Pond lining options 4

[murdock]

I have a storm pond that needs to be excavated well into the water table (GW @ 3' BGS, pond floor at 12' BGS). I don't want to introduce all this groundwater into the receiving water, so I'm investigating pond lining options. So far I've seen bentonite (used in a geotextile 'sandwich' as well as tilled into the underlying soil) and plastic liners (HDPE and PVC).

What do you have for preferences for pond lining and why? Thanks in advance.

--
Shane Mullen, Staff Engineer
Llewellyn-Howley Incorporated

http://www.lhinc.net

 

[DMcGrath]

Choosing a lining system is mostly based on application, with a lesser degree of designer preference. A few decision points:
1. How important is it to keep the water in the pond without percolation to the subgrade (and water table) - are there compliance issues?
2. Will the pond need to be cleaned of accumulated sediment? (liner damage during cleaning)
3. How will the high water table affect the liner (installation difficulty, "floating", etc.)? Most likely this will require some form of dewatering.
4. Will there be any requirement for leak detection?

From your description, it sounds like the hard part is going to be keeping the groundwater out of the pond and preventing your liner system from floating. These are not insurmountable problems, but the scope of work for this "pond" will probably surprise those that have to write the check to pay for it. You may also want to verify that the depth into the groundwater is OK with the government environmental folks (EPA or the like in the US + state agencies).

Good luck!

 

[cvg]

normal pond liners are designed to prevent water from leaking out of the pond. Your problem is the reverse and I believe difficult to solve. For example, how will you prevent the 9 feet of hydrostatic pressure at the bottom of your pond from blowing the bentonite liner out and flooding the pond? Floating was already mentioned and would be difficult to overcome. Full concrete lining could work, but would need to be designed for hydrostatic uplift.

 

[bltseattle]

As noted by CVG, you need to account for the buoyancy uplift on the liner by (a) ballasting the liner (sufficient weight overlain to hold it down against buoyant forces) or (b) drain the water table to reduce uplift or (c) provide a groundwater cutoff (e.g. slurry wall) designed to eliminate uplift.

I prefer clay/bentonite liners because of their self-healing capability, but they are more expensive I believe than say PVC.

 

[murdock]

Thanks everyone.

Looks like I should minimize the amount of groundwater that has exposure to the pond to reduce uplift as much as possible. I thought of putting a curtain drain along the perimiter of the pond to do this until I became concerned with introducing the groundwater to the discharge point (since the pipe would have to daylight somewhere).

I'm not too familiar with slurry walls. Would they be a lot more expensive as opposed to a curtain drain?

--
Shane Mullen, Staff Engineer
Llewellyn-Howley Incorporated

http://www.lhinc.net

 

[dicksewerrat]

How deep will the water be on the inside of the pond? You can subtract the inside depth from the outside depth. This will give you the actaul forces to counteract in keeping the liner on the bottom. You can place small stone on the liner, then larger and larger stone. Then when it comes time to clean out the sediment, once you hit rock, STOP.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bltseattle]

Slurry wall I've seen used in this application consisted of a trench filled with controlled-density fill. This sounds economical to me (lean concrete, no steel, minimal forming), but I don't know the cost. Thickness was 12 or 18 in, I can't recall now. This element should be designed by a geotech since the depth will depend on the soil permeabilities and water table elevation(s).

 

[DMcGrath]

Installing a slurry wall may be a good option if site conditions are favorable for this. If the slurry wall has nothing to "key into" at the bottom (i.e. a relatively impermeable layer), then the groundwater will merely flow under and restore the groundwater level on the inside unless the water is actively pumped or drained out. The slurry wall I have in mind is a bentonite-amended material. A good geotechnical investigation will go a long way to help you decide a course of action, perhaps to the point of rethinking the pond depth or reducing stormwater runoff volume to avoid costly underground retrofitting.