Study on the Effect of Flexible Layer on Support Structures of Tunnel Excavated in Viscoelastic Rocks

Support is one of the most important aspects that contribute to long-term stability and tunnel safety. Lining cracking and even failure often occur during tunneling construction, especially for soft rock tunnels. To address such a problem, a supporting scheme that a flexible layer is adopted and placed between the first lining and second lining is proposed in this study. An exact closed form solution is derived for the mechanical behavior of a circular tunnel supported by such a supporting scheme. Analytical formulas are provided for displacements and pressures of linings, taking into account the installation delay of the second lining. Numerical results of the finite element method validate the results calculated from these formulas. Further, a parametric study is performed to investigate the effects of thickness, Young's modulus and Poisson's ratio for the flexible layer on supporting effect. It is concluded that the flexible layer plays a good role in absorbing deformations due to rock rheology. The displacement and pressure can achieve a good improvement, and this improvement is dramatically affected by the thickness and deformability of the reserved flexible layer.