Advice for Tank Foundation on Clay

[coastalforu]

I have 4 70ft diameter x 40 ft tall tanks, 4 ft apart, applying 5,000 lbf/ft2 each on clay down to 120 ft. No ringwall. Tanks have sunk 4ft over 40 years and need to be raised and leveled. What type of foundation is recommended?

 

[JStephen]

I'd suggest consulting your local geotechnical engineer.

If you're planning to put foundations under the existing tanks, that may limit your options considerably.

 

[Bagman2524]

I would recommend pilings down to a hard clay or sand layer. Surprised they didn't have pilings to begin with. Most southern Louisiana clays can only handle about 1000 psf

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet

 

[hokie66]

I would think, for what it is worth, that you have achieved a stable condition for these tanks after 40 years of preload. But if you remove the preload for reconstruction, you may allow the material to rebound. You need good geotechnical advice as to how to proceed.

 

[dhengr]

Coastalforu:
I agree with Hokie, and would like to know the settlement/time frame history of these tanks. Has the settlement settled down over the years, or gotten worse? If this settlement has not done any damage to the tank bottoms, or to the joint btwn. the bottom and the tank shell, what’s the problem? And, to raise the tanks, what is your plan of attack? That can’t be a simple project. You could add a reinforcing plate ring around the bottom of the shell (maybe a rolled, bent, angle) and put small screw/auger type piles at regular intervals around the tank to spread the load out a bit, and to a lower strata of the underlying soils.

 

[charliealphabravo]

Sounds like you have another leaning tower of Pisa on your hands. With the clays compressing like that I would get foundation recommendations from a geotechnical engineer. I would say your foundation design depends on whether you think you are near the end of settlement or if you expect another 2-4 ft in 40 years.

 

[oldestguy]

There are firms in the business for fixing situations like this. I won't try to list any here, but check for a nation wide firm that has a wide variety of methods of underground improvement. In general I would expect they will be in the compaction grouting field. Do a search for nation-wide geotechnical pressure grouting contractor as an idea. You will need a list of references with a similar problem. Ordinary mudjacking firms likely are not suitable, due to the complexity of this. You will want a form of guarantee, since these methods can cause unintended damage, such as filling sewer lines nearby, lifting the wrong area, etc. While installing piles may be thought of, I question that option, since you might as well tear down the tanks and start over then.

 

[FixedEarth]

Don't assuming all the settlement have taken place! Get a soils firm to drill few borings say down to 80 ft or so and ask them to run settlement analysis. If the total settlement is 6 ft, re-leveling will not help you for long. I agree, I am also surprised they haven't used piles or piers on the original design. Further, the circles are too close, so you induce intersecting pressure bulbs. That is is each tank will induce vertical stress to the tank on the left and to the tank on the right. So are the corner tanks leaning inward?

http://www.soilstructure.com/