Study on the mechanics of construction of large section continuous arch tunnel in fly ash pile

There is no precedent of using concealed excavation method to construct large cross-sectional continuous arch tunnel in fly ash accumulation in China, which brings great challenges to the design and construction of the tunnel. Based on the Yanpingba tunnel project, we conducted an indoor similar model test of large-section continuous-arch tunnel through fly ash accumulation, and explored the evolution of large-section continuous-arch tunnel collapse arch and the mechanical behavior of excavation construction under this stratum. The results show that: the continuous arch tunnel excavation in the fly ash pile is highly susceptible to destabilization, and the destabilization process is divided into four stages: local collapse phase, main arch forming phase, intense destabilization phase, and stratigraphic penetration damage phase, and the damage pattern is characterized by abruptness, intensity, symmetry, and synchronization; the height of the collapsed arch is about 0.4 times the tunnel excavation span, and the longitudinal and transverse cracks inside the arch " Cutting" causes the soil body to be damaged by tension-shear, and the dense annular cracks outside the arch have a horizontal angle of 34°~72° with the excavation center point and penetrate with the longitudinal cracks near the excavation contour line, resulting in the overall destruction of the strata; the bearing capacity of the fly ash pile is seriously insufficient when the single sidewall guide pit method is used for excavation, and the settlement of the left and right vaults are 16.86 cm and 14.91 cm respectively according to the similar conversion relationship. The bearing effect of the diaphragm wall can effectively reduce the displacement of the strata, and in the process of conversion from single-hole pressure arch to double-hole pressure arch, the diaphragm wall is easily deflected to the side of the first hole by bias pressure, so it is suggested to add supports to the left side of the diaphragm wall in the excavation of the first hole.