Backyard Aquaponics
A place to discuss aquaponics

TC have you thought about raising the ground level. The water table here gets high in a hurry. That's why our septic fields are raised. Seems to me if you built the whole area up a few feet, it wouldn't be a problem.

I expect that a little bit of build up can be achieved with the fill taken from the holes dug to make the pond/tank.

Each location has it's own set of concerns. Here I can dig at least 4 feet down without hitting water even the day after one of those 4" rain events. I understand your property has water hiding just under the surface in all but the driest years. I don't think I would be trying to do anything more than a foot in ground in your location but here, I'm up on the ridge.

If we ever get to build the system at the "new house" (assuming we ever get to build the house) I intend to build up the ground around the tank area and hence the sump off to the side. I don't think I'm going to bring in enough fill for the purpose to have the whole system essentially above ground with dirt fill around it though. Then again, we will be spraying shotcrete in the construction of the house so I suppose we could make shotcrete tank and sump to go with it but I do kinda like the liner method for ease of design alterations.

Morning star uses a bunch of those concrete vaults for tanks. The handy thing about tanks of such a consistent shape is you can make dividers with pvc and netting for separating fish from eachother and grading ans such things. Tempting for those reasons though I'm sure they cost a fair bit, are heavy to move, and require sealing.

I've been looking for ways to introduce more void space under the gravel, if big enough this can be the sump. Think of it as an artificial water table. To make the void space you could use milk crates under the gravel, blocks, corrugated septic chambers, etc. As long as there is a mass of gravel on top I don't think rain water pressure could stress this much.

http://www.hancor.com/press/2008_04march.html

An idea I've been toying with has been to have the top of my sump level with my gbs so that when my gbs drain it is impossible for the sump to over flow and the gbs will empty at the rate of the pumping from the sump to the FT.

The uplift force when the soil is completely saturated with water (ie water table has risen to the surface) can be calculated by multiplying the bottom area of your hole times the depth times the density of water. Say you have a three foot hole with a 3x8 area, that will give an uplift force of 4500 lbs. Shocrete may help prevent the water from coming in on the sides, but it's not heavy enough to keep your basin in the ground.

Has anybody looked at (or heard of) structural fill? It's made from fly ash, a coal power by-product, and can be a very inexpensive way deal with fill. Even mixing it in with the sand would provide some cohesion that would in effect be creating a large cement lined sump.

Just a few thoughts, sorry I missed the action on this thread, I've just now discovered this forum...

Hey Jon, can you spell out that equation for us non-engineers.

Sure thing, in fact, a picture is worth a thousand words.

Still learning the ropes around here it seems, basically, if you let the water pressure build up under your sump, it will pop it up. One certain way to prevent this is to fill your sump up with water during a high water table event. You don't need the entire height since there is some friction on the sides that helps keep it down, but if you are just using a liner and no box, it's probably not a bad idea to keep the sides from caving in.

This is certainly a bigger problem in sandy soils, since water (and hence pressure) travel some much more quickly than in clays. If for some reason, your buried sump is in an area that floods often and you don't want to bother filling it every time it rains, consider either replacing it with something heavier, use some sort of friction adding material to the outside (like big nails sticking horizontally and sealed out of the box), or add a perforated pipe at the bottom to drain the water table. Keep in mind that if you add a pipe, you need to keep in clear or else you'll ahve problems.

Ask youself, if the tank wasn't in the hole during the flood, what would it look like. Now imagine the effort it would take to push an empty sump into that pond, and hold it there. Either weigh it down or nail it in if you're content to have a pond in your sump hole.

Simple equation:
water weighs 1kg/l. (approx 8.5lbs/gal.)

Uplift =
Mass of water displaced (sump tank volume * density)
minus
the mass of your sump tank

For example:

200l barrel weighs approximately 2kg, displaces 200l => empty barrel under saturated ground = 198Kg uplift.

IBC buried in saturated ground - 1000l = 1000kg - 20kg (mass of IBC) = 980Kg uplift.

If you fill the same IBC with 900kg of rock, and 50l of water, you have 1000kg-(20+900+50)=30 kg uplift.

It seems that this is an important consideration for those with large systems. The tendency it seems would be to place the sump in a low lying area, but if you bury it, all these issues seem to pop up.

It seems that to prevent system damage, it's a good idea with buried a sump and a pliable liner to fill the sump with water to prevent wall cave in if the water table rises.

I am tinkering with such a design and I'm not sure what the most cost effective way to deal with this is on a consistent basis. One cheap solution that comes to mind, at least for the clayey soil in my area is improvement with fly ash. It's cheap, perhaps free if you're clever, and almost makes me think that you could forgo the liner if not for leaching concerns. I've done a literature review on the subject and if you're interested in the more technical aspects, let me know. The same principle also works for sands, but you need a higher quality fly ash which I think would make it cost prohibitive.

Interesting DJ!
I have seen a 40' container rise from the ground in a clay soil, also pushed up
a large concrete slab on top of it ( 40 sq mtrs - [400 sq feet])
and a few ( hydrostatic sealed ) septic systems also pop up.
...but as yet not with sandy soils ( Australia, in particular Western Australia)
has very sandy soils near the coast.
But I am interested in what you are saying!

Sand is generally well drained, but if you're talking about a cyclone+low lying area+lots of obstructions (foundations) you might have a recipe for disaster.

And for the record, while we don't get cyclones in Michigan, we caught two hurricanes in a row a couple weeks back (Lowell and Ike), one from each ocean.

How much rain annually do you get DJ?
We here in the west ( south of Geraldton) get about 1100mm just less than 4 feet.

a bit less of rain, plus about the same amount of snow on top of that. we tend to get it in storms though, localized flooding isn't rare.