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# Base and Bedding Strength

The shear strength of the discontinuity planes in RocTopple can be modeled using one of the following strength criteria: Mohr-Coulomb or Barton-Bandis. These strength parameters are defined in the Base and Bedding Strength tab of the Input Data dialog. To open the tab:

1. Select Input Data on the toolbar or the Analysis menu or press F3.
2. Select the Base and Bedding Strength tab.

You can specify strength criteria independently for each discontinuity plane (Base Joints and Toppling Joints). Although the dialog refers to the discontinuities as "joints," the actual discontinuities in your slope model can be any type of planar discontinuity due to bedding, faults, joints, schistosity, etc.

NOTES:

## Mohr-Coulomb

The Mohr-Coulomb model relates shear strength, , and normal stress , according to Eqn.1:

Eqn.1

where is the friction angle of the joint plane and c is the cohesion.

## Barton-Bandis

The original Barton equation for the shear strength of a rock joint is given by Eqn.2:

Eqn.2

where is the basic friction angle of the failure surface, JRC is the joint roughness coefficient, and JCS is the joint wall compressive strength [Barton, 1973, 1976]. On the basis of direct shear test results for 130 samples of variably weathered rock joints, this was later revised to Eqn.3:

Eqn.3

where is the residual friction angle of the failure surface [Barton and Choubey, 1977]. Barton and Choubey suggest that can be estimated from Eqn.4:

Eqn.4

where r is the Schmidt hammer rebound number on wet and weathered fracture surfaces and R is the Schmidt rebound number on dry unweathered sawn surfaces. Equations 3 and 4 have become part of the Barton-Bandis criterion for rock joint strength and deformability [Barton and Bandis, 1990].

NOTE: For more help on the shear strength of discontinuities, including a discussion of the Barton-Bandis failure criterion parameters, see Practical Rock Engineering (Chapter 4: Shear Strength of Discontinuities) on our website.