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CONS Cr Calculation Error

In late February 2021 a customer alerted us to a possible problem with the calculation of Cr in CONS. After investigation we determined that there was indeed an error in the program's Cr calculation.

Error Description

In one specific circumstance the program would erroneously include the slope of the first curve segment past the end of the recompression interval (i.e., a segment on the virgin compression portion of the curve) as part of the slope average. Since the virgin compression curve typically has a steeper slope, this error would tend to bias toward larger Cr values. The problem is illustrated in the following figure:

Cr Slope Averaging Overrun Example

Note that this problem affects CONS versions 4.0.0.0 to 4.0.0.37. Older versions (e.g., version 1.x) stopped the reload curve averaging calculation at the last point at or before the unload start pressure.

Mitigating Factors

There were three factors that could reduce the severity of, or entirely eliminate, the problem:

  • Compression curves without a loading increment with a pressure identical to the unload start pressure. Since the bug was triggered only when the reload curve included a point with a loading pressure identical to the pressure at which the unloading was begun (illustrated in the figure above), the problem did not manifest if the reload curve did not include such a point (see the figure below).
    • Cr Curve Without Slope Averaging Error

  • Selecting the option to calculate Cr as the average of both the rebound and reload slopes. Since the rebound portion of the curve tends to closely track the reload portion's slope, the effect of the additional curve segment's slope would be reduced (because the rebound portion would add additional bias towards the "proper" slope value.

  • Entering an overburden pressure. When an overburden pressure is entered, and a Pc value can be calculated, the program calculates Cr as the slope of lines drawn between the curve value at the overburden pressure and the curve value at the preconsolidation pressure. The bug we found does not affect this calculation.

Correcting the Problem

Version 4.0.0.38 of the program, posted on March 1, 2021, correctly halts the Cr slope average when the recompression curve is reloaded to the same pressure at which unloading was initiated. Your program may have automatically updated itself: to check, start the software then select "Help" > "About This Program...": If your CONS version is 4.0.0.38 or later, (or if you see a date instead of a version), you're good to go. If you do have an out of date version, follow these steps to update:

  1. Close the "About" dialog, then click on the "Click here to check for updates" link on the program's opening screen. If software reports that updates were found, let it download them then, when the "download complete" notification appears, close then restart the program.
  2. If the software reports that no updates were found, or if your CONS version doesn't change after you download updates then restart the program, try step 1 again after starting the program using Administrator privileges (right-click on the shortcut that you use to start the software then select "Run as administrator...".
  3. If you still cannot get your CONS version to change, visit our Manual Updates page and, following the page's directions, install the manual CONS update.
  4. If the steps above failed you likely do not have the Windows security permissions necessary for updating files in the program's installation folder; however, one last step can be tried: Visit our Downloadable Installer Request page and request a new installer program. You can use the program to install a fresh, up to date, copy of your licensed Geosystem software directly over the top of your existing installation. (Don't uninstall the existing installation first.)