Gradations from the same hole and nearly the same depth not matching?

[Lemonandapples]

Hello all

I'm trying to figure out if this is a testing error or if there is something I am not understanding. See the attached drill log.

The fines content between a "UU Trim" and "bag" are different by about 40%. This happens twice which makes me think its not just a coincidence. Is there reason why the trimmings from the UU sample would be so different than the bag gradations? Is a lab error?

Edit: To clarify I am referring to samples 68 - 71. Where the UU trims shows values in the 40%'s, and the bags show values in the 80%'s.

 

[MTNClimber]

How was the bag sample collected? Is it from an auger, split spoon, or something else?

 

[Lemonandapples]

Not 100% sure how the bag sample was collected. However the drill log states that the tube was 2.5 long. Since the bag sample seems to be consistently AFTER 2.5 feet, I'm thinking that means the bag was not taken from the tube, but grabbed with some other method. It wouldn't have been auger cuttings right? See attached.

 

[MTNClimber]

There isn't text in the report describing how they were collected?

There's a couple of reasons why the UU samples and bag samples differ. Auger cuttings can be easily contaminated by getting mixed with other soils from 30' above. There's also the possibility that there is some time of alluvial, glacial-fluvial, or some other type of layering going on within the soil stratum itself. My guess would be the UU should be more accurate, if collected correctly. Depending on implications of the fines, say for dewatering purposes, this may warrant further investigation or otherwise maybe just neglect those samples that are in question.

 

[BigH]

A possibility is that the stratum is layered and the bag is a mix of the various layers; if from a split spoon, the various layers within the sample would have been separated for "purity" . . .