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Capital Square Case Study

CSP Offers Storm Sewer Solution

CSP system maximizes usable lease space, enhances site aesthetics, provides long service life

By Mike McGough, P.E., Chief Engineer, NCSPA and Keith Fraase, Sales Engineer & Marketing Manager, Johnston Fargo Culvert, Inc.

When Lexstar Development and Construction began development on a new $17.5 million commercial office park in Fargo, N.D., top goals for the complex included maximizing usable lease space while implementing a strong aesthetically-pleasing design that would lure top businesses along the I-94 and I-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” diameter HDPE detention pipe. However, after viewing several poorly maintained and unattractive storm water detention ponds near the proposed location, Lexstar asked their design firm Ulteig Engineers to find alternative products that would meet their project goals to improve the aesthetics for future tenants as well as better utilize the site space.

Flexible CSP Provides Underground Solution

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, while also meeting a hydraulic head limitation that restricted detention pipe from exceeding 66” in height.

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

Another site challenge that made 78” diameter CSP an attractive solution was a cut of nearly 8’ 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’ 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 this owner originally desired for the site.

When installing this CSP system, the contractor discovered the pre-fabricated manholes, storm drain and large diameter arch detention pipe connected with ease. The pre-fabricated manholes complete with lifting hooks are lightweight and attached quickly to other pipe conduit. The large diameter detention pipe was also equipped with lifting hooks that provided a method to safely lift the 65’ 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 cubic feet 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” 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” diameter pipe and pipe stubs that connect the storage system were banded together with 24” 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.

The general contractor Kindred Plumbing and Heating (KPH) soon found out numerous other benefits of a corrugated steel pipe 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.

Placing all of the site’s storm water storage underground also added 25 percent more usable lease space with the addition of two new building units, and 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.

At the end of the day, the corrugated steel pipe system:

  • added 25 percent more usable lease space
  • greatly enhanced the aesthetics by eliminating the pond and adding a park
  • provided greater ease of installation
  • allowed the contractor to better utilize labor
  • increased the service life of the project to a minimum 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 & marketing manager for Johnston Fargo Culvert, Inc. Fraase can be reached by e-mail at kfraase@johnstonculvert.com.

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