Casagrade Chart (A-Line) question

[305Gurl]

If I use values to plot on this chart and it's near the A-line (USCS method) and the A-line equation (PI= 0.73(LL-20) gives a PI value that is larger than the given PI I found from LL-PL, am I suppose to choose the soil that is below the A-line; Is the opposite true?

For example: if PI=LL-PL= 26 and my LL is 56 and my calculated PI= 0.73(LL-20)= 26. Will my soil be a CH or MH.

What if PI= 0.73(LL-20) yielded 27; what if it yielded 24. what would I choose off the chart?

Just not clear in this area

Thank you

 

[YMZ PE]

In practice, if your soil plots on the borderline, we would call it CH/MH. You could further refine it if you had the gradation available from a hydrometer test. Exam problems will likely never be that ambiguous.

Think of the A-line as a cutoff. Draw the line (not just the particular point corresponding to your LL), then plot the LL and PI obtained through testing as a point. If your test shows PI=26 but the calculated line is PI=27, you would be MH because your actual soil is below the A-line. If the A-line equation shows PI=24, your actual soil plots above the line, so it would be CH. Does this make sense?