Swell Percentage Vs Swell Index 3

[pelelo]

Hello,

There are two different ASTMs for obtaining the Swell % and the Swell Index (at this time I don't have them handy).

My question is, is it possible in real practice to have negative values of swell percentage?, same question applies to the swell index.

In my experience, whenever I get either a swell% or a swell index value, it is always positive which I interpret as the volume of the expansive soil sample is expanding (instead of shrinking).

Please let me know your thoughts.

 

[MTNClimber]

We use ASTM D4546 Test Method C for determining swell %. It is possible to get a negative value, its called hydrocollapse. In those soil types you are worried about settlement due to wetting of the subsoils instead of swelling soils. Keep in mind that sample disturbance of non-expansive or even expansive soil can produce negative values as well.

Swell index isn't used in my area so I won't comment on that.

 

[pelelo]

thanks a lot MTNClimber,

Do you know any reference that can provide information about the " hydrocollapse" behavior?.

thanks,

 

[MTNClimber]

I don't think I have anything that just generally talks about hydrocollapse prone soils. Most of the references I have are for specific geologic regions.

Generally the collapse prone soils in my area are relatively dry (or commonly referred to as "slightly moist" in my region) clays and silts. The soil will have "pinhole" or "porous" structuring where the voids in the soil will close up, densifying the stratum, upon wetting under a constant load. Typically you can tell before even opening a California sample liner because it's significantly lighter in weight (DD around 70-90 pcf with a MC around 4-8%) than a normal soil sample. Sometimes you have to look closely at the sample to see the holes, other times its very obvious, more like pencil holes.

Pencil Holes:

Pinholes:

 

[EireChch]

My generic paragraph on collapse soils.

Collapse soils are soils which pose a risk of settlement and collapse on inundation with water. At their dry, natural state, they possess stiffness and high apparent shear strength; but upon wetting, they could exhibit a significant decrease in volume. Such soils, which exhibit this phenomenon at fairly low levels of stresses are called collapsible soils (Tadepalli et al. 1992; Rollins and Rogers 1994). Loose fills are considered as collapse susceptible soils.

If i recall correctly, ASTM D5333-03 (Collapse potential of Soil) is based on Jennings and Knight (1975)

 

[Okiryu]

I use CBR tests to check swelling. I sometimes get negative percentage values for swelling index. However, my soils are residual medium stiff/stiff clays. No exactly sure what is causing the negative value results but what it is clear is that my local soils are not collapsible soils.