Foundation Ideas for Raised Pond

[MoonShadows]

I've been giving thought to what foundation I will use for my raised pond, and am seeking feedback.

The pond is a 425 gallon hexagon pond. 3 of the 6 sides have flat benches attached at the top.

Dimensions:
Edge Length: 35.5"
Short Diagonal: 61.5"
Long Diagonal: 71"
Perimeter: 213"
Area: 3274.5" (22.7 square feet)
Height: 30"

If I have done my calculations correctly, the unit when full will weigh @3850 lbs. That's 425 gallons x 8.36 lbs. per gallon plus about 300 lbs. for the frame, filter, etc. If I divide that weight by 22.7 square feet, I come up with a weight of 170 lbs. per square foot. (Not as much as I imagined before I did the math.)

These are the three possible foundations I am thinking of:

Ground: Nice compacted earth where I want to put it...no problem

Slightly raised platform (deck material) of @10 x 10 to created an area around the pond and benches. However, with this foundation, any idea how close my footings should be to assure stability? I will probably float it on solid concrete blocks, each placed on a bed of gravel. That is the same method I used when I built my 8' x 10' chicken coop on just 4 corner footings and it has stayed level for over 4 years now...but is much less than 170 lbs. per square foot.

Timber framed ground level paver, slate or pea gravel base. Thinking that might blend in the best with my landscape, but will it radiate and hold too much heat in the summer?

Any thoughts?
Thanks.

 

[sissy]

I have a 300 gallon stock tank sitting on the ground full for over 4 years now and did nothing to it or the ground and it is fine .But if there is any kind of erosion of soil it may be a problem or sandy soil which PA is not known for or back fill soil .But freezing could pose a problem for your pond and fish .

 

[MitchM]

It really depends on water, frost level and soil conditions.
Make sure that water is draining away from the pond in all directions.
Right on the ground would be the simplest, but you should do it on a bed of crushed gravel, the depth of the crushed gravel depends on frost, water and soil.
Slightly raised raised platform, four corner blocks would be fine, again on crushed gravel, a little deeper though.
The paver option is the most difficult because over time the pond will eventually push the immediate pavers down, unless you do a really good job laying the base gravel/sand layers.
My personal preference would be a raised platform or on a concrete slab.

 

[MoonShadows]

Thanks for the input, Mitch.

Soil conditions are hard and compact. The company says you can put it right on the ground no matter the frost conditions. In case of a problem, while the pond will be level, the water would drain away from the house and right down into a field below where we plan to put it. The ground would be the simplest, but I like an idea of a "border" around it to create an outside "room".

So you think a slightly raised platform with just 4 corner supports would be enough on a @ 10 x 10 when the pond (without benches is @ 7 x 5) would be centered on that 10 x 10? No need for additional support right under the pond itself?

My thoughts exactly on the pavers...if they become uneven, a corner could eventually compromise the liner.

While the ground would be the easiest, we really like the slate paver idea, but because of the potential problems with a slate bottom the raised platform seems to be our direction right now.

 

[MitchM]

Your soil is clay? Topsoil on clay? Clay expands when it gets wet, topsoil will simply squish out of the way and decompose/reduce over time.
And fyi, British Columbia to the rest of Canada is what California is to the rest of the USA... BC has a fairly mild climate.

If I put that pond on the ground here, guaranteed it would freeze solid every winter and the outside frame would be moved around by the frost.
A 10" joist platform would hold that size I think. I would check some lumber span tables or look at some sites that explain deck construction for hot tubs.
My indoor 600g aquarium is supported with 4 6 x 6 posts on an engineered lumber frame that I built. I chose engineered lumber because it doesn't have the variations that dimensional lumber does. Your pond will have a much higher tolerance for flexing of it's base than my aquarium has.

 

[MoonShadows]

To be honest, I have no idea of my soil composition, but where I want to put the pond the soil is very stable. If I put it directly on the ground, I plan to dig down and put a layer of gravel, followed by a layer of sand, followed by the liner protector. I really am not too worried about heaving in such a small space, and the outside frame fits together with no hardware (cedar 2 x 4's interlocked with dado cuts), so there is plenty of tolerance.

But, like I said, I want to create an outdoor room around the pond, so since slate is bad for the liner and heat retention, I will most likely will go with a raised platform. Thanks for the suggestion to look at span tables.