Could someone please explain how a cutoff trench on the back side of the dam would be constructed? Could that trench be lined with plastic? What if gravel is hit in making the cutoff trench, (the cutoff trench has a gravel bottom)?

Bump. My pond holds well in the "bowl". It seems to leak out under the base of the dam.

I would be interested in information on this subject also. I tried rebuilding my dam but I guess I didn't get down deep enough due to water still being in the pond.

Are you all referring to a core trench? If so, the lack of response may be due to your subject line.

There, I changed the subject. I am referring to a trench, dug into the dam on the backside, and lined, or packed with impervious material. I was wondering what to do if said trench had a gravel bottom. Would you continue to dig deeper? Could the trench be lined with heavy plastic, then backfilled with the dirt that came out of it?

There needs to be a continuous impervious wall keyed into an impervious underlying base layer to keep water from leaking. A new dam behind an existing dam can be created, but the water level in the existing pond will make creating a new dam more difficult because of the hydrostatic pressure it puts on the leak. I cannot speak to the use of plastic or liners because I am not familiar enough with their use. If the water can get down to gravel, then it can leak out. Gravel layers can be sealed with a continuous clay liner, but water must be prevented from reaching the gravel. A complete continuous seal up to the full pool level is needed to maintain water level. I have personally installed a core trench in the dams and lined the complete bowl with clay on my three fish ponds and half dozen wetlands. The new pond is not yet full, but the rest hold water very well. The new pond has not gone down yet, but it needs another 4' of water to reach the overflow pipe. If you can drain the pond, you could core behind the existing dam down to good clay, and then use the old dam soil to top it off above the full pool level and behind the new dam. Folks use different terms for the same thing in different places. I just wanted folks to chime in and help you out. Having built my last pond alone, I know how hard it can be not to take shortcuts. This forum has ben good for keeping me honest while building new ponds. I still hope to build another 3 ponds before I kick the bucket.

When the pond leaks down about 30-36 inches from full, the water stops coming under the dam, so the pond wouldn't have to be drained. The "bowl" or excavated part is about 8 feet deep. I don't know if it's roots left in the dam, or lack of an original core trench. A trench dug on the back side of the dam would let me discover the reason for the leak. As you have probably already read, the pond was renovated in August 2015, but about 2/3 of the dam was left in place. There were many trees on the dam between 6-14 inches diameter; ash, basswood, willow, elm, and cedar. The root balls were pushed out, but probably many side roots were left behind. The inside wall of the dam was scraped clean and left. If it's dead roots, there probably is only a few inches of clay covering them on the inside face of the dam. I need to do something about it this year. I'm tired of pumping it up several inches in a day, only to have it leak back down in about nine days.

When you say water stops coming under the dam, does the water level also stop dropping in the pond? If so, could the leak be at the level that the pond stops dropping? Maybe a shallow recoring on the top of the pond down below the level that the pond stops dropping would seal it up?

The pond was down 15 inches from full this morning. I hand dug three test holes at the toe of the dam. The first 16 inches or so was silty clay "topsoil" full of small roots. After that, I hit hard, solid clay that was barely diggable and very sticky and pliable. The water leak is going along the top of this clay layer and saturating the silty clay in the top layer above it. It appears that the old part of the dam either has no core or is penetrated by many roots.

Could a core trench a few feet up from the toe of the dam be cut with a mini excavator? There is enough clay in the dam there to tie in and make the trench impervious.

How will you keep the water out of the trench so that the new clay core will pack?

I might use a plastic trench liner keyed into the clay in the bottom. At any rate, I will wait for the pond to leak down so there is no water in the trench. When it leaks down to the excavated portion, the leak stops. I would try soilfloc again, but am unsure if it would be the proper remedy for the kind of leak I have, plus there is almost no suspended clay in the pond water, plus lots of FA on the bottom.

Edit: I drove 3" PVC standpipes about 4 inches into the hard clay in the bottoms of the test holes, and dipped he water out of the pipes. The bottoms of the pipes have not accumulated any water. The water is standing about 16" deep on the outsides of the pipes. I guess this proves that the water is seeping under the dam between the bottom of the clay that forms the dam and the impervious layer about 16 inches below. Also, if that lower layer was pervious, the water wouldn't still be standing on the flat behind the pond over two weeks later.

Probably the old part of the dam built about 60 years ago was never cored.

Does anyone else have something to add here? Is a trench as described on the back side of the dam likely to be successful?

John, I have seen a contractor dig a 6 or 7'wide and 12 foot deep trench in the dam (while the pond had water in it) and then add clay (hauled in from another area on the property). He would add the clay and every so often he would use a drivable vibrating sheep's foot vehicle. He tried a smaller (3 to 4' wide unit) remote controlled version of a sheep's foot but it was to slow to do the job he wanted. He rented the sheep's foot equipment. He told me later where he used the remote control unit he was still seeing some seepage. I am not sure if this will help you but thought I might pass this along. Hope this helps

Thanks. I was thinking of digging a 16 inch wide trench with a backhoe along the back side. Sounds like it probably wouldn't work. It seems to be leaking at the bowl-dam junction, although that area appears to be solid clay on the inside of the dam.

Posting a drawing.

Here is a cross section sketch of my pond. I have dug a half dozen test holes at the toe of the dam. All indicate it is probably leaking under the dam. Of course, the drawing is not to scale. The sides are not that steep.
Thoughts?

Will new core extend above full pool?

It would tie into the existing clay in the dam at a point just below or right at full pool, so there would be a seal. The seep only has the pressure of three feet of water at the bottom. It doesn't take much clay to hold back a few inches of water. Going all the way to the top of the dam would be unworkable as the trench would be too deep to work in. I would be happy if we could get it to hold at one foot below full pool. It looks OK at that stage.

John if I am understanding your problem correctly I think it might be just as cost effective to entirely take off the top part of the dam down to the old dam. Then cut a core down into the old dam however far you need and replace the top of the dam while compacting it properly.

I may be all wet in that suggestion. But I am thinking of machine operation and ease of the job. It may take no more machine time for a dozer to take the top off, a backhoe to core a couple feet into the old dam, then have the dozer and whatever compaction method you can come up with to recreate the top portion of the dam. The dozer could simply push the top off to the backside slope so a very short push with minimal material movement.

I'm just thinking you may be overthinking this problem (based on your sketch). If it is much harder to core and compact within a narrow core all the way to the top of the dam, it may be just as cost effective to redo the entire upper portion of the dam. I'm just thinking of machine time and ease/difficulty of the job.

Just an idea. Worth every penny you paid for it.

Looking at your drawing again, I think the dozer could remove most of the top of the new part of the dam while leaving a foot or so on the inside, creating a slope towards the back slope. Then with the dozer go in and cut a vee shape roughly in the middle of the dam down into the old dam (to create the core seal). Then rebuild the dam while compacting it.

It is hard to know without knowing the width there is to work with, but a six way blade on a dozer can do quite a bit to make a vee shaped trench as long as you do not need it over a couple feet deep. If the dam is wide enough I think it would be all doable with a dozer plus whatever you can come up for compaction.

That is pretty much what I did with my old refurbished pond. Scuffed the top of the old dam off, then cut a vee in the middle of it with the dozer with the blade angled and tilted to tie the old dam together with the new material. then layered and compacted the rest of the new portion of the dam with a loaded 12 yard rubber tired dolly style scraper. It does not leak at that new dam/old dam junction and I probably have about 3' of water like you above the top of where the old dam was.

This is my renovated pond (done Aug 2015). It's only about 18 feet from the toe of the dam to the property line on the south and west sides. I would need to get permission from the neighbor to the south to have the dozer temporarily move over the edge of his land...it's swampy anyway from my pond leak. The dam is all original except for the east side. That part was moved and extended about 30 feet eastward during the renovation process. The highest part of the dam is only about 5 feet above the original soil.

Here's a picture. The property line is the faint purple line on the south and west sides. North is at the top. This was taken Feb 2017, before the new pond filled. It's the excavation on the NE of the pond in question.

I met with our pond guy this morning. We are going to try to do a new core or "cutoff wall" of clay on the back side of the dam at about full pool level using a dozer and full sized backhoe. He thinks the problem is likely the rotting roots from trees and trumpet vines that were on the dam at the time of renovation. At the time, the back side toe of the dam was inaccessible due to brush, an old woven wire fence and the previous neighboring owner. The brush and fence have since been cleared, the line established, and the property changed hands to a friendly owner.

The dozer / excavator guy still has not started on our cut off wall project. Here are a couple of pictures of my two ponds from this evening.

First the leaker. It would be much lower if I had not pumped over 6" of water into it in the past two months to support the BG spawn. Note some of the structure showing.

The second picture is the newer pond, about the same size when full. It's only down about 10 inches. I have not pumped any water into it, and it has smaller watershed.

John, did they give you a date to start yet?

No, they may have forgotten me. They said it's only a one day job, and they would start right after finishing a job 2.5 miles up the road. I was by there Tuesday afternoon, and they had finished and were gone. May not really want the work.

I don't know any other dirt movers that have the proper equipment. I'll give them a couple more weeks, then try to contact them. I guess the lower my pond gets, the better to fix the leak.

I hope it don't take too long for you to get it done.

I would not wait but would get back with them. If they are not interested then you need to locate another dirt contractor or at least find out what's up .

I got in contact with him and he thinks it will be sometime late next week. Pond will likely be even lower then, and easier to get clay from the end opposite the dam if we need extra to pack the cutoff trench. If we find some huge roots, we will need the extra clay.

Still nothing done. No show.

Dangit John I think I would call him and bug hin

Just going to wait and see if he shows. If he doesn't show in the next three weeks, I'll try again early next summer with someone else. The spot needs about six weeks before frost time for grass to get grown in before winter. I've been grooming it and now the dam is smooth enough to mow at three inches, on top and back.