Monitoring And Construction Methods Numerical Simulation For Large Section Tunnel In Lanzhou Yellowriver High Terrace

Yellow river terrace in Lanzhou area is the most typical river terrace with complicated geological conditions. Combining with the tunnel construction of Lanxin second double line in Kongjiaying tunnel. Thorough and systematic monitoring measurement have carried out on typical section (loess formation, loess and thick pellet soil formation, thick pellet soil and soft rock formation, loess thick pellet soil and soft rock formation). Comparative analysis the characteristics of stress deformation on large section under different geological conditions.The monitoring results showed that, the stability of surrounding rock has large difference in different stratum assemblage, especially the stability of tunnel is worst when there is thick pellet soil in vault; there is significantly bias in tunnel, the effect of bias on the tunnel can be well controlled through secondary lining.The peripheral convergent displacements of tunnel section all are small for various geological constructions, which are much less than the regulatory requirements. The crown settlement from small to large are in the order of loess and thick pellet soil formation, loess thick pellet soil and soft rock formation, loess formation, thick pellet soil and soft rock formation. The convergence in order of thick pellet soil and soft rock formation, loess and thick pellet soil formation, loess thick pellet soil and soft rock formation, loess formation.Overall, shotcrete force is good. The stress of steel frame, between steel frame and lining change fast, convergence change fast too in loess formation; while they change and converge slowly in thick pellet soil and soft rock formation. Due to the impact of bias the stress between steel frame and lining turns to tensile stress in loess formation. The stability of surrounding rock is good by the measurements of lining and steel frame. The deformation in spandrel is small, other positions are very stable. It can be illustrated that after primary support, closely followe invert and secondary lining can improve the situation of force and deformation, maintain the stable of tunnel.Numerical simulation for tunnel under different stratum structures and construction methods has been conducted with FLAC3D, and the difference and change of displacement and stress at monitoring points are obtained which can find the safest construction method with the smallest impact to surrounding rock and further improve the safety of the construction. The simulation results showed that: during the construction process in loess formation, settlement value of each monitoring point change small, and finally converge, when the tunnel face is less than45meters from the invert. But when the tunnel face is more than45meters from the invert, the displacement and stress of the surrounding rock increase with a large speed and meanwhile sedimentation value increase linearly and can’t achieve a convergence. So we can drawn that the distance between tunnel face and invert can increase to about45meters, footage2meters at loess formation. The distance between tunnel face and invert can increase to about45meters, footage2meters at loess and thick pellet soil formation. The distance between tunnel face and invert can increase to about45meters, footage3meters at loess thick pellet soil and soft rock formation. The distance between tunnel face and invert can increase to about45meters, footage3meters at loess thick pellet soil and soft rock formation.