A four-barrel, 96-in.-diameter reinforced concrete pipe (RCP) detention system was designed for a Lexus automobile dealership in Nashville, Tenn. Each of the barrels had a length of 160 ft connected with a precast concrete pipe and precast junction box manifold system. Storm water had to be detained on site, then slowly drained through a storm water quality unit. The system has 33,000 cu ft of storage capacity.
After the structure had been designed by an engineer, an alternative design was presented by a distributor of 96-in.-diameter corrugated metal pipe (CMP). The alternative had a lower capital cost of pipe.
To strengthen the design by the engineer, it was necessary to present design considerations of a CMP detention structure to compare the options based on engineering and not a price point. The core of the design comparison was service life versus design life and the performance of materials according to design standards. It is not enough to consider only storage capacity; other factors include structural integrity, load-bearing capacity and maintenance over the design life of the system.
To determine durability of CMP, one must evaluate the pH and the resistivity of the soil. CMP service life (years to perforation) is a function of delaying inside abrasion and corrosion, as well as the harmful effects from aggressive soils surrounding the pipe. The Handbook of Steel Drainage and Highway Products contains helpful information for selecting the proper gauge of steel and the proper protective coating. To determine the structural strength of the CMP system, one must evaluate the gauge of the pipe, along with the type and placement of the backfill needed to help the flexible pipe carry the load. The gauge of the steel determines pipe stiffness. A heavier gauge (greater thickness) is required to allow the pipeline to function for its design life. The supplier of the alternate pipe material suggested 14-gauge (very thin) steel and only a plain galvanized (the minimum) coating.
A conservative assumption of a soil pH of 6.0 and a resistivity of 5,000 ohm-cms indicates that 16-gauge galvanized metal will last 20 years before the first sign of perforation (small holes abrading through the steel). If the design life is 50 years, a 10-gauge galvanized metal thickness (multiply 2.3 by 20 years) is required to last 46 years. The CMP structure would require the extra cost of additional crushed stone to 12 in. over the top of pipe, while the built-in strength of RCP required crushed stone only to the “springline” to give the owner an RCP service life of 100 years. The CMP was under-designed and destined for replacement long before the 100-year concrete system.
After a report from Sherman-Dixie Concrete Industries Inc. that supported the original detention system design by Barge, Cauthen & Associates, the precast concrete structure was installed by Sunrise Contracting of Nashville. The contractor installed 644 ft of 96-in.-diameter concrete pipe in about 12 hours.
