I talked with the contractor that built my neighbors pond about building my pond. He said he didnt have to core trench the dam on the neighbors because the clay was so good. I thought you always had to core. So I`m asking you, the experts. My pond will be about 100 ft from the neighbors. I expect the clay to be about the same. Sould the dam be core trenched?

I've never claimed to be an expert but here goes.

Good clay, mashed against other good clay, still doesn't make the leak tight seal that you want. Every time I find a leaking dam, I ask if it was cored. I've only found one cored one that leaked. Of course, I haven't seen them all or even very many. I've seen a lot of non leakers that were cored. You can almost raise mosquitos behind my non cored dam. But it is dry behind my cored one.

Of course, like Mike Otto says, "all dirt leaks". But, if I ever do it again, mine will be cored. It's just not worth the possible problems.

Wear a helmet. Wear a seat belt. Cook the pork long enough.

Core the dam.

Hey, nashfireman...welcome.
We whoop on this one now and again. I always send folk to my distress signal when it came time to core or not to core . If ya haven't yet been there, skim thru this thread for openers.

I am a Geotechnical Engineer and a Geologist and do Soils Engineering for a living. In additon I have bulit six ponds on my farm, none of which had a core trench, or leaked very much. Typically I only use core trenches where clay is in short supply.


If you are on good clay, the most important aspect of embankment construction is to use clay that has the moisture content adjusted to a couple percent wet of optimum. Then scarify the the surface of the native soil and insure that you get good compaction of the lifts. Typically you should be looking for 90% of modiefied Proctor. You get alot better sealing with the clay slightly wet, than at optimum or on the dry side.

I would always core trench. I have two ponds that seep a little under the dike. And guess what? I have plenty of clay in the soil and the contractor said, " No need to core trench with this clay." He was wrong.

Boy, this got long.

JAS, I've never talked with anyone who has both educational credentials and practical experience in this matter. If you ever make it to the D/FW metroplex, I would love to sit down and talk about clay with you. You will do most of the talking and I can draw a pretty good crowd of listeners from this area. BTW, I have no idea what soil types or weather conditions exist where NashFireman is going to excavate and build. Nor do I know his tolerance for small leaks.

However, for practical reasons, I have to disagree with you on the need to core. In your part of the world, a pond that you allude to as not leaking very much can be a disaster in my drought stricken part of the world. I've not seen a leak slow down when it runs between natural ground and piled and mashed clay. I think of it by saying that I buy health insurance before I get sick and life insurance before I die. The uncored, slightly leaking pond is the pre-existing condition that the insurance guys don't like hearing about.

In a perennially dry area which we usually have, the % of clay that goes into making up sandy loam is critical. If too little clay, the sand all runs off the face of the dam into the pond. If too much clay we get cracks in the face when water recedes. And, it darn sure recedes in 100 + temps. The cracks close when we get a good rain but somehow I feel that the integrity has been compromised.

In certain areas around D/FW we have black gumbo clay soil. When it dries, it shrinks and when it gets wet, it expands. When it shrinks, we get huge cracks in the ground. This happens every summer. Thus, we get a lot of busted concrete slab foundations and cracks in walls. Even after repairs, you almost can't sell the house. I've never tried to build a dam in that type of area or talked to anyone who has. But, I've always wondered what would happen, with or without coring. What are your thoughts? If you make it to D/FW, I'll buy the beer and and listen.

Dave:

I used to work in your part of the world and you are right the clays are a little different down there in good old Texas. I buildt two large dams on the San Gabriel, one at Granger and one at Georgetown Texas. I have first hand experience with those so called gumbo, "expansive" clays. The Dam I built at Granger was two miles long and used 17 million cy. These large dams were cored but we could have skipped the coring because the material used for the the zones outside the cores was such high quality inpervious clay. They were so large skrinkage cracks were not a consideration in the design.

Smaller dams may be a problem using these clays because the skrinkage cracks that form as the clays desicate can be 15 feet deep. I think to build a good small dam in that gumbo you may want to have upstream and downstream zones of clay with a low expansive index to protect the gumbo from desication.

In my part of the world, (upstate New York), we don't get drought conditions like you do and our clays are not near as expansive as your clays.
Here I can scrape the topsoil away and expose the clay subsoil and start building the dam. Typically I have built my personal ponds where the clay strata was thick, so I excavate maybe 10 feet into the ground and built a dam 10 feet high. This results in a 20 deep pond.

I have gotten a little lazy in building ponds for myself due to having great pond building conditions. I can get 90% compaction from my dozer treads so I haven't even been using my sheepsfoot roller. The clay I have been using comes out of the ground a little on the dry side but since the entire embankments are solid clay, I have skipped using any moisture adjusts to the clay. So far I have completed six pond with only one small visable leak in one of them.

nashfireman, you started a fun discussion.

in my mind, the necessity for a core trench depends on the risk variables relative to yer situation. to clarify yer situation, add some specifics such as:

what kind of surface area and depths are we talking about for pond....how big?
will it be a dug pond or damned ravine?
do you have year round water supply?
how deep is the water table groundwater?
any bedrock in area?
have you done any test pits?

answers to questions like these will dictate whether its appropriate to core the dam or not. as you can see from the posts above, in some areas like jas's a core trench might be overkill, however, special "just right" areas are less common then marginal pond building areas, and thus building a dam with a keyway is usually the best way to go.

DIED, marginal pond building area is an overstatement for the central basin of Tennesse. It isn't that good. We have more caves than almost all states.

The post has left out too many details. If it is a little bitty cow pond or something, he might get away with no trench. If it's going to have any real pressure, it's 50/50 chance that it will work no matter what he does.

My thoughts are to do everything just right, from the beginning, by the textbook, and that way if it fails, you know you did the best you could do.

It's what I did.

I`ll try and answer the questions that Dave asked. Surface area,, about 1 acre. More if I can keep the wife away while digging. Various depths to 10 or 12 ft? I will be a dug pond on a slight slope with a long dam along the back side. Water supply is from runoff from the slope. Aprox 5 acres of runoff. Bedrock,, we have dug 10 ft so far in areas and seen NO rock. Amazing for Wilson Co TN. Its been 24 inchs topsoil then red clay. Test pits. We dug 10, saw no ground water. I will add a picture that will show you alot more if someone will tell me how. Thanks guys for all your input.

Maybe this will take you to the pictures. The shiny stuff is clay. Some photos are of the pond area and some are of the drive way. http://i178.photobucket.com/albums/w245/nashfireman/LAND057.jpg If this line wont work go to photo bucket and login as Nashfireman. Password is Nashville.

When I built my pond, I cored it because I thought that was important to stop it from leaking. Laying one layer of dirt over another creates a seam that would be easier for water to find a way through it then the virgin soil below the dam, or the compacted soil above.

After finishing it, I was in a discussion about it with a soils engineer who told me that the real reason to core a dam was that the weight of the dam is what holds it in place, and the core improves the integrity of the dam by locking it into place "AND" adding to it's weight.

Depending on how much water height you are holding back, dictates how much weight you need in the dam. The angles of the dam are figured in increasing the overall weight of the dam, and not some angle of holding back water.

The same is true with how much freeboard the dam has above the water. It's not so much to hold back extra water, but to increase the amount of weight of the dam to hold it in place.

I did it right, but for all the wrong reasons.

Eddie

We finished our pond at Thanksgiving last year. Our soil is deep sand, but we built our pond in a draw where the sides of the ravine had clay just below the surface and we had the dam span these two sides of this wide draw. The dam is nearly 600 feet. My contractor planned on coring the base of the dam, but decided against it when he determined that the trench depth required was not reasonable: at the lowest point of the ravine he would have had to dig down 15 feet or more to reach the clay. Further complicating it was the fact that there were active springs above that clay. Instead he pushed as much sand as possible to the back of the dam site and dug as deep as he could with the bulldozer. (actually he got his D8 stuck for 2 days in it) He then started working from the sides and back pushing the sand/clay mixture towards the dam and kept building it up higher and higher. With this method the dam actually extends halfway into the center of the pond and is at about a 40 degree angle. At that center point he was deep enough to now be at clay level. All told, he dug about 12 feet down and the dam was 10 feet above original soil level for a total of 22 feet of depth at the midpoint of the pond. Overall the pond is about 2 1/2 acres. The dam is probably as wide as 100 feet at the base. Steep on the back, very slight angle on the inside of the pond. I was concerned about the lack of coring, but I must say, we are half way full now and we are losing maybe a half inch a week in depth, if that. Based on Eddie's post about the weight of the dam creating the seal, the logic of this design could be the increased downward pressure on the dam because of a more gradual slope. The dam surface has no less than 3 feet of clay on the top of it. I'll try and figure out how to put the construction pictures on this site to illustrate the concept. Hopefully this will continue to work. Our depth is about 10 feet at the deepest point - I now hope we have enough rain to fill it another 10!

I live in the Nashville basin probably not far from where you are. I recently finished my pond and here's what I'll say about core trenches. I really don't think it's necessary to core your dam for the following reason. I built my own and when I was ready to start building the dam I thought long and hard about whether or not to core. I had argued in the past on this forum that coring wasn't necessary on a good clay base and not even practical in our part of the country with such thin soils. Here you have to keep the water from going into the basin soil or it hits the rock and will bypass any core unless the core goes halfway to China. That being said when it was time to build my dam since I have my own machine I decided I would cut a core trench since it was only more spare time and diesel and the core couldn't hurt anything whether it helped or not. I cut about 4 ft into the subsoil when I started hitting limestone bedrock and had to stop. I then compacted the core with my pull behind roller as it was filled with my best susoil (48-51% clay, Mimosa silt loam). I then constructed the dam in 8" lifts with my best soils in the middle (all of my subsoil has adequate clay). I finished up the dam last fall but before I could compact the basin I got very busy with work and had to postpone pond work until later in the fall. Then in late Oct I had to go out to CA for a week and when I came back the pond drain had gotten clogged and the spring and a rain event had put about 5' of water in the pond. I drained it out but that late in the year it was impossible to get it dry in the bottom. Being anxious, I packed the upper part of the basin and plugged the drain with my fingers crossed. The pond picked up another 6 ft of water pretty rapidly, but was leaking about 1/2 inch/day, which bothered me because I wanted it to hold like a bucket from the get go. So, with no rain in sight due to El Nino, I opened the drain back up in mid Feb and determined that I would wait until it dried enough to pack it proper in the basin. I had to go out to CA again in early March and when I got back home the pond was fairly dry except for a 30' x 30' patch in the very bottom. I rented an Ingersoll Rand self propelled vibratory padfoot compactor over a weekend and pounded the hell out of the pond basin with 56,000 lb force on the pad feet. I then closed the valve back up and watched the spring start fillin it back up. I've had 2 moderate rain events since and the pond is now showing 6.5 ft on the stick and appears to be holding like the plastic bucket I was pining for. Research your soil type and I will bet that one of the limitations listed for it by the soil service is 'difficult to pack' like my Mimosa silt loam, but it is still perfectly capable of holding water as long as it is packed properly and a dozer simply can't do it, not enough ground pressure. Unless you have the $50,000 it would take to core down 50 ft into the rock and fill it with concrete I'd save my money on the core trench (might work) and spend $500 on a padfoot compactor for a weekend (will work). Core trenches are only practical in certain situations where you have impervious soils overlain with pervious soils. If your subsoils are thin and prone to seepage because of the soil structure (not lack of clay), then you are wasting your time and money on a core. Compacting the subsoil in the basin works on the same principle as a plastic liner. If the water can't seep into the soil, then it doesn't make it down to rock, and if it can't seep into the rock, it can't get past the dam. My common sense told me this was true before I started and my experience thus far has borne it out. I'm going to post some pics one of these days.

Hillbilly

Hillbilly, sounds like the best answer is "It all depends".

Nashfireman,

This is a great site for soils. Obviously there is no substitute for digging your own samples but one thing I liked about this site is it allows you to see what type of soil is under your neighbor's ponds. You can look for sites similar to your's nearby and talk to those folks about their construction.

Good luck, I know of a lot of nice ponds in your part of Middle TN.

http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx

please help! im building a one acre pond on my land. my dam will be 180 feet across and about 12 foot high. it will join two bottoms together. i started digging my core out. it was going great until i got close to the spring feed creek. i started hitting limestone about 2 foot down. i tried digging past it. after about 8 foot it started getting harder to dig but it's still limestone. water is running in the sides as i dig. what now?

If you want the pond to be as large as planned, you gotta keep digging. Chiseling out the rock if necessary. There's an article in one of the Pond Boss Magazines in the past year or 2 of exactly that scenario and what they did to keep digging the pond.

also when trying to dig the limestone out the side walls keep falling in. i have water coming in at about 15 gallons a minute. i have been pumping the water out. the water table is high do to the amount of ran that we have gotten this summer. would i just be better to wait until it drys out? or will the water keep coming in because its a undergroundspring?

Disclaimer: I am neither a soils expert, nor even a digger of holes (apart from a few post holes) I am regurgitating what I have read, and am a hobby geologist.

I would take a guess that water is running horizontally between limestone layers, thus why you have springs. As you are digging, you are exposing this flow. It is possible that there is good news in this: The flow is where you can reach it, rather an straight down into the Mines of Moria.

Around us in western NY the limestone is pretty solid, more like dolomite. It is more or less impervious after you get below the weathered layers. However in some parts, limestone is very porous, full of holes, sinkholes, caves, etc. I suspect you have the first type, where once you pass the weathered layers, you have a solid base. This is based on that you actually have a stream and ground water.

If you have a means to divert the water downstream while you work rather than pump it, I would do so (siphon?). However it sounds like you are in a really tough spot of needing to pump.

I think the only thing you can do is bust out the rock down to solid unweathered layers along the entire length of the dam before backfilling with very tightly compacted clay making a core that connects the clay to reliable stone. When you are done, you have to hope that inflow is greater than outflow, and that there are no other stony escape paths for water inside of the basin.

Good luck with your work. I think it is common to run into these issues where bedrock is just below the surface. In my experience, projects never are as easy as hoped.

thanks for the information. the water is coming in right below the old creek. i have put in a 10 inch pipe going under the dam and turned the old creek over to the new pipe to divert the water on top. about 40 foot of the lowest ground is when i started having water coming in. it was slow at first but when i started digging up about the last 12 foot of core is when the water started coming in hard. the thing i don't understand is it's coming in from the down hill side, not the up hill side towards the pond.

Hmm, that is odd it is coming from the downhill side other than perhaps it is just draining out of other areas uphill and making its way through a different path than you would expect. Maybe continuing building the core will reveal an older creek bed where this is coming from?

Water will flow to the point (area) of lowest pressure even if uphill.

when i started digging the core at the highest point i only had to go down about 2 feet before i was able to get through the top soil. the ground turned to a dark grey clay that was hard to dig. when i hit that i came up with red clay that i used to compact for the core. as i got about 30 feet or so from the creek i had to start going down to about 6 feet. water would start coming in about 3 feet down. i would dig down past the water to hard shell rock. it was really hard so i stopped their. the water came in slow enough that i could put the core in. it wasn't until i tried to finish at old creek that i ran in to the water coming in. i was thanking maybe the water ran back to the holl because it was the lowest point. i was able to dig down about 9 or 10 feet. the water was coming in around 4 feet from the top. we have had a lot of rain this summer so that isn't helping much. i could have probably finished the core but i new that if i didn't do something about the water it would just keep coming up in the old creek bed and wash the back side of the dam out over time. so i am at a stand still. i am undecided on what to do. i could try digging up the creek as it goes down hill and see what i can find. i was able to look at the holl that i dug yesterday and after 4 days water is still coming out and running down the ol creek bed. but we did have about 1.5 inches of rain over that time. sorry for long post lol

i was able to make it back out today and look again. i was able to see the water coming up from inside the holl. i took a measurement from the water coming up to property line. i can move the dam back about 12 feet. i could put a new core in behind the spring. my question is this, about 4 foot of the front side of the dam will be over top of the spring. would that be ok seeing the spring would be on the front of the dam and the core behind the spring? i could divert the spring over to the new creek and it can flow through the new pipe

That is a very tough question. You need a good experienced dirt man who has seen it to even guess at that. I would not want an active spring at the front of my dam.

With a spring inside the dam I would be worried about the dam becoming saturated and failing.

last summer I had cray fish dig into the inside of the my dam during low water and once the water came up the dam became saturated and collapsed.

i was able to talk today with the man thats going to help finish building the dam. he has built about 25 ponds. he said that the spring that i hit when putting in my core is a real good spring. he said that he can put the front side of the dam behind the spring. the dam was going to go straight across but now it will have a horseshoe look to it. we will have to do the core over again. Will this be ok? i can't move the entire dam back do to the property line and another creek that goes by the dam. ( that has just run off water)

I am assuming that you mean the spring will be in the pond. If this is so, then if the source of the spring drops below the level of the pond at full pool, the pond level will also drop. Think of it like two buckets of water connected by a small tube from the bottom of one bucket to the other. It could help all the time or just sometimes, and it could hurt sometimes. Surrounding the spring with something to contain the water, during a dry time, may help you predict how it will behave. This could be a culvert or earthen wall depending on the way things are arranged in your situation. You may even be able to drive a pipe in and see how high the water rises in the pipe if you can seal it off well.

i was able to talk today with the man thats going to help finish building the dam. he has built about 25 ponds. he said that the spring that i hit when putting in my core is a real good spring. he said that he can put the front side of the dam behind the spring. the dam was going to go straight across but now it will have a horseshoe look to it. we will have to do the core over again. Will this be ok? i can't move the entire dam back do to the property line and another creek that goes by the dam. ( that has just run off water)

i thank i understand what you are talking about. as of right now i have about 4 springs feeding into the pond sight. water off those 4 springs a total of about 11 gallons a minute. i have 4 more springs inside the pond sight that are coming up and running over to the new pipe. i have walked down the creek behind where the dam is going to be and i did not see any more springs coming up. i am sure the springs don't stop on my land. the man that has land behind me had a 2 acre pond right behind where mine is going. he said his always stayed up and never had a problem until a bad storm came in 1998 it rained so hard that water came over the dam and washed it out. he didn't want to spend the money to rebuild it.

That is what emergency spillways are for. We got a 100 year rain two weeks after my pond was built. The pond builder thought I went a bit overboard on my design but came back right after the rain and said that I was the only one of 5 that he built in the last 3 years that was not going to need repair.

so is it a bad thing to have springs inside your pond? i guess it would be ok as long as the water pressure coming in is stronger than the water pressure that th pond will have when it is full?

It is usually a bad thing. You're betting that the water table won't drop below the level of the springs.

If you can have the water in the spring enter above the full-pool of the pond, then it is a good thing, otherwise it depends on the spring and the elevation of the water that feeds it, especially during dry times.

is there anything that i could look for at the pond sight that maybe would let me know if the springs will be ok in the pond? i could move the dam up hill about 400 feet but i would have to clear more trees off. also not sure if i would hit more springs. the first spring is about 800 foot above the pond sight and there are more coming down making about 15 gallons or so a minute before it would come into the pond sight. so i thank it's a real good chance ill hit more. also there is about 30 acres off run off coming down into the pond sight

Is there a way that you could surround the pond with dirt to see how high it pushes?

If the water table drops you don't have a spring, you have a drain.

i don't have a way to put dirt around the spring. iv have never seen any of the springs dry up or slow down very much even in the dry part of the summer

You just might be OK.

Thanks. i guess I'll go on and give it a go and pray for the best. probably b next summer unless the ran stops. I'll update when im done. thanks for everyones info!

I have a pond that is leaking. Have you ever heard of using dust from the quarrying of limestone to seal the pond? thanks Flitchman