A Comparison Of Viscous Material Model Mechanics And Boundary Conditions

In underground projects time-dependent deformations can occur and subside during the construction process or continue over a long period of time. The nature and impact of time-dependent behavior is related in part to the particular combination of rockmass mechanical properties, in-situ stresses and local boundary conditions (for a particular element of rock). Considered simply, this behavior can occur either under sustained loading (constant stresses) and manifests as creep strain or it can occur as a relaxation process under conditions of fixed strain. Numerous models are available in commercial software that are combinations of visco-elastic, visco-elasto-plastic and truly visco-plastic models. An additional case of strength degradation (without impact on transient stress or strain) is also possible. The long-term behavior of the excavation damage might be affected by these boundary conditions both in a tunnel or a shaft environment. Thus, it is crucial to analyze and further understand the mechanics of the different constitutive viscous models being used and adopted for time-dependent simulations. This paper serves to investigate and illustrate the individual mechanical analogues of these visco-models utilizing both 2D and 3D analysis methods. A sensitivity analysis is presented of a rock specimen that is being subjected under stress-controlled and strain-controlled conditions under both uniaxial and triaxial tests. In addition, a tunnel case analyzed and examined in 2-Dimensions in order to understand the various mechanisms of the models in tunneling conditions. Drawing on the results, this paper highlights the mechanisms of the viscous models and the implications that may arise from the different space conditions by giving practical guidelines.