There are so many technical aspects related to tunneling in weak rock because of encountering rock mechanics problem such as high strain rate, squeezing,…together with practical problems. Tunneling in this condition requires the considerable use of a combination of support systems such as shotcrete, rock bolts, wire mesh and steel sets to stabilize the tunnel. Using of top heading and benching down excavation method in large cross section tunnels, stability of support systems installed in heading section while executing benching step is one of important issue which has to be taken to account. In this study, several methods for benching in weak ground condition have been studied and the method used in long penstock tunnels of Roudbar dam waterway as a case study has been explained. The method including excavation of two side wall of benching section beneath crown support system and filling excavated area with reinforced concrete piles has been investigated through numerical models using FLAC2D software. Results show that using piling system, deformation of tunnel wall and displacements of crown support decrease comparing to benching without additional treatment. But according to horizontal displacement magnitudes, it would be better to use another additional support such as invert struts combined with piling for completely controlling of horizontal displacements.

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