Equivalent Mohr-Coulomb Parameters

An important feature of RocData is the calculation of equivalent Mohr-Coulomb parameters for non-linear failure envelopes. This is very useful because many geotechnical engineering analysis programs require Mohr-Coulomb parameters (cohesion and friction angle) as input, even though actual strength envelopes are often non-linear.

The equivalent Mohr-Coulomb parameters are automatically calculated if you are working with one of the non-linear strength criteria in RocData:

• Generalized Hoek-Brown

• Barton-Bandis

• Power Curve

Mohr-Coulomb parameters are calculated by determining a linear envelope which provides a "best-fit" over a given stress range of a non-linear envelope. The stress range is specified using the Failure Envelope Range option in the Sidebar (see below for more information).

• The results of the Mohr-Coulomb fit (cohesion and friction angle) are displayed in the Sidebar and on the failure envelope plots.

• The equivalent Mohr-Coulomb envelope can be displayed on the failure envelope plots by selecting the Mohr-Coulomb Envelope option from the toolbar or the Analysis menu.

Equivalent MC Envelope (blue) for non-linear shear-normal envelope (red)

Display of equivalent Mohr-Coulomb parameters in Sidebar

Failure Envelope Range

The Failure Envelope Range option in the Sidebar has a direct effect on the equivalent Mohr-Coulomb parameters for non-linear failure envelopes. If you change the stress range, you will obtain a different Mohr-Coulomb fit.

Application

The appropriate stress range (maximum sigma3 or sigmaN value) over which equivalent Mohr-Coulomb parameters must be obtained depends on the application, i.e. the type of geotechnical excavation being designed or analyzed. There are four possible selections: General, Tunnels, Slopes and Custom.

• For details about the Mohr-Coulomb fitting procedure, and the significance of the Failure Envelope Range option, click here to view the Hoek, Carranza-Torres and Corkum (2002) paper. (See section 4: Mohr-Coulomb Criterion, and section 6: Determination of Sigma3 max). Based on the principles outlined in this paper, RocData estimates maximum sigma3 values for tunneling or slope design applications.

• The General option is available only for the Generalized Hoek-Brown criterion. When the Failure Envelope Range option is set to General, sigma3max will automatically be set to sigci / 4. This is based on the empirical observation that the stress range associated with brittle failure, occurs when sigma3 is less than about one-quarter of sigci.

• When the Failure Envelope Range option = Custom, you may enter any value of sigma3max or sigNmax. TIP: this can be used to customize the stress range of the failure envelope plots, even if you are not interested in the equivalent Mohr-Coulomb fit.

• NOTE: the Failure Envelope Range has NO effect on the calculated parameters of a non-linear failure envelope.