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Old 03-26-2007, 09:21 AM   #1
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Concrete shed footings in a marsh/wetland?


My next project is a 10X12 shed to store my yard maintenance junk. I have 4 - 8" sonic tubes, 4 - 4X6X8 treated posts, and 14 - 10X12X12 treated boards leftover from my deck that I will be using as a base floor.

How easy is it to dig footings and pour concrete in wetland areas? The proposed location does not have sitting water but I know there is probably water 2" under the surface. Footings will have to be 48" deep min since I live in the midwest.

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Old 03-26-2007, 12:12 PM   #2
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Concrete shed footings in a marsh/wetland?


Sounds interesting, to say the least.....pretty hard to place concrete and achieve strength in water....I would use the sono tubes, if it meads your code, and dig holes, placed the tubes, and pour.....you might want to dig a test hole and see what will actually happen. If it does fill with water immediately, then you might want to discuss this with our local inspector or a contractor and seek advice from someone familiar with your specific area.

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Old 03-26-2007, 12:35 PM   #3
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Concrete shed footings in a marsh/wetland?


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Sounds interesting, to say the least.....pretty hard to place concrete and achieve strength in water....I would use the sono tubes, if it meads your code, and dig holes, placed the tubes, and pour.....you might want to dig a test hole and see what will actually happen. If it does fill with water immediately, then you might want to discuss this with our local inspector or a contractor and seek advice from someone familiar with your specific area.
Well, I did a little researching and it sounds like one possibility is to dig the footing, wrap two thick garbage bags around the sono tube and then put the tube in and pour the concrete. Sounds like it might work.

Just have to dig the holes and try I guess.
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Old 03-26-2007, 03:01 PM   #4
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Concrete shed footings in a marsh/wetland?


Cement will set just fine under water. In fact it will have a higher end strength than normal concrete. It will require longer to reach that strength, which matters for stripping the forms, etc. If you have access to a big enough pump, you can hydroset the sonotubes by setting them where you want them, then using the hose to scour beneath them.
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Old 03-26-2007, 07:44 PM   #5
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Concrete shed footings in a marsh/wetland?


I agree Tscar with the point of concrete in water...but for a DIY project? I have never had to contemplate doing this, but how would you get at the mix you want?
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Old 03-26-2007, 09:08 PM   #6
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Concrete shed footings in a marsh/wetland?


Nothing different from doing the same thing on dry land, other than the issues of digging a hole that fills with water faster than you can dig, hence the hydraulic method of "driving" the form.
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Old 03-26-2007, 10:18 PM   #7
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Concrete shed footings in a marsh/wetland?


To pour the concrete below water, do the same think they do on bridge seals but on a smaller scale. On bridges they may pour 10 to 20 feet or more below water. You just the same principle, but on a smaller scale.

With the hole dug, the 8" Sonotube in place (vertical and set to height), and water in the Sonotube, insert a large hose or pvc pipe (3-4") down to the bottom of the Sonotube form. Pour concrete(3/4" maximum aggregate size) down the tube - it is heavier and will force the water up. You must keep the bottom of the pipe well below the top of the concrete. As you pour the concrete, the water will be displaced and forced out of the top of the form. When you get the form filled, scrape off the extremely wet concrete and pour more into the pipe to get the better mix to the top.

Insert some vertical rebar and a Simpson bracket that you can attach your wood to.

No wood in the water to rot!

The amount of vertical support you get depends on whether you can get the bottom of the sonotube down to firm ground. You can check first (before pouring) by driving something down to measure the firmness on the soil.
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Old 03-26-2007, 10:22 PM   #8
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Concrete shed footings in a marsh/wetland?


You should do that when placing concrete 4' + into a sonotube, anyway, wet or dry.
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Old 03-27-2007, 07:37 AM   #9
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Concrete shed footings in a marsh/wetland?


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Originally Posted by concretemasonry View Post
To pour the concrete below water, do the same think they do on bridge seals but on a smaller scale. On bridges they may pour 10 to 20 feet or more below water. You just the same principle, but on a smaller scale.

With the hole dug, the 8" Sonotube in place (vertical and set to height), and water in the Sonotube, insert a large hose or pvc pipe (3-4") down to the bottom of the Sonotube form. Pour concrete(3/4" maximum aggregate size) down the tube - it is heavier and will force the water up. You must keep the bottom of the pipe well below the top of the concrete. As you pour the concrete, the water will be displaced and forced out of the top of the form. When you get the form filled, scrape off the extremely wet concrete and pour more into the pipe to get the better mix to the top.

Insert some vertical rebar and a Simpson bracket that you can attach your wood to.

No wood in the water to rot!

The amount of vertical support you get depends on whether you can get the bottom of the sonotube down to firm ground. You can check first (before pouring) by driving something down to measure the firmness on the soil.
Thats the method I was looking for! Thanks! I will post pics when I am done.
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Old 03-27-2007, 06:26 PM   #10
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Concrete shed footings in a marsh/wetland?


Anyone see any issues with using only qty four 4X6 posts on four 8" footings as a base for a 10X12? Will it be enough?

I never questioned it until someone that I worked with suggested using a min of 6 posts.
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Old 03-27-2007, 07:17 PM   #11
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Concrete shed footings in a marsh/wetland?


The size of the concrete post/pier would probably be dictated by strength of soil you hit for solid bearing. A 10x12 is not big, but you could get excessive settlement with a 8" diameter Sonotube if it is not too good.

You may not need posts if you build directly on the concrete.

You could jump up to 10" or 12" if you felt the solid ground was a little soft. The procedure would be the same.

In most areas, a shed under 120 sf would not require a permit, but you should check.
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Old 03-27-2007, 07:37 PM   #12
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Concrete shed footings in a marsh/wetland?


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Originally Posted by concretemasonry View Post
The size of the concrete post/pier would probably be dictated by strength of soil you hit for solid bearing. A 10x12 is not big, but you could get excessive settlement with a 8" diameter Sonotube if it is not too good.

You may not need posts if you build directly on the concrete.

You could jump up to 10" or 12" if you felt the solid ground was a little soft. The procedure would be the same.

In most areas, a shed under 120 sf would not require a permit, but you should check.
Yes, 120 or under doesnt have a required permit. I think I will upgrade to 12" columns and just have four. Judging from the slope of the grade from door entrance to back of shed the entrace side floor joists will probably rest on the 2 footings and the back two will require about 2 foot posts. Starting on footings tomorrow if weather holds...

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