It's not "osmotic pressure", it's "pressure". Osmotic pressure is about water flowing through a semi-permeable membrane (eg cell-wall) from a more dilute solution into a more concentrated solution.
/me remembers Mr Ford's biology lessons from >40 years ago.
It doesn't actually "float" it is water pressure below the liner raises it.
That is my worry. I know a few large rocks will stop it floating fully, but if it starts bulging & bubbling in could be a problem
It's also about solution pressure differential ... if you have a 'dry' area with zero water pressure then that is low osmotic pressure, and the ground around has a high water pressure, the pressure differential will cause water to flow across the ground to hole transition point.......... it's the science behind land drains.
If that works (with a few large boulders to help) then I'll be happy.
Otherwise I have dug a damn big hole I have to fill in. :-(
Your use of the term 'osmotic pressure' is incorrect. Osmosis, and the pressure resulting from it, arises as a result of differing salt concentrations across what is known as a semi-permeable membrane, in effect a super-fine filter, that allows water molecules to pass but not soluble compounds such as sugar or ions such as sodium, potassium, chloride, sulphate etc. Where two different salt concentrations exist on either side of a semi-permeable membrane, water diffuses from the low concentration to the high concentration in an effort to establish equilibrium and make the two salt concentrations equal. If the higher concentration is in a closed container, very high pressures can build up amounting to over a hundred atmospheres in extreme cases.
What you have is simple drainage from a moist or saturated soil into a large void. There is no semi-permeable membrane and no difference in salt concentration between the water in the soil and that in the hole. Call it hydraulic pressure or water pressure if you like, but what it isn't is osmotic pressure.
As I and others have said already, if the pond is full of water then the water pressure in the pond onto the liner from above balances the pressure from below and the liner won't float. The liner will only float if the level of water in the pond drops below the water table at any time, which is unlikely if the OP is raising the edges of his pond above the surroundings and maintaining a good level of water in it.
I can assure you it can float. The big pond that was at marconi had several large bubbles in the liner, to the extent they looked like whales trying to get out.
All sorts were tried to get rid of them including chucking something like fifty concrete fence posts on the bottom and drilling holes in to let the air/gas out but nothing worked for long.
Presumably methane from rotting vegetation under the liner, or even a rotting underlay. Harry's talking about water pressure.
No, it's not osmosis, it's just a pressure differential. Apart from requiring a semi-permeable membrane (we're talking on a molecular level here, not a 4 inch pipe with holes in) osmosis needs two solutions, one with a higher concentration of solute than the other. In your example the dry side isn't a higher concentration of anything, it's just dryer (and downhill).
That is floating.
Andy
The liner is denser then water so it can't float in the conventional sense. It is is hydrostatic pressure raises it from the bed. Or gas as someone else suggested.
This 'term' features in exactly this context in several Building text books ... on Ground works, so maybe it has more than one use ?
That is how they describe the design & action of land drains ... The perforated pipe as the semi-permeable boundary between water laden soil and the void within the pipe .... and how the pipes will also draw water from below the pipe as well as from above.
I'm not a scientist .... but the reference texts on ground drainage specifically state movement of water into perforated pipes in land drains is due to Osmotic pressure.
It that is the case, and I'm surprised to hear it, then it is a misuse of the term IMO. A simple perforated pipe could never act as a semi-permeable membrane in true osmotic terms. Can you give a reference to a specific textbook, or are you quoting from memory?
Fascinating. AFAIK Osmosis only has one meaning, and that isn't it.
Andy
IMO the OP means 'hydrostatic pressure', but he's mis-remembered it from somewhere. The Wiki entry on land drains makes no mention of osmotic pressure, nor would one expect it to. See
In message , Chris Hogg writes
I knew I was right to stay away from osmosis:-)
I was warned by the supplier of my liner that *sub-surface* water will share space with that contained in a flexible liner. Apparently they had a lot of complaints from owners of swimming pools dug in Thames Valley gravels.
For a flat bottomed pond, laying a few inches of pebbles on top of the liner might help but the sides can still bulge inward.
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