Free swell test 4

[phuduhudu]

I have been asked to look at a house built on clay showing considerable distress. I am a structural engineer but the client had a geotech report done. It has free swell results on 2 samples of soil of around 80%. That seems like a lot to me but I am not sure how to interpret it. The soil also has a PI of 22-23 with about 30% of the material passing a 75 micron sieve. I don't have much experience with clay soils. Can anyone shed any light on the significance of the free swell test results.

Carl Bauer

http://www.bauerconsultbotswana.com

 

[fattdad]

What's the liquid limit?

I'm not too sure what the "free swell" test would do for me; however. The fact is that if you have a crawl space the only thing that matters is the foundation performance (and the chimney foundation if you have one). You can lower these an extra few feet (it costs more) and get into the zone where there is negligable moisture change throughou the year and many of these shrink swell problems go away. You can also address some of this by controlling your landscaping.

The foundation provides a bearing stress. To realize foundaiton distress, the swell pressures have to exceed the foundation stress. Free swell will overstate the extent of the swelling. No doubt, the free swell test will give you some measure that the soil is prone to swell. I'm just not sure how you convert free swell into a design value for the foundation. I'd be interested to see what others do. . . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[phuduhudu]

There is no crawl space. The slab bears on a layer of imported gravel on the existing clay soil.

Carl Bauer

http://www.bauerconsultbotswana.com

 

[AlmostPE]

what is free swell test? I am confused.

 

[emmgjld]

There are a variety of 'swell' tests, unfortunately, some of the terminology is similar but, the definitions are not. Some tests just indicate whether the soil is expansive and provide a general designation, such as slight, low, moderate, severe. Such tests are very limited for design purposes. I have seen some attempts at correlation with local conditions which appear to hold water.

For a free swell of 80%, the test may be placing a sample in a vertical faced flask to a known height, adding water and then measuring the final height of the 'swelled' clays. Unfortunately, without any local correlations, I can offer no advice, other than the clays are pretty expansive and probably nasty to work with.

For actual interpretation, I will assume your free swell test is, or is similar to, ASTM D-4546, method A. I am also going to assume the test includes a reloading of the sample after the free swell has been determined.
If this test method has been used, I believe the free swell should be 8%, assuming the Liquid Limit (LL) is less than 50. If the Liquid Limit is over 50, a significantly higher number of free swell is appropriate but, I have not seen any that approach 80 but, I have led a sheltered life.

Using this test method, after the free swell has been measured (at a nominal 'seating pressure, 1 kPa [20 psf] or somewhat greater) the sample is then loaded, with readings to obtain load vs. displacement. The loading should continue until the original sample height (void ratio) is obtained. At this point the 'swelling pressure of this clay can be obtained.
The resulting load curve could be used to estimate amount of potential soil heave at given load.

 

[dirtsqueezer]

USCS defines clay/silt soils as being over 50% clay/silt. You are probably looking at a silty/clayey sand or gravel, which is a lot of silt/clay, but most of what you're looking at in terms of aggregate, well, won't swell. As far as the PI is concerned, I believe that's done on only the clay/silt portion of the material, again being only 30%. This being said, the plasticity/liquidity won't determine the overall behavior of the larger grained soil. I assume the same would apply for the swell potential as well. Have you done any other building in the area that you could compare the soil types to? It also helps if you know or can contact the geotechnical engineer. I'd be surprised if his/her number wasn't somewhere on the report.

 

[dirtsqueezer]

Just as a caviot, I'm looking up ASTMs for the above statements. I've found ASTM D4546-03 for shrink/swell potential that might explain your 80% value. As far as the Atterbergs go, after reflection, it might include sands. I'll just write when I know for sure.

 

[BigH]

India has a "standard" on the free swell test - it is similar to the one that emmgjld described first. They don't use water, though. I'll try to see if I have a copy of the test or procedure or lab form - but it would be at home.

 

[BigH]

Still looking for the test method and the liquid used - but in Indias Ministry of Transport, they do not use a clayey material with a free swell more than 50% for embankments except in special cases.

 

[MiketheEngineer]

Call the geotech guy and ask him for help!!

 

[msquared48]

In Colorado where there are a lot of these expansive soils, I have seen styrofoam used under portions of the strip footings, and some spread footings.

I am not a geotech, and have never designed such a footing system, but can only assume that the purpose of the styrofoam is to direct the potential for the swell of these soils into weaker areas created by the presence of the styrofoam. I can only guess that the percentage of styrofoam useage is related to the percent of swell.

Just thinking out loud here.

Mike McCann
McCann Engineering

 

[Patgeotech]

carlbauer,based on your free swell test results, clay content and plasticity index value, your clay would classify as probably potentially medium in terms of expansiveness (free swell between 50 to 100). Often when the structure is double storey in size and the free swell is less than 100, the structure shows no sign of distress, however I am guessing that your structure can be regarded as light (possibly single storey). A medium potential for swelling would put you in a swelling potential range of between 1.5 to 5% of the layer thickness. Did the geotech report do any swelling pressure testing. As your structure is showing signs of distress, I would guess that the sweling pressure is in the range of 15 to 20 kPa (not good for a single storey house).

Another question - are there any big trees next to the structure? Any trees been removed? Anything that could have lead to drying out of the clay? Conversely, any leaking buried services?

Hope this is of assistance to you.