Acceptable Settlement Limits

[Ynot2009]

I am currently designing a foundation for a crude storage tank, 80+ metres in diameter and approx. 20 metres high.
Based on preliminary geotech. data, an annular crushed rock foundation should suffice.

The question i have is how are maximum settlement tolerances nominated? I believe i should pass this information on to the geotech. engineer to ensure the ground material suffices these defined tolerances?
API 653 nominates tolerances to assess in-service tanks based on evidenced and measured in-service settlements over a noted settlement radial area, however, is there any information/standard that specifies a foundation's performance specification for maximum settlements for geotechnical design? i.e. maximum uniform settlement shall be....?, maximum planar-tilt settlement shall be....?, maximum uneven settlement shall be....?, maximum differential settlement shall be....?, maximum edge settlement shall be....?, e.t.c

Other than EEMUA 159, i cannot find any other information relating to the above.

API 650 nominates construction tolerances but not foundation design performance tolerances/criteria.

Any valid assistance/commentary/guidance will be greatly appreciated.

 

[BigH]

Tanks will settle and the amount of settlement will depend on the soil type, its overconsolidation (yes, even sands), if any ground treatment is done, etc. If the settlements are uniform (relatively - yes, I know the centre will settle more than the edges, etc) in the general sense where the materials affected by the tank are homogeneously the same, then settlement magnitude likely won't be a problem so long as you don't have shear or edge shear failures. Of course, you will need flexible connections. I remember a case history years ago where Dames & Moore was sued (successfully, sadly) as they had indicated a tank settlement would be about 24 inches or so (memory so don't quote exactly). Client got nervous when the settlements reached 39 inches. He took down the tank and drove piles and rebuilt the tank. Dames & Moore's argument was that even though the settlement was more than predicted (and remember that consolidation settlements - if you are within about 30 percent of true, you had a good day) - the tank was still serviceable - it didn't lose its ability to function. D&M lost but not because of logic.

 

[Ynot2009]

Appreciate the response BigH, however, it has only touched on the query. The client has requested that varying maximum settlements be nominated (for those based in my initial query) for reference to the geotechnical engineer to enable the geotech. to specify an appropriate founding base material and/or to check the existing material suffices these criteria. I have tried to explain that settlements cannot be identified by exact values and that API 653 and EEMUA 159 relate to the assessment of existing tanks.

Is there information / standards / industry best practice e.t.c. that specifies maximum settlements that could be nominated in a specification to enable a geotechnical design to validate a foundation system to satisfy these criteria?

Again, any valid assistance/commentary/guidance will be greatly appreciated.

 

[ishvaaag]

http://www.petrospection.com.au/Tank Settlement Page.htm

http://www.api.org/meetings/proceedings/upload/Braune_EdgeSettlement.pdf

http://www.eng-tips.com/viewthread.cfm?qid=238207&page=3

http://www.ecedesign.com/pdf/Software/Coade/BrocherLowRes_TANK.pdf

http://www.geo-sys.com/ulsd.htm

http://www.geocon.net/pdf/paper42.pdf

http://www.eng-tips.com/viewthread.cfm?qid=112023&page=5

http://www.chempute.com/tank.htm

http://www.api-u.org/documents/2009ASTTrainingBrochure.pdf

http://www.ais-engineering-books.com/books/pc/viewPrd.asp?idproduct=11

http://www.allbusiness.com/manufacturing/chemical-manufacturing/936980-1.html

http://www.vakbladgeotechniek.nl/pdfs/Foundation-Option-for-LNG-Tanks.pdf

https://eprints.kfupm.edu.sa/1592/1/P019.pdf

http://www.ast-forum.com/index.asp?year=2004

Hope some of these help, google it.

 

[BigH]

ynot2009. What you are really trying to imply is that you wish ground treatment to be used (surcharge loading with wick drains, for example; or piled foundations; or other means) in order to limit a settlement to some specific value that you determine ahead of time? Unless there are specific tolerances on due to piping or other considerations (tilting as an example of a clarifier), personally, I think this is putting the cart before the horse. Let your geotechnical engineer study the strata, determine the settlement (and differential settlement); then make a judgment as to whether the settelement is "too large". If so, then you can look at the tools to limit the settlement by ground treatment of other means of more positive support.