City of Lincoln, Nebraska
Biosolids from Lincoln’s Northeast Wastewater Treatment Facility (WWTF) are pumped to a location 1.5 miles north of the plant prior to land application. The pipeline must pass below Interstate 80. With the widening of the Interstate 80 corridor between Omaha and Lincoln, the relocation of this pipeline became a priority. Based on the new Interstate design, the existing force main will no longer have sufficient cover. In addition, the force main casing would not provide sufficient protection to span the entire width of the proposed six lanes of the interstate. The depth of bury and span of the steel casing did not meet Nebraska Department of Roads (NDOR) minimum standards. This deficiency required the City of Lincoln to reconstruct the undercrossing and WLA was tasked with the engineering design services.
During pre-design, a topographic survey, wetland delineation, and geotechnical soil investigation were completed. Preliminary design consisted of pipe route selection, analysis of sludge pumping head conditions, material selection, and preparing an order of construction. As analysis of the feasibility of horizontal directional drilling versus “jack and bore” installation methods was also completed. The alignment and profile of the pipeline was coordinated and approved by the NDOR for the roadway undercrossing, as well as with the Nebraska Department of Environmental Quality (NDEQ) for required permitting.
The completed project consists of approximately 425 linear feet of jack and bore underneath Interstate 80 of an 8″ diameter ductile iron pipe through a 20″ diameter steel carrier pipe, and connections to the existing force main.
When the new pipeline was constructed, the force main could not be offline for more than 24 to 48 hours. WLA prepared a detailed order of construction checklist to address the interruption.