Problems with C_c and P_c Results

Problem 1: CONS Not Reporting C_c and/or P_c

The software uses two construction lines in its calculation of C_c and P_c values. Although the construction lines can be adjusted on all three compression curve types (percent strain, deformation and void ratio, all vs. log(pressure)), the calculations behind the C_c and P_c results are by definition based on the void ratio vs. log(pressure) curve, so if you haven't entered the data necessary to produce a valid initial void ratio, the software will not display the construction lines, nor will it report C_c and P_c results.

The following values must be entered into the "Sample and Test Info." window before the software can calculate void ratio values:

• Height of the specimen at test commencement
• Specific gravity
• Tested specimen diameter
• Either initial density test data or initial specimen weight and final moisture content test data

What's more, the software will perform "sanity checks" on all of the above values, meaning that the software will not accept values that do not make sense (e.g., specific gravity values higher than 4.5, density test data that results in dry densities greater than 300 pcf, or moisture content readings that result in negative moisture content percentages). If you think you've entered the necessary values and you're still not seeing construction lines appear on the compression curve display, you might be running afoul of one of the program's checks. To see if the program's rejecting your values, load the test, select "Window" > "Sample and Test Info" then "Window" > "Reports": if your sample data contains an error, the program should flag it for you.

Problem 2: The Pressure at the Red Construction Line on the Compression Curve Chart Does Not Correspond to P_c

The vertical construction line is not intended to mark the preconsolidation pressure (P_c): Instead, per Casagrande's graphical method of estimating preconsolidation pressure, the vertical line is to be located at the point of maximum curvature on the virgin portion of the compression curve. With this point, along with the slope of the other (blue) construction line, the program uses Casagrande's method to determine P_c.

• For more information on adjusting the construction lines, see this page of the program's online help.
• For information on the Casagrande graphical method, see the program's Documentation of Calculations, ASTM D2435, or any geotechnical engineering textbook.