Fat to lean clay in a single, discrete soil deposit

[JJBDMALLC]

Is it possible to have that range of plasticity from an otherwise homogenous clay source? I'm talking range of plasticity index from less than 10 to nearly 80, and range of liquid limit from 30 to greater than 100. Is there a method of deposition, mineralogy, or chemistry that could explain the wide variance, or do I have suspect data?

 

[TDAA]

How many data points do you have? Have you rerun any to verify? Do they show a trend? Are they consistent with other tests near by?

As I started reading I thought, "Sure, why not?" You do have quite a range, allthough it might be correct. Also to consider is what sort of geologic setting are you in?

 

[JJBDMALLC]

Approximately thirty data points, reruns are in progress. To explain a bit better: the data is from depths of 80 to 200 feet below ground surface on the North Slope of Alaska, possibly (likely?) within the Skull Cliff Member of the Gubik Formation. Individual samples are from a total of 34 boreholes in an area less than five acres square. Lab analysis run by two different laboratories. Have you ever seen such a variance over such a small area? I have yet to find a pattern in the results that sheds any light, hopefully reruns will help. Pore water salinities of the same samples range from 10 to 40 ppt. Any further thoughts? And thank you for your time.

 

[emmgjld]

If I am understanding your situation correctly:
This is the same geologic formation? or Soil Unit?
Very little visual differance in the samples?

I have dealt with such extreme changes in clay plasticity in the lowest members of the Mancos Shale (Km) in Western Colorado. The variation is within 20 feet vertical (differant strata) and 1000 feet horizontally (facies change).

 

[JJBDMALLC]

Same and same- same apparent soil unit and same geologic formation (though samples are still to be considered unconsolidated seds). Little to no- and definitely with no obvious pattern- visual variation; dark gray to blue gray, occasional fine to very fine sand micro-lenses. I mentioned depth before but missed a point- depth is below mudline, not ground surface. We are beneath <1 to more than 20 feet of water in a broad, active river channel and immediate floodplain. I don't know if that is of consequence or not. Any idea how much mineralogy and/or chemistry play into the physical (engineering) properties of the material? I know a little, but only from very far removed empirical data that doesn't add much but additional questions. Thank you, I appreciate any help you can offer.

 

[TDAA]

Well, for sediment, the rivers historical course, changes in flow rate, etc, will effect how the soils were deposited. This will certainly change the grain size distribution. Also, the parent material that was eroded may change the mineral content. Being in the flood plane, there is likely more stratification than you can determine from relatively small samples and mixed up cuttings.

This may account for a lot of the variation, and lack of a distinct pattern.

I have seen this on a much smaller scale on a landfill project. I had the benefit of watching the excavation though, and we had a rather defined channel from a dried creek. Nice fat clay soil liner material in the old bed, and nasty sandy, non-plastic silts on the sides. If only the contractor would have used a trackhoe vs. scrapers, we would have had enough material.

Anyway, this is the same concept on a lot smaller scale. You are dealing with a much longer sedimentation period, and wider area with flood plane, etc.

 

[fattdad]

I'd say the range in data is reasonable. What I would be equally interested in is the comparison between the natural moisture content and the liquid limit. If you have a consistent ratio between LL and Wn, then you are dealing with a soil deposit with similar stress history.

Where are you on the Slope? 20 years ago, I spent a fair bit of time up there, Barrow, Kaktovic, Nuisqet, Ataqsit, Point Lay, etc. (I've forgotten how to spell the village names, oh well. . . ).

Good luck.

f-d