sedimentation pond issues 4

[jyf]

Hi, All

I’m designing a sedimentation pond now and three issues here.

The first one is that client want to built the pond in this winter. Then, he said we don’t need worry about the high ground water table. I am still worry about that. Since the pond is 12 feet deep, the frost depth is about 6 feet. I am worried it will be still some water come out during the excavation.

The second one is client want 3 feet depth compacted clay liner for the pond. Then, the loader can drive in to clean the sediment out. I couldn’t find the recommend compact rate. Anybody knows that?

The third one is client said in their site, clay is very good. He suggest me go 1:1 slope for the side slope of the pond. But the book that I found said 3:1. Anyone did 1:1 before?

That’s the first time that I do the sedimentation pond, any input will be much appreciated.

Thanks.

 

[tsgrue]

.

Where are you located?

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[msquared48]

Important thing to remember here:

YOU are designing the pond and are solely responsible for the function and durability of the system. Do not let the client manipulate your design to the detriment of your liability. This is NOT good engineering.

Mike McCann
McCann Engineering

 

[jyf]

I am in Alberta, Canada

 

[jyf]

thanks,msquared48

One thing that I didn't mention is, that's a coal mine site, the client did a lot excavation jobs. If he said 1:1 slope is OK, then he should did that before.

 

[msucog]

1:1...wow, hope it's rock (or something very close to it)

msquared is 100% correct. i suggest you play CYA with a client like that because if something fails, you'll have your neck in the noose. if you're not sure about slopes or something else, find someone who does and recommend accordingly. document anything that deviates from those recommendations.

 

[PELS]

1:1 would be too steep for the front end loader to drive down into the pond and definitely too steep to get out.

 

[jyf]

Thanks, msucog

I am looking at some documents are said 3:1 is recommend for clay liner. I am not sure why 3:1. I do have experience do 2:1 slope excavation for clay(road, not pond). That works very well. What's your suggestion or experience?

Unfortunately, in our form, nobody has that experience and I will ask some friends to see if they have that kind of experience or not.

 

[jyf]

Thanks for reply, PELS

I am planning to do a ramp for sediment clean up(15% for the ramp). backhole or loader can use that ramp reach the bottom of the pond then do the digging. Is that OK?

 

[PELS]

Yes, 15% works for that purpose. I hope you have enough room to position the ramp outside of the actual pond dimensions, otherwise it will occupy too much of the volume of the pond itself (unless you enlarge the pond). At 15%, you will have to have an elongated ramp leading into the pond. At 12 feet deep, that translates to an 80-foot long ramp.

 

[cvg]

for wet clay, 15% may be slick. we usually stick to 10% max for access ramps.

1:1 may be suitable for excavation slopes, but probably not for the clay liner. How will you place the liner at the steep slope and will the clay slope be stable? A geotechnical engineer could address this design issue for you.

 

[jyf]

Thanks PELS and cvg.

Any body knows compact rate and lift for the clay liner?

I want go 96%S.P.D. and every 6" lift, is that OK?

 

[jartgo]

Why do you need a liner? Normally you wouldn't necessarily be concerned about keeping water in a sedimentation basin.

 

[toones]

jyf
Having personally worked in Alberta, someone building a 'sedimentation' pond with a clay liner, at 1:1 slope seems to me like a fantasy scheme.

It sounds like a evaporation pond is likely what you are attempting to design. In that case, a clay liner built on a 3:1 slope at 95% standard Proctor is pretty much industry standard. Steeper slopes risk failure without conducting a geo materials investigation.

You really need to get a geotechnical engineer involved with this. Alberta has high standards in geotechnical engineering and constructing or designing this pond without geo involvement has very heavy liabilty potential.

 

[jyf]

thanks jthompson and toones.

jthompson
I think liner is a typical standard in industry . for residential we do not need a liner. Correct me if I am wrong.

toones
We do not have a geotechnical engineer in our firm. But since that's not sounds good. I will certainly ask a geotechnical engineer's help for that.

Thanks again.

 

[toones]

I might add that winter conditions is not the time of year to build this. A clay liner constructed in frozen conditions would be useless.

 

[jyf]

hooooops,

toones, why is that? So, we cannot do it in winter?

 

[jartgo]

I see, this sed. basin is part of a treatment process, not just a sediment reducing measure. Thanks.

 

[gravel]

I would suspect, with clay embankment soils retaining pore water longer than pond draindown, you will need at least 2:1 or flatter slopes to prevent slumping & slides. If the face was free draining to keep up with drawdown, then you could maybe go to 1:1 without stability issues.

 

[roncity]

.

Ask the client to show you some existing 1:1 slopes in the area with similar soils.
Did the slope re-vegetate?
Did the slopes erode?
Are the slopes mowed ?

The client may have experience, but unless he has actual examples, be careful....be very careful