Numerical modeling and parametric study of NATM tunnels supported by umbrella arch

In this numeric study, the behaviors of tunnels supported by the umbrella arch method were investigated parametrically through two-dimensional (2D) finite element models. Firstly, the numerical modeling concept was validated principally by comparing the results of the models with the real measurements taken from two different twin tunnel sections excavated with the New Austrian Tunnel Method (NATM) and supported by the umbrella arch (UA) located in the Çekmeköy-Sancaktepe-Sultanbeyli Metro Line project. For the validation, the models were created with three different approaches called plate (P), dummy plate (DP), and individual pipe (IP) in the literature, and their appropriateness was evaluated. All three approaches succeeded in predicting the surface settlement with an acceptable error margin. In comparison with the two others, DP approach predicted the real site data more precisely, and it was adopted to proceed with the parametric studies. Based on the validation study, by using constant deconfinement ratios, a total of 162 models were created for the parametric study with and without UA as a combination of three tunnel diameters, three overburden depths, three clear distance ratios between twin tunnels, and three soil conditions. These models were analyzed comparatively in terms of forces and displacements. The results indicated that maximum vertical displacement governs the maximum resultant displacement in tunnels, and surface volume loss (VS) increases as inner tunnel volume loss (VL) increases, and VL increases as ground conditions worsen. Their ratio (VS/ VL) decreased with the reduction in tunnel diameter (D) or overburden depth (C). The results showed that the twin tunnels converge differently due to geometry and ground conditions, and this difference causes asymmetric settlement profiles, which may also include two peaks as the distance between tunnels increases and C/D decreases. In the aspect of forces on support elements, the factor of safety increases as tunnel diameter gets smaller, overburden depth gets shallower, clear distance gets larger, and ground condition gets better. The results implied the importance of deconfinement ratio to simulate UA effect in 2D.