Direct Shear Test Data

Direct shear 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. normal / shear stress data pairs) in order to determine the "best fit" strength envelope and the associated strength parameters, for a given strength criterion.

To analyze direct shear data:

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

Direct shear data cannot be analyzed with the Generalized Hoek-Brown criterion.

Select the Use Lab Data option from the toolbar or the Analysis menu.
You will see an input data dialog:

If you are using the Mohr-Coulomb or Power Curve criteria, select Data Type = Direct Shear.
If you are using the Barton-Bandis criterion, direct shear data is in effect by default. However, you must enter the residual friction angle (phir) as an input parameter. If you do not know the value of phir, it can be estimated from the Residual Friction Angle dialog.

Select a Curve Fitting method:

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

Enter the direct shear data (i.e. normal stress / shear stress 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.

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 Barton-Bandis criterion to direct shear test data

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 direct shear data

After a curve fitting analysis of direct shear 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 direct shear data analysis

	Input Data	Output Data
Mohr-Coulomb	sigN/sigT data pairs	c , phi
Barton-Bandis	sigN/sigT data pairs , phi(residual)	JRC , JCS
Power Curve	sigN/sigT data pairs	a , b , d

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 Direct Shear Test Data

The Plot Test Data option can be used to show or hide the direct shear test data on the failure envelope plot in the main RocData view. For direct shear data, the data is plotted as points on the shear-normal stress plot. Direct shear data does not appear on the principal stress plot.