FOUNDATION DESIGN FOR TANK FARMS 3

[17237]

A client wishes to construct a tank farm (ten 80-ft diameter tanks) on silty sands. The fine sands (medium dense) extend all the way down to 60 feet from ground surface. The client wants to place the tanks directly on the sands without footings. Does anyone have any experience doing this? Especially, when an impermeable layer has to be placed below the tanks as per SPCC regulations.

 

[stewbaby]

Think you'll find this practice in almost any tankfarm over 20 years old...and API 650 allows earthen foundations without ringwalls if engineering analysis and practice prove a ringwall unnecessary. BUT...We are continually jacking our older tanks to install ringwalls and replace in-situ sand with clean structural fill to correct settlement and corrosion issues (our location consist of clayey sand that tends to eat tank bottoms).

Read up on API 650, especially appendix B.4

The impermiable layer can be worked into either case, althought the ringwall makes it much more straight forward.

 

[17237]

Thanks very much. I not only found API 650 but also found Duncan and Orazio's paper on stability of oil storage tanks. Found them both to be very useful.

 

[BigH]

To be honest, I have worked in a number of such refineries and have never used a concrete ring foundation. We always put directly on a tank pad - a built up pad of about 1.5m high about 5m larger in radius than the tank. The pad would be select granular and we'd use 3inches of crush screenings. On some projects where we had fill, we did use a granular ring wall at the edges. In some of our cases we were on firm to stiff clays but your sands should pose much of a settlement problem and will be built out during your initial water test of the tank integrity. Of course, you didn't indicate if the sands were "loose" or "compact". Sounds, though, like you have good conditions for a tank farm. The only other thing that you will need to address and do so with a degree of seriousness is the containment issue. With silty sands, if the tank were to rupture, you have a major problem with contamination of the soils below and out. You will need a clay liner or impermeable geomembrane for containment along with containment dykes. If you use clay, remember that if it dries out it may develop shrinkage cracks. Check out your state/province requirements for tank rupture mitigation. Ontario has a good one from what I rememeber.

 

[pmkPE]

The diameter of the tank is not the concern, it is the tank height and the specific gravity of the tank contents versus the bearing capacity of the sands and what amount of settlement you can accept as well as the issue of containment. What are the contents with regards to toxicity?

 

[17237]

The site is in Texas. N values are from 14 to 20 all the way down to gravel at 60 feet. Fine silty sand is encountered from 0 to 14 feet. Medium grained sand from 14 to 60 feet. Height of the tank is 60 feet. As for settlement, the client is still debating the issue with their engineers. Since no clays are available, what would be the best way to construct dikes around the tank farm?

 

[BigH]

I don't see a major concern with the settlement. Tanks will operate okay even with 6 inches of it. If large dia tanks put the centre up a bit as it will settle more. However, I do always recommend flexible jointing of piping leading in/out of tanks.

For containment - you will probably go with a geomembrane buried a few feet below the site grade - your sand on top. Similarly, the bunds will be of sand with the geomembrane taken up the sides - need to think through the detail of 'holding' the geomembrane at the top -how to turn down to ensure it does slide back on you. Rememeber to have a system to drain the bund if you get a rupture - you'll have to collect the spilled liquids. Remember to post no-smoking signs everywhere. etc.

 

[Focht3]

Where in Texas? Gravel at 60 feet?

If you're on the Gulf coast, you will want a tank ring in case of a hurricane and the associated surge tide. Not to mention the 130+ mph (209+ kph) winds. I know, I know - we haven't had any tanks undermined in over 30 years. Hmmm, we haven't had a major hurricane in Texas in over 30 years, either!

We are long overdue...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[17237]

This is a pipeline breakout facility in El Paso (just north of Ft. Bliss). Fine sands followed by caliche and medium grained sands, in turn underlain by gravel. I don't think we need to worry about hurricanes. You are not THE FOCHT from Mclelland Engineers, are you?

 

[Focht3]

Ah, El Paso! You wouldn't know what to do with all that rain! You only have to worry about small earthquakes, but a tank ring may or may not be needed.

I'm not THE FOCHT from McClelland Engineers - but I know him awfully well...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"