Triaxial Lab Test Data

Triaxial strength data from lab tests of rock or soil samples can be analyzed with the Use Lab Data option. This allows you to perform a curve fit of the data (i.e. sigma1 / sigma3 data pairs) in order to determine the "best fit" strength envelope and the associated strength parameters, for a given strength criterion.

To analyze triaxial lab data:

First select the desired strength criterion and stress units in the Project Settings dialog. The analysis of triaxial lab data can be performed using the following strength criteria in RocData:

Triaxial data cannot be analyzed with the Barton-Bandis criterion.

Select the Use Lab Data option from the toolbar or the Analysis menu.
You will see an input data dialog. In the dialog, select Data Type = Triaxial. NOTE:

The Data Type option is only applicable for Mohr-Coulomb or Power Curve strength criteria. For the Generalized Hoek-Brown criterion, triaxial data is in effect by default, since direct shear data cannot be analyzed.

Select a Curve Fitting method. NOTE:

The available curve fitting methods will depend on the strength criterion which is in effect. See the Curve Fitting Method topic for more information.

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 the 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 the 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.
If incorrect data pairs (i.e. data for which sigma3 exceeds sigma1) are accidentally entered for triaxial data analysis, the curve fitting calculations are not carried out, and an error message will be displayed in the dialog.

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 for each strength criterion, see the table below.

Fitting of the Hoek-Brown criterion to intact rock lab 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 screen.
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 lab data

After a curve fitting analysis of triaxial lab data, the following output parameters are calculated, according to the currently selected strength criterion. The calculated strength parameters are the "best-fit" values.

Summary of input and output parameters for triaxial lab data analysis

	Input Data	Output Data
Hoek-Brown *	sig1/sig3 data pairs	mi , sigci
Mohr-Coulomb	sig1/sig3 data pairs	c , phi
Power Curve	sig1/sig3 data pairs	a , b , d

* Hoek-Brown criterion for intact rock

It is important to note that for intact rock, the parameters of the Generalized Hoek-Brown criterion are as follows:

s=1
GSI=100
a=0.5

therefore the Generalized Hoek-Brown equation reduces to the following:

This is the equation which is used for the curve fitting of triaxial lab data (intact rock) to the Hoek-Brown criterion. Note that the curve fitting variables are sigci (the uniaxial compressive strength of the intact rock) and mi (the Hoek-Brown "m" parameter for intact rock).

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.