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#485502 01/23/18 08:57 AM
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Hey all, I have a core trench depth question...
Soon will be building a 1 acre pond on roughly 8 percent slope, so it will be fairly long and narrow. I am very familiar with the construction process, but would like clarification on trench depth. Seems that many mention that bottom of trench needs to be 2 feet below the bowl bottom. Others say just dig into virgin ground a good four feet and call it good. Is that two feet mentioned above, level with the bowl bottom or does that two feet follow the 8 percent slope? If I am following the 8 percent slope, then I could easily be over 8 feet deep on the core trench (depending on how deep I cut on the bowl)

I just don't want leaking water following the natural strata and moving beneath my core trench.

How do the rest of you interpret the rules of thumb for core trenches? Thanks in advance...

John

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I make sure mine are dovetailed a few feet into an impermeable soil layer that hopefully extends under your pond. That layer may start just below the topsoil or may be too deep to reach. This does not depend on if your pond is a 2' deep wetland or a 20' deep pond. The water needs to be blocked from getting out of the body of water by the core trench. The depth of the water does affect the pressure exerted below the water surface and the width of the trench needs to be wide enough to account for that water pressure. Test holes should help determine your particular geology.

Last edited by RAH; 01/23/18 12:18 PM.
RAH #485573 01/26/18 09:37 AM
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RAH, thank you...That helps clear up one of my questions. Good to know that I won't have to go that deep with my core trench. Do you think that a 1 foot layer of compacted clay (spread in two layers, then compacted) from the core trench extending under the bowl, would be enough? I will have a 12 ton sheepsfoot roller on site. I also intend to line the entire bowl with a 1 foot layer of compacted clay (two lifts as well). Do you think pushing extra material up onto the cut (upper side) of the bowl and compacting that, is overkill? Or could I get away with just compacting that upper cut area (not laying extra material in there)? Hoping this all makes sense, and truly appreciate your thoughts.

By the way, I did dig two test holes in the upper bowl area, down 10 feet and found a very consistent hardpan below a very thin layer of topsoil. I was able to roll a reasonable "worm" in my palm, with the material at all depths.

John

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I do spread the clay up the inner side of the dam. While no expert on the depth of the clay liner, I tend to over engineer and would want at least 2 feet of good compacted clay everywhere that would be underwater. I also compact 6" lifts. For me, it is cheap insurance to overbuild before the pond fills. I have two successful ponds with very little leakage plus a wetland that holds water like a drum. My most recent pond is not yet full, so I cannot claim success yet. I did install a drain just in case, but really had to due to water infiltration even during a moderate drought. I ended up digging and packing trenches (down to grey clay) almost entirely around with a backhoe due to fear that springs might operate in reverse if not sealed up to full pool. In this pond there was a very thin layer of sand between the red and grey clay beneath. So far the water level has only increased, but it has a ways to go. Good luck!

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As RAH mentioned, the core trench "depth" is entirely dependent upon how deep you need to gp to tie into good clay (or other impermeable material, not only in the core, but on the sides as well till it is all above full pool level. The core also need to be high quality COMPACTED clay to a foot or more above full pool level, the entire length of the dam, and about 10 feet thick. It's also suggested the bowl and inner dam face be dug out 2 feet deeper than planned grade, and then lined with a well compacted clay blanket laid in 6" lifts....The worst soils go on the backside of the dam as filler to slope your grade.

Bare in mind, tracked equipment is designed NOT to compact soils. As fpr pressure, water creates a little under .5 PSI per foot of head (height), so a 30 foot deep pond generates 15 PSI at that depth....at a 4 foot depth, it is only 2 PSI...




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