Uplift factor of safety for clay liner 1

[vollEngineer]

Anyone have info on acceptable factors of safety for uplift in detention ponds (I got a 2' clay liner with a small layer of soil above and 1.5' head difference) Water Mgmnt wants FS of 2. I get 1.3 or so. Anyone know of generally accepted standards or calc procedures that allow the FS to go lower? Anyone have a standard saying the FS for uplift can be as low as ...?

 

[GeoPaveTraffic]

My question would be "what is the potential for the water head to be greater than 1.5 feet?" or "what is the potential that thte clay will be thinner than 2 feet or weigh less than you are using in your calcs.?"

 

[vollEngineer]

You're absolutely right. I'm only willing to review the normal operating conditions, though. I'm not designing. I'm reviweing the uplift factor of safety under design conditions.

 

[GeoPaveTraffic]

I think you missed my point. If the "normal operating conditions" are so well know that there is no chance of the water head being greater than 1.5 feet or that the unit weight of the clay will be less than you are using or that the thickness will be less then you only need a factor of safety of 1.0. However, since it is very unlikely that you know these things to that level of precision, then you need a factor of safety greater than 1.0.

I realize that your question is how much greater, but you and your client are the only ones that can answer that question. An appropriate factor of safety depends on many factors, including: risk aversion, consequence of failure, knowledge of the input parameters, exactness of the calculations, etc. As a profession, we have chosen to forget this in favor of a magic number that works all of the time.

Look over the above list, add to it, and talk to your client about the situation. Only then can you make a good professional judgment about what the factor of safety should be.

 

[vollEngineer]

Gothca. Taking a breath is sometimes a good way to find a solution. I appreciate your thoughts.