FEA analysis on Roto Moulded PE underground Rainwater Tanks
FEA analysis on Roto Moulded PE underground Rainwater Tanks
(OP)
Hi All,
Im just about to embark on designing some underground cylindrical rainwater tanks roto moulded using Polyethylene. The tanks will be backfilled with pea gravel. I have tried to do as much research as i can on loading of undeground tanks as this is a brand new area for me.
I have found an earth pressure equation which takes into consideration the angle that the backfill is at in relation to the side wall of the tank. Was planning to split the faces of the tank up in equal heights and work out the pressure for each height increment. I would then just apply thsi normally?
I dont know if this is the right approach. Could you even point me to some examples of where this sort of analysis has been done?
Tank wall thick nom 10 mm. Will use shell elements, large displacments and Cosmos Advanced Prof.
Thanks in advance
Grill
Im just about to embark on designing some underground cylindrical rainwater tanks roto moulded using Polyethylene. The tanks will be backfilled with pea gravel. I have tried to do as much research as i can on loading of undeground tanks as this is a brand new area for me.
I have found an earth pressure equation which takes into consideration the angle that the backfill is at in relation to the side wall of the tank. Was planning to split the faces of the tank up in equal heights and work out the pressure for each height increment. I would then just apply thsi normally?
I dont know if this is the right approach. Could you even point me to some examples of where this sort of analysis has been done?
Tank wall thick nom 10 mm. Will use shell elements, large displacments and Cosmos Advanced Prof.
Thanks in advance
Grill





RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
Could you use a hydrostatic pressure loading and just apply a really high fluid density that somewhat matches the curve that would be generated by your other equation? Can you publish the equation that you found?
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
I dont know how to scan and attach but it is
Ph=Koyh
Ko an K1 i cant type out teh equation but they take into consideration the angle of the material against the side of the tank.
Grill
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
I agree with namklof that the soli pressure is a lot more complicated then the equation you give. The basis for the equation you give is a hole with vertical walls supporting a surrounding soil with a horisontal ground level.
Then the pressure can be calculated as: Pressure = constant * depth * density of soil * g
Since your tank is cylindrical the pressure constant will depend on the inclination of the supporting wall. And since the tank is flexible the pressure will also depend on the deformation.
There are ways to simplify this but I you might need help from somebody with geotechnical experience to do it.
GBor mentioned fluids and there are similaritys but one huge difference. Fluids typically don't have any "inner" strengt, soils have. Ever heard of a pile of water?
"Hydrostatic" can be conservative but also unconservative depending on deformation.
I hope I haven't confused you to much but part of the problem is the flexible tank. If it was rigid it would be simpler.
Good Luck
Thomas
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
I think you got my point, but just to make it clear, I was simply trying to make it easier to apply a smooth load. Even the equation you provided, my point was to multiply the "constant" times the soil density (also a constant, I think) to give a modified density. Then, the pressure really is rho * g * h (depth), which is what most processors use to apply hydrostatic pressure.
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
I got your point. And you are correct, there are similaritys between soil pressure and hydrostatic pressure. In some respects they are the same for a vertical rigid surface. But for inclined nonrigid surfaces they can differ significantly.
My point, and I think you agree, when you analyze this you have to understand the mechanics / physics of the problem.
Regards
Thomas
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
From further discussion my two loading conditons are as follows.
(1) Concrete backfill with a high water table - I have to analyse the tank when it is being installed due to weight of wet concrete on it. Also once set i will analyse with Hydrostatic pressure
(2) Dry site with Pea gravel backfill - Again this is a soil loading problem.
I got the equation from the Roark book which is is Formulas for Stress and Strain. Im trying to find the correct page.
Hope this makes sense
Sean
RE: FEA analysis on Roto Moulded PE underground Rainwater Tanks
I believe there is some method of simulating creep within Cosmos Works ADV Prof that i have but i havent worked out how to do it yet.
Grill