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A cost-effective installation of the University of Nebraska-Lincoln chilled water system

Wednesday, August 16, 2017

Ben Day, Water Industry Expert

Construction is underway at the University of Nebraska-Lincoln (UNL) to incorporate thermal energy storage to its City Campus chilled water system. The project’s purpose is to produce chilled water at night during off-peak power periods, and store it for distribution during the day. This strategy will reduce peak power charges and is potentially save UNL $850,000 to $900,000 per year. The project’s budget is approximately $12 million.

The project consists of three components: an 8.15-million-gallon chilled water storage tank, a pump building, and chilled water main piping to convey chilled water to and from the facility. The focus of this article is the chilled water system.

The thermal energy storage tank was sited to be near an industrial area, but out of the way of City Campus. The tank’s location is south of the Devaney Sports Center near a concrete plant and grain elevators. To move chilled water to and from this location, twin 36-inch water mains need to cross North Antelope Valley Parkway and Antelope Creek. The section of parkway to be crossed consists of an elevated roadway including a mechanically stabilized earth wall. The creek was part of a large flood control project partially funded by the United States Army Corps of Engineers (USACE). Because of this funding, USACE had jurisdiction to review and approve the proposed water main crossing. The creek is also managed by the Lower Platte South Natural Resources District (LPSNRD).

Several methods of crossing were considered: open cut trenching, aerial crossing attached to an existing or new bridge, and trenchless installation. Open cut trenching was deemed unfeasible because of site constraints, potential impacts to the roadway, USACE 404 permitting requirements, and costs. Aerial crossing was a reasonable alternative but would potentially impact the flood plain. As an exposed pipe, it would be a security risk to flooding or vandalism. It would also impact aesthetics for adjacent bike paths and pedestrian bridges, and appeared to be slightly more expensive than trenchless installation. Trenchless installation by method of Horizontal Directional Drilling (HDD) was evaluated and determined to be the most economical solution, but it required a USACE 408 permit.

HDD consists of a pilot rod being pushed through the ground along a set horizontal and vertical path. The pilot rod needs to be on the mark for the installation to progress. Once the pilot rod is in place, a series of reamers are pulled through to expand the hole to a size larger than the utility being installed. Finally, the hole is cleaned and the pipe is pulled through the hole. The process includes bore gel to stabilize the hole and prevent collapse during the reaming and installation process and grouting of the space between the pipe and the hole after installation.

It was important to understand the exact USACE requirements for the crossing and the 408 permit submittal. A meeting was held with all involved parties including the LPSNRD, UNL, Olsson, and USACE. The HDD installation required a significant geotechnical evaluation to determine the required depth of bore to prevent fracking of drill fluid (migration of the fluid to the ground surface), to determine acceptable grout pressures, and to complete the 408 process. The crossing was designed to be 30 feet below the flow line of the creek and used a combination of data collected in the field, the geotech evaluation, and a Vermeer BoreAid software package. Information from the design was compiled into a 408 permit report and submitted to the USACE. The permit was approved with minimal comment.

The reaming process for the first 36-inch high-density polyethylene (HDPE) pipe installation started the week of May 8, 2017. After several reaming passes and cleaning of the hole, the first pipe was pulled in on May 18. The preparation for the second pipe began on May 19 with the second 36-inch HDPE pipe pull completed on May 26.

Every project has challenges and solutions. Trenchless technologies are ever evolving and should be considered as alternatives when site limitations, disruption to public access, or surface restoration requirements become prohibitive and costly. When considering trenchless technologies, one must consider permitting and regulatory authority requirements. It is advantageous to meet with these authorities up front and gain a complete understanding of the requirements so the design and permitting process can be as efficient as possible.

If you have any questions, call me at 402.458.5693 or email Ben Day at jday@olssonassociates.com.  

  

 



Ben Day, PE

Civil Engineer

Ben has extensive experience in project management, planning, design, and construction phase services for water/wastewater projects and municipal civil engineering projects. His focus is in water and wastewater system designincluding water conveyance and trenchless technologies. He is well versed in hydraulic modeling and water and wastewater system studies and master planning.

  

 

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