Design of membrane-lined ponds 1

[MRM]

I've been asked to assist in finding the solution to a problem involving membrane-lined ponds for a WWTF. The pond was recently constructed (during some very wet months here). The general design features 3:1 side slopes, smooth geomembrane, topped with a consistent 8 inches of "sand,", and then with a consistent 4 inch layer of "topsoil."

The pond has been filled and approximately 50-75% of the side slopes well above the water line have sloughed in significantly, sliding down the membrane.

We went out to the site, did some checking, collected some of the sand samples, and found that:

a.) The sand+topsoil depth was consistently under the expected total of 12 inches in areas regardless of whether sloughing or no sloughing occurred; i.e., there was no discernable pattern of performance vs. adherence to the specs.
b.) After some testing, the sand used is very fine; approx. 60% pass the #40, over 30% passes the #100, and 8 to 10% loss by wash using the #200. The particles within the fine sand fraction appear to be subangular to subrounded, and the angle of repose of the washed portion is around 34 deg.
c.) After checking with the civil engineer, I found that there were no specs (at least none that were strictly enforced) for the sand material to be used above the membrane.

I'm in the process of finding some more details out regarding the membrane itself.

At this time, I suspect the sand is not as well drained as a sand should be for this application. I think that rain/possible sheet flow or run-off fed a lot of water to the material. Considering the gradation, I think a partial liquefaction of the material occurred causing the sloughing observed and down-slope movement over the membrane.

Has anyone else had an experience similar to this with a lined basin?
What type of membrane should typically be used for this application?
Is strain/elongation of the membrane material usually a consideration for the downward forces typically expected? (I wonder if this may have initially caused the formation of small tension cracks near the crest of the slope.)
What reference is typically used for lined basin design?
Any case histories about using a rather poorly drained material for this application?

Thanks.

 

[DMcGrath]

Looks like this project was doomed to fail from the start. A smooth geomembrane at 3:1 slope is unlikely to hold any type of soil material for very long, especially when it is fully saturated. Some interface shear testing would determine the maximum angle you could expect to be stable (GM vs sand) in both saturated and unsaturated condition.

As soon as the water travelling down slope in the sand material hits the waterline of the pond, it has nothing to do but saturate the entire thickness. Regardless of the permeability of the soil layer, when it hits the waterline, it will be full saturated.

A more appropriate liner would have been a textured liner or perhaps a geocomposite clay liner (GCL). I have seen ponds lined with smooth geomembrane, but the GM was left exposed and no attempt was made to cover it with soil. Liners covered with soil typically have at least 18" thick. That begs another question: how did the contractor deploy 8" of sand on top of the geomembrane without damaging the geomembrane? Typically you need enough layer thickness above the GM to reduce the equipment pressure on the GM to less than 5 psi. I have seen the track pad "grousers" on a bulldozer seriously damage a 60mil HDPE liner through 12" of pea gravel. Any concerns about the integrity of the liner?