Pond Boss Forums POND CONSTRUCTION Soil questions Healer, heal thyself

Calling all dirt-guys...
Hey Tn Hillbilly, Dirtguy, Meadowlark (I'm shinin' the dozer sillouhette into the nite sky!), Bob or and any other PB'er with dirt in their teeth.
Let's talk core...my core. Last summer, after clearing a ton of timber for the pondsite, one of the last things my excavator did before wrapping it up for 2005 was to cut a portion of the core at the damsite (we stopped the project here so I could catch my $ breath and think a whole bunch 'til summer '06 to complete). When they did so, I thought I kinda understood what they were doin', but then maybe I didn't. I just spent 4 hours this early a.m. (in 5 degrees + w/c) with him discussing a bunch of new twists and ideas that I have for the project. Then I asked him to explain, again, what they were doing with core.
He said that the portion that they dug, in the very center-bottom of the valley where the dam will lie, is now complete and will not be re-disturbed. (by the way, the entire length of the dig was about 75' long; the dam will be about 265' long at the top, stretching across a perfect valley). He said that it gives them a small head start and a look at the soil to verify the previous test hole. I said "is that it?" He said no...that's why we staked the short area that we did. We will come back this summer and pick up at both sides, completing out to the ends of the dam. I told him I thought a core was constructed all at once, packing a layer at a time from one end to the other. He said no, that the main reason for the core was to cut across the path to disrupt any potential for piping within the subsoil. I added that I was concerned that he did not remove the layer of topsoil when he rolled it back in. He responded that the blend of the t-s and clay was a non-issue and besides, the soil tends to go back in at the order is was removed, more or less, with the best clay at the bottom anyway. I was having some issues with his wisdom and he knew it. He assured me that this is exactly the procedure he follows for all his embankment ponds without failure over 20 some years.
NOTES: I trust him implicitly. He comes highly recommended, has done many ponds in the area, was the NRCS agent in this county before he started excavating, and has done nothing less than make me feel like my project is his own. This is the first time I have even questioned him as his presentation and work is so impeccable. I am posting 5 pic's that kinda show the progress of this core section. If you are like me and in an area that doesn't yet have hi-speed, go get a cup of coffee or just change the channel.






I should also add that he said that he would have no problem if I want him to dig it all back up again and re-pack it as one long core when get to diggin' this summer.

The design of your core trench looks fine to me. The topsoil layer however looks pretty sandy in the pictures. If it is sandy, make sure the core cuts if off all the way across the dam. I have seen a number of ponds leak because a porous layer was not cut completely off.

Brettski, I've never run a dozer but have a leaky dam on one of my ponds due to not having my dam cored. I didn't know to insist on it at the time it was built.

Essentially, a core should be cut through one side, down through the bottom, as he has partially done and through the other side. It should be one continous trench. Then it should be packed with good clay.

I've been asked about a core on several occasions and have a problem verbalizing it. Once you see it done, it makes perfect sense. An internet search has not given me a good picture that I can relay. Maybe someone else can find a good illustration. Can anyone post one that they have?

By way of illustration. Get three wood blocks. Place two vertically to represent the sides of your valley. Then place the other horizontally between the other two to represent the bottom of the valley. That will somewhat, figuratively, represent what you are starting with prior to doing any dozer work. Now take a fitted piece of plywood and wedge it between the 2 vertical sides and down to the horizontal block. This will represent the dirt that will be used on your uncored dam. The weak or potentially leaky spots is where the plywood meets the horizontal and vertical blocks.

Now cut slots in all three blocks and push the plywood down into the slots. This ties the plywood (your dam) to the 3 blocks. See the difference in the fit?

The same concept applies to completely coring a new dam. It should be cut through both sides and down through the bottom. A lot of leaks occur where new dirt is stuffed next to existing dirt.

Think about it. A glass aquarium holds water because the sides are glued together. However, you really can't apply any kind of glue to dirt. Thus, the whole thing, when properly done, is cored to tie all of the 3 pieces together to prevent leaks.

Brettski,

I'm talking with ZERO dam building experience but I would of thought a core would be one continuous band of material from one end of the dam to the other.

Thanks for posting the pictures and please keep us updated.

Nice woodworking analogy, DD - the core trench as a dado!

I just got a mental picture of Norm Abrams on a dozer.

Brettski,

I have to ask. If the dirt that came out of the trench was put back in, why dig a trench in the first place? The main reason to excavate a trench is to replace the dirt that was removed with good clay dirt to effect a seal. If the dirt is good clay anyway, why remove it? Another point. If you are going to pack the trench with a dozer, it needs to be at least twice as wide as the dozer, otherwise, how do you ever get a track on top of the center of the trench? If the trench is cut as wide as the dozer, you need a roller to pack it.

Bettski,

Have him push all the top dirt out of the way for dam construction, once the dam is complete, use it to dress the dam. I suspect that he is just pulling your leg, and thinks you don't really even need a trench, otherwise why do that little bit of work on it, without removing the top dirt? All of the top dirt should be removed from the entire dam site, not just the trench.

DD1...thanks for the quick response. I am pretty much past core-101, but truly appreciate the concern. Your subsequent point regarding a continuous, monolithic seam is right where I am gettin' stuck with this contractor's approach. Follow me...
TnH...You responded as I expected, and it does not fall on deaf ears. That's why I solicited it...I want all definitive postures on the table as I decide a course of action (or reaction). I am not going to question his decision to cut a core, even if that clay down there appears to be in excellent condition, based on sampling. His decision to do so is to make absoulutely sure that there is no unseen seams, lenses, or piping thru the entire length of the dam. We can debate this decision, but it will be moot for this project. OK, so the core is gonna happen. This is where I climb on board with you. If you are gonna pack a core, everything I have learned to this point is specific application of the best clay substrate available thru the entire mass...period. I was also taught that it is best applied by a methodical layering and compaction thru the entire length of the dam. (Meadowlark would disagree with the this...I respect his posture). I also had the same thoughts regarding tracking it in on a one-blade wide trench, but will admit that I have had more than one NRCS agent tell me that this practice would be acceptable. Pub 590 stays fundamental by saying a core trench the width of the blade at 8' min is fine, then goes on to say that thorough compaction should occur in layers 9" thick...no mention of the equipment. As I noted at the end of the pics, he said that he would have no problem with pushing it all out and doing it at once...not being a wise guy or pulling my leg but sincerely wanting to cooperate with my desires so long as they don't compromise what he considers good practice. I already anticipate pursuing this option. This dam site is backed up into a corner of the property and the valley behind it is about 1 ac. The plan is to waste about 12,000 cu yards back there, taken from in front of the dam and filling in the valley behind to create a very nice gentle slope behind the dam...in the range of 14-1. The silty clay topsoil is about 30" thick at the base of the valley and diminishes to 12" at the upper edges. The clay below is excellent and goes a long way. We pulled a sample 13 feet down and it doesn't stop. I pulled the well drilling records for 4 wells surrounding (all within 2000 ft) and they show 2 ft of t-s, then 30 feet of blue, yellow, or gray clay. There is a 450 ac hunt club across the street from me. There are 6 ponds on it, up to 6 ac. He built 5 of them. 2 years ago, he just finished a 2 ac that goes 20 ft deep at the dam. It's a beauty.

If you are going to remove all topsoil from the pond basin to the centerline of the dam I wouldn't worry about a core trench. If he plans on wasting dirt (topsoil/silt) on both the front and backslope then yes I would do a core trench with a clay centerline.

Is he planning on doing the whole project with just the dozer? Or will he also use the trackhoe/haultrucks, scrapers?

Several issues strike me here. First, the purpose of a core trench is to create an underground waterproof barrier, similar to a vertical wall of compacted, processed dirt. Key word here is 'processed.' Otto has explained to me many times, shown it to me many more times. Look at the top photo and notice the layers of soil, down to clay. Pushing or excavating the dirt from the core trench, then 'processing' the dirt simply means to mix the dirt with an adequate amount of clay to make it compactible. Then, the pulverized, mixed 'processed' dirt is returned to the trench and compacted in layers, usually six to nine inches at a time. Unless the local soil is too porous, or there's not enough clay to mix, the soil removed from the trench is acceptable to put back in it, as long as it's mixed well and compacted properly. That's why core trenches are wide, so heavy equipment can get in there to compact the returning soils. Compaction is done with a sheepsfoot roller or loaded rubber tired scraper. Bulldozers aren't heavy enough. The core trench should be long enough, and deep enough, to create that underground barrier needed to minimize seepage. So, as long as the core trench does that, and the dam is built on a strong clay foundation, you should be fine.

"Pub 590 stays fundamental by saying a core trench the width of the blade at 8' min is fine, then goes on to say that thorough compaction should occur in layers 9" thick...no mention of the equipment."

The key phrase there is 'thorough compaction' Can't get that with a dozer. I wish I had your money Brettski! Wait, I wish I had your contractor's money!

One thing about 'processed' dirt. That's the kind an excavator makes money on. The more dirt you 'process' the more the excavator makes on a given job.

TN H,
I think Bob is refering to processed dirt as the mixing of availible materials to acheive a substance that will compact properly.

Tn H states:
That's the reason I reached out to you...for your respected opinion. I have a window here to adjust at virtually no loss.
Tn H adds:
ouch.

Bob,
Thanks for kickin' in. My guy was making that exact point regarding "processed" soil as he was explaining it to me...just a different delivery. It was this point that concerned me since all I have been exposed to speaks of the importance of good clay as a core structure. I assumed that it meant going down 30", extracting the real good stuff, and packing only that virgin clay into the core. I am sure that his method of rolling it back in is his "processing" procedure of a soil type that is very common in this region. As mentioned, this guy has big success in the area as a pond builder. If I read you right, this processing method can be acceptable if the final mix is acceptable.
I still maintain concern for two main points beyond this. a) performing the coring operation in seamless strokes across the entire core, end to end b) proper compaction. He operates with a back-hoe, D6 hi track, and an earth mover. I am certain that his immediate plan does not include specific compaction equipment. Is it out of the question to increase the width of the core to 1-1/2 blade widths and track it in with the D-6, by tracking side, center, other side? What do you think of the practice of doing the center/valley first, then coming back to do the edges and up later? It bothers me. Also as mentioned, he knew it and will have no problem recutting this center section and doing it all at once if that's what I want. I have 30 minutes of dozer time on the core work they have already done...zip cost.

Brettski,

Just kidding about the money. I know it costs a lot and you are intent on doing it right, obviously. I suspect that by the time it's said and done you could have owned that dozer in the pictures. I can tell from the pics it will be a beautiful sight when finished.

What I see as an injun ear and amatuer dozer operator is this. Let's assume you let the contractor come back in and work the ditch as two seperate pieces joining what he has already completed. Since a dozer has to dig itself in on a ramp, no longer than the trench he has already dug is, he will have to dig virtually all of it back out just to get the dozer down to the elevation of the original ditch, to start the new ditches. Call me suspicious, but this tells me that the trench work he did was nearly 100% unproductive and leads me to believe he was just burning what money you told him you had to spend until he had to pull out. You do need to insist though, that they use a roller of some sort to put it back in, that dozer just ain't going to get it. I'd defer to the 'proper compaction' statement in Pub 590. Unless your contractor can bend the laws of physics to his own purposes a dozer just can't pack. I'd say the comment by the NRCS agent speaks volumes on his level of expertise. (Not to say it hasn't worked for them in the past)

If you're not going to remove the top dirt from below the dam then you do need a trench, but I'm removing all of mine as it's too valuable to bury under a pile of clay, and I'll need it to dress the dam. And obviously you need a good clay core in the dam itself, unless the entire dam is built out of clayey material.

I understand what you mean by 'processing'. Mixing the heavy clay with the top dirt still gives a high enough percentage of clay in the overall mix to effect a seal, I'm just of the opinion that removing the topdirt down to clay and building from there gives a better seal.

Brettski- Don't make the same mistake I did. We removed ALL of the topsoil and used just clay for the dam. Clay by itself contracts way too much when in drought conditions. My pond didn't get a chance to fill before a drought hit us and the dam cracked completely thru. My NRCS agent to me we should have mixed some topsoil with the clay to allow the dam to expand and contract more without the major cracking. I have spent 2 years trying to seal the cracks. Have made great progress, as only have one small seep now.

Uh-oh...h20 kicks in with:

From a level of book smarts, this makes sense. From a level of street smarts, ask the man who owns one. Thanx for adding your (unfortunate) experience. This certainly adds credance to the practice of "processing" soil during dam construction. Is it possible that my soil conditions parallel yours to the extent that my guy's practice of processing is reasonable and even wise? When all the smoke blows away, it is difficult to argue with the success this guy has had in this region. I am still un-convinced and question his method of compaction. h20, what about this construction technique on your project...?

Seamless strokes are a good idea, to make sure the entire core trench is bound with good soils, end to end.
Compaction is very important. Water always seeks the path of least resistance, always. While there is always some degree of settling in a dam, the core trench needs to be compacted. His scraper is heavy enough to do it end to end, rather than side to side.

B-ski's question:
Bob's response:
OK, I get it on the weight of the scraper. I believe I may have painted a fuzzy pic on my question, quoted above. Both the scraper (e-mover) and the dozer transmit weight at both edges of the equipment. There is no compaction at all at the center of the equip. My question/proposal was to cut the core 1.5 times the equipment width to allow it to make multiple, parallel passes thru the core trench, adjusting the path to hit ALL areas within the trench. Lastly, are we nix'ing the dozer for suitable compaction regardless of the number of passes...period?

There are several things you can do on the core trench depending on how deep it is. I would say if are from 1 to 4 feet deep on it the best method is to cut the core sides to a 1.5 or 2/1 slope. This gives alot better seal on all sides versus just the bottom. Over 4 feet deep it is alot easier to bench back into the sides as you apply each lift. I would just be sure that the core trench is included in the overall bid. There is alot of times around here that the core is additional because of the unknowns and it is twice moved dirt.

As far as compaction there is a 20% volume of earth differnce between a dozer vs a scraper, according to the nrcs. So if I have a pond to build for the nrcs and I go in and tell them I will be using a scrpaer they will reduce the total yardage by 20%. We recently just built a 3800 yard pond with dozers only and they put in a 7/10's of a foot settling allowance in the deepest part of the fill, as you move out towards the end of the dam the allowance decreases. My opinion is yes a dozer is suitable for compaction if the lifts are put in 6 to 8 inch lifts. Is using a scraper or sheepsfoot better than just a dozer, yes.

Obviously you shouldn't build a dam out of pure clay. That's why pure clay cores are inside the dam, to keep the core moist, and if your dam soil has a high clay content the dam needs to be dressed with top dirt to keep it from drying out. Think about what is going on underneath the dam if you don't remove the top dirt from the base. The water permeates the base of the dam until it reaches the core, that means the front half of the dam is 'floating' on water soaked soil. If that isn't the case then you don't need a core in the first place. h2o did the right thing in removing the top dirt, but the wrong thing in using pure clay for the dam. Not only should the dam be dressed with top dirt but also the sides of the basin down to at least the lowest water mark anticipated in future droughts. Otherwise you will develop leaks as the clay in the banks dries out and have a pond that slowly receeds over the years. Brettski, just do the math on the dozer. My D5 weighs about 26,000 lbs but has about 3200 square inches of 'foot'. It's not a matter of opinion, it's a matter of fact. It won't exert any more pressure than me walking on the dirt. Dozers make quite a fuss and mess driving over raw dirt with the grouser bars stirring things up but they are intended not to pack by design. If you packed that trench with a dozer and then built a brick wall on it it would crack all to pieces. I'll quit beating this horse.

When h2O noted use of clay only for the dam, I kinda assumed that he meant the core. Now, reading back, it certainly appears that he means the entire dam structure. Yikes...major fundamental boo-boo. If this is the case, sorry h2O...didn't mean to rub it in.
My dam plans will, indeed, follow standard procedure of core construction and dressed with the saved t-s.

TN Hillbilly,
Please don't ever stop beating that horse!

We did dress the dam with top soil, but a few inches of topsoil is not enough to prevent the clay from drying out too much.