Optimization on Highway Tunnel Construction in Upper-hard Lower-weak Stratum with Intensely Weathered Granitic Migmatite
Aiming at the special geological conditions of Lincang-Yunxian expressway tunnel,where the geological layers consist of fully and intensely weathered granite mixed rock with upper-hard lower-weak stratum,the original excavation and support scheme would lead to significant deformation and convergence issues in the surrounding rock during construction process.This could potentially result in local collapses,roof caving,and other problems.Therefore,in the conjunction with tunnel deformation monitoring result,the targeted proposal was made to optimize the tunnel step length,anchor rod length,and arrangement.The comparative analysis was conducted through finite difference numerical simulation software to assess surrounding rock pressure,tunnel deformation,anchor rod axial force,and depth of plastic zone under different excavation and support conditions.The result indicates that when the tunnel is constructed by using step method and excavated near the boundary between hard and soft rock stratum,the weak nature of surrounding rock under arch foot prevents the formation of reliable support,leading to significant settlement and convergence deformation of overlying soil,with the minimum surrounding rock pressure occurring at boundary position.In the excavation process of tunnels in upper-hard lower-weak stratum,the excavation and support during middle step excavation have the greatest influence on tunnel deformation convergence.As the length of middle step shortens,tunnel deformation convergence gradually decreases,while surrounding rock pressure increases.The short anchor rods have weaker control influence on the deformation of surrounding rock.Increasing the length of anchor rod can better enhance its suspension effect,fully utilizing the bearing capacity of surrounding rock,and reducing the deformation of tunnel after excavation and support.The excavation and support scheme with step lengths of 4,6,2,2 m for upper,middle,lower steps,and anchor rod lengths of 4.5,6.0,6.0,4.5 m respectively,which meet the requirements for tunnel deformation convergence and are convenient for practical construction,ensuring the safe and rapid progress of tunnel.
tunnel engineeringconstruction optimizationnumerical simulationstep lengthanchor rod layoutupper-hard lower-weak stratum
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