In the description of the Hoek-Brown model, the definition of the mobilized dilatancy is a function of σ’3 based on two input parameters ψ and σψ.
However, internally, this definition is not followed for PLAXIS 2D 2018 (and earlier) and PLAXIS 3D 2017 (and earlier). Instead, the evolution function shown in red in Figure 1 is used.
Figure 1. Intended evolution (dashed line) of mobilized dilatancy angle as a function of σ’3 and the actual evolution (red) for PLAXIS 2D 2018 and PLAXIS 3D 2017
This means, with the standard setting, you will not see any dilatancy effect in the tension zone, since the defaults settings are ψ = 0° with σψ = 0 kN/m2, see Figure 2.
Figure 2. Default dilatancy settings with ψ = 0° and σψ = 0 kN/m2 for the documented behaviour (dashed line) and the actual behaviour for PLAXIS 2D 2018 and PLAXIS 3D 2017 (red line)
This issue is now solved since PLAXIS 2D 2019 and PLAXIS 3D 2018.00: the dilatancy behaviour based on the confining stress σ'3 now follows the description in the manuals:
For older versions (PLAXIS 2D 2018 and earlier, PLAXIS 3D 2017 and earlier):
When you want to get similar behaviour as described for the standard behaviour (ψmob = 90° at σ'3 = σt, and ψmob = 0° at σ'3 = 0 kN/m2) you need to define these parameters: ψ close to 90 ° and σψ = 0 kN/m2.
Figure 3. Workaround: Dilatation angle settings for Hoek-Brown for documented "default" settings