Hyperbolic
A Hyperbolic shear strength envelope is defined by the following equation:
It is important to note the definition of the parameters 
and 
in the Hyperbolic shear strength equation. The following figure illustrates a hyperbolic shear strength envelope.
Cohesion is defined as the shear strength at 
.
Friction angle is defined as the friction angle at 
.
The Hyperbolic shear strength model has been found to characterize the shear strength of soil/geo-synthetic interfaces, and other types of interfaces [ Esterhuizen, Filz & Duncan (2001) ]. For example, it could be used to model the shear strength of:
- a concrete/soil interface
- a geotextile/soil interface
You may wish to use the Hyperbolic shear strength model, to model the failure mode of "direct sliding" along a Geosynthetic/Soil interface. In this case, you will have to define a narrow layer of soil along the geotextile, and assign a material type which uses the Hyperbolic shear strength model.