Reinforcement design and stability analysis for large-span tailrace bifurcated tunnels with irregular geometry

• New reinforcement analysis method is based on 3D nonlinear stress distributions.
• Solving for normal stresses in the concrete lining by the Cauchy Stress Formula.
• Calculating the nonlinear stress distribution in the section using the Lagrange interpolation.
• New method can be applied to design the reinforcement system for any arbitrary section.
• Reinforcement parameters were used to successfully guide the construction of Guandi Hydropower Station.

The stability analysis of large-span tailrace bifurcation tunnels with complex 3D geometries and the associated challenges are presented in this paper. A new nonlinear reinforcement analysis method is firstly proposed for the design of reinforcement systems, i.e. steel-reinforced concrete lining and rock bolts, for large-span tailrace tunnel bifurcations with complex 3D geometries. The proposed method is then implemented into FLAC3D using the FISH language to calculate the reinforcement parameters and the resultant nonlinear stress distributions. After that, it is validated by comparing the concrete lining stresses and reinforcement obtained by the proposed method with the analytical solutions for the ideal case of a circular tunnel in an infinite elastic body. Finally, the new method is applied to calculate the distribution of deformations, stresses, plastic zones and design the reinforcement system for the large-span tailrace tunnel bifurcation in the Guandi Hydropower Station during the construction, operating and maintenance periods, respectively. The calculated reinforcement parameters successfully guided the construction of the tunnel. The proposed nonlinear reinforcement analysis method may also be beneficial to the design and construction of other large-span tunnel bifurcations with complicated irregular cross sections in similar hydropower projects currently ongoing in southwest China.