A Storm Sewer Vision

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When Lexstar Development and Construction began development of a new $17.5-million commercial office park in Fargo, N.D., top goals for the complex included maximizing usable lease space and implementing a strong, aesthetically pleasing design that would lure top businesses along the Interstate 94 and Interstate 29 corridors.

In the project’s original designs, the storm water system included a traditional above-grade storm water detention pond, concrete manholes and 60-in.- diameter high-density polyethylene detention pipe.

After viewing several poorly maintained and unattractive storm water detention ponds near the proposed location, however, Lexstar asked its design firm, Ulteig Engineers, to find alternative products that would meet their project goals.

Challenges & Solutions

Johnston Fargo Culvert, Inc. experts worked with the designer on a large-diameter corrugated steel pipe (CSP) system, complete with prefabricated corrugated steel manholes, small-diameter storm drain and large-diameter underground detention pipe. The great diversity and flexibility of CSP products allowed the design of an underground detention system that easily compensated for the loss in storm water storage after removing the above-grade pond option. It also met a hydraulic head limitation that restricted detention pipe from exceeding 66 in. in height.

Overcoming this height restriction was accomplished by using 78-in.-diameter equivalent corrugated steel arch pipe. After the aching process, the 78-in.-diameter equivalent arch pipe dimensions were established to be 87-by-63 in., well within the height restriction. By using 1,200 ln ft of the 78-in. arched CSP, designers were able to achieve an storage capacity equivalent to that of the original design (approximately 40,000 cu ft).

Another site challenge that made 78-in.-diameter CSP an attractive solution was a cut of nearly 8 ft across the site (invert of detention system to final surface pavement elevation). This depth allowed a minimum fill above the pipe to at least 2.5 ft over the majority of the CSP. The larger pipe filled more of the excavation, allowing the native soils to be used exclusively for site balancing and landscaping. The CSP option created an opportunity to complete the vision the owner originally desired for the site.

Installation

When installing this CSP system, the contractor discovered the prefabricated manholes, storm drain and large-diameter arch detention pipe connected with ease. The prefabricated manholes, complete with lifting hooks, are lightweight and attached quickly to other pipe conduit. The large-diameter detention pipe also was equipped with lifting hooks that provided a method to safely lift the 65-ft-long sections from the transport trailers into a staging position.

A combination of 22 manholes and inlets will collect the surface drainage for the site. As storm water accumulates in the 40,000 cu ft of underground detention, storm water is released into the city of Fargo storm water system at a predetermined rate. The entire corrugated steel pipe storm water quality system was designed to release its contents into the city storm sewer though a 12-in.-diameter outlet.

An internal CSP expanding band and mastic joint compound was used to connect the manhole to the outlet conduit. This outlet conduit was bored under an adjacent street to reach an existing storm water manhole. The remaining 18-in.-diameter pipe and pipe stubs that connect the storage system were banded together with 24-in.-wide, two-piece bands with a neoprene gasket. These positive joints resist pipe separation, and the silt-tight gasket minimizes infiltration of the granular bedding and backfill.

General contractor Kindred Plumbing and Heating soon found numerous other benefits in a CSP storm sewer system. With this long, large-diameter pipe, the contractor was able to keep two excavators busy on the project during installation. One of the excavators was used for excavating and the other was used for placing and backfilling, providing a more economical and faster installation.

Project Goal Checklist

Placing all of the site’s storm water storage underground added 25% more usable lease space, allowing for the addition of two new building units. It also greatly enhanced the aesthetics of the site by eliminating the unsightly pond and providing space for a park. The centrally located park includes a waterfall and picnic areas, and a gazebo is planned as expansion progresses over the next construction season. The additional building units are to be constructed over a period of time as tenants occupy the completed space.

In addition, Lexstar was sold on the service life provided by the 12-gauge Aluminized Type 2- coated steel. This coated steel will provide a minimum service life of 100 years. A normal service life cycle in this area of the country is 50 years.

The corrugated steel pipe system:

  • Added 25% more usable lease space;
  • Enhanced aesthetics by eliminating the pond and adding a park;
  • Provided greater ease of installation;
  • Allowed the contractor to optimize labor; and
  • Increased the service life of the project to a minimum of 100 years.

Having a vision to investigate alternate products for storm water storage and conveyance made all the difference for this developer.

Mike McGough, P.E., is chief engineer for the National Corrugated Steel Pipe Assn. McGough can be reached by e-mail at mmcgough@ncspa.org. Keith Fraase is sales engineer and marketing manager for Johnston Fargo Culvert, Inc. Fraase can be reached by e-mail at kfraase@johnstonculvert.com.

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