In the elasto-plastic analysis, the flow rule has a great influence on the magnitude of the calculated displacement, so the selection of the flow rule is very important. In this study, the effect of flow rule on elasto-plastic analysis was investigated by calculating the convergence of a circular tunnel excavated in the generalized Hoek-Brown rock masses of varying GSI values. For this purpose, the ABAQUS UMAT subroutine was used to implement the elasto-plastic constitutive law using the generalized Hoek-Brown failure criterion for yield function.
In the elasto-plastic formulation, the flow rule determines the direction and magnitude of plastic flow by assuming the appropriate plastic potential function. The associated flow rule is assumed if the plastic potential function is identical to the yield function. On the other hand, the flow rule is non-associated if two functions are different. It is known that when the associated flow rule is applied, excessive volume expansion takes place in the course of the plastic deformation. For this reason, the selection of the flow law is very important for elasto-plastic analysis. In this study, the response of flow rule selection on the elasto-plastic behavior of a circular tunnel in the generalized Heok-Brown rock mass (Hoek et al., 2002) was investigated. The elasto-plastic constitutive law of Clausen & Damkilde (2008) using the generalized Hoek-Brown yield function was executed in ABAQUS by use of UMAT subroutine.
