How to determine if soil is expansive?

[Tstruct]

I am not from geotechnical background so I would like to understand the following questions:
1) What are the limits/values (from geotechnical test) to determine if soil is expansive or not? And if it is expansive, are there any tolerable limits of expansion to safely place structure on. Please provide references from codes.

2) Please see the attached bore log and comment on the expansion of this soil. Foundation has to be placed at 2m level.

Thank you.

 

[sdz]

"Please provide references from codes." What country?

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Ooo eee ooo ah ah ting tang walla walla bing bang

 

[Tstruct]

Preferably from US codes. But you can refer any country.

 

[sdz]

AS 2870—2011, Australian Standard®, Residential slabs and footings
Section 2 Site Classification.

A shrink swell test of site soils is required.



Ooo eee ooo ah ah ting tang walla walla bing bang
Ooo eee ooo ah ah ting tang walla walla bing bang

 

[kieran1]

At 2.0m the SPT is 4. This is poor for a building foundation bearing.

I usually consider soild with a PI>15 to be expansive



Kieran

 

[MTNClimber]

Colorado firms use ASTM D4546, Standard Test Methods for One-Dimensional Swell or Collapse of Soils [URL unfurl="true"]https://www.astm.org/d4546-21.html[/url], to determine if a soil or bedrock sample is expansive.

The 1D Swell tests are typically performed on intact samples of soil collected from a California sampler. But if you are just curious to see if your disturbed samples are expansive, you can remold them to a desired density and flood them with water. This is sometimes done on projects where imported fill needs to be checked for a swell potential specification. This video shows one method of checking swell potential of a remolded sample. Please note that the more water added to a sample (for compaction), the less the soil will swell.

Although I don't find it helpful, Peck, Hanson, and Thornbrun (1974) has a table that suggests a general relationship between PI and expansion potential: 0-15 = Low, 0-35 = Medium, 20-55 = High, >35 = Very High. As you can tell, a PI of 10 could have a Low to Medium Expansion Potential. It could be non-expansive... or it could be moderately expansive. Who knows. Which is why the 1D swell tests provide a better answer.

 

[GeoEnvGuy]

To your first question on limits of how to determine if a soil is expansive or not. Is the material free draining if yes then its not expansive, most clay soils are not free draining therefore have the potential for expansive behavior. The second part of your first question tolerable deflections are usually related to differential settlement and that is determined for each foundation type. There is no bonus allowable deflection or differential settlement because the site has expansive soils, it just means more mitigations required in the foundation to limit deflections.

To your second question based on the below chart from 1964 PI of 9.6, clay fraction of 64.9% low potential, this really doesn't mean much though as the minerology for various clay minerals have different swelling potential. This is why the local geotechnical engineer would be best suited to inform you of the potential risks from expansive behavior of the clays found in the region.

 

[howardoark]

If you're required to do expansion tests you should probably do them. We wouldn't consider a PI of 9.6 to be expansive, but your geotechnical engineer didn't do Atterberg limits on the material your foundation will be resting on, so you really don't know. We have expansive shales in my area.

Do you have collapsible soils in your area? If so, you should also have them look into that.

 

[RazvanGherasim]

Hello, Tstrut!

In Romania there are some indexes that can determine if a soil is expansive or not according to old romanian standards and eurocode 7.
First would be the activity index which is calculated by IA=Ip/A2(niu) (Ip - plasticity index and A2(niu) would be the percentage of clay (particles smaller than 2 microns). If a soil is expansive, Ia would have a greater value (0.75...1.25). If Ia>1.25, that soil sample is very active.

The second indicator would be UL (free swelling, i hope i got that right). This is mostly a chemical test made in a cilinder. UL=(Vf-Vi)*100/Vi (Vf - final volume, Vi - initial volume). The soil samples with UL<70% are considered unexpansive. If 70%<UL<140% the soil samples are considered expansive.

And to determine the pressure of soil expansion (Pu), this can be determined in oedometer test. Increase sigma (vertical effort) to the design value (geological+future structure effort), let it consolidate at every loading stage. After the final load stage consolidation, put water into the cell, and if the soil is expansive, you will not see an additional settlement, but a decreased settlement.

 

[morecoffeeplz]

@MTNClimber: Many firms along the front range use the Denver Swell procedure in place of the ASTM method. I do not recommend the procedure in that video as there are potential issues.

 

[MTNClimber]

More coffee -

Please elaborate. My understanding is that the Colorado/Denver swell test is very close to ASTM D4546.

The procedure in that video is just a way to understand if your soil is expansive and not a method used in my practice. I agree the way it was executed can be problematic, but in a pinch, it could tell you if you have expansive soil.

 

[dik]

With a SPT = 4, I'd likely look for a different site.

They have the abbreviations for everything except the SPT, it appears.

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-Dik