ARMA 12-413 Comparison of approaches for modelling dilation of brittle rockmasses around circular excavations and associated issues

摘要

Conventional numerical modelling tools for analyses of underground excavations are well suited for modelling elastic behaviour and yield response of geomaterials with very simple constitutive behaviour. Non-dilatant elastic and perfectly plastic behaviour is most commonly used for preliminary analyses in the absence of the data required to consider more complex behaviours such as strain-weakening and dilation. This results in inaccurate predictions (normally non-conservative) of tunnel wall displacements and support loading which leads to a tendency of arbitrary conservatism in support design. For truly brittle models, this deficiency is more acute. This problem is largely a function of a poor understanding of how inelastic dilation occurs in different types of rockmasses, the rate controls on strength loss and volume change during yield, and the complexities of rock-support interaction in yielding conditions. Recently, efforts have been made to understand and quantify the complexities of dilation angles in rock, including dependency on confinement stress and plastic shear strain incurred. These current approaches will be reviewed and compared. The effects of dilation on numerical model results are examined for different material types, most notably those exhibiting brittle behaviour. A dilation angle model based on lab testing data is compared to an empirically determined method for estimating wall displacements around an excavation. Deficiencies in existing strategies for addressing dilation in numerical models are analyzed critically, and key areas for future work are identified.