Triaxial Field Test Data

Triaxial strength data from field tests (e.g. strength tests on in-situ rock masses, or observed rock mass failures under known stress conditions) can be analyzed with the Use Rock Mass Data option. This allows you to perform a curve fit of triaxial data (i.e. sigma1 / sigma3 data pairs) in order to determine the best fit strength envelope for a rock mass using the Generalized Hoek-Brown strength criterion.

To analyze triaxial field data:

First select the desired stress units in the Project Settings dialog, and make sure that the Generalized Hoek-Brown strength criterion is selected.

Select the Use Rock Mass Data option from the toolbar or the Analysis menu.
You will see an input data dialog. Select the desired Curve Fitting Method (Levenberg-Marquardt or Simplex).
The curve fitting of field data using the Generalized Hoek-Brown criterion, requires entry of intact rock uniaxial strength (sigci), and disturbance factor (D), as input. If you do not know the value of these parameters, they can be estimated from dialogs within RocData.
Enter the triaxial data (i.e. sigma3 / sigma1 data pairs). The data can be entered in the spreadsheet within the dialog, or imported from a file:

To enter data in the spreadsheet, set the Number of Tests to the desired number, and type the data directly into the spreadsheet, or use the Paste button to Paste the data from another application (e.g. Excel).
To import data from an existing file, select the Import button. The data can be imported from RocData (.roc), RocLab (.rlb), comma delimited (.csv) or tab delimited text (.txt) files.

As soon as the data has been entered in the spreadsheet or imported, the curve fit is performed using the selected Curve Fitting Method, and the best-fit results will be displayed in the dialog. For a summary of the calculated output parameters, see the table below.

Fitting of the Generalized Hoek-Brown criterion to triaxial field test data

Mogi's Line can be displayed on the failure envelope plot by selecting the checkbox in the dialog. For the definition of Mogi's Line see the Display Options topic.
When you are finished entering the data, select OK in the dialog, and a summary of the curve fitting results will be displayed in the main RocData viewing area.
To display the test data on the failure envelope plots in the main RocData plot area, select the Plot Test Data option.

Calculated output parameters for triaxial field data

After a curve fitting analysis of triaxial field data, the following output parameters are calculated. The calculated strength parameters are the "best-fit" values.

Summary of input and output parameters for triaxial field data analysis

Strength Criterion	Input Data	Output Data
Generalized Hoek-Brown	sig1/sig3 data pairs, sigci (intact uniaxial compressive strength), D (disturbance factor)	mb, s, a, mi, GSI

Residuals

The Residuals value is a measure of how well a strength criterion fits a given data set. It is equal to the sum of the square of the vertical distances of the given data points from the fitted curve. The goal of the curve fitting computation is to determine the strength envelope which minimizes the value of the Residuals. The Residuals value for the best fit strength envelope is displayed in the test data dialogs and the main RocData viewing screen.

Display of Triaxial Test Data

The Plot Test Data option can be used to show or hide the triaxial test data on the failure envelope plots in the main RocData view. For triaxial test data, the data is plotted as points on the principal stress plot, and as Mohr circles on the shear-normal stress plot.