering Consultants (PEC|
|General Contractor:||BRB Contractors, Topeka, Kansas|
|HDD Contractor:||Brooks Construction, Burden, Kansas|
Special consideration was required during the design process to ensure that the specified pipe was capable of withstanding the installation stresses that would be associated with a 2,000-foot directional drill installed 75 feet under the river. The project was designed and specified to allow 12-inch and 14-inch DR18 Fusible PVC® pipe or 14-inch and 16-inch DR9 HDPE pipe. The upsizing of the HDPE pipe was required to maintain inside diameters consistent with the Fusible PVC® pipe based on the required hydraulic capacity of the system. The low bid contractor selected Fusible PVC® pipe as the most cost-effective material for the drill. In addition, the Fusible PVC® pipe option provided ease of connection to the bell-and-spigot PVC pipe on each side of the drills. According to Ken Burkhead, project manager for BRB: “It made sense to utilize Fusible PVC® pipe for this installation. Not only was the material more cost competitive than the HDPE, the installed cost was significantly lower due to the smaller borehole requirement. We’ve utilized Fusible PVC® pipe on other projects and were confident in the material’s ability to provide a successful installation.”
The geologic formations present in the bore alignment were varied, consisting of sandstone overlaid by sands and gravels. The design required the alignment to be within the sandstone formation while crossing under the levee on the north side of the river in order to prevent groundwater migration under the flood control levee.
To provide consistent control of the drill bit while crossing beneath the Kansas River, Brooks Construction utilized a wireline steering system. This provided the ability to meet the horizontal and vertical alignments required by the engineer and owner. Although drilling through the sandstone was more difficult and time consuming than standard dirt drilling, it provided extremely stable boreholes
While twin 2,000-foot HDDs are more common today than in previous years, they still require skilled contractors and significant coordination during the design, drilling, and pull-in stages. The City of Lawrence has adopted HDD as an effective way to install water and wastewater pipelines when open-cut is not feasible or would result in unacceptable impacts to residents. Both the 12-inch and 14-inch force mains were successfully installed, pressure tested, and brought into service in June of 2014.