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Allowable soil bearing capacity problem?-from boring test data

Allowable soil bearing capacity problem?-from boring test data

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.


RE: Allowable soil bearing capacity problem?-from boring test data

How high is the existing bin - and what bearing pressures are involved with it.  The boring data you have, obviously, was done before it was placed.  I am presuming that no piles were driven.  Does the soils log have anything on Atterberg limits - undrained shear strengths?  This appears to be a classic example of a desiccated crust. The fact that you have a bin already placed and it has been in place for 14 years or so - this would create an increase in the undrained shear stength of the soft to firm clay - but it is difficult to determine based on your information.  I would strongly suggest that you retain an experienced geotechnical engineer in the area and carry out another programme or one or two borings to obtain better information.  For your situation, you might try to compute the allowable bearing capacity from a two layer theory (as you will find in many texts - Button or Srinivasan); however, one aspect that needs to be considered is if settlement will be a problem - short term - and long term.  How stiff is the raft on the "doughnut" (or ringwall) foundation?  

Messing up can be very messy - see the Strathscona grain Elevator collapse.  
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:

RE: Allowable soil bearing capacity problem?-from boring test data

To add to BigH ---
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

What you have is a soil boring log and not a soil engineering report. Your second soil layer looks a lot like Fat Clay that has moisture content near the liquid limit.  This would make the soils highly compressible and not suitable for shallow foundation support.

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

With all this - it would be interesting to know what the status of the existing bin/foundation is.  The OR said that the ring foundation was already there - so something was supported.  Bin?  How high was it?  Any settlement records?

RE: Allowable soil bearing capacity problem?-from boring test data

the use of a safety hammer or automatic hammer seems to be a secondary problem, if it's a problem at all.  SPT testing (in any form) is not the best for sensitive clay.

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!



¡papá gordo ain't no madre flaca!

RE: Allowable soil bearing capacity problem?-from boring test data

Thanks a lot for all you guys precious responses. Recommandations are taken. And I already told the customer to find a Geotechnic firm to do the soil test. I will definitely not take any liability on the soil.

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.


RE: Allowable soil bearing capacity problem?-from boring test data

there is strength in the soft "silty clay" layer.  You cannot consider it a zero-strength horizon - after all, it's supporting the 12 ft of overlying soil.


¡papá gordo ain't no madre flaca!

RE: Allowable soil bearing capacity problem?-from boring test data

Winnipeg is known to be located west of Transcona to Russians... because shortly after Terzaghi came out with his famous bearing failure mechanism there was a large grain elevator, founded in varved laclustrine clays that decided to lay down on it's side... <G>

Best you get a good geotekkie involved!


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