Topsoil as a Lake Liner? 2

[bidrite]

I am working on a project in an area with a topsoil layer resting on top of sand and gravel. The proposed pond is likely to need a liner to maintain a constant water level during dry seasons. Due to the lack of clay soils, the idea was proposed to use topsoil as a lining material in lieu of bentonite or a mechanical liner. Does anyone have any experience doing this or reasons why this would not work?

 

[roncity]

Most top soils contain organic matter that will rot and decay and dissolve when submerged.
So even it the soil is compacted, eventually the loss of organic material will allow water to pass thru and drain the pond.

Unless the top soil tests as a high clay/ silt content and low organic, I’d suggest looking for a better lake liner.

 

[CarlB]

"organic matter that will rot and decay and dissolve when submerged"

Not true with highly organic soils, peat bogs go centuries submerged with little change, it's when they are not submerged that they decompose. I've heard of very old logs being retrieved from the bottom of the Great Lakes and are in perfect shape.

 

[cvg]

Peat may not decompose for centuries however, it does not provide the qualities you need for a lake liner. The liner must have a low hydraulic conductivity. For soils, this usually means high clay / silt content. You need to sample your proposed topsoil and determine if it is suitable. If not, maybe it can be blended with some bentonite to make it perform better (if it is marginal quality).

 

[civilperson]

Bentonite is a cost effective solution since a small amount will suffice. The expansive clay flows to the leak location and the administration technique can be two men in a rowboat with 50 pound bags of bentonite sprinkled on the water. A second application is sometimes required.

 

[dgillette]

CarlB and cvg are correct that organic material can last an awful long time (witness the iron-age bodies dug up from peat bogs in the UK, peat found deep in karst features in FL, etc., as well as logs and shipwrecks in the Great Lakes), but it has to be kept submerged, away from oxygen, the whole time. If exposed to the atmosphere, it can go quite quickly, so it appears to me that it would be a poor choice for a lake that might empty under foreseeable conditions.

 

[bidrite]

Thanks to all for your input. I think I will try to separate the most highly organic material for disposal and use material with less visible organics blended with bentonite and see what happens.

 

[BigH]

dvg is correct that peat normally does not have the permeability requirements for a liner. However, when loaded (as in tailings dam sediments), the coefficient of permeability drops considerably - two to three orders of magnitude. There was some good work done in the mid-70s on this - my own group carried out similar experiments. I know of one uranium tailings dam where the decrease in the coefficient of permeability under load was taken into account and the peat/muskeg was actually left in place to act as a liner.

 

[oldestguy]

Will topsoil rot? Hey fella, any current day rotting is long complete. I hear that rotting and settlement theory many times by so called geotechnical engineers, yet I would dare say they never saw a case of this happening and causing a problem. I don't hesitate to call for foundations on topsoil, provided that topsoil soil is compacted. This came about by watching contractors "get away with it" and it worked. Only I apply the usual rules for foundations on low organic soils, neglecting the color of the stuff.

Anyhow, topsoil run-off is common in settling in the bottoms of glacial lakes "lining the bottom" and many an excavation right near that lake can be dug and only local ground water needs handling.

At Madison, WI where ground water supplies the city wells, the groundwater table has been lowered by well pumping in town. You can dig right near those lakes and you are in the dry, below the lake level. The mud bottom forms a seal of sorts. The wells don't appear to affect lake levels.

So, to say it won't work is refusing to recognize that at times it does work.

 

[fattdad]

returning to the original post: Your statement is tha the pond "is likely" to need a liner. I'm just wondering what the field conditions are such that you are not sure? Will you excavate to create the pond? Where's the water table? How thick is the sand and gravel layer? How deep will the pond be when completed.

I'm working on a similar project and we are looking at using a pond liner. One item of note is what is the consequence of failure? If the leakage is more than you "calculated" what's the loss?

Consider the following: A pond that is 6 ft deep and "failure" is when water drops to 4 ft deep; a liner that is 0.7 ft thick (i.e., 8 in) with a permeability of 2x10^-6 ft/min (i.e., 1x10^-6 cm/sec) and a water table that is below the liner. Just how long will it take for the pond water to drop the two feet?

Using Q=kIA and taking the intermediate head as 5 ft (using a 1sf area) you get
Q=2x10^-6(5/0.7)1=1.43x10^-5ft^3/min

If you then complete the math, that relates to 97 days. So it would take 3 months for this drop to occur (less evaporation).

How much water are you directing to this pond? How many rain free days are you considernig?

As it stands, we are commenting on whether topsoil can be used in a liner - well sure, I guess. First it would help though to know what the liner requirements need to be. . . .

f-d


¡papá gordo ain’t no madre flaca!

 

[BRIS]

Top soil by definition is permeable otherwise there would be no water movement for plant growth. Even when compacted it will have an infiltration rate of at least 2mm/hour. So you lose at least 48mm a day. It will likely be much more. You also lose say 5mm a day to evaporation.

 

[CarlB]

I don't think it's reasonable to compare permeabilty of a submerged topsoil layer with exposed, unsaturated topsoil supporting lawn. Plants/grass also promotes infiltration.