Free swell test
Free swell test
(OP)
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
www.bauerconsultbotswana.com





RE: Free swell test
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!
RE: Free swell test
Carl Bauer
www.bauerconsultbotswana.com
RE: Free swell test
RE: Free swell test
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.
RE: Free swell test
RE: Free swell test
RE: Free swell test
RE: Free swell test
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RE: Free swell test
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
RE: Free swell test
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.