5.1.6 Hoek-Brown Rock Plasticity

The Hoek-Brown criterion defines the stress condition under which a rock mass will deform inelastically and, if not supported adequately, collapse. This criterion can be applied in plane stress, plane strain, axisymmetric and solid elements. See §17.1.8 for background theory.

    (syntax)

YIELD

HOEKBR specifies that the Hoek-Brown rock plasticity model must be used.

YLDVAL

specifies the parameters for the plasticity model: sci is the unconfined compressive strength $\sigma_{{\mathrm{ci}}}^{}$ of the (intact) rock sample, mi is the Hoek-Brown constant m_i deduced from test results for a particular (intact) rock type, gsi is the geological strength index GSI according to Figure 17.14, dilat is the dilatancy angle $\psi$ (in degree) to define the non-associated plasticity.^5.2

Figure 5.8: Hoek-Brown yield condition boundings

There are two optional parameters [Fig.5.8]. By default DIANA assumes unbounded stress in the compressive regime. [cap = 0] However, if you specify cap then compressive stresses will be bounded. If you specify parameter tensio then DIANA will adopt the tension cut-off criterion for the stresses in the tension region.^5.3The default is tension cut-off at zero stress. [tensio = 0] Note that if you specify a tension cut-off value tensio then you must also give a value for cap.

HARDEN

STRAIN applies the strain hardening hypothesis, which is the only one available for the Hoek-Brown rock plasticity model. The input syntax for the hardening parameters is analogous to the input for the Rankine Principal Stress model. See the description of the HARNAM, HARVAL, CMPNAM, and CMPVAL input data records in §5.1.3.

    (file.dat)

'MATERI'
   1  YOUNG  42000.
      POISON 0.2
      YIELD  HOEKBR
      YLDVAL 148.0 18.0 75.0 11.5
      HARDEN STRAIN
      HARNAM LINEAR
      HARVAL 0.09