Design flexibility meets long-term solutions

December 2009 » PROJECT CASE STUDY
Underground detention system and rehabilitation project provide long-lasting solutions
Michael McGough, P.E.

Projects
East Village stormwater detention, Roswell, Ga.; box culvert repair, Carroll County, Mo.; and service life study, Greene County, Ill.

Participants
Haines, Gipson, and Associates
Missouri Department of Transportation
National Corrugated Steel Pipe Association

Product applications
Aluminized Steel Type 2 corrugated pipe provides cost-effective and long-lasting solutions in stormwater detention and roadway culvert applications

The final design for East Village was a two-tiered, 144-inch-diameter, 10 gage, Aluminized Steel Type 2 corrugated pipe detention system.

In today’s highway construction industry, designers are not only trying for the most economical and sustainable solutions, but many are still looking to the right side of their brain for unique project designs and quick, safe rehabilitation solutions. When development of the East Village project began in the spring of 2007, detention was a major point of debate. The site is located just north of Atlanta in Roswell, Ga., in a highly developed corridor in need of redevelopment. Several detention options were considered by the engineering firm of record, Haines, Gipson, and Associates. Offsite drainage to adjacent ponds, chambered systems, and the concrete vault all had issues that included cost, footprint size, drainage easements, permitting, and construction time.

Micrometer measurements of pitting on trepan coupons cut from a 50-year-old culvert in Illinois indicate that the projected service life of the 14 gage, Aluminized Steel Type 2 corrugated pipe exceeds 100 years.

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. However, because of a considerable elevation change in the outfall of the system, the idea of a tiered CSP detention was considered. When all of the numbers were on the table, the tiered CSP detentions system was the most economical.

The final design for East Village was a two-tiered, 144-inch-diameter, 10 gage, Aluminized Steel Type 2 (ALT 2) CSP detention system. Once fabrication was complete, 3,500 feet of 144-inch pipe was delivered to the job site. The contractor spent the next two months installing the pipe in conjunction with a massive retaining wall that was constructed next to the tiered system.

The detention system is designed with 3 feet of backfill separating the two 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 load pressure than a single system at the same depth because the pipes above create less pressure than a typical 120 pounds per cubic foot of compacted soil.

The contractor backfilled the systems using a quarry sand to achieve acceptable levels of compaction. The pipe joints were also wrapped with a geotextile fabric to eliminate any risk of infiltration of the backfill.

Today, construction of East Village is on-going and new projects are emerging all around it breathing reviving 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 system unlike any other.

Rehabilitation of a deteriorating culvert
A structurally deficient concrete box culvert on Route 65, a major four-lane highway in Carroll County, in North-Central Missouri was in desperate need of repair and rehabilitation. With horizontal cracking and bowing on the exterior walls, there was concern that the structure would cave in.

Missouri Department of Transportation (MODOT), the project owners, initially considered a design for the project that called for removal and replacement of the existing twin 12-foot by 12-foot concrete box culverts. This option would have cost approximately $379,000, taken two months to construct, and required use of a temporary roadway bypass.

MODOT District Two engineers developed an alternative solution using ALT 2 coated CSP to reline the existing culverts. This solution saved $41,000 in materials costs and cut construction time to only three weeks. The improvements took place inside the existing box culvert structure. Therefore, it was not necessary to build a temporary bypass.

MODOT provided its own labor for this alternative, which exposed its crew to new replacement alternatives and aided in controlling the overall cost on the project. ALT 2 has a long history of proven service life in Missouri and other states. The first installation in Missouri was in 1952, and it is still performing today.

Installation studies demonstrate long service life
Since the 1950s, numerous CSP installations have been the subject of evaluation to establish accurate, predictable service life guidelines. One example is a study of an ALT 2 CSP installation in Greene County, Ill.

This evaluation, conducted 50 years after the CSP was installed, examined the environmental conditions — soil resistivity and water resistivity — along with a trepan sample. There were three soil samples taken from various positions around the pipe, and a single water sample was taken from inside. The results from these samples, along with the overall condition of the pipe, were analyzed to determine the remaining projected service life of the CSP.

Soil sampling results showed a soil resistivity of 1,930 ohm-cm, a pH of 7.0, a chloride count of 40 ppm, and a sulfate count of 68 ppm. Results from the water sample showed a water resistivity of 550 ohm-com and a pH of 6.9. It had been approximately five days since the last rainfall. The overall condition of the 36-inch-diameter ALT 2 CSP was good. The culvert was visually round, had no apparent buckling, and almost no red rust at the 6 o’clock position of the invert.

The trepan coupons were bead blasted to remove loose oxides. Images of the coupons’ remaining surfaces were recorded. Micrometer readings were taken using a ball micrometer to find the average thickness of the original material and a point micrometer was used to determine the deepest pit depth. The overall starting thickness was 0.114 inch. The original micrometer results were: 0.113 inch, 0.114 inch, and 0.113 inch. The deepest pit depth was 0.018 inch, leaving 0.096 inch of remaining material.

Based on conservative pit penetration extrapolations from this Greene County study, the projected service life of 14 gage ALT 2 CSP will exceed 100 years in this type of environment.

These studies prove that CSP can provide design flexibility with economical, long-lasting solutions.

Michael McGough, P.E., is chief engineer at the National Corrugated Steel Pipe Association. He can be contacted at info@ncspa.org.


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