Pond Boss Forums GENERAL QUESTIONS Help Pond dam slumping

Need help! I recently bought property with a pond. The pond is ~1-1.5 years man made new. We have had a lot of heavy rains this year and I noticed cracking at the top of the damn wall. Upon further investigation, I discovered that the pond overflow drain was not properly draining away from the base of the outside dam wall. The water was pooling at the base; even appeared to go back in. I corrected that, yet now I believe I’m experiencing slumping (believe that is the correct term). Where the crack occurred the earth is now sinking and the base of the outside wall is pushing out. I have lowered the water level significantly until I can get this fixed.

Could this be settling from me fixing the drainage issue? Or something more concerning? Never owned a pond before. Looking for guidance and help. Thank you.

Do I understand you correctly that the dam seems to be moving away from the pond? I think maybe the dam was not properly cored and compacted when it was constructed.

Lots of possibilities.

Yes LFish, that looks like a red flag problem. (Sorry for the bad news.)

Hard to tell from photos, but that does look like a slump to me. Perhaps showing translational movement of over 12" to the right (downslope)?

Also, the slump mass appears to be lower than the scarp (the remaining stationary wall)?

Approximately how long is the dam, and how deep is the pond at the front of the dam?

Was the pond constructed by a "pond" guy, or an earth mover? If the dam was not constructed properly, then a slump is a common failure mode.

However, even dam construction experts don't have 100% success rates. I would highly recommend you have your pond builder come out and look at the damage.

The water not draining properly from your overflow outlet also could be the cause. If the dam was properly constructed and compacted, but water did some undermining along the route of the outlet pipe, then there could have been a pretty large cavity created in the center of the dam. Even after fixing the drainage problem, it would be almost impossible to backfill and compact material back into that void just by working from the backside of the dam.

Finally, is there any water showing on the downstream side of the dam when there is zero water going through the overflow pipe? If there is, then you have some water moving through your dam. That water will both remove some dam material, and "lubricate" the fill material which makes it much easier to slump.

Did they install anti-seep collars on your outlet pipe? That is a common failure point for water to leak through the dam. Especially if your outlet washed out some fill along the pipe (or even up to an anti-seep collar).

Just throwing out some ideas for you. Your type of problem is very difficult to diagnose over the internet.

Hoping you get things fixed there! We hate to see dam washouts on Pond Boss.

Looks like the dam was not properly compacted, and even possibly not constructed with the proper type soil, or it was too wet or dry if it was the proper soil. In any case I would say it wasn't properly compacted at the very least.

I wouldn't put any more $$ into the pond fish wise until you get the problem fixed. It might take cutting out a section of the dam, letting the water out and rebuilding the whole dam, with a proper core trench and properly compacted soils.

(You cannot properly compact the dam the way it is now. You properly compact soils thicker than 8" or so and knit that layer of soil into the previous 8" of soil.)

I have fixed a dam that a guy had built, dont know by who, but it was reasonably easy to fix from the back side, first of all you will have to start at the base of the back side of the dam and cut a key way several ft deep to keep it from sliding, then compact another layer up the back side of the dam with a longer slope then what you have there, I have seen dirt slump like that on the side of the highway when they try to leave it too steep.
The biggest cause for your problem is the steepness of the back side of your dam and the moisture gathering at the base of it. it is imperative to add enough soil to the base area to push that water on out and away from the dam and then also add to the slope, 3.5 to 1 minimum slope and probably more like 4 to 1 since you already are experiencing issues and the soil has already separated.
It might not be as much of a compaction issue as how steep and wet it is, wont matter how hard its compacted if its too steep.
Good Luck!, and keep us posted as to what it took to fix it if you don't mind. maybe some of us can learn from it.