New Creep Constitutive Model for Soft Rocks and Its Application in the Prediction of Time-Dependent Deformation in Tunnels

Abstract

Creep often occurs in tunnels excavated in soft rocks and it poses a risk to the long-term stability of the excavation. To derive measures for the effective mitigation of the risk, it is important to understand the time-dependent damage mechanism governing the deformation behavior of the surrounding rocks. A new creep constitutive model for soft rocks was proposed in this study to help achieve this understanding. The new creep model was developed based on the disturbed state concept, for example, the relatively intact state of rocks was described by the Burgers model and the fully adjusted critical state was represented by the generalized Bingham model. The analytical solution from the proposed model was given and the parameter calibration based on uniaxial compression creep experimental results were described. The deformation estimated by the proposed model was demonstrated to be in better agreement with experimental data than that estimated by the Burgers model alone. In addition, a three-dimensional creep constitutive model was proposed based on the one-dimensional form with the assumption of the constant bulk modulus. A case study for a tunnel with extensive time-dependent deformations was conducted using FLAC3D with the incorporation of the proposed three-dimensional creep model. The results indicated that the proposed creep model performed well in the prediction of the time-dependent deformations and failures in the section of the tunnel being studied. This model provided a very useful tool for engineers when determining the design for excavations and ground support schemes for tunnels in soft rocks where significant creep behaviors must be considered.