In the NATM method, the tunnel is excavated in parts over a certain length; hence the tunnel is in various stages of completeness over the entire construction. Part of the stability in the NATM method relies on the fact, that the ground if allowed to deform slightly, is capable of contributing to its own support. For this reason, the NATM is a method where the surrounding rock or soil of a tunnel are integrated into an overall arch or ring, becoming themselves part of the support structure. The NATM method utilises this effect systematically for its development, but also uses modern means of monitoring and support, such as shotcrete and rock bolts, to guarantee the stability during the tunnel construction.
In practice, during construction, a tunnel is unsupported over a limited length behind the advancement front. However, in a 2D model a tunnel excavation would mean that the ground is left unsupported over an infinite length which might lead to failure in the absence of three-dimensional arching effect. To overcome this limitation of 2D modelling and to account for the effects of the third dimension on the excavation within finite element analysis framework, the Convergence-confinement method or Beta-method is widely used.
The Beta method in PLAXIS 2D can be performed using two approaches:
Figure 1: NATM construction process
Figure 6: Schematic representation of Deconfinement method vs. Incomplete Staged construction method (p: pore pressures; σ': effective stresses)
PLAXIS 2D Tutorial 05: Excavation of an NATM tunnel [Tutorial]
Coffee Corner: Tunnelling simulated by the convergence-confinement method in PLAXIS 2D [Webinar]
Support capacity evaluation of a tunnel lining in PLAXIS 2D [API / Python scripting]