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Questions and answers from the GMA Techline.
I need your advice on a reservoir we are designing. It is an irrigation reservoir with a 40-mil HDPE geomembrane liner on a 3:1 slope. The client wants to cover the liner with 1ft of silt loam soil. The water elevation in the reservoir will fluctuate about 15ft through the season. I am not sure the best way to get the soil to stay on the slope if it goes through seasonal saturated and unsaturated conditions.
Your advice is much appreciated.
Pete | Wisconsin
Pete,
There is trouble ahead!
At the minimum, be sure to use a double-sided textured geomembrane. If a geotextile will be used above the geomembrane and below the cover soil, it should be a needle-punched nonwoven of adequate weight. This combination should be statically stable with a silty loam cover soil on a 18-degree slope but the F5-value will be nominal at best.
Now the really ugly part and that has to do with water. Your commentary of saturated vs. unsaturated is appropriate but of greater concern is drawdown of the reservoir level that adds a hydrostatic pressure notorious for causing sliding, so much so that such failures are called “veneer failures.”
The rapidity of the drawdown vis-à-vis the permeability of the cover soil is the key parameter in this regard. Next, and indeed of importance, is the saturated shear strength of the cover soil. Here proper laboratory similitude of direct shear testing is necessary.
The point in the above discussion is that both testing and design are absolutely necessary for a safe slope against veneer failure.
Pete, please do not just try it just to see if it works as you describe … it won’t!
Bob Koerner | GMA Techline