Segmental Retaining Wall as Part of Detention Pond Wall - Earth Dam 3

[RFreund]

We are the specialty engineer for the segmental retaining wall and I'm wondering how to approach this situation. We have a wall that supports a detention pond. The top of the wall is at 863. The bottom of wall is 854. The HWL of the pond is 862 and the bottom is at 857. The pond is behind the wall. See attached sketch.
[ol 1]
[li]Would you design the wall to resist a full hydrostatic pressure?[/li]
[li]Would you apply hydrostatic pressure from 857 to 862 or from 854 to 862?[/li]
[li]Would you apply a buoyancy force to all the resisting soils below 862?[/li]
[li]Going a different route - would you use a free draining aggregate throughout the entire reinforced zone with a drain at the back of the reinforced zone to completely eliminate hydrostatic pressure?[/li]
[/ol]

Section here: LINK

EIT

http://www.HowToEngineer.com

 

[r13]

Is the retention pond lined? Is it existing? Your geotechnical consult shall provide water table and proper lateral pressure for wall design. Also, the consultant shall recommend method of excavation and proper backfill material to ensure stability of the retention pond, which is quite close to the proposed wall.

 

[oldestguy]

Sorry to put a damper on the idea, but it is loaded with questions. I'd defer the design of a dam or other retaining structure to a consulting engineer who specializes in thee sort of projects. Seepage in the soil underneath can destroy it in one day if not properly handled. You need a geotech on this one.

 

[r13]

I forgot the seepage/flow-net business

 

[RFreund]

Thanks for the replies. While I'm still interested to know more about how this 'earth dam' is evaluated, your responses have been helpful. Part of the question is really who's scope should this be in. We are really only an engineer for the wall system. But I don't think the wall can be designed without input from the project geotechnical engineer.

At some point though someone has to run such stability calculations and I'm curious to know what that involves.

It is a proposed wall

EIT

http://www.HowToEngineer.com

 

[r13]

Depends on who's the pond owner, you might have legal, and environmental issues. A geotechnical consultant shall be hired to lead and oversee the efforts.

 

[PEinc]

I would consider a steel sheet pile wall with an aesthetic block facing attached to the front of the sheet pile wall, that is, as long as the sheet piling can be driven deep enough.

http://www.PeirceEngineering.com

 

[r13]

The wall is too close to the pond. A seepage cutoff wall maybe required. The slop stability of the pond during construction must be investigated/considered.

 

[MTNClimber]

It can be done with enough consideration during the design and tight control over materials. There's not enough information provided to give you a good answer to your questions though. You write "clayey" soils as the retained material. How clayey is this material? What storm interval does the HWL correspond to? As retired13 already asked, is there a liner? What would happen if the wall failed? How long does the wall need to be in service for? Regulators may or may not see this as a dam. You should chase that down ASAP.

As for the internal drainage questions. I would absolutely put a rear drain if you don't use washed crushed stone as backfill. I might still put a small rear drainage system if stone is used depending on the answers to some of those questions above. Regardless, the reinforced fill should have a low percentage of fines. Drains should be sized larger than normal to handle a large flow in cause of piping/failure.

 

[RFreund]

Thanks again for the feedback.

I think I have set this situation up slightly incorrectly. There is a geotech involved and eventually we will coordinate. But for now I think the approach is that the berm behind the wall is constructed of Clay soils which prevents leakage (Not sure if there is a liner). The wall will be designed with free draining material so that any water can pass through the wall structure and prevent hydrostatic forces on the wall mass. The wall can be designed using saturated retained soils and dry granular weights. This is atleast a starting point. Then a more global study can be done by the geotech to see if that all works. Also they will need to evaluate/specify the clay layer and if any liner is needed.

MTNClimber - thanks for the input and a couple responses:
The pond is normally dry. I'm beleive the HWL corresponds to 100year (is that specified in the building code or just typical for commercial construction?). Service life is 50years I believe. I don't beleive it has been classified as a dam.


EIT

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[oldestguy]

Expect that the geotech will need test borings and other samples. Lab tests maybe required. You are premature to be deciding ANY of the factors needed for proper planning and construction. I'd take all of the comments very lightly until the report is in hand.
Edit: I see you have a helper below.

 

[r13]

I'll perform preliminary design on the proposed wall assume worst cast scenario. Then modify after getting recommendation from the soil consultant.

 

[cvg]

whether this is regulated or not, it is a small dam
recommend that the wall be set back away from the dam embankment so it is not considered a part. the wall should be able to fail without affecting the dam.

some of the potential failure modes for dams are:

overtopping (100 year storm or greater)
piping failure (rodents, cracking)
settlement
foundation collapse (saturated foundation)
seepage through foundation
stability and seepage analysis under the following cases:
a)under construction (dry)
b)normal operating
c)flood stage
d)rapid drawdown
low level drain pipe through the block wall
flood inundation area if it fails

 

[oldestguy]

cvg: I trust that outline of the supplemental dam is not to scale. Not wide enuff space here for the full dam, right? What side slopes? Not 1:1 or steeper I hope. Also for earth dam, with inevitable seepage, where is your toe drain? Gradations?

 

[geotechguy1]

I'd start by preemptively notifying your insurance of the possibility of a claim

 

[MTNClimber]

It's a tough situation but you can design for this situation to make it work. Ultimately, I would consider going very conservative with all design assumptions and then go from there. I can't say this enough but make sure the regulators do not classify this as a dam. Not sure I would personally take this job but I don't have enough information (nor do the Negative Nancy's in this thread). Definitely a odd one. Best of luck to you!

 

[cvg]

"oldest", sketch meant to be very rough but to illustrate a point. It does not include any details and certainly not to scale. Toe drains are not always required, the seepage analysis will provide the data to evaluate that. without knowing the geotechnical data, side slopes cannot be determined, but 1:1 is far too steep, this will be evaluated by the slope stability analysis.
my point is that the dam should be separate and designed by others that know what they are doing.

 

[RFreund]

@geotechguy - wow, Bold statement. Do you have any technical information you could provide to backup your concerns? I don't mean to call you out here and I do value others' opinions but I believe this site is best served provide providing technical information.

To be honest there are a few negative comments on here which kind of surprises me for a detention pond which is normally dry. I suspect this is due to lack of information provided which I apologize for. Typically when I see a detention pond formed with berms there is 24" of clay liner. I would think if you provided a 24" clay liner or even less since this is a temporary condition that the detention pond would hold the water for a couple days. If the berm is made with relatively good stable play soils then the wall could be designed as "normal" with possible rapid drawdown And use of a free draining material in the reinforce does own would be appropriate.

Also, side slope is 4:1 currently.

I kind of see 2 different analysis's one which is more of a global perspective and follows the limit States which CVG mentions above. And the other is more internal and local to the retaining wall. I'm more concerned with the latter.

CVG - thanks for the comments.


EIT

http://www.HowToEngineer.com

 

[r13]

How the pond in operated? Is it collects and retain the excess street runoff? Does the water elevation controlled by a release gate, or has means of draining out? lastly, is this pond privately owned, and is it included in the local flood control plan? Until you have answers to all, do not touch the pond, include attempts of alteration/modification, which will involve shut down of the pond during construction, and result in reduced water retention capacity.

 

[RFreund]

It is a detention pond that fills to the HWL during rain events. The water is released via a large storm pipes and also over land flow via a weir which is located 1 foot below the top of wall.

But again I'm more interested in the soil mechanics behind this situation and CVG/MTNClimber have tried to assist. I do appreciate the other comments and warnings and for sure will heed this advice.

EIT

http://www.HowToEngineer.com