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Green building ancillary. Pond construction.

user-757117 | Posted in General Questions on

Here’s a something I’ve had a hard time getting good advice on. Hopefully someone can help:

I’ll soon be digging a pond that will rely on a dam built of clay excavated from the from the deep end of the pond.
I’ve read that clay should be compacted at “optimum moisture content” – whatever that is. At this time of year the excavated clay will be quite dry.
What is the “optimum moisture content” for compacting clay?
If the clay is compacted dry, will the level of compaction be very far off from where it would be if it were compacted at “optimum moisture content”?

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  1. GBA Editor
    Martin Holladay | | #1

    The best person to ask this question is an experienced bulldozer operator.

    Plenty of excavation contractors in rural areas have broad experience building ponds. You want to find a guy in his 50s or 60s who has built dozens of ponds. (Definitely not a young guy who has built just one.) Ask him for advice.

    Without a doubt, he won't be able to tell you what the moisture content of the clay should be. But if he's any good, he'll tell you something like, "It's too dry right now to build a dam," or "Let me look at the site and tell you what I think."

  2. Riversong | | #2

    From the Soil Compaction Handbook (

    Moisture content of the soil is vital to proper compaction. Moisture acts as a lubricant within soil, sliding the particles together. Too little moisture means inadequate compaction - the particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens the load-bearing ability. The highest density for most soils is at a certain water content for a given compaction effort. The drier the soil, the more resistant it is to compaction. In a water-saturated state the voids between particles are partially filled with water, creating an apparent cohesion that binds them together. This cohesion increases as the particle size decreases (as in clay-type soils).

    A quick method of determining moisture is known as the "Hand Test". Pick up a handful of soil. Squeeze it in your hand. Open your hand. If the soil is powdery and will not retain the shape made by your hand, it is too dry. If it shatters when dropped, it is too dry. If the soil is moldable and breaks into only a couple of pieces when dropped, it has the right amount of moisture for proper compaction. If the soil is plastic in your hand, leaves traces of moisture on your fingers and stays in one piece when dropped, it has too much moisture for compaction.

  3. user-757117 | | #3

    That's good advice. Thank you both.

  4. Peter Powell | | #4

    Lucas, I'm assuming you are building a fairly large pond, say 1/4 acre or more with a dam of 8-10 feet or more in height, then compaction of the clay is irrelevant. If you have what we call around here "sticky clay" it should only be used as the pond liner in a layer of about a foot thick and in the core trench. You absolutely do not want to use that material for the dam breast itself which should be built of suitable material, properly compacted. Once the dam is built and the core trench is filled, the clay should be spread as dry as it comes out of the excavation. The normal equipment movement will compress it more then adequately and it will expand and seal up as soon as the pond fills. If you try to install the clay too wet, it will will not spread evenly, will stick to the the equipment tracks and generally result in a mess. The most critical issue in pond design is the water source. If you are relying on a spring, be careful that your clay does not seal off the the water , forcing it to exit under the dam which can undermine it.

  5. AllanE | | #5


    I had a 2 acres pond that never held water, as Martin mentioned, finding the right "pond guy" who has experience and the right equipment is half the battle. If you find the "right" clay I don't think the moisture content at application matters. We also used a little bentenite, rolled into the clay to assure that ours would hold, and after 3 years it has.

  6. AllanE | | #6

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