High plasticity silt

[Terin]

Have a project with some interesting soil behavior. High plasticity silt, consistently with liquid limit of about 62, plasticity index of 30. Little or no clay. Compaction curves are flat, comparable to the shape of silty sand. Maximum dry density of 95 pcf and 22% optimum moisture content. Compacts extremely well, even at moisture contents 10% over optimum. We did a series of remolded samples compacted to 80-85-90 percent relative compaction at optimum and got a straight line relationship of unconfiend compressive strength, 8600 psf at 90%, 3100 psf at 80% compaction.
No complaints about it, just unusual behavior properties. Has anyone experienced soil like this?

 

[msucog]

which method did you perform for gradation? did you use deflocculating agent, soak and aggressive agitation? 10% wet seems excessive to achieve "good" compaction. it does sound a bit strange.

 

[dgillette]

Based on Terin's numbers, maybe it's not so crazy to see good compaction at 10% wet of w-opt.

Assuming sp gr = 2.7, 95 pcf gives solids volume in a cubic foot equal to 0.56. 22% moisture gives water volume = 0.33. That leaves air volume = 0.10, so at max density and optimum water content, the soil is farther below the zero-air-voids curve than is typical.

Changing the dry density to 90% of 95 pcf, and the water content to 32 percent, I calculate air void volume as 0.05, still well below the ZAV line. At 32% moisture, the ZAV line corresponds to about 95% compaction.

Old timers have told me to expect W-opt for standard Proctor to be pretty close to PL as a first estimate. In this case W-opt is 10 percent less than PL. Terin says the moisture-density curve is pretty flat, which would fit with being able to compact well even way wet of optimum.

Regards,
DRG

 

[fattdad]

To say, almost no clay implies that you all did a hydrometer test and are basing the statement on grain-size criteria. Is there any sand content? What is the geologic setting?

I did a job in Panama with a silt and the maximum dry density was in the upper 70s. No sand, didn't do a hydrometer (didn't see the need), but the behavoir (i.e., Atterberg limits) was an elastic silt. I thought the local lab was all gacked up. Didn't believe it at all. Turns out the soil was derived from volcanic rock and the lab was just fine. We did an earthwork project, it placed well and is doing just fine holding up a vacility that breeds 150 million flys a week.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

ok, I'll bite - breeding flys?

 

[fattdad]

Yes, then you sterilze them and release them into the wild, where they try to reproduce to no avail. End result, reduce the fly population. This is a specific fly though, one that carrys the screw worm parisite, which wrecks havock on livestock. This has been going on for at least 30 years and we designed a few facility in Panama - better control on the push of the screw worm from South to North America.

f-d

¡papá gordo ain’t no madre flaca!