Time-dependent solution for non-circular tunnels considering the elasto-viscoplastic rockmass

Abstract

The time-dependent ground response of tunnels in squeezing ground requires a thorough understanding of the creep mechanism. Several studies conducted assume that this ground response can be characterized by elastoplastic and viscoelastic behaviour. However, the elasto-viscoplastic behaviour associated with the creep mechanism can predict this ground response realistically. Thus, this paper, presents a solution considering the elasto-viscoplastic behaviour for the ground response estimation of a non-circular tunnel under hydrostatic stress field in an isotropic and homogenous rock mass. The proposed method is an extended form of the closed-form solution based on the fractional-order derivative viscoplastic constitutive law. Thereafter, the proposed method is applied to the horseshoe tunnel ground response and support structure capacity estimation in squeezing ground. It is ascertained that delayed behaviour responsible for squeezing can be estimated realistically about 14% convergence and 24 m yield extension. Whereas the conventional solutions underestimate the convergence range between 2.5% and 5% and the yield zone extension between 12 m and 16 m. In comparison to these conventional solutions, it accounts for 10% tunnel convergence and 50% extension of the yield zone in the long-term. Additionally, the solution determines the suitable long-term safety coefficient for support structure installed at the right time and location behind the tunnel face in squeezing ground.