Shear Strength
The shear strength of the joint planes in RocSlope2 can be modelled using any of the following strength criteria:
- Mohr-Coulomb
- Barton-Bandis
- Power Curve
To define a joint’s shear strength:
- Select Home > Joints > Properties
- The Define Joint Properties dialog will open. Select the Joint Property you would like to edit using the left-side menu.
- Select the Strength tab in the main dialog area.
- Choose the Strength Type (Mohr-Coulomb, Barton-Bandis, or Power Curve) and enter the joint strength parameters.
- For a Deterministic Analysis, define the shear strength of a Joint Property to be applied to a single joint set or multiple joint sets, including Basal Joint Set (if enabled).
- For a Probabilistic Analysis, it is possible to define statistical variability of the joint strength properties. See the Joint Property Statistics topic for more information.
![Define Joint Properties dialog](https://static.rocscience.cloud/assets/help/rocslope2/figures/define-joint-prop-dlg.png)
The shear strength models are described below.
Mohr-Coulomb
The Mohr-Coulomb model relates shear strength, τ, and normal stress σn, according to Eqn.1:
τ= c + σn tan ϕ + σn tan ϕw
where:
𝝉 = shear strength
c = cohesion
𝝓 = friction angle
Barton-Bandis
The original Barton equation for the shear strength of a rock joint is given by Eqn.2:
![barton-bandis strength equation initial](https://static.rocscience.cloud/assets/help/rocslope2/figures/barton-bandis-strength-equation.gif)
Where 𝝓b 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:
![barton-bandis equation revised](https://static.rocscience.cloud/assets/help/rocslope2/figures/barton-bandis-pt2.gif)
Where 𝝓r is the residual friction angle of the failure surface [Barton and Choubey, 1977]. Barton and Choubey suggest that 𝝓r can be estimated from Eqn.4:
Ï•r = (Ï•b - 20) + 20 (r / R)
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].
For further information on the shear strength of discontinuities, including a discussion of the Barton-Bandis failure criterion parameters, see this Practical Rock Engineering resource (Chapter 4: Shear Strength of Discontinuities).
Power Curve
The Power Curve model for shear-strength [Miller, 1988], can be expressed as:
τ = c + a (σn + d)b
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.