Underground Detention System Concerns 1

[ChipperB]

I am reviewing at set of plans that use an underground detention system to control post development flow. An open field will become a large parking lot. I have concerns because the soils are clay with a high PI. This system is actually more of a retention system because the outlet pipe is 2.5 feet above the floor of the chamber, which is 9" of clean, crushed angular no. 4 stone. There is a 6" under drain system. My concerns is the subgrade will remain wet and effect the bearing capacity of the underlying soil that supports the detention system. Also, a major road is about 15-feet away and parallel to the underground detention and I am not sure if it would be affected also. Does anybody have experience with underground systems like this?

 

[3feethighandrising]

If they are using infiltration for flow control, they need to prove and show how they calculated the design infiltration rate, and have supporting evidence that the soils will support infiltration (depth to ground water below bottom of infiltration system). Many stormwater regulation manuals identify the accepted techniques for calculating infiltration rate and calculating additional safety factors. Test pits to verify groundwater depth are usually mandatory, and a soil sieve analysis is a good idea.

If they are using an underdrain system, where does it drain to, and what prevents short circuiting? This sounds like a StormTech Chamber type of system. In this case the underdrain is really a cleanout.

There also needs to be a draw down calculation for the system, to show how long it takes the system to drain, at the infiltration rate they calculated. If this is a vendor type system, you could notify the vendor and they will be happy to provide supporting calcs, tech, and even a demonstration to their products.

 

[beej67]

I pinkstarred 3feet because he said everything I was going to say.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[EngWade]

The bearing capacity of the soils should only be affected if the underlain soils do not have the capability to infiltrate the runoff. In this situation, the only thing that would affect the effective stress (bearing capacity) of the supporting soils is if an increase in the pore pressure (water pressure within the soil) were to occur.

Since we are talking about a detention system below a parking lot, I'd assume the depths we are using are in the magnitude of 3-8 feet, and with this little head, little pore pressure fluctuations will occur.

Also, the major road should not have an effect so long as the underground system (infiltration units?) is designed to withstand the expected traffic loading (which if these are pre-fabbed units) is an easy check with the manufacturer.