Umbrella arch method performance, structural behavior and design elements utilizing in collapsing zones

The umbrella arch method is a pre-reinforcement technique that ensures tunnel stability in poor ground conditions or shallow overburden. Quantitative assessment of this method is still challenging due to its complexity, there is no accurate method to model the behavior of reinforced arch forming with steel pipes and grout. This paper uses finite difference numerical simulation to model and analyze the effectiveness of the umbrella arch method in Qazvin–Rasht railway tunnel as well as in-situ measurement. Umbrella arch has modeled considering pipe, grout and soilcrete as pipe umbrella components; each pipe simulated individually. The structural behaviour of pipes in the excavation sequence is analyzed as well as the importance of geometrical and physical design elements, on tunnel stability using FLAC 3D software. Results indicate that the umbrella arch method has reduced tunnel convergence by 49%. By continuing the excavation step, residual pipe length ahead of the face and tunnel arch, the location of maximum pipe displacement, force, and moment are changed. Furthermore, by considering executive limitation, an increase in pipe angle, diameter and thickness, length and overlapping length, as well as pipes distance decrease, center to center grout penetration radius, and water-cement ratio 0.5 have had a positive effect in providing more strength arch and tunnel stability.