Help with foundation preparation

[nutbutter]

I was wondering if anyone can give me shed some general knowledge on preparation of soils underneath a building. I do the civil site work for commercial developments, however, we always sub out the structural and geotechnical, so my knowledge of building construction is very limited.

Anyway, it seems to me that the soils found onsite (I'm in the KC metro area) are rarely suitable as a building material and fill material has to be brought in. What kind of fill material do they usually use under buildings? Is it an aggregate mix like CA-6? What is a low-volume change layer and when is it needed?

I know clay under the slab can lead to shrink and swell, but can you eliminate the problem by placing a large enough coarse soil layer on top (assuming that piles aren't needed). Also, what is the difference between fat clay and (lean) clay? Is one more suitable to building?

Obviously, you can see that my knowledge of soils is very limited. If you could steer me to a reference (or better yet a free webpage) that explains any of these questions in more detail then I would be very grateful.

Peace,
Stoddard AKA Edwin

 

[ACtrafficengr]

It's not free, but check to see if any of your local colleges offer a basic geotech class. I learned a lot that way.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[cvg]

http://www.eng-tips.com/threadminder.cfm?pid=256

try reading through the posts on the geotechnical engineers foundation engineering forum for starters.

 

[nutbutter]

Does anyone have any quick and dirty answers or to my very broad questions above?

Any general information or advice would be appreciated.

Peace,
Stoddard

 

[SlideRuleEra]

Try "Foundations in Expansive Soils"

http://www.usace.army.mil/usace-docs/armytm/tm5-818-7/

and "Bearing Capacity of Soils"

http://www.usace.army.mil/usace-docs/eng-manuals/em1110-1-1905/toc.htm

Both are free .pdf downloads from the US Army Corps of Engineers.

http://www.SlideRuleEra.net

 

[Ron]

stoddardvila...

You should specify fill material that is free draining, having no plasticity, and no more than 10 percent material passing a No. 200 sieve (fines) as structural fill under buildings. Other materials will work, but evaluate them on a case-by-case basis. If such materials are not readily available in your area, you can use materials with higher fines contents, provided you create a capillary barrier beneath the floor slab, with coarse sand or small gravel, and make sure that the material is well compacted.

The material should be compacted to at least 95 percent of the maximum dry density as determined by ASTM D1557 (modified proctor) or 98 percent of the maximum dry density as determined by ASTM D698 (standard proctor), if you cannot use material with less than 10 percent fines.

In general, you should recommend proofrolling the building area with overlapping passes of a compactor appropriate to the soil type, so that soft or yielding areas will be made evident and can be removed and replaced with select fill.

Further, fill materials should be placed with a moisture content as close as practicable to the optimum moisture content determined by the appropriate Proctor test.

That's the quick and dirty.

 

[nutbutter]

Thanks Ron, that was helpful.

Is "pre-loading" an option? My dad is developing a site a few states over and said that they are going to use "pre-loading" instead of piling down to bedrock to build over these poor soils. It sounds like they just bring in a bunch of extra fill that will be a heavier load than the building and put it in the pad area. Then it sits for awhile and will consolidate the clays underneath so they will be more structurally sounds once construction starts. Is this basically how it works? How long, in general, do you need to let the excess load sit over the pad?

Also, I have one more quick question. What is the difference between fat clay and (lean) clay?

 

[ACtrafficengr]

We're using preloading on a bridge project. You need a layer of sand and /or wick drains to allow the water to escape. We're preloading for something like 9 months.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[Ron]

stoddardvila..."Pre-loading" or surcharging a site is done to force consolidation of soft zones, whether clay, silt or other. It should not be done in the absence of adequate geotechnical information and with the guidance of a geotechnical engineer familiar with local conditions. Time ranges depend on the consolidation potential of the materials, which can range from a few months to a year or so. Surcharging for periods longer than a year are unusual and usually not cost effective.

As for the clays, depending on who you ask, fat clay is generally a clay with high plasticity and a lean clay has low to moderate plasticity. It can also refer to the actual amount of clay in the material (which also influences the plasticity), for instance a material with more than 50 percent fines in the clay size range is termed a "clay". If the material has 55% clay particles it is a "lean" clay as compared to one that has, say, 80% clay particles.

 

[BigH]

Stoddardvilla - why not go to eBay or Amazon books. Look for a book by, say, Tomlinson on Foundations - look for the 4th edition. This will go a long way to explaining and in a reasonable manner many of the questions you are posing. I found that the Tomlinson book is an "easy" book to read and provides great explanations simply. You could go for Bowles or Das but they might end up confusing you more than many would think (they have gotten "fat" with lots of info - information overload for someone just starting out. Peck Hanson and Thornburn, Terzaghi and Peck are other works that would be essential.
Preloading does NOT always require the use of wick drains as alluded to. A lot depends on the nature of the soft soil you have - its structure. In the NJ coastal area, the compressible layers typically have thin sand layers/seams in them - as a result, the expulsion of water (consolidation) occurs fairly rapidly without the need for vertical drains.
I've use wick drains and stage loading to build rather high embankments on soft soils of inadequate undrained shear strength. But there is an art to it.