A New Strain-Softening Constitutive Model for Circular Opening considering Plastic Bearing Behavior and Its Engineering Application

Geomaterials generally show strain-softening characteristics after peak-load. Based on the triaxial test for sandy mudstone, a simple elastopeak plastic-strain-softening-damage model (EPSDM) was proposed. Compared with the traditional strain-softening model, EPSDM shows obvious plastic bearing characteristics before strain softening. Then, the closed-formed solution of circular opening was deduced based on the newly proposed model. A plastic shear strain increment was introduced as the extension constraint condition of peak plastic zone. The solution correctness of EPSDM was also verified by comparing with other research results. In addition, the solution based on EPSDM could degenerate for a series of results obtained by elastobrittle plastic model (EBM), elasto-strain-softening model (ESM), and elasto-perfectly plastic model (EPM) under certain conditions. Hence, it could be regarded as a unified solution. Finally, the research results denoted that when the inner pressure was fully released, the maximum postpeak failure radii and surface displacement of surrounding rock indicated the characteristics of EBM>ESM>EPSDM>EPM. Therefore, the plastic bearing behavior could effectively decrease the postpeak failure zone radii and surface displacement. The dilation coefficient noticeably influenced postpeak failure range and surface displacement, particularly the damage zone radii and tunnel wall convergence. The research results can provide very important theoretical bases for evaluating the tunnel stability and support design reliability for underground engineering.