For the majority of applications, corrugated metal pipe (CMP) is the “go to” material for subsurface infiltration. This in turn, leads to a lot of design questions. Below is a list of six questions and answers we received from a recent webinar on designing CMP detention systems.

1.  What are the design considerations for construction beneath buildings and material sorting areas?

Generally, detention systems are designed to be under some sort of flexible pavement / rigid pavement, and Contech prefers not to install them under buildings. However, if a system is to be built under a building, all live load and dead loading will have to be accounted for. This may increase the gage and backfill material of the system. Special attention must be given to the zone of influence of the foundation, and access for maintenance must also be addressed.  These projects should be evaluated on a project by project basis.   

2.  Can you use CMP detention systems in high water table environments?  How do you counter buoyancy in high water table environments?

Yes, CMP detention systems can be used in high water table environments. Tiedowns and special backfill may be recommended to counteract buoyancy issues.

 3.  Is pipe cover normally measured from bottom of asphalt section or to bottom of entire pavement section (i.e. bottom of rock subbase).

CMP pipe cover is measured from the bottom of flexible pavement (i.e.: asphalt) or the top of rigid pavement (i.e.: concrete.)

 4. What is best, a single manifold system or a double manifold system?

It depends on site conditions and what is needed. Each has their benefits, but having a single manifold is more cost effective as it reduces the amount of fabrication needed and can make installation easier, by not having the concern of having the pipe barrels to match up perfectly on both manifolds.

5.  What is the preference for the location of risers - on the edge of the storage pipe or in the center and why?

The only limitation on riser placement is that they cannot be placed on welded seams. Other than that the riser locations are up to the EOR.

6.  Can CMP detention systems be used in coastal environments?

CMP detention systems can be made with different materials and coatings to accommodate different environments and service life requirements. Galvanized steel, Aluminized Steel Type 2, aluminum, asphalt coated and polymer coated options are available for different applications.  For coastal environments we usually recommend CMP pipe made from aluminum because of the potential of salt water and/or more harsh soils. Giving the geotechnical report to your local representative can help you choose the right one for your specific project.