As previously discussed in our “Construction Cover vs. Post Construction Cover” blog post, the total cover required for a system is the combination of the pavement, measured from the bottom of flexible pavement or top of rigid pavement, the granular road base, and the backfill material. But is 12” really enough post construction cover for your CMP detention system? The answer is YES! (Most of the time).
When designing an underground detention system, depth is your best friend. Utilizing all the depth available on-site can reduce the overall price per cubic foot; this means smaller system footprints, less material, and happy customers. With this focus on depth, the total cover requirements for a system need to be brought into the discussion. Too often has the conversation occurred where it’s assumed 2ft, 3ft, or even 4ft of cover over is needed for a CMP system. Using more cover than is required eats into the available depth and negates all the benefits of a smaller system size. In reality, for a 96” diameter or less system, only 12” of cover is needed. Now, where do these cover values come from, and what applications do they apply to? For the design of most underground CMP systems, we turn to the Corrugated Steel Pipe Design Manual (NCSPA | National Corrugated Steel Pipe Association). As mentioned, 12” of cover is the answer MOST of the time. Things like diameter, loading, etc., can influence and often increase the cover requirement
Another factor that impacts the total cover is pavement type. The road base section can be included within the total cover requirement. It is generally suggested to have 6” of free-draining angular washed stone above the pipe before the road base. However, this can be adjusted to incorporate the 6” of stone into the road base. If a rigid pavement like concrete has been specified, the pavement thickness can be included within the total cover requirements. If a flexible pavement like asphalt has been specified, the cover would need to be measured to the bottom of the flexible pavement.
Ultimately, in designing a CMP underground detention system, the cover requirement tends to be less than anticipated. Taking the time to review total available depth, pavement type and cover requirements can save time and money as a project is designed. Contech offers a variety of tools, including our Design Your Own Detention System Tool, that can help determine the optimal system for your project. Reach out to a Contech Stormwater Consultant can also help answer the question of “Is 12” of cover really enough?”