In order to study the unsymmetrical load effect in geological bedding strata for the Muzhailing tunnel on the Lanzhou-Chongqing passenger dedicated line in China,we investigated the deformation,mechanical response and pressure of the surrounding rock and the mechanical characteristics of bolts of the tunnel.The results suggest that open zones appear at arch and invert where joints open up,when layered stratum is horizontal,or when the dip angle of inclined bedding is small.Open zones occur perpendicular to a joint.The failure mode is bending disjunction at the arch.What's more,with the dip angle increasing,tunnel excavating would induce shear zones where joints experience a certain shear displacement,and lead to obvious geological bedding unsymmetrical load.The failure mode is shear damage.For the joint dip angle in the range of 75-90°,the failure mode is flexural crushing at the wall and vertical shear rupture above the arch.The restraining effect of two sides weakens for vertical dip.On the whole,shear failure instability trend would occur and the tunnel collapses evenly.When the angle between the bolt and structure plane is greater than 23°,bolts can enhance the shearing stiffness of joint plane.Unfortunately,in the general purpose graph of tunnel for 250 km/h of passenger dedicated lines,the bolts have equal length and spacing.The rationale behind this is worthy of further study.For inclined bedding,the surrounding rock pressure at the left wall is more than that at the right wall.In addition,lining is likely to be damaged at left shoulder and side wall.With the dip angle increasing,the unsymmetrical load gradually achieves symmetry.Asymmetry design for support is recommended to reduce the unsymmetrical load on lining disturbed by excavation.
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