Impact of Ringwall size on center to edge differential settlements - API 650 Tanks

[kapoA]

thread256-327302

I am designing a 144' by 60' API 650 tank on firm to stiff clay soils.

I read that it's common practice to size the ringwall width such that the bearing pressure is same as that under the tank base - to minimize differential settlements. I wonder how much impact does width of ringwall have on differential settlement between tank center and edge?

Behaving as a flexible footing and following elastic theory, the settlement at tank center and edge will be different (center almost twice edge) even if the bearing pressure under ringwall is made same as that of the tank base.

Need clarity on this issue. Thanks

- KapoA

 

[JennyNakamura]

Do you have a soils report? You might be able to just excavate the top two or three feet and backfill/compact with structural base course and top it off with a a sand cushion (if it's ok with the geotech), or a similar scheme. Ring walls are intended for use where you have crappy soil at the surface but competent soil a few feet below. The ring wall will confine the bad soil in the upper layer allowing the vertical pressure to be transferred to the competent soil and the ring wall will go into hoop tension. I know some folks think it's a good idea to always use ring walls. Either way, use a cushion layer of sand at the top and crown it at the center to account for the greater settlement at the center. The soils report should be giving information on expected settlements (at least the ones I'm used to looking at).

If you haven't already, you should read "Oil Storage Tank Foundations" by Boberg. Here's a link:

http://www.slideruleera.net/TankFoundations.pdf

Jenny

 

[kapoA]

Jenny,

Geotech recommendation says ringwall foundation and have given settlements. What I was really trying to know is how much impact does width of ringwall have on differential settlement between tank center and edge?

KapoA

 

[eea]

Sounds like you should pose this question to the geotech and when you get their answer, verify with the tank supplier that their tank can take the stress atrributed to this if it ever develops?