Pond Embankment Design

[lindbls]

I will have a stormwater pond constructed on gently sloping ground, bounded in part by a soil embankment, the other sides will be cut into existing ground. The interior and exterior slopes need to be between 3:1 and 4:1 to obtain the storage volume I need and the max slope for safety reasons (per our local drainage manual). The top of the berm is expected to be 6 feet wide at minimum. The max water depth will be about 4 feet, with one foot of freeboard.

How extensive of a design is typical for such a soil embankment? Or, following "standard guidelines" (of which there may be some??), is a detailed design not typically performed (e.g., flat slopes, wide crest, low berm). If a detailed design is standard practice,(which I'm suspecting it is) what are some of the more common standard design approaches?


(I work in a goverment office that is short on experienced engineers and long on managers...it provides nobody to bounce things off of.)

 

[bushel3]

Does anyone at your firm know how to use a slope stability program? If so, it should not take too long to do a quick check of the slope. Given the gentle slopes that you are considering I would pick a slope that you'd like to use and check it using the program. I would estimate values using conservative values for c and phi based on other testing that you have done in the area on siimlar soils. If no one in your firm is fimilar with the slope stability program you could check the slope using Talyor stability numbers. After your quick checks if the F.S. you get is very high (over 1.75 to 2) then I would not do any further study. If it is lower I would refine the study.

In my eperience working with the good glacial till soils in Chicago the quick check will show factors of saftey well over the F.S.'s given by the Corps of Engineers for all of the loading conditions. That may or may not be true else where.

 

[cvg]

get a copy of the USBR "Design of Small Dams" which will give you a wealth of advice useful for your project. If you want FEMA to recognize your reservoir, make sure the embankment is certified to meet their requirements.

 

[SlideRuleEra]

"Design of Small Dams" that cvg recommended, and other good information, is available for download (free 65 MB .pdf) here

http://www.usbr.gov/pmts/hydraulics_lab/pubs/index.cfm

It is listed under "Engineering Manuals"

http://www.SlideRuleEra.net

 

[GeoPaveTraffic]

If I read the orginal post correctly the total height of the "dam" will be 5 feet, maybe a little more due to the slopeing ground. While you need to pay attention to any dam, a 5 foot dam requires very little analysis. Unless the soil conditions are very poor a 3H to 1V slope 5 feet high is stable by inspection.

The most important item to pay attention to is the outlet structure. Make sure it is big enough to pass a significant design storm, you only have 1 foot of freeboard. You may want to armor a slightly lower section to serve as an emergency spillway.

 

[saruman26]

I agree with GeoPaveTraffic. For projects like this where a detention basin is to be built, standard details and design specifications for soil compaction, soil composition, etc are acceptable. Just make sure your plan clearly states that the soils are to meet the required specs and that a minimum 95% compaction is achieved as measured by Standard Proctor or any other applicable tests.

Also, an emergency spillway is a must have for any basin. Size it to pass the complete 100-Year design flow and protect the spillway embankment (both sides and top) from erosion. This can be accomplished with riprap, concrete checker blocks, mechanical turf stabilization products (i.e. North American Green products or similar).

 

[bushel3]

I agree with Geopave. If you are not positive the slope is going to be okay I would perform one of the quick checks that I mentioned in my original post. That should set your mind to rest and provide a little something for the file if anyone ever asks.

 

[lindbls]

Thanks for the great feedback. Unfortunately, the downstream slope of the embankment will be 2H:1V, not 4H:1V as mentioned in the original post. The upstream side of the pond will still be 3H:1V. A pretty important "typo"!!

The embankment constructed using DOT Standard Specs for Select borrow, compacted to 95%. The embankment will have a key dug about 4ft below existing grade, 10 feet thick.

While I'm focused first on embankment safety, I also don't want go so far overboard being conservative that it costs 3x what it needs to. The 10ft thick "key" seems mighty big for this size embankment. The key size was based on design guidance from a stormwater manual based on designing storm ponds.

I'll take a look using the Taylor chart and see what FS I come up with.