Ringwall Fdn Settlements

[SeanMD]

Design of a ringwall tank foundation, (ringwall required by owner).

I have proportioned and designed the fdn so the soil pressure under the fdn is the same as the soil pressure under the tank (at the same depth as b/fdn), which I think is pretty standard practice to attempt to limit differential settlement. Thing is, these pressures are totally dependant on fluid height, and in the long term the fluid height will not be constant. For example, right now they balance at 3/4 height, but they are about 500 psf off at full fluid height.

Does it make sense to try and balance the pressure under the max fluid level condition where the most settlement will occur, or at some average fluid height, at say 3/4 full, since in the long term the fluid height is always changing?

 

[FixedEarth]

I Would continue your approach to differential settlement analysis. For example, compute the differential settlement between the center and the tank perimeter when tank is full and also when it is empty.

Also consideration should be given to heave if soft clay is present at bottom of footing. I would also look at effective stress values if Sand and GWT are close to the bottom of the excavation.

 

[oldestguy]

I'd do the settlement analysis with full load. That then is likely to give the most settlement and, for most soil areas (not peat), I'd assume that this settlement will not rebound noticeably with lighter tank loads.

More info on soil types would help discussors to advise.

 

[SeanMD]

Soil type is loose sand and silty sand. The majority of the settlement (3" or so) is expected during the one month hydrotest phase. After that the tank will hold gasoline with a lower SG and since the majority of the settlements will have already occured, I would think balancing the pressures at a later date will be less important than during the hydrotest phase.

I think I have it pretty close, at the full hydrotest level the interior soil pressure under the tank is ~350 psf higher than the soil pressure under the ringwall, and bearing pressures for all load cases are within allowables. If I make the ringwall narrower (12"), I can balance them out at the full hydrotest load, but the soil pressure for the seismic load exceeds allowable by a whole bunch.

 

[cileng]

Can you share how you calculate the settlement under the ringwall and the center of the tank? Do you use the constraint modulus and change in the pressure due to loading under the the tank and the ringwall foundation to calculate it? Do you also look at the bearing capacity? Thank you.

 

[fattdad]

adjusting the bearing pressure of the ringwall to the anticipated fluid contact pressure will not really address differential settlements. Irrespective of the foundation bearing pressure, the seat of settlement (under an empty tank) will just not go to that great of depth. When the fluid is added to the tank, the seat of settlement will extend to a much greater depth and additional settlement will occur (i.e., you say 3 inches or so).

I'd review a paper by J. M. Duncan that relates to his case studies on tank settlements and proceed accordingly. (I'll try to find citation.)

f-d

¡papá gordo ain’t no madre flaca!

 

[SeanMD]

cileng - I'm not sure, the numbers come from the geotech.

Fattdad, If you can find the citation, I would appreciate it.

I finished the project, and basicly just checked the bearing pressure under the ringwall at full water condition, and compared it to the pressure under the tank. It was pretty close. I decided to not really look much into the variable operating loads with fluid lighter than water, since so much of the settlment will occur during the hydrotest, and rebound will not occur.