The Hoek-Brown model is the most used failure criterion for rock masses. With the release of the PLAXIS CONNECT Edition, a side panel of the Parameters tabsheet of the Hoek-Brown model has been introduced to assist with the definition of each of the material parameters.

The pre-procession tool can be used to guide the determination of the Hoek-Brown parameters:

• Analysis tab: shows the Hoek-Brown failure envelope in the plane of principal effective stresses σ'₃ - σ'₁, in order to visualise the effects of changing of rock mass parameters on the failure envelope.
• Hoek-Brown parameter tab: allows to determine the parameters |σ_ci|, m_i, GSI and D using a graphic interface and interaction with the left mouse button (LMB).

Figure 1: Uni-axial compressive strength of the intact rock  (σ_ci) for Hoek-Brown model on the side panel

Uni-axial compressive strength of the intact rock (σ_ci)

One of the input parameters of the Hoek-Brown model is the uni-axial compressive strength of the intact rock (σ_ci). Users should be aware that, although normally expressed in [MPa], the unit used for σ_ci in Plaxis is based on the global project units. When the global project units are set to [kN] for Force and [m] for length, this uni-axial compressive strength of the intact rock (σ_ci) will be expressed in [kPa] = [kN/m²]. These global project units are used consistently throughout the program for e.g. stress, strength and stiffness, therefore be sure to enter the correct value here.

Note that it is also possible to change the displayed units within the material dataset. The material database will then convert the values internally to the defined project units. To change this, click on the unit icon at the top: