STORAGE TANK SETTLEMENT ALLOWANCE.

[Mselva]

Dear Engineers:

I would ask according to your experience, what is the maximum settlement to be considered as "permissible" for a storage tank.

We need performing a soil improvement. If we do it until 40 feet we will get a settlement of 2.5 feet but if we do it until 80 feet we will get a 1 foot settlement.

The difference in cost is substantial so we wouldn't like to have to reach the 80 feet for improvement.

I would appreciate your feedback

Sincerely,

 

[IFRs]

Either way you have large settlement and your piping network will need to provide for it. You may want to design the foundation and tank for routine periodic jacking and grouting.

 

[Mselva]

Thank for answering.
I guess there must be a limit to the settlement.
I know I can use flexible connections for connecting tank - pipeline but what will be the most reasonable gap for that?

 

[IFRs]

Tanks with large settlements need special piping arrangements, which are unique to each layout. You can probably design to tolerate 4" to 6" of movement before some adjustment is needed. You will have to monitor the settlement and make provisions to adjust the piping or jack the tank as needed. It will become routine maintenance. If the piping is 2" maybe more, if 36" maybe less.

 

[Mselva]

Hi IFRs:
The inlet pipe is 16 inches and the outlet pipe 12 inches. It is possible to think in flexible connections with settlements up to 2 feet?

 

[JStephen]

There's really not a limit on total settlement, but if you have large total settlements, you'll likely have large differential/out-of-plane settlements that have to be dealt with as well.

 

[Mselva]

Someone knows about flexible connectors for pipes of 16 "and 12" that allow us to handle the tank settlement up to two feet?
Thank you,

 

[LittleInch]

The Flexible connection is the least of your issues in my opinion - Use marine hoses which are 15 m long. The issue with these sorts of settlements is that they are rarely even over the diameter of the tank (you don't say what diameter of tank you are looking at) and hence there is a considerable risk of tank collapse or failure due to uneven settlement and the tank creasing or collapsing. Either you need a small tank with a very strong foundation to prevent one soft area from undermining the tank structure or you need to pile your foundation down to solid ground or have a factor of safety on your pile design.

I am asusming that the settlement you are talking about is the settlement of the tank once you've built it and filled it with something, but I stll think this is a big amount.


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[IFRs]

If you have time, consider pre-consolidation so the settlement is mostly done by the time the tank is built. One way is to put in wick drains with runoff channels, pile dirt up to a height that equals twice the tank load. Let it settle and drain. Then scrape off the excess and build your foundation / tank. Takes time but is pretty simple and proven.

 

[Mselva]

Thank you.

The two proposal are designed to prevent differential settlement. The situation is that one (the most expensive) proposes an improvement to achieve a settlement of one foot, the other proposes a minor improvement, preventing differential settlement but subject to a settlement of 2 feet.

For a cost issue, we would like to implement the proposal with a settlement of two feet, only if you can handle them with pipes.

The tanks are 50 feet high * 150 feet in diameter.

Thank you,

 

[foreigner]

You might want to check out API 653 Annex B.

 

[Mselva]

Thank Foreigner. I will.
Best Regards,

 

[Bluecat46]

Hi if you use EEMUA 159 section 7 tank foundations give you some good information to work with.

 

[Mselva]

Thanks Bluecat.
Best Regards,