Study on the large deformation control technology of a single-hole double-track railway tunnel

Due to the influence of soft rock with high geostress, large deformation is prone to occur during tunnel construction. Since it relies on the Yuntunbao Tunnel of the Chengdu − Lanzhou Railway, regarding the large deformation of soft rock, a control concept was proposed in this study through theoretical and on-site investigations and testing; the large deformation control technology involved an advanced support, which is a combination of yielding and resistance and coordinated bearing, with a corresponding support system. The applicability levels of two and three phases of a large deformation tunnel were compared and analyzed. The research results showed that with large deformation, the presupport of forepoling umbrellas could effectively control the settlement of the tunnel crown and ensure the stability of the excavation face in the soft rock tunnel. By assembling long bolts, double-layer initial supports, and grouting behind the supports, the problems of rock intruding the limit and the initial support failure of large deformation tunnels could be effectively solved. Relative to the two phases, although the construction process of the three phases was more complex, the cumulative deformation of the tunnel was only 63 − 70% of that of the two phases; this phenomenon was more conducive for controlling tunnel deformation. The stress of the steel frame at the tunnel spandrel was smaller than that of the two phases. Consequently, three phases were more suitable for constructing large deformation tunnels in soft rock strata.