Pond Boss
Posted By: Littlestfish Compacting soggy dam - 11/11/08 02:20 AM
In the first week of September this year I had an excavator dig out a roughly 20 by 20 foot area on a gentle slope. Test holes that I had dug showed clay and a water table within a foot and a half of the surface all summer long. As this pond was dug on a slope we built a clay berm on the down side. This was created by removing all the topsoil, digging down into the clay and then using the clay from the main hole of the pond to create a dam. The main berm is of good clay about 6 feet wide and rises above the original ground level about 5 feet and is then backed by about 10 more feet of clay top soil which creates a gradual slope to the original ground level.

The hole is only about 6 feet deep at most, but about 3 feet up the side that is dug into the natural clay there is a couple small springs and these filled the pond with about 3 feet of water in the first 3 weeks. Then the water level stopped coming up even though the springs continued to contribute a small amount of water from several inches above the water level.

The water level remained constant for a few weeks and then we began getting fall rains. When we had a good rain fall the pond came up 6 inches but then over a couple weeks of dry weather it lost all this. We just had a few days of heavy rain, about 6 inches in total, and the pond came up a foot, but I am seeing it going down by about an inch a day, even though there is still a lot of visible seepage from the bank at the back plus what has been coming from the two springs.

I guess I have a leak.

Today as I was walking around on the dam I think I found where this is happening. Most of the dam is now quite firm to walk on and seems to have settled some but one area of about 3 by 3 feet is very soft and when I walked on it and noticed this and tried to stomp it down to compact it some more, it got almost jelly like.

I don't see any obvious puddle at the bottom of the berm showing where water is running out.

The guy who did the excavating did try to compact the clay but I think it was difficult as it is a small area with some fruit trees he had to avoid and even at the dry part of the summer the clay was quite wet.

What I am wondering is should I walk a bit every few days on this jellylike spot to compact it, or could this make the problem worse? I am hoping this leak will repair itself as the clay settles. Is it likely our heavy winter rains will cause the clay berm to settle and seal better over the next year? And would it help if I stir up the loose clay on the bottom of the pond to create a lot of clay particles in the water?

I really would like the pond to fill to the top and stay full.
Posted By: Rainman Re: Compacting soggy dam - 11/11/08 05:58 AM
Welcome to the forum Littlefish!

If your clay was "wet" when it was pushed into the dam, it could NOT have been compacted properly. It does have to have moisture to compact, but too little or too much won't work.

I had a 10 foot tall dam for a catch pond built in 7/07 and it had not rained in over 30 days beforehand. The clay was surprisingly wet like your's, and I did not fill it for a year.It settled over 2 feet as it dried. When the track loader would try to compact an area, it was like you said, Jello. One area would sink while another rose. Given time to dry and possibly settle, it may seal.
Posted By: Scott Trava Re: Compacting soggy dam - 11/11/08 12:42 PM
Let the pond winter over the jelly mond is due to water trapped in the clay and it pumps if still there next summer aerate that spot with a backhoe and repack
Posted By: bobad Re: Compacting soggy dam - 11/11/08 01:30 PM
 Originally Posted By: Littlestfish
What I am wondering is should I walk a bit every few days on this jellylike spot to compact it, or could this make the problem worse?


That's a typical "jelly hole", and walking on it doesn't help much.

Jelly holes are caused by light, dry soil floating atop wet, heavy mud. The dry soil acts as a moisture barrier, preventing the mud from drying out. I know it sounds counterintuitive, but the cure is to soak the dry upper soil with water so it weighs the same as the underlying mud. Once all the soil has the same moisture content, it will quickly settle and pack nicely. If you can wait it out, time and winter rains will do the job for you.

Most jelly holes are easy to prevent. All you have to do is avoid burying heavy mud under dry, light soil. Sometimes that's hard to avoid, because a pond fills from the bottom up, soaking the soil at the base of the dam before the top layers have time to get soaked.
Posted By: Littlestfish Re: Compacting soggy dam - 11/11/08 03:16 PM
Thanks for your replies and reassurance that things will probably settle and seal. It seems most other stuff I don't want to seal, like drainage trenches, eventually plugs up and other piles of clay I have dug and just left sit seem to compact over the years with all the rain we get here. That jelly hole does feel very unstable though it is located a couple feet away from the rim of the pond. The top soil will get saturated real soon so it's good to know things will probably settle down.

I have been reading about how springs can sometimes suck water away. There is a lot of springs in this area as there is a square mile of 800 - 1000 ft high mountain behind the pond, which is where the springs and seepage is coming from. As the water is clearly originating from a land mass above the pond I am hoping these springs will be the contributing kind and not the siphoning type. In the winter there has always been some seepage and flow into a shallow drainage trench that is about 50 feet away from and below what is now the lower edge of the pond. There is similar seepage all along the base of the mountain, though I have never noticed a spring where the jelly hole is now located.
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