Creep constitutive model considering the overstress theory with an associative viscoplastic flow rule

Tunnel excavation in squeezing ground is very challenging due to the difficulty in making reliable predictions at the preliminary design stage. Tunnel response in squeezing ground is made possible by employing creep constitutive models. However, literature outlines the limitations of the conventional creep constitutive models in estimating delayed deformations due to the squeezing mechanism. Hence this paper presents, a fractional-order derivative viscoelastic viscoplastic (FDVP) constitutive model capable of estimating delayed deformations characterized by squeezing. The FDVP constitutive equations are derived as an extension to the Burgers model and adjusted Perzyna overstress function with an associated viscoplastic flow rule. The constitutive model validation and verification are conducted by using the experimental data obtained from literature and monitored tunnel convergence data, respectively. Thereafter, the constitutive equations are implemented in FLAC(3D) and applied to simulate deformations responsible for squeezing within a tunnel employing in-built constitutive models for verification purposes. The constitutive model shows very good agreement with experimental data and yields close results with monitored tunnel convergence data. The model can be successfully used in numerical code for tunnel stability analysis in squeezing ground.