Time-dependent longitudinal deformation profiles for circular tunnels in squeezing ground conditions: a Seri Nala case study

Abstract

Tunnels excavated in squeezing ground conditions often experience time-dependent convergence, significantly challenging their stability and demanding robust support systems. Therefore, timely installation of supports is crucial for ensuring safe operations. The Convergence Confinement Method (CCM), which utilizes Longitudinal Deformation Profiles (LDPs), offers a practical tool to determine optimum support system installation points. However, many studies overlook the creep effect when analyzing LDPs and designing a preliminary support. To address this, present study investigates time-dependent LDPs for four tunnel chainages of the Seri Nala region, Atal tunnel, India. A visco-elastic perfectly plastic (VEPP) model was utilized to effectively capture the creep behaviour of the surrounding rock mass. A 2D plane-strain finite element back-analysis was conducted to evaluate the VEPP parameters using a field-instrumented data. Subsequently, these parameters including shear modulus, viscosities, and retardation time; facilitated the construction of LDPs through axisymmetric analyses, revealing the intricate deformation characteristics along the tunnel sections. Moreover, study was further extended to examine the effects of constitutive models, excavation rate and anisotropic stress ratios on LDPs. The findings reveal that different constitutive models produce distinct LDPs, with the Kelvin-Voigt and VEPP models producing significantly higher time-dependent displacements than traditional elastic and elasto-plastic models. This discrepancy underscores the importance of considering viscous and plastic behaviour into deformation analyses. Additionally, the study emphasizes the critical role of excavation rates in managing tunnel stability, as lower rates resulted in increased deformation due to creep effects. The transition from elastic to visco-elastic and visco-elastic perfectly plastic behaviour, influenced by anisotropic stress ratios, further highlights the complexities involved in tunnel design. Overall, this research provides valuable insights for researchers and practitioners, aiding in the safe and cost-effective construction of tunnels.