Equipment Type

"Gravi-Force" Project Provides Lowest Cost

The city of Lincoln, Neb., has outgrown infrastructure systems for critical areas of the community. To solve the problem the Lincoln Wastewater System (LWWS) undertook a strategy to accelerate development of the utility infrastructure, nearly doubling the rate for the affected areas. They determined that their most critical need was to upgrade their wastewater collection system to prevent overf...

February 11, 2008

The city of Lincoln, Neb., has outgrown infrastructure systems for critical areas of the community. To solve the problem the Lincoln Wastewater System (LWWS) undertook a strategy to accelerate development of the utility infrastructure, nearly doubling the rate for the affected areas. They determined that their most critical need was to upgrade their wastewater collection system to prevent overflows both now and in the future. Their secondary need is a system that will accommodate future wastewater flows resulting from development to full build-out capacity.

There were restraints on the design slope of the sewer and therefore field control of grade was critical.

LWWS developed a Wastewater Facilities Master Plan in conjunction with the Lincoln-Lancaster County Comprehensive Plan to proactively address wastewater issues and assure that the city is well prepared to meet wastewater service needs when they occur. These plans identified future utility needs for 25 years (Tier I) and 50 years (Tier II) using population projections provided by the Lincoln-Lancaster County Planning Department.

The project delivery method was to design, bid, build. Kent Prior, P.E., project manager for HDR Engineering of Lincoln, and Scott Aurit, P.E., HDR project engineer, presented the concept of "gravi-force" during the interview for the project. The LWWS engineer, Brian Kramer P.E., looked at the Capital Improvement Plan (CIP) for the city and the proposal by HDR for the project. Previous recommendations to LWWS were to design a gravity sewer in 2004 and add a second parallel sewer in 2024. These sewers would be parallel to an existing sewer that was constructed in 1974, so the final solution would include three parallel sewers.

The project started when LWWS issued an RFP. HDR proposed and was selected based upon their innovative approach to design the North 31st Street Lift Station Outfall Improvements, a vital part of the system. HDR's creative design proposal to LWWS was to construct a "gravi-force" main to achieve the full range of capacity with a single cost-effective project.

HDR presented two alternatives as a solution to the systems capacity problem. Alternative one, for a single gravity outfall sewer along with the existing sewer, could meet existing flow capacity requirements. In the future, this new 36-inch gravity sewer main would be surcharged in keeping with the "gravi-force" concept. This would increase the capacity to the levels necessary to meet future requirements.

"Gravi-Force"

The "gravi-force" concept allows the new sewer to operate under gravity flow conditions until 2024, when flows become greater than its capacity. Then it is allowed to surcharge to become a force main, creating additional capacity. The concept, which exhibited creative planning for the long term, was what won the project.

In addition, the "gravi-force" concept optimizes construction costs. The design is a gravity sewer system sized for near-term capacity that can be pressurized in the future to function as a force main. Aurit said "We can achieve a capacity sufficient for the area's requirements after build-out, when we operate it in the 'force main' mode."

HDR analyzed the "gravi-force" concept alternative utilizing the existing 24-inch force main beneath Salt Creek to deliver flows for Tier I to a new diversion structure which would be constructed to replace the existing receiving manhole. The diversion structure would split flows between the existing 24-inch gravity sewer and a proposed new sewer. A new 36-inch sewer would then extend south along an abandoned railroad right-of-way, where a second diversion structure would be constructed to convey flow to either of the existing 30-inch or 42-inch trunk sewers. A second 24-inch force main would need to be constructed in approximately year 2036 beneath Salt Creek to convey Tier II flows to the diversion structure and through the existing 24-inch gravity sewer and proposed 36-inch "gravi-force" sewer.

A second alternative for a force main analyzed by HDR would utilize the existing 24-inch force main beneath Salt Creek for Tier I flows. A new 24-inch force main would connect to the existing force main and extend south along the abandoned railroad right-of-way to a diversion structure that would be constructed to convey flow to either the existing 30-inch or 42-inch trunk sewer. A second 24-inch force main would be needed in approximately year 2015 from the Lift Station to the existing 30-inch trunk sewer to convey Tier I and Tier II flows.

Alternative two would require a new 24-inch force main immediately and a second 24-inch force main in the future. LWWS chose alternative one for the "gravi-force" sewer.

Aurit explained, "As population growth occurs within a service area, upgraded and expanded facilities are required to meet service needs. The timing associated with future system improvements within each drainage basin depends on the rate of development within the basin. An important part of the project, the Little Salt Creek drainage basin, is a growing service area, which includes over 3,000 acres of developed land and will ultimately be 5,300 acres in all.

"In addition, several capacity-related problems existed with the current collection system downstream from the 31st Street Lift Station.

"We eliminated the need to construct a parallel gravity sewer or force main now, followed by a third in the future, which would be in a more congested area and at an inflated cost. The 'gravi-force' main project solution accomplishes the same effective result with only a single project constructed now that delivers a substantially lower life-cycle cost." A single, larger gravity flow system was not considered due to the congested area and the presence of other utilities.

Based upon the parameters evaluated, Kramer concurred that a new outfall sewer could be constructed to convey flows along the abandoned railroad property and along State Fair Park Drive right-of-way to the existing 30-inch and 42-inch trunk sewers. This route would maximize utilization of the existing capacity in the 30-inch and 42-inch trunk sewers and eliminate the need to have a new sewer constructed to convey flow all the way to the Theresa Street WWTF. The higher elevation of the railroad grade provided sufficient cover and permitted pipe slopes needed to move the flows into the existing 30-inch trunk sewer.

Only Pressure-Rated Options

LWWS considered only pressure-rated products for the line, which would ultimately operate as a low-head force main system. The specified pipe options included HOBAS CCFRPM, PCCP, SW-HDPE and C-905 PVC. The installation contractor, Roloff Construction Company, of Omaha, Neb., chose HOBAS because of its overall economic advantages, the long 20-foot joint lengths and its simple gasket-sealed joint.

"On a direct bury project like this one, we needed a simple slip joint," said Monty Habrock, project superintendent with Roloff. Once the line was installed, the pipe was tested. "The air test held so well that we decided to plug the manholes and test up to 2,000 linear feet at a time and it never dropped even a pound," Habrock added. The entire line was pressure tested to 6 psi for a duration of two hours without a single leak or pressure drop.

Even though Habrock had never installed HOBAS pipe before, he found it user-friendly and easy to install. "HOBAS field service personnel contacted us several times during the job to offer assistance and they visited our job site, but we never needed their help," admitted Habrock.

"The HOBAS FWC coupling is utilized for both gravity and pressure installations. Even the so called 'gravity' couplings easily withstand 50-psi internal pressure and a similar coupling design can be used in pressure systems operating up to 250-psi," said Rene Garcia, senior engineering associate with HOBAS in Houston.

Lowest Total Cost

HDR developed capital costs for the sewer alignment alternatives. The lowest project cost to meet the current need was alternative two. However, this would not have delivered the lowest life-cycle cost. To determine the lowest life cycle for the project, LWWS believed it was necessary to use a projected growth rate, projected flows and an estimate of the present value of both alternatives. The calculations, based on present values, showed that the "gravi-force" sewer option would have an overall saving of $300,000 compared with the force main option.

The project is now completed, and the complete construction cost was approximately $1.5 million. The savings resulted primarily from not having to install a new sewer 20 years down the road. Major redevelopment will be occurring in this area for the next 10 years, thus the $300,000 is likely an understatement of the total cost savings. The project was completed within the CIP budget the city set for the first sewer project, thus the city received a 50-year solution for the cost of one project versus the planned two projects.

Apparently, this is the first identified application for this concept. The authors could not find other documented instances where a utility had designed specifically for this concept. Construction on the project was begun in 2005 and completed within nine months and is operating successfully. The improvement project for the "gravi-force" main was accomplished within the original budget proposed for the Tier I flow condition, i.e., a 50-year solution was provided for the same budget as the proposed 25-year solution.

Garcia said, "HOBAS pipes have been chosen for many projects such as this Lincoln installation where the decision was based on the total installed cost. HOBAS pipes' inherent corrosion resistance produces a long service life. In typical sewer environments, the design life is 100 years. The quality, materials and sophisticated manufacturing produce consistent, reliable pipes."

For more information, please contact HOBAS at (800) 856-7473 or visit www.hobaspipe.com.

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