Compacting pond liner with high water table 1

[chiggy]

We have excavated a small backwash settling pond and are planning to install a clay liner. When we reached the bottom depth we hit water and the soil (clayey sand) is very unstable and is pumping . It's basically like walking on a water bed in places. At this point I'm not sure a roller could get in and out without burying itself. I don't expect it to dry out on it's own very quickly. My first thought is to dig a sump, pump it, wait, and pray for no rain. We may have to mix portland or something to stabilize it? The contractor wants to install a poly liner. I am not too fond of this idea if the water table ever reaches above the pond water level.

Thoughts?

 

[fattdad]

You can often remove soil faster than the ground water can recover. When excavation occurs too fast you create a transient condition where there is a vertical upward gradient, which can otherwise disturb firm soil. Now I'm not saying I got the story right, I'm just giving you something to consider. . .

In an ideal world, you would have known the positon of the water table prior to digging (i.e., had a well installed). You would have known the various soil layers below the water table and you would have performed dewatering or installed a cutoff in advance of excavation.

Where is this project located?

What is the function of the liner (i.e., to facilitate a permenant pool; to separate contaminated waters from the natural ground water system, etc.)?

In the US I just can't think of too many jurisdictions that would allow contamination to be directed to a pond situated below the ground water table. There are exceptions; however (e.g., I designed an inward gradient landfill over 15 years ago in Virginia).

The ground is now disturbed. Do you really want to pay to muck it out? Was there dewatering specificaitons in the contract? Is a membrane liner really that risky (i.e., how often will the lined pond be empty and the uplift forces be a concern)?

Just a few comments during coffee break. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Consider stabilizing the bottom and then adding the liner Stabilizing commonly is done by adding breaker run rock, with sizes maybe 4 inches max and smaller. You may have to excavate some before doing this, because the end result is you are adding solid volume. Stabilization depth required can be determined by testing one area, with a tapered amount from say one foot to three foot thickness. Then drive on that and use the thickness that this test provided for the whole job base. You may have to waste the part of the treatment area done with the lesser thickness of rock. During the test use what ever compacting devices you plan to use for the whole job.

Chances are that by the time this stabilizing is done, you will have most of the rock voids filled with the soil from below, so hydraulic conductivity of the original soil won't have changed much.

I have used this tapered test method for many other areas needing stabilization, such as pavements. It gives you the most economical thickness needed and you are not guessing.

I doubt that any form of sump drainage alone will work. There also may have to be a drainage layer added before you place the liner, so that your liner won't be affected by excess water from below. Single size aggregate for the drainage layer will work only for a short time before it is plugged up. Instead use a less permeable well graded layer, such as concrete fine aggregate, with perforated pipes at no more spacing than 15 feet. Concrete sand does not plug up. A clean bank-run sand may substitute, but give that a test also before committing to it.

 

[BigH]

Oldestguy has some good thoughts on this which are in line with a situation we have where we have a naturally occuring "hole" in a rock elevation with seeping water into it that we need to fill with a clayey silt material.