slab on grade

Site Grading & Site Design Vignettes and Multiple Choice

slab on grade

Postby fixybopixy » Sun Oct 13, 2013 11:26 am

Ballast practice exam #6

6. what type of soil would be best for slab-on-grade construction?
A. Clayey sand
B. organic silt
C. poorly graded gravel
D. lean clay

Correct answer C. This confuses me bec per my attachment from Ching section 3.18, the base course of a slab on grade is gravel but then the soil beneath it can be anything else. Wiki says "Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas." (http://en.wikipedia.org/wiki/Shallow_foundation) so i am not sure if the question is asking which is the best base course or which is the best soil below the base course.

any ideas?
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Re: slab on grade

Postby verygnar2meetU » Sun Oct 13, 2013 1:30 pm

You are getting the reasoning confused.

Where there is a lot of slab-on-grade (by virtue of little to no frost level), there are happen to be clay soils. This is a circumstance.

In terms of the actual construction, gravel is the best choice.
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Re: slab on grade

Postby fixybopixy » Sun Oct 13, 2013 1:57 pm

tx for reply. and poorly graded gravel is better than well graded gravel for slab on grade?
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Re: slab on grade

Postby verygnar2meetU » Sun Oct 13, 2013 3:42 pm

Gravel is well draining regardless of gradation.

Well-graded gravel under a slab is best, considering it will compact better than a gap/open/poorly graded gravel. This is because there are many different sized gravels that work together, filling in against each other, and providing strength.
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Re: slab on grade

Postby Coach » Mon Oct 14, 2013 2:52 pm

fixybopixy wrote:and poorly graded gravel is better than well graded gravel for slab on grade?

Do you see "well graded gravel" on the list of possible answers?
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Re: slab on grade

Postby DaveW » Mon Oct 14, 2013 7:24 pm

It's not uncommon to stratify base to sub-base layers.

Base Layer - GP
Sub-Base - GW
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