Load Transfer Mechanism And Anti-pull Stability Of Tunnel-type Anchorage In Soft Rock Strata

The tunnel-type anchorage is one of the main forms of anchorage of the suspension bridge and recently being used in the large-span suspension bridges due to their small size,high bearing capacity and low environmental disturbance.Although the existing researches on the tunnel-type anchorage have provided some insights into the deformation and failure characteristics,the anti-pull mechanical behavior and load transfer mechanisms it has not been fully explored.In particular,the united bearing characteristics of the plug body,interface and rock mass,the stress distribution and evolution characteristics of the interface between the plug body and the rock mass,and the crack initiation and evolution characteristics,spatial distribution and failure modes in rock masses are not studied thoroughly.Furthermore,the factors affecting the bearing characteristics of tunnel-type anchorage are mostly limited to the plug body design parameters and mechanical property of rock mass in the anchorage area,and the interface mechanical properties between the rock mass and plug body are often ignored.In the meantime,facing the trend of tunnel-type anchorages extending to soft rock stratum,the understanding of stability control technology and mechanisms of tunneltype anchorage installed in soft rock strata is still lagging behind engineering applications.With the methods of similarity model test,theoretical analysis and numerical simulation,this paper mainly investigates the joint bearing and failure characteristics,the influence of the interface roughness on the anti-pull mechanical behavior,stability control technology and mechanisms of tunnel-type anchorage installed in soft rock strata.The main research contents and results are as follows:(1)A set of similarity model test system is developed to investigate the load transfer mechanism and anti-pull Stability of tunnel-type anchorage in soft rock strata.The distribution and evolution of the displacements field,major strain field and interface stress are also discussed.The results indicate that vertical displacement of the ground surface is mainly concentrated in the middle and front part of the vertical projection of the plug body on the ground surface,which has the characteristics of the stage,regional and hysteresis.The horizontal displacement field of surrounding rock of tunnel-type anchorage in soft rock strata experienced the evolution from asymmetric trumpet-shaped to spindle-shaped during loading,and the vertical displacement field experienced the evolution from inclined layered distribution to vertical strip distribution for the rock located in the upper part of the plug body and approximate arc distribution for the rock located in the lower part of the plug body.The major strain concentration first occurs in the rock mass near the rear end of the plug body and is accompanied by the rotation of the major strain axis.The points at the tip of the concentration band of the major strain all rotate clockwise.Rotation characteristics of the major strain axis of the rock mass outside the concentration band are related to whether there are new concentration zones on both sides of the existing concentration zones.The stress distribution of the interface sequentially changes from a double-peak shape with large two ends and small middle to a double-peak shape with the peak at the rear end shifting forward,and finally into a trapezoid-like shape with an overall decrease in stress.(2)Based on the similarity model test,the ground surface cracks and inner cracks of rock mass were investigated respectively to discuss the progressive failure characteristics of tunnel-type anchorage during loading.The results indicate that the cracks in the surrounding rock on the upper part of the plug body are distributed one by one from the rear end of the plug body forward in a multi-layer trumpet-shaped after breaking the tunnel-type anchorage.These cracks change from small-angle shear cracks to tensile-shear composite cracks,and then to large-angle tensile cracks during propagating.The surrounding rock of the lower part of the plug body is dominated by shear cracks approximately parallel to the axis of the plug body,and finally,a thin shear band is formed.The ground surface cracks propagate in a direction approximately perpendicular to the vertical projection of the arch of the plug body on the surface.The ground cracks experience the evolution from tensile cracks to shear or tensile-shear composite cracks,to tensile cracks,and then to tensile-shear composite cracks during propagating.The length of the crack increases gradually from the back end of the plug body forward,and the properties of the crack gradually transit from tensile crack to tensile-shear(or compression-shear)composite crack.The block shape cut by the fracture surface inside the rock mass gradually transits from strip-shaped to blockshaped from the back of the plug body to its front end.The vertical dislocation value between adjacent blocks decreases from backward to forward.The deep cracks in the rock mass are gradually annihilated,and the damaged area gradually shrinks to the shallow surface and the front end of the plug body.(3)Based on the surrounding rock failure characteristics,distribution and evolution of the interface stress,and the load-displacement relationship of the plug body obtained from the similarity model test,the load transfer characteristics of the cluster composed by plug body,interface between the plug body and rock mass,and the clamping effect of the rock mass was investigated.The failure characteristics of the tunnel-type anchorage at different stages were also analyzed.The results indicate that the tunneltype anchorage installed in soft rock strata with a rough interface experiences four states in turn: approximate linear-elastic deformation state,slow nonlinear deformation state,plastic deformation state of the accelerated loss of bearing capacity and failure state of bearing capacity loss entirely.The joint bearing cluster is composed of plug body,interface between the plug body and rock mass,and the clamping effect of the rock mass was formed initially when the tunnel-type anchorage was in an approximate linear-elastic deformation state and a slow nonlinear deformation state.The bearing body is gradually transformed from the weight of the plug body and rock mass to the interface and clamping effect of the surrounding rock.In the plastic deformation state,the tunnel-type anchorage has undergone successively the failure stage Ⅰ,which is shear cracks in the surrounding rock near the back face of the plug body,and the stage Ⅱ,which is a double failure of compression-shear(or tension-shear composite failure)of the inner rock mass and sliding failure of the interface.The proportion of bearing capacity derived from the interface and the clamping effect of the surrounding rock is gradually low.In the failure state,only the weight of the rock mass and the frictional resistance caused by the gravity of the rock on the interface are left in the joint bearing cluster.The tunnel-type anchorage enters the failure stage Ⅲ,which is the shear(or tensile-shear)cracks of the rock mass located on the upper and lower part of the plug body propagating gradually and debonding of the interface.(4)Three similarity model tests with different interface roughness were designed to research the influence of the interface roughness on the load transfer and failure characteristics of the tunnel-type anchorage.The results indicate that the ultimate loadcarrying capability of tunnel-type anchorage decreases with the decrease of interface roughness.The tunnel-type anchorage with low roughness interface enters the hardening stage after experiencing the approximate linear-elastic deformation state,and the corresponding load decreases with the decrease of interface roughness when entering the hardening stage.The main cracks in the upper surrounding rock of the plug body gradually move to the front end of the plug body.There are only some small cracks with large angle in the surrounding rock near the back face vault of the plug body.With the decrease of the interface roughness,the failure mode of the surrounding rock in the lower part of the plug body changes from the failure mode of the thin shear band to the slip failure mode along with the interface.The surface failure area is approximately 1/4 ellipse,and the range decreases with the decrease of interface roughness.The three-dimensional failure mode of the tunnel-type anchorage with low interface roughness is an approximately triangular pyramid,which is different from the approximate bell-shaped three-dimensional failure mode of the tunnel-type anchorage with high interface roughness.(5)The principle of anti-pull tie to improve the bearing characteristics of tunneltype anchorage installed in soft rock strata is studied using the similar model test method and finite difference method.The results indicate that the plug body with an anti-pull tie improves the load-carrying capability of tunnel-type anchorages mainly due to the normal squeezing of the surrounding rock against the tie.This leads to on the one hand,increase the friction resistance of the interface between the anti-pull tie and the surrounding rock,and improve the shear capacity of the interface between the plug body and the surrounding rock.On the other hand,expand the bearing range of surrounding rock at the low part of the plug body.The yield load and the ultimate uplift load of the tunnel-type anchorage increase approximately linearly with the increase of the height,width,length and number of the anti-pull tie,and decrease approximately linearly with the increase of the slope angle of the anti-pull tie and the distance between the anti-pull tie and back face of the plug body.The yield load and ultimate uplift load of the tunnel-type anchorage are the most sensitive to the width of the anti-pull tie,followed by the height,length and slope angle of the anti-pull tie,and the position of the anti-pull tie and the distance between the anti-pull tie and back face of the plug body are the least sensitive.