detention structure walls, surcharged, on clay...

[scherry]

I have some detention structure walls. they are not particularly tall less than say 7 feet to the base. They are retaining filled earth over fat and lean clays, over moderate to highly weathered limestone. the general management of the soils for the building is to remove and replace some or all of the clay. Of course that is not practical for the paved areas. one of the walls is surcharged by auto parking. the civil interpretation of the drainage condition for the wall (in this office)is that it will not get water behind it because the paving causes a fast runoff into the detention pond. OK. Except I am not sure I completely believe that. my first question is do I design these as undrained... according to the soils report there was no water found but they point out that this type of soil structure may have perched water. If i design them as drained, then theoretically when I have my detention period i will allow water to get into the backfiull behind the wall. which seems undesirable...I also have another wall which at this point appears not to be a retaining wall, but a freestanding wall, presumably with the sole purpose of making the detention pond, thus I would tend to design it for wind seismic, and or the water during the maximum flow event. no worries about drained or undrained. I guess my other question is does there need to be protection for the base of the walls. water is entering the detention basin from two directions over the walls. I have been assured that the maximum detention period is very short - a matter of minutes even, not hours. and i guess, answering my own question, if they leave the clay soils in place at the bottom of the detention pond the percolation is so low that the water should have flowed out before it causes any problem with my foundation.

Right?
any help or information about standard proceedurs and requirements for this type of structure would be greatly appreciated.

 

[cdh61]

scherry,

1st, I would not rely on no water collecting under the parking slab. You will have to do a site assessment around the parking lot since subsurface seepage can occur through the sides. I have encountered lots of parking lots with paved surfaces and encountered significant water in the underlain granular base materials after rain storm events.

2nd, drains in higher retaining walls are typical and a good practice to reduce the potential of porewater pressure.

3rd, you should evaluate erosion at the base of the wall if water is going to be running over the wall even if the runoff will only be for a short period of time.

4th, you should call the geotech engineer to discuss the above since misinterpretation is not a good thing. Also you could then obtain additional advice related to your questions from the person who completed the geo-investigation.

regards

 

[Focht3]

Good advice. Definitely call the geotechnical engineer -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[cvg]

with retaining / flood walls expected to retain water, it would be good practice to assume that soil behind the wall could be completely saturated.

also, as far as erosion at the base of the wall, the civil should probably evaluate and address that part of the design (with data obtained from the geotech). standard practice would require calculation of scour depth and construction of a toe down or cutoff wall or placement of some sort of erosion protection