Evaluating the loads acting on the tunnel supports correctly is one of the most important factors in the design of the tunnel supports for NATM. In this study, two-dimensional FEM analysis using the monitored crown settlement and convergence data was performed to evaluate the modulus of elasticity and the coefficient of lateral pressure of the ground where a tunnel was excavated, and the load acting on the tunnel supports. The results show that the relationship between the overburden depth and the load acting on the tunnel supports are classified into some patterns according to the rock type.
The tunnel supports including shotcrete lining, rock bolts, steel sets and concrete lining are designed for NATM using various methods including:
the rock mass classification method that involves classifying the rock mass into a number of class with its seismic velocity and the condition of boring cores as main indices, then mechanically applying a combination of tunnel supports suited to the rock mass class that has been determined based on past experience;
the method of setting the scale of the tunnel supports using numerical analysis such as FEM or other analysis method;
the method of combining tunnel supports with reference to examples of design under similar geological and topographical conditions.
With the exception of tunnels with extremely large overburden heights and those in special rock types such as squeezing rock, most road tunnel supports in Japan have been designed using the rock mass classification method. Regarding the design of the concrete lining for road tunnels in Japan, the thickness of the lining is, in line with past experience, set at 30 cm regardless of the rock type in order to obtain a safety factor for unknown phenomena and as interior finishing.
