Investigation of tunnel excavation numerical analysis method for the combined finite-discrete element method

Tunnel excavation was a typical three-dimensional space problem. As a two-dimensional numerical method, the combined finite-discrete element method did not demonstrate the spatial effect during the tunnel excavation simulation. A new excavation path for the combined finite-discrete element method was proposed based on the spatial evolution characteristics. Then, the tunnel excavation simulation was carried out under different excavation paths (linear softening curve, exponential softening curve, power softening curve, and new softening curve). The results showed that the advantages of the exponential softening curve and power softening curve were inherited by the new softening curve. The new softening curve demonstrated the spatial effect during the tunnel excavation simulation. Moreover, both the maximum softening rate and peak value of model kinetic energy occurred near the intermediate moment, which was an advantage over other softening curves. The discussion indicated that both the softening rate and elastic modulus range should be considered when determining new softening curve parameters. And the optimal total softening steps can be estimated by the proposed equation with an error tolerance. Finally, the new softening curve was well applied in the numerical analysis of tunnel excavation and support design.