CSP Detention System Goes Vertical
Unique CSP double-tiered detention maximizes usable space, meets cost and timing needs, provides long service life for retail center.
By Mike McGough, P.E., Chief Engineer, NCSPA and Mark McCord, Civil Engineer, Southeast Culvert, Inc.
When development of the East Village retail center project began, detention was a major point of debate. This site, just north of Atlanta in Roswell, Ga., is in a highly developed corridor in need of some redevelopment. Haines, Gipson and Associates, the engineer of record, considered several options for the detention during the design process. The original design was a concrete vault, but cost and timing brought other ideas into the arena. Offsite drainage to adjacent ponds, chambered systems, and the concrete vault came with a slew of issues such as cost, footprint size, drainage easements, permitting, and construction time.
During a value engineering session with the general contractor, a corrugated steel pipe (CSP) detention system was proposed. As a result of the volume required for storage, the system quickly grew and expanded outside of the useable area for detention, but due to a considerable elevation change to the outfall, the idea of a tiered CSP detention was brought about. When the smoke cleared and all of the numbers were on the table, the tiered CSP was the most feasible.
The material cost of the CSP system reduced the initial budget by approximately $200,000, not to mention time and labor costs that were saved by choosing a system that was quick and easy to assemble, while staying within the originally designed footprint. Simply stated by Scott Samuel of Haines, Gipson and Associates “at the end of the day, the pipe system won out in cost and timing.”
CSP East Village Detention – By the Numbers
The final detention design for East Village was a tiered 144”, 10 gauge, Aluminized Steel Type 2 CSP detention system. With timing as a major issue, Southeast Culvert, Inc. of Winder, Ga. went to work. The project required in excess of 800,000 pounds of steel. With the help of AK Steel, SEC, Inc. was able to procure the balance of the material needed to complete the project. From the arrival of the steel, Sec, Inc. spent three weeks producing the pipe and fabricating the required fittings for the detention.
Structural reinforcement measures were taken to insure that the bottom system maintained its integrity. SEC, Inc. performed Finite Element Analysis of the manifold systems to ensure long-term structural performance under dead and live loadings. Once fabrication was complete SEC, Inc. took on the sizeable task of delivering 3,500 feet of 144” pipe to the job site. The system arrived on time and the contractor spent the next two months installing the pipe in conjunction with a massive retaining wall being constructed next to the tiered system. The detention installation was slowed dramatically due to the close proximity of the wall but total time spent on construction of the system was approximately two weeks.
The detention system is designed with 3 feet of backfill separating the tiers. The top and bottom systems are offset to distribute the loading and to allow access risers to the bottom system. The lower system experiences less loading than a single system at the same depth because the pipes above cause less loading than the typical +/- 120 pounds per cubic foot of compacted soil. The contractor backfilled the systems using a quarry sand in order to achieve acceptable levels of compaction. The pipe joints were wrapped with a geotextile fabric to eliminate any risk of infiltration of the backfill. Due to the contractor’s diligence and careful monitoring by Southeast Culvert, Inc. the installation went smoothly and, most importantly, met all expectations.
Benefits of CSP Exceed Project Needs
The strength and flexibility of the CSP system made it an attractive option. 144” diameter, 5” x 1” corrugation profile, 10 gauge pipe is structurally capable of accepting up to 39 feet of soil cover assuming that sound installation practices are used. In addition, strides in material understanding and technology over the last few decades have produced more durable products. Studies show that 10 gauge Aluminized Steel Type 2, installed in normal soil PH and resistivity, will provide a service life beyond 100 years.
An often overlooked advantage of using CSP is the well-documented recycled content in every foot of pipe manufactured, a bonus for LEED certified projects. The combination of strength, flexibility, durability and economics made the CSP detention a difficult product to compete with.
Today, construction of East Village is complete and new projects are emerging around it, breathing new life into an area in need of smart development. Thanks to a few progressive thinking engineers and contractors, East Village has a functional and durable detention unlike any other.
Mike McGough, P.E., is chief engineer for the National Corrugated Steel Pipe Assn. McGough can be reached by e-mail here. Mark McCord is civil engineer for Southeast Culvert, Inc. McCord can be reached by e-mail here.