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  • Results 1 to 10 of 10

    Thread: Will this cinder-block wetland filter be strong enough?

    1. #1
      Hop-frog is offline Member
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      Will this cinder-block wetland filter be strong enough?

      I am building a wetland filter using cinder blocks, built directly on flat ground. It must be raised, to protect it from regular flooding.

      I'm concerned if the construction is strong enough to support the weight of the water and rocks. Here are the details of constructions:

      1. The cinder blocks measure 16"x8"x8", will be stacked 3 blocks high (24"), and reinforced with 4' rebar driven into the ground.
      2. The entire structure measures ~7' wide by ~33' long.
      3. Then the holes in the cinder blocks will be filled with Quikrete Conrete Mix, mixed with 50% gravel.
      4. The blocks will be glued using Quikrete Maxon Mix.
      5. For added strength, a neighbor suggested I divide it into chambers. Will this help? That means added complexity to the plumbing, but is doable.
      6. Water is pumped from the pond bottom down a series of PVC pipes and splits, pumping the water equally to the bottom of each chamber.
      7. Chambers are filled with ~4 matrix blocks, covered in larger rocks, then a layer of gravel, and finally a layer of pea gravel. So they must support a great deal of weight!
      8. Each chamber is lined with a 45 mil 15x15' EPDM pond liner.
      9. In theory, the water should be forced upwards through the matrix blocks, layers of rocks, to the surface. The entire structure is set on a minor include, of about 5" per 25', so I think the water is forced upwards, then will flow from the highest chamber, down the lowest. The water then returns to the pond via a pipe or chute.


      Name:  construction1.jpg
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      Does this construction seem strong enough to support the weight of the water and rocks, plus potential freezing during the winter?

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    2. #2
      batman is offline Senior Member
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      You should have some type of footer under the block even if it's only leveled and compacted gravel 4 to 6 inches deep and it needs two rows of horizontal re-bar. Dig out the top layer of soil including grass and roots and compact with gravel. Skip the glue and use 100% concrete mix fill. Extra labor little additional cost
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      Last edited by batman; 02-23-2021 at 07:55 PM.
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    3. #3
      Hop-frog is offline Member
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      Here is an additional picture, I tried posting earlier, but my account is new so I could not:

      Name:  construction3.jpg
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Size:  45.6 KB

      When finished, it is capped with 2" stones.

    4. #4
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      Last edited by Hop-frog; 02-24-2021 at 08:17 AM.

    5. #5
      Hop-frog is offline Member
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      Quote Originally Posted by batman View Post
      No, you should have some type of footer under the block even if it's only leveled and compacted gravel 4 to 6 inches deep and it needs two rows of horizontal re-bar. Dig out the top layer of soil including grass and roots and compact with gravel. Skip the glue and use 100% concrete mix fill. Extra labor little additional cost
      I want to make sure I understand you right...I'll dig a trench under the shadow of where the blocks go...not needed over the whole area under the pond, right?
      Should I mix cement in that gravel/rebar foundation too?

      What do you mean by using 100% concrete mix fill? You mean to fill the holes in the bricks, don't mix with 50% gravel?

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      batman is offline Senior Member
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      Typically the walls are put on a concrete footer with re-bar but I've seen short wall (4 high) water lily displays built on compacted stone under the block. Just a shallow trench under the block with gravel. They held up long term. Vertical rebar every other cell. Horizontal re-bar between 1st and second block and on top block. Buy notched block or cut and knock out with hammer for horizontal re-bar. All cells filled with wet concrete mix such as Quikrete.

      Look up one of the dry stacked block pond builds here such as ICU2's.
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      Hop-frog is offline Member
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      My cinder blocks don't have any grooves for putting the rebar in horizontally. Can I chisel that out? Drill holes through it? Or is that just going to weaken the blocks? Would you recommend welding the rebar together or is that unnecessary?
      Last edited by Hop-frog; 02-24-2021 at 11:21 AM.

    8. #8
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      Quote Originally Posted by Hop-frog View Post
      My cinder blocks don't have any grooves for putting the rebar in horizontally. Can I chisel that out? Drill holes through it? Or is that just going to weaken the blocks? Would you recommend welding the rebar together or is that unnecessary?
      Mine didn't have notches either but a diamond blade for a cut off saw will make quick work of them. I just cut down one side and then the other and took a
      hammer and tapped out the pieces.



      Just over lap the rebar about a third of the length iirc... no welding needed.

      I'm confused by the comment "It must be raised, to protect it from regular flooding." Why would it
      need to be raised?
      Also, don't drive the rebar into the ground. All rebar should be encased in cement. If it's touching the
      ground and then into the concrete, it'll rust and eat all the way into the rebar into the cement.
      --Steve



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    9. #9
      Hop-frog is offline Member
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      Thanks very much for your help! I've been studying the pictures of your pond construction.

      Quote Originally Posted by icu2 View Post
      I'm confused by the comment "It must be raised, to protect it from regular flooding." Why would it
      need to be raised?
      We experience flash flooding in the summers. It does not just bring fresh water, but also tons of mud from nearby yards. My first pond did not survive this, so I've rebuilt the pond with a 20" stone rim, plus added French drains in areas where the flooding is worst. As a result, I can't simply dig a hole next to the pond to create a wetland-filter...it too must also be raised.

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      batman is offline Senior Member
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      Was afraid of the concrete dust killing my good circular saw so I bought an inexpensive one for block cutting. Wet and make the block damp but not dripping wet and dust is minimized. Got good life out of the Dewalt brand of diamond blade.
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