Tunnels constructed in soft rock and high field stress show significant gradual convergence with even after the excavation. This paper proposes a new visco-elastic-plastic solution for deep circular tunnel under squeezing conditions. The model is composed of Mohr-Coulomb plasticity and Burger's model for visco-elastic material. The state of the stress around the tunnel boundary and the tunnel wall displacement due to elasto-plastic behavior is determined using a conventional closed-from solution. Additionally, Burger's creep material model is used to determine the viscous behavior of tunnel wall convergence with time. Burger's model and Mohr-Coulomb's parameters are determined from unconfined compressive strength tests, triaxial tests, and creep tests on the cylindrical specimen of the host rock. The results of the proposed solution are compared with tunnel convergence data from laboratory-scale model test. The comparison show that the proposed solution reflects the tunnel's time-dependent behavior and adequately predicts the tunnel wall displacement compared to the experimental data.
The analytical solutions for the design and analysis of tunnels consider either elastic-perfectly plastic, elastic brittle plastic, strain-hardening and strain-softening behavior of the surrounding rock (Brown et al., 1983; Hoek et al., 2002; Sharan, 2008; Park, 2014; Fahimifar and Zareifard, 2013; Wang et al., 2011; Zou et al., 2015; Zhou et al., 2016; Li et al., 2016; Pan and Dias, 2016; Xiao et al., 2016). The elasto-plastic behavior of rock is commonly defined by either Mohr-Coulomb or Generalized Hoek-Brown criteria (Hoek et al., 2002) and the model parameters can be determined from triaxial tests. However, these constitutive models neglect the time factor, which is critical in squeezing ground conditions.
Squeezing ground conditions causes a considerable convergence of the tunnels that increases for a long time (Terzaghi, 1946; Barla, 2001). The design of tunnels excavated in such conditions a relatively difficult process. Squeezing behavior is associated with low rock mass strength compared to the in-situ stress and at the tunnel location (Wood, 1972; Jaeger and Cook, 1976; Jethwa et al., 1984; Barla, 2001 and others). The onset of yielding zone around the excavated tunnel in squeezing ground conditions causes a significant increase in the tunnel convergences (Barla, 2001). Therefore, prior assessment of tunnel convergence and support capacity is crucial for a tunnel support system's reliable design.
