Soil's load bearing capacity - Ballast #92

Structural Layout Vignette and Multiple Choice

Soil's load bearing capacity - Ballast #92

Postby fr3d_astaire » Sun Feb 22, 2015 4:32 pm

I need some further clarification on Ballast question #92:

A soil's load bearing capacity:
A. increases with depth
B. decreases with depth
C. is not related to the depth
D. is related to only the soil type

I thought the correct answer is C because it really depends on the site conditions, type of soil, water content etc. (ie a layer of clayey soil under a later of sandy soil). Ballast says the correct answer is A, is this because typically the deeper the soil the more compact it gets?
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Re: Soil's load bearing capacity - Ballast #92

Postby Design intent » Sun Feb 22, 2015 4:51 pm

If you read Mike-SE's notes on lateral soil pressure he discusses how the soil pressure increases with every foot of depth.
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Re: Soil's load bearing capacity - Ballast #92

Postby fr3d_astaire » Sun Feb 22, 2015 6:03 pm

Design intent wrote:If you read Mike-SE's notes on lateral soil pressure he discusses how the soil pressure increases with every foot of depth.

Correct, however the question is asking about the bearing capacity of the soil, not how much lateral pressure it exerts. ;)
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Re: Soil's load bearing capacity - Ballast #92

Postby mick2 » Tue Feb 24, 2015 1:09 am

The deeper you go, the closer you get to stable bedrock (away from the slits and fine grains) 'A' is the only correct answer, soil load bearing capacity increases with depth.
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Re: Soil's load bearing capacity - Ballast #92

Postby Mike-SE » Tue Feb 24, 2015 3:15 pm

none are correct. And all could be correct. this question is subjective, skip it if it comes up on the exam
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Re: Soil's load bearing capacity - Ballast #92

Postby fr3d_astaire » Tue Feb 24, 2015 4:42 pm

Mike-SE wrote:none are correct. And all could be correct. this question is subjective, skip it if it comes up on the exam

I figured, thanks Mike!
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Re: Soil's load bearing capacity - Ballast #92

Postby Coach » Tue Feb 24, 2015 4:47 pm

Mike-SE wrote:none are correct. And all could be correct. this question is subjective, skip it if it comes up on the exam

Actually, guessing is better than skipping.
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Re: Soil's load bearing capacity - Ballast #92

Postby hnQ_9999 » Thu Feb 26, 2015 3:54 pm

Now that I know the correct answer is A, my guess is "A".
C is definitely wrong.
D is half correct.
That left A and B. Imaging bedrock at 48 inches below finish ground and
bedrock at 48 feet below finish ground. One could "guess" 48 feet depth
of soil would provide more bearing capacity, assuming only soil
is responsible for load bearing.
E. I can be wrong
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Re: Soil's load bearing capacity - Ballast #92

Postby Mike-SE » Fri Feb 27, 2015 7:18 am

C is definitely correct in Mississippi, parts of Texas and other states. Soil in northern Mississippi consists of 300 feet of highly expensive clay, no change in capacity with depth. There are areas of the country where bedrock is so deep that It should be considered not there
D is definitely correct, that is why IBC Chapter Chapter 18 provides load-bearing capacity of soils base on SOIL TYPE
bedrock 48" down will provide higher load capacity than at 48' below grade. If you place the footing directly on the bedrock you have the same capacity. IF you bear your footing 36" below grade you have 12" and 47' of compressive material between the footing and bedrock.
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Re: Soil's load bearing capacity - Ballast #92

Postby rifkamattman » Sat Feb 28, 2015 11:19 pm

not sure if this helps, but just going over Thaddeus (again) and this is what he says:

Bearing Capacity (Ballpark):
Bedrock = approx. 10,000 psf
Gravel
Sand
Silt = 2,000 psf
Clay = 1,000 psf
Organic

This of course affects foundations systems (shallow versus deep).

I know that soil pressure is completely different, but in the same breath....

Soil Pressures:
Gravel has a large vertical and small horizontal component for pressure (angle of repose)
Dry Sand has a large horizontal and small vertical component for pressure (angle of repose)
Wet/Moist Sand has an even larger horizontal and small vertical component for pressure (angle of repose)
Saturated Sand has the largest horizontal and small vertical component for pressure (angle of repose)

And by the way: Fp = Allowable Soil Bearing Pressure, which dictates type of foundation
Light Load/Good Soil = Shallow Foundation
Heavy Load/Poor Soil = Deep Foundation
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