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Project At A Glance:
Name of Project: Elkhorn WWTF Decommissioning & Interceptor Sewer Extension
Construction Category: Sanitary Sewer Collection
Name of Project Owner: City of Omaha, NE
City/State Project: Omaha, NE
The City of Omaha hired JEO Consulting Group, Inc. to develop a wastewater preliminary engineering report (PER) for the Elkhorn Wastewater Treatment Facility (WWTF), which is owned and operated by the City of Omaha, Nebraska. Much of the population growth that is occurring, and is projected to occur, in the Omaha metropolitan area lies within the Elkhorn WWTF drainage basin. With all the growth that has occurred within the basin, the WWTF was operating at design capacity (1.0 MGD) and would not be able to support the future growth that is expected to occur in the next 20-30 years within the drainage basin.
After reviewing a variety of WWTF expansion and replacement alternatives, it was ultimately determined that the most cost-effective option was to decommission the Elkhorn WWTF and redirect flows to the City of Omaha’s Papillion Creek sewage system basin. The new sewer main was approximately 4,865 ft in length built with 30-in. OD/27-in. ID vitrified clay pipe. It was constructed on city-owned property, street rights of way (ROW), permanent sanitary sewer easements, and temporary construction easements. Omaha-based contractor L.G. Roloff Construction Co. was awarded the project and issued NTP in October 2019.
The pipe route crossed Omaha’s 192nd St., which experiences heavy vehicular flow and paralleled historic Old Lincoln Highway, which limited access for construction crews. A 42-in. steel casing crossed 192nd St. that had very tight tolerance to a large diameter water main crossing above the new sewer’s alignment. The bore was extremely flat and because it was in the middle of the job, missing grade would jeopardize the effectiveness of the entire project.
L.G. Roloff subcontracted the bore to Exeter, Nebraska-based Horizontal Boring and Tunneling Co., which used the pilot tube/guided boring method to complete the 204-ft long tunnel.
In addition to these challenges, Papillion Creek also dissects the project site. Two inverted siphon structures, depressed sewers, consisting of two 18-in. and one 16-in. diameter barrels, carry wastewater under the channel. Due to creek setback requirements, these are two of the longest siphons in the entire Omaha system. Horizontal Boring and Tunneling also completed horizontal directional bores for siphon construction.
The project had a milestone date of June 19, 2020, which included completion of the sewer interceptor extension and siphons, such that all wastewater flow that would have went to the Elkhorn WWTF, would instead be conveyed to the Papillion Creek Water Resource Recovery Facility (WRRF) via the new interceptor extension.
Sewer construction in soft/wet alluvial soils during the winter months in Nebraska was a challenge that the contractor had to overcome to meet this key date. Cold weather concrete construction was also a factor on the four cast-in-place siphon structures. One of the siphon structures was approximately 24 ft deep and was constructed under high voltage power lines.
L.G. Roloff used its Cat 349 excavator for the open-cut installation, as well as support equipment for backfilling and loading from Cat and Doosan. Lincoln Winwater furnished appurtenances as well as the Logan Clay VCP, which was shipped from Arizona and California, requiring careful coordination to ensure it arrived on time and without damage.
L.G. Roloff project manager Jesse Walz said that there were several challenges on this far-ranging project, particularly the tight grade tolerances in difficult ground conditions. “Laying pipe along an existing creek bed with soils that were soft and wet was a challenge, especially considering how flat the alignment was,” he said. “Additionally, the 7-ft lengths of the VCP required even more care in assuring the profile was on plan. “In doing work like this, you need to ensure that your surveying equipment is calibrated accurately.”
After pipeline and manhole construction, all facilities required a final epoxy costing for additional corrosion protection. Crews worked from November 2019 to April 2020 to substantially complete the project ahead of the June milestone date.
The final phase of the project was to decommission the existing wastewater facility except for facilities planned to remain. Eriksen Construction Co. was hired to decommission the wastewater treatment plant. Their scope included grit removal and disposal, demolition, abandonment, and modifications along with installation and abandonment of onsite utilities.
There were also items to be removed and salvaged or reused from the existing wastewater treatment facility. One of these items was a belt filter press which happened to also be in a building that was to remain. The City of Omaha has not finalized the future use of this property but has several options available.
Benefit to the Community
The long-term effects of this project will save both the client and community as there will be far less maintenance on the interceptor sewer. The interceptor sewer was also sized for the complete build out of the Elkhorn sewer shed and is projected to serve a population of approximately 28,000 people. The project also eliminated a source of unpleasant smell which would have otherwise slowed growth and develop in the vicinity of the plant.