Allowable soil bearing capacity problem?-from boring test data
Allowable soil bearing capacity problem?-from boring test data
(OP)
Hi, everyone,
I am involved in a challenging project and need your help please:
A customer has an existing grain bin foundation and he wants to put a new grain bin as high as possible. So I am asked to determine what is the maximum vertical pressure we allow to apply on this foundation. After Analysis, the foundation structure itself is not an issue, but the soil bearing capacity is a big question, the customer now can only find the soil boring test data (done 14 years ago) and I summarized these soil boring data in attached document. No complete soil test report was found (which will usually contain soil engineer's analysis and the allowable soil bearing capacity recommended by the soil engineer).
My worry is:
From the soil boring test data (see attached document), top 12 feet soil layer has an allowable soil bearing capacity of around 3000psf (I am using Fig 8.10, relation between standard penetration resistance and allowable bearing pressure, in R.F.Craig's Soil Mechanics TextBook and the raft case), but beginning from 12' depth, the next 40' thickness soil layer has nearly no structural strength at all (standard penetration resistance is only 1 to 2 blows/per foot). Because the foundation diameter is 64', so according to soil mechanics, the pressure at 12' depth will still has 0.9P (P is the pressure at bottom surface of foundation. Therefore I am worried that if P=3000psf will make the whole 64' diameter foundation punch through top 12' soil layer and sink completely into earth (because the next layer has no strength at all).
So my question is: based on attached soil boring test information, what allowable soil bearing capacity can assigned to this soil?
Thanks for your help.
I am involved in a challenging project and need your help please:
A customer has an existing grain bin foundation and he wants to put a new grain bin as high as possible. So I am asked to determine what is the maximum vertical pressure we allow to apply on this foundation. After Analysis, the foundation structure itself is not an issue, but the soil bearing capacity is a big question, the customer now can only find the soil boring test data (done 14 years ago) and I summarized these soil boring data in attached document. No complete soil test report was found (which will usually contain soil engineer's analysis and the allowable soil bearing capacity recommended by the soil engineer).
My worry is:
From the soil boring test data (see attached document), top 12 feet soil layer has an allowable soil bearing capacity of around 3000psf (I am using Fig 8.10, relation between standard penetration resistance and allowable bearing pressure, in R.F.Craig's Soil Mechanics TextBook and the raft case), but beginning from 12' depth, the next 40' thickness soil layer has nearly no structural strength at all (standard penetration resistance is only 1 to 2 blows/per foot). Because the foundation diameter is 64', so according to soil mechanics, the pressure at 12' depth will still has 0.9P (P is the pressure at bottom surface of foundation. Therefore I am worried that if P=3000psf will make the whole 64' diameter foundation punch through top 12' soil layer and sink completely into earth (because the next layer has no strength at all).
So my question is: based on attached soil boring test information, what allowable soil bearing capacity can assigned to this soil?
Thanks for your help.





RE: Allowable soil bearing capacity problem?-from boring test data
Messing up can be very messy - see the Strathscona grain Elevator collapse.
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Tschebotarioff (1951) has another excellent treatment of your condition and how a grain elevator failed. Also see the article if you can find ASCE SM Journal 1970 as described:
http://cedb.asce.org/cgi/WWWdisplay.cgi?7000826
RE: Allowable soil bearing capacity problem?-from boring test data
I can think of several things which could have been mentioned in the original report which should concern you;
-- The sampling may have been accomplished with an 'Automatic Hammer' which would affect how these blow counts should be interpreted.
-- Was site drainage improved or compromised by the final construction?
-- Was the report actually accomplished for this type of structure & loading conditions?
-- Was any ground improvement accomplished as part of the original recommendations?
-- What are the actual soil properties? The short written description provided leaves far to much to the imagination. There are Silts and then there are Silts.
As BigH recommended, You really need a new Geotechnical Evaluation.
RE: Allowable soil bearing capacity problem?-from boring test data
As a rule of thumb, we like to see a crust that is 2 times as thick as the compressible layer. However, in this case you ony have 1 Boring and it show the crust is 30% instead of 200% of the compressible layer.
As mentioned above, you need a new soils investigation with several Borings, adequate lab testing program and comprehensive geotechnical engineering report. You are governed in this case by excessive settlement and not by bearing pressure.
RE: Allowable soil bearing capacity problem?-from boring test data
RE: Allowable soil bearing capacity problem?-from boring test data
You need to do a boring, take a few shelby tubes and determine the undrained shear strength and consolidation characteristics for the soft soil layer.
You use the classification "silty clay," but this is likely a colloquialism. The actual zone of silty clay (in ASTM D-2487) is very small and the maximum liquid limit for this zone is way below your natural moisture content.
To answer the question you posed in the OP, do more field work, get good samples, determine the undrained shear strength (for the confining stresses of the sample depth), now the actual soil classification (i.e., do Atterberg limits and percent sand contents), get an odometer test performed with time readings and a rebound curve and look at bearing capactity and settlements.
J. M. Duncan has published a great reference on the design and settlement of steel tanks. Go get this and learn about it too!
f-d
¡papá gordo ain't no madre flaca!
RE: Allowable soil bearing capacity problem?-from boring test data
However myself is also interested in this topic and would like to learn something new. To me, I feel the very weak 40' thickness layer can be neglected for its supporting function and considered as similar to a pond of water, and the 12' soil layer punching shear failure will control the soil capacity. Being left school too long, I can not remember exactly how to calculate the soil punching shear, but just wondering if we can use a same way when calculating the punching shear load for concrete slab? Please correct me if my logic is wrong.
I will keep you guys updated about this soil issue.
Thanks.
RE: Allowable soil bearing capacity problem?-from boring test data
f-d
¡papá gordo ain't no madre flaca!
RE: Allowable soil bearing capacity problem?-from boring test data
Best you get a good geotekkie involved!
Dik