Restraints on the design slope of the sewer made field control of grade  critical.
Restraints on the design slope of the sewer made field control of grade critical.

The 230,000 residents of Lincoln, Neb., are straining the city's aging sewer system, and there's no relief in sight. The population increased by 30% since the system's last major expansion in 1974, and is expected to double by 2050.

Not only do the managers of the Lincoln Wastewater System (LWWS) have to accommodate their current customers, they have to consider future residents.

To do this, they created a master plan that identifies utility needs for 25 years (Tier I) and 50 years (Tier II) using population projections provided by the Lincoln-Lancaster County Planning Department. Based on the plan, LWWS issued a request for proposal for a design-bid-build project to significantly increase existing capacity and accommodate future waste-water flows to full build-out capacity.

Previous planning recommendations led LWWS to assume the solution would involve two separate projects. Thus, most design proposals recommended a gravity sewer for near-term capacity needs and then adding a parallel sewer 20 years later. Because the two new sewer lines would be parallel to the existing line, the final solution would involve three parallel sewers.

Another Approach

A proposal from Omaha, Neb.-based HDR Engineering Inc., on the other hand, recommended a solution requiring just one project. The concept: a main sized to meet current needs, but that can be pressurized to function as a force main, thus creating additional capacity—and providing the city with a 50-year solution in one step. The gravity sewer system would integrate with the city's existing 24-inch diameter sewer for optimum flexibility. HDR called the design “gravi-force.”

This proposal resulted in an innovative approach for improving the outfall sewer from a lift station that was experiencing capacity-related problems due to an overloaded area located upstream. Though not part of a combined sewer system, this area was prone to receiving inflow and infiltration during wet weather.

The Essence Of "Gravi-Force"

An important part of the plan for growth in Lincoln is the Little Salt Creek drainage basin service area, which includes more than 3000 acres of developed land and will ultimately be 5300 acres in all. Basin flows were exceeding the capacity of the pumping station and outfall sewer.

To accommodate current and projected growth in Little Salt Creek basin, a 36-inch gravity outfall sewer was built alongside the city's existing 24-inch sewer located on the south side of Salt Creek to meet existing and Tier I flow capacity requirements. To split flows between the old and new sewer lines and minimize the flow discharged to the existing outfall sewer, a receiving manhole was replaced with a diversion structure. The new sewer extends along an abandoned railroad right of way, where a second diversion structure was constructed to divert wastewater to existing 30- and 42-inch trunk sewers leading to the wastewater treatment facility.

LWWS engineer Brian Kramer, PE, determined that this route would maximize the existing capacity in the 30- and 42-inch trunk sewers and eliminate the need to build a third sewer to the treatment plant. The elevated railroad grade provides the necessary sloping required to deliver flows into the existing 30-inch trunk sewer.

In the future, the gravity sewer main can be surcharged to increase capacity and meet Tier II flow requirements. “We eliminated the need to construct an additional parallel gravity sewer now, followed by a third in the future, which would be in a more congested area and at an inflated cost,” explains Kramer.

A second 24-inch discharge force main may also need to be constructed under Salt Creek to deliver Tier II flows in a little more than 30 years to the diversion structure of the newly placed outfall sewer system.

Because the main will be later reconfigured into a low-head force main, contractors used pressure-rated pipe, supplied by Houston-based HOBAS pipe USA—the sole manufacturer of centrifugally cast, fiberglass-reinforced, polymer mortar pipe (CCFRPM; see sidebar The Pipe Connection).

Calculating Lifecycle Cost

Before finalizing the project design, HDR analyzed a second alternative for a force main beneath Salt Creek: a new 24-inch force main for Tier I flows, and a second 24-inch force main in about 10 years for Tier II flows. This alternative represented the lowest capital cost to meet current needs, but not the lowest life-cycle cost.

To determine that figure, LWWS used projected population growth and flow rates and an estimate of the present value of each alternative. The calculations showed that the “gravi-force” sewer option would save $300,000 compared with the force main option, primarily by eliminating the need for a second sewer line.

The $1.5 million project was completed in nine months. Because it was completed within the capital budget set for the first sewer project, the city received a 30- to 50-year solution for the cost of a 25-year one.

— Prior is a project manager and Aurit is a project engineer with HDR Engineering Inc. in Omaha, Neb.

Read the sidebar, "The Pipe Connection"