Estimation of Rock Load of Multi-Arch Tunnel with Cracks Using Stress Variable Method

In multi-arch tunnels, the increased rock load on the concrete lining of the main tunnel and side walls due to the excavation of adjacent tunnels is critical and must be considered in the design stage. Therefore, this study estimates the rock load of a multi-arch tunnel using two-dimensional numerical analysis, considering rock mass classifications, overburden, and construction steps. The rock load is estimated using two criteria: the factor of safety and stress variable. The rock load is underestimated when the factor of safety is applied to rock mass class III. However, the stress variable method reveals a reasonable rock load as overburden increases. Particularly, the rock load is estimated to be equal to the overburden in shallow tunnels and approximately 0.7 times the tunnel width in deep tunnels. Additionally, the crack-induced rock load is computed using back analysis at the excavation completion stage of adjacent tunnels, yielding the relation between the rock load height and the deformation modulus of the rock mass. Therefore, an accurate estimation of the rock load of multi-arch tunnels emphasizes the importance of a more economical and realistic design and must be addressed in the process of performance-based tunnel design.