By Michael D. Kusch
How stormwater detention systems are designed and installed determines compliance with the objectives of The Clean Water Act(1). As stormwater regulations tighten in the months ahead, along with many new municipal combined sewer overflow (CSO) projects, precast concrete underground products – compared with competing materials – are the best choice for the environment. This study of the installation of a stormwater detention system in Tennessee is a case in point.
A four-barrel, 96-in.-diameter reinforced concrete pipe (RCP) was designed for a Lexus automobile dealership in Nashville, Tenn., as part of an Underground Stormwater Detention System (UGD). Each of the barrels is 160 ft long and connected with RCP and a precast concrete junction-box manifold system. The barrels were connected with 24-in. equalization pipe. Stormwater had to be detained on site then drained very slowly through a quality stormwater unit. The system had nearly 33,000 cu ft of stormwater storage capacity.
So what is the need for this well-designed UGD system? Easy answer: It’s the law to protect the environment!
Environmental stewardship informs stormwater designs
Stormwater is rain runoff from surfaces such as rooftops, paved streets, highways and parking lots. As water runs off these surfaces, it can pick up pollutants such as oil, fertilizers, heavy metals, pesticides, soil, trash and bacteria. From here, the water might flow directly into the local watershed comprised of streams, creeks, rivers, lakes or wetlands. Or, it may go into a storm drain and continue through storm pipes until it is released untreated into these waterways.
In addition, large, impervious surfaces in urban areas, like this Lexus dealership site, increase the quantity of peak runoff flows. These in turn cause adverse hydrologic impacts such as scoured streambed channels, in-stream sedimentation and loss of habitat. Thus, underground precast UGDs, such as the one discussed here, are used to pretreat and slow the discharge volume of stormwater to an acceptable, healthy level.
It is good environmental stewardship to control the quantity and quality of stormwater discharge from developed sites to protect human life and property. Responsible stormwater designs help to maintain overall watershed quality and comply with EPA, state and local government regulations under The Clean Water Act.
How to compare RCP with CMP stormwater systems
After the Lexus project was bid, an alternative design was presented to the general contractor by the flexible CMP (corrugated metal pipe) distributor. Because this CMP distributor considered only volume capacity in the design, this alternative had lower capital costs.
To strengthen the benefits of the engineer’s original RCP design, Sherman-Dixie Concrete Industries presented a valid analysis (comparison) of a CMP versus an RCP detention structure. The general contractor needed to compare the options of the two pipe materials based on sound engineering – not on a bid’s price point or a polished sales pitch.
The comparison focused on service life versus design life and the performance of materials according to design standards. It is not enough to consider only stormwater storage capacity. Other critical factors include structural integrity, load-bearing capacity and maintenance costs over the design life of the system.
Post-installation CMP durability and strength considerations
To determine durability of CMP, you 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 (acidic) soils surrounding the pipe. The “Handbook of Steel Drainage & Highway Construction Products”(2) contains very helpful information for selecting the proper gauge of steel and protective coating. To determine the structural strength of the CMP system, you must evaluate the gauge of the pipe along with the type and placement of the backfill needed to help the flexible pipe attain its design strength and ability to carry the load.
- Steel gauge – The gauge of the steel determines pipe stiffness. All steel will rust. A heavier gauge (greater wall thickness) is required to allow the pipeline to function for its design life. The supplier of the alternative pipe material suggested 14-gauge (very thin) steel and only a plain galvanized (the minimum) coating. Who would want to risk millions of dollars worth of Lexus inventory parked over a huge paved footprint of 96-in. diameter flexible CMP? It is a fair question, and one that Sherman-Dixie proposed to the project’s engineer of record, John Gore, P.E.
- Service life – A conservative assumption of a soil pH of 6.0 and a resistivity of 5,000 Ω-cm indicates that 16-gauge, galvanized metal will last 20 years before the first sign of perforation (small holes through the steel caused by abrasion). If the design life is 50 years, a 10-gauge galvanized metal thickness is required for a design life of 46 years (multiply 2.3 times 20 years).
- CMP installation requirements – The CMP structure proposed would require the extra cost of additional 12-in. depth of crushed stone over the top of the pipe. By comparison, the built-in strength of RCP would require a crushed-stone backfill only to the spring line – or center – of the pipe, and would provide the owner with RCP service life of 100 years. The CMP proposal was under-designed and destined for replacement long before a stronger and more durable 100-year precast concrete UGD. For the Lexus owner, premature replacement would translate into replacement costs and loss of business.
- EOR’s decision – A report from the RCP producer supported the engineer’s original design for a precast concrete stormwater system by detailing the required CMP design specifications mentioned above. Because these required specifications were not originally considered in the CMP bid, the engineer of record denied approval of the CMP submittal.
Consequently, Sunrise Contracting of Nashville installed 644 ft of 96-in.-diameter RCP, supplied by Sherman-Dixie Concrete Industries, in about 12 hours.
Note: A similar version of this article appeared in Stormwater Solutions in April of this year: http://www.estormwater.com/calling-reinforcements
Michael D. Kusch, technical marketing director for Sherman Dixie Concrete Industries, Nashville, Tenn., has a bachelor’s degree in environmental design and has been involved in the promotion of precast infrastructure products with Sherman-Dixie Concrete Industries for 21 years. Contact him at [email protected]
Stormwater Regulations Tighten
As state and national stormwater regulations tighten in coming years, designers will be charged with designing sites that capture and hold the first inch of rainfall and keep it on site. This opens up similar projects – like the Lexus case study – for underground detention or retention systems manufactured of precast concrete pipe, precast box culverts or precast vaults.