# Shear Strength

In **UnWedge**, three different strength models are available for defining the shear strength of the joints used in the analysis:

- Mohr-Coulomb
- Barton-Bandis
- Power Curve

## Mohr-Coulomb

Eqn.1

Where is the friction angle of the joint plane and *c* is the cohesion.

### TENSILE STRENGTH

For the Mohr-Coulomb model, you may also define tensile strength. If defined, the tensile strength will be applied to wedge faces which are in tension (for example: all planes of a falling wedge, or the non-sliding plane(s) of a sliding wedge). The tensile strength will apply a passive resisting force normal to the wedge plane, equal to the area of the wedge plane multiplied by the tensile strength.

## Barton-Bandis

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

Eqn.2a

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, Barton and Choubey revised this to Eqn.2b:

Eqn.2b

Where is the *residual* friction angle of the failure surface [Barton and Choubey, 1977]. Barton and Choubey suggest that can be estimated from Eqn.2c:

Eqn.2c

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 2b and 2c have become part of the Barton-Bandis criterion for rock joint strength and deformability [Barton and Bandis, 1990].

For further information on the shear strength of discontinuities, including a discussion of the Barton-Bandis failure criterion parameters, see Practical Rock Engineering: Shear Strength of Discontinuities) on the Rocscience website.

## Power Curve

The Power Curve model for shear-strength [Miller, 1988], can be expressed as:

Eqn.3

Where *a, **b,* and *c* are parameters typically obtained from a least-squares regression fit of data obtained from small-scale shear tests. The *d* parameter represents the tensile strength. If included, it must be entered as a positive value.