Effects of forepoling, nailing and micropiling on the behaviour of a two-storey tunnel

This study investigates the effects of three improvement methods, the umbrella arch method (UAM), soil nailing and micropiling, on the performance of a new Austrian tunnelling method 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 the Modares tunnel one of the most challenging parts of this project. This uncommon square shape was chosen due to 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 the initial model. Combining the forepoling and nailing systems decreased the settlement by 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 the settlement by 47.5%.