Cored Dam vs Uncored Dam?

I'm new here but wanted to see if anyone has had any experience with dam building. I have an uncored farm pond that is 50 years old and 6' deep in the middle. I would like to raise the dam height 4 more feet to make the pond 10' deep. But I'm being told by the guy with the equipment that he will need to breach the dam, drain it, put a core in it and THEN finally build it up 4 more feet. Of course, he wants $35,000 to do all that instead of only $9,000 to just add clay and compact it to build it up another 4'. He says the dam will leak if he doesn't core it. But it's never had a core in it so that logic doesn't make any sense to me. Does anyone have any similar experience like this? Or any opinion on it? The soil is 90% clay I was told. It's exceptional pond building dirt in my opinion even if it's not 90% clay. By that I mean that I can make a breech in the top of the dam that is 1 foot wide x 1 foot high and then throw a couple of shovel fulls of dirt in the hole and the dirt plugs the leaking hole almost immediately and without any compaction and it never leaks after that. And the existing dam has never leaked a drop either. That's all I can think of to say. Thanks to all.

Hi Wayne;
I'm not experienced on this subject at all,  but I do think you are correct.
And I know you are the guy with the checkbook !  
That makes you the boss and if he want's to make $9000 he will do as you ask.

Maybe he knows more , but what's the worst it could do, leak?

Here is some information from Australia that will help you.
earth dam embankments

Another tip is to talk with other contractors.
There are tests you can carry out yourself to ascertain the soil qualities.
Do these before you meet the contractor and it may help him also.

     Howdy J Wayne and everyone else,


             I have no experience building dams, however when I figured out the load for my roof on the cabin I built I researched on the internet and although there was some math involved it was nothing any regular guy or gal couldn't handle. It would seem to me, as you are questioning the guy with the equipment, and you should, you could figure out just how much more pressure the added water will create on your dam. It has to do with how much more volume you're adding to the existing pond and how much that volume of water weighs and then how much more pressure will be on the damn. Of course, you should know the dimensions of your existing damn too because that's the other half of the relationship. It appears to me, if you know the original water weight, volume and pressure, then dam size and weight too, you can determine the relationship between the two and move from there. In addition, can you increase the thickness of the dam on the back side?
             All of this is moot though if you simply take those numbers to a civil engineer and get their feedback. I don't imagine it would be as much as a new dam the guy with the equipment is proposing. It's certainly worth a call to check. Last, the engineer will give you definitive answers and reasons and then you know where you stand and if you choose to you can figure out adaptations like making the pond wider or deeper etc.

       Best of luck in your project,

        Thomas

That's what I was thinking too haha. Yeah. But it never leaked before so I'm not sure why it would suddenly start just from adding more clay to it.

what is an uncored pond?

Thank you for the info. I'll check it out.

That was a fantastic article. Thanks so much.

Excellent thoughts. Thanks.

If it is 6 feet deep then at the bottom it is pushing out with roughly 3 psi water pressure.  Adding 4 feet for 10 feet total means it will be pushing out with 5 psi so almost twice as much is why it might leak.  Now I can tell you the area I am in is nearly pure clay so I know nothing about cores.  Our dams can be 25 or 30 feet high with nothing but clay soil for fill.  Now one of my questions is where is the fill coming from to raise the dam that much?   Your soil might be clay but if you are digging the fill out of the pond it might be washed in sandy soil is another possible reason for the cores.  Also remember when the dam reaches a certain capacity the rules change for how high the dam is supposed to be.  After it reaches X acre feet permits, inspections are required and a taller dam height is required(I think ours is 2 acre feet).  To be sure the waves don't wash over after it something like 4 or 5 feet above the water line dam height is part of our requirement in WY and the dam is inspected every 10 years.  So if you have similar rules you might find you had a foot of dam height above water line under the old stock pond but the bigger pond requires the 4 feet added  for water and 5 feet added to prevent wave action from spilling over.  Earthen dam slope in this soil is flatter than 45 degrees so it takes a lot of fill over a large area to raise the dam by much.  In less well bonded soils with some sand that slope gets flatter still.  The core might reduce that slope slightly.  If you think the contractor is taking you for a ride get other bids.  If they all require the same thing then likely he knows what he is taking about.  If some will do it your way then likely he doesn't.

While my work is different I do know in my work there are certain corners I simply refuse to cut anymore because they come back to bite me in the butt.  If I am going to cut them(rare) then I provide a written explanation of the risks and the customers signs for it.  Since this is something that depending on location that could kill people he may be even more leery.

Good points. I have neighbors with dams within a half mile of mine who have dams that are 10' and 14' high. Also, our soil is very clayey which is why my original dam is made of nothing but clay. And the additional clay will come from the site just like the original clay did. Lucky it will small enough not to need any permits. And I will be getting more bids. Finally, we don't have any neighbors below our dam for 4 miles so I would think no one would be in danger.

I am a Civil Engineer.
Pressure on a dam wall comes from the DEPTH of the water only, it has nothing to do with the volume of any dam or pond.
A cored dam, has a central section that rises from the base to the top of the dam, the core is made from IMPERMEABLE clay to prevent any water working its way through the wall thus causing a washout and collapse.
If it collapses that is when the VOLUME of the dam becomes a problem.
The link I gave above gives very useful information to anyone interested.

So if the current dam is 6-7' high and uncored but I want to add another 3-4' of height to it, does it sound "right" that I would need to drain the pond and core the whole dam just to add 3-4'? Would it be better/easier to dredge the bottom of the pond 3-4' instead? I just don't know anything about the bottom of the pond. Dredging it could cause it to leak. Or maybe I'm just stuck with what I have? I could understand coring the dam "again" if it was cored to start with but since it's not, that's where it gets me confused. Thanks for the previous information.

If the material within the dam wall is impermeable clay, then it does not need a core. The whole wall is a core.

Ordinarily soil analysis would be carried out by relevant people to ascertain what is on site.

I guess the question has to be, what is the reason for the enlargement?

I have seen dams leak after being dredged, because it cut into a layer that could leak.
Leaks can be sealed with bentonite or a plastic layer installed in the dam and covered with a protective soil layer.
Both these cost money.

Right. I'm getting a soil analysis right now. I've read that an ideal bass pond is 10-12' deep. So I need another 5' or so.

NEW INFORMATION. After much research, I am going to try to add the 5' of height to the dam using a hydraulic fill process. It is an extremely cheap but surprisingly effective method if you have good soil. I will only spray on a 3' layer first and then see if the first foot of new layer leaks once it gets above the original freeboard before I continue on up. It may take me a month or more though to get it done so I will eventually post the results.

Can you explain this process please?

I did a quick search, here is the link hydraulic fill dam embankments

I have to say from the quick read oif some papers I am not sure if I would do it.

Knowledge of the soil and the process is important.
BUt Fortune favours the Brave!

I am definetly no expert on this, but i think that if you increase the height of the dam you would usually need to increase its witdh at the base also.
I think this requires the pond to be drained, which raises the issue of keeping the dam at the correct moisture level while the work is done in order to prevent cracks.
Also the process of increasing the witdh might be tricky, as usually the dam is compacted in horzintal layers, i do not know whether you can just pack material on the sides.

Anyways depnding on the consequences of a failure of the dam, you should consider hiring an experienced consultant before doing earthworks on your own.

If your only goal for the pond is to fish bass, maybe it is feasable to just excavate some deeper parts in the pond.
As for the bass habitat i would consult with the people on pondboss

I'm not 100% sure of the correct process but what I've been doing for the last few days is to suck up mud from the bottom of my pond and pump it on to the top of my existing dam in ~3" layers that are about 5' wide. So I've added about 12" total so far and since it has rained every day here for the last 4 days, the depth of my pond has risen about 6" so far and there are no leaks, yet. But then again, I have beavers helping me which I'm sure has helped a lot. I'm going to add another 12" by the end of this weekend so we'll see.

The base of the dam is already 40' wide which to me is excessively wide for such a short dam. My pond has also experienced cracks in the dam during droughts but those cracks have only been about 6" deep. I thought about excavating the bottom but then I run the risk of piercing the bottom layer of clay and having a major leak. That's why I'm adding to the height of the dam and so far, it's working as planned. And luckily, if the new dirt somehow fails, there are no homes or yards or roads etc that are downstream from me for 7 miles so I'm lucky there.

I forgot to add that I'm using a trash pump and excavating the sides of the pond near the dam that are under water and pumping that slurry to the old dam. I lay 3" of "mud" on top of the old dam and by the next evening, that dirt has dried out and settled and then I start the process over. The pond depth is now a little over 7' deep so I only have 3' or less to go because I've decided to stop at 10' deep. I'm also going to make the top of the new dam 12' wide.

Update: This has been a very slow process but so far, it is working well. But I've only added another foot to the dam so far since I only have the weekends to do it now and it's rained several weekends since I started. The pond is now about 8' deep and not leaking a drop. It's going to be interesting when I get up another 2 feet or so and need to connect to the old spillways without washing away the new dirt on my dam. I think I may need to build concrete retaining walls on top of the existing spillways. I'll eventually post before and after pictures. I'm surprised this technique isn't used more often. Maybe because it's very slow. Maybe I could go faster. I'm going to lay on 4" this weekend and see. That would decrease my time substantially but it's working and at least it's a very cheap process.  I'll update again in a month just in case anyone ever comes across this in the future haha.

If it’s already core quality soil for the whole dam wall, I’d have to agree with John, who clearly knows more the I do anyhow.  What will be done about a level sill and or monk for overflow comes to mind though. I’d like to have at least two failsafe passive overflows for ponds.

I am glad to hear its progressing.
You could think about loose rocks instead of a concrete spillway, loose rocks are less likely to be eroded out.
Concrete only works if large areas are down and no edges exposed to soil are available for he water to scour.