Effects of forepoling, soil nailing and micropiling on the behavior of a two-story tunnel

This study investigated the effects of three improvement methods, the umbrella arch method (UAM), soil nailing and micropiling, on the performance of a New Austrian Tunneling Method (NATM) tunnel in Tehran, Iran. This tunnel passes under Modares freeway, a major artery in Tehran and is part of the Arash Project. The construction of a tunnel with a large, approximately square cross-section on a disturbed soil stratum and the existence of adjacent embedded canals (qanat) have made Modares tunnel one of the most challenging parts of this project. These challenges required the use of a nearly square two-story tunnel. This uncommon square shape was chosen because of limitations caused by adjacent facilities. All combinations of three supporting systems (forepoling UAM, soil nailing and micropiling) were investigated using the numerical method. The influence of these methods on surface settlement and the adequacy of the initial tunnel lining are discussed. In short, Forepoling was the most effective with a 24% decrease in settlement compared to Initial model. Combining the forepoling and nailing systems decreased settlement 37.5% and was the most effective synchronic system. The best results were for the model incorporating all supporting systems, which was a decrease in settlement of 47.5%.