Mechanical Properties And Stability Analysis Of Surrounding Rock Mass Of Tunnel Anchor Employed In Xingkang Bridge In Tibetan Area Expressway

Tunnel anchor is a special type of anchor employed in suspension bridge and the stability of tunnel anchor plays a significant role in the safety of the suspension bridge.Compared to the traditional gravity anchor,tunnel anchor possesses the advantages of less excavation,less influence to the environment and higher cost performance,however,the application of tunnel anchor in practice engineering is very sensitive to geological condition and rock properties.The issues regarding to the interaction between tunnel anchor and surrounding rock mass and the creep deformation and the stability of tunnel anchor under long-term load,have not been well addressed.The Xingkang Bridge is a key engineering project of the Yakang Expressway connecting Ya'an City and Kangding City.Due to the limitations of geography and geology,the tunnel anchor constructed at the Ya'an bank of Daduhe River for the Xingkang Bridge has become the longest tunnel anchor worldwide.In this paper,the studies mainly focus on the mechanical properties of surrounding rock mass of the tunnel anchor,the bearing capacity of the tunnel anchor,the long-term stability of the tunnel anchor,and the monitoring of the surface displacement of the tunnel anchor in the Xingkang Bridge project.The primary conclusions are shown below.(1)By collecting the literatures about tunnel anchors employed in the existing suspension bridges,especially the research on the engineering geological conditions,the physical and mechanical properties of surrounding rock mass,the scaled model,and the creep deformation of tunnel anchor,we have revealed the relationships between the creep deformation of tunnel anchor and the size of tunnel anchor,the location of model anchor and creep model on the basis of the geological conditions of Xingkang Bridge.(2)Combining with the geological mapping of model anchor,test adit and real bridge anchor,and the statistical analysis of weathering characteristics of fractures,the main factors affecting the quality classification of tunnel anchor rock mass are obtained.The most important factors are the integrity degree of rock mass and the characteristics of controlled structural plane.Finally,the quality of the surrounding rock mass have been evaluated by using a variety of rock mass quality classification methods.(3)Based on the mechanical test of tunnel anchor and rock mass quality classification,a numerical analysis mode was established in the software FLAC^3D.The recommended values of rock mass physical and mechanical parameters are then obtained by using LSSVM and PSO intelligent displacement inversion method in this model.(4)Through the deformation and strength variation characteristics obtained from the in-situ test of model tunnel anchor at a scale of 1:10,it is determined that the main deformation of anchor body and surrounding rock is elastic deformation and the deformation shows the elastic-plastic characteristic.The deformation value of anchor body is approximately equal to the sum of deformation value of surrounding rock and dislocation deformation value of interface between the anchor body and surrounding rock.Meanwhile,the dislocation deformation value between the anchor and surrounding rock is far less than the deformation value of surrounding rock,accounting for 15%of the deformation value of the anchor body.The main factor controlling the deformation of the tunnel anchor is the deformation of surrounding rock.(5)Based on the Mohr-Coulomb strength criterion,the pullout factor of the tunnel anchor of the Xingkang Bridge is derived,and the approximate calculation value is 3.48.The in-site test result shows that the pullout factor of the model tunnel anchor is 5.25.Obvious wedge-shaped effect is existed in the stress of the tunnel anchor.The anchor body is controlled by the yield strength of surrounding rock and the characteristic point of yield load is 5.25N_P.(6)A modified creep model is proposed on the basis of the Burger's model.The modified Burger's creep constitutive equation is then suitable for deformation analysis of tunnel anchor by introducing the additional tension of surrounding rock on the anchor.The creep characteristics of the tunnel anchor and the surrounding rock are determined as follows,i.e.the additional tensile force on the tunnel anchor from the surrounding rock increases with the increase of the main cable load,the concrete Poisson's ratio and the unit elastic resistance coefficient of the surrounding rock mass,and the additional tensile force decreases with the increase of concrete elastic modulus and radius of anchor body;the larger the main cable load is,the more significant the effect of the additional tensile force is.(7)The main affecting factors on the creep deformation of tunnel anchor are obtained through the analysis of FLAC^3D three-dimensional creep numerical model.The numerical analysis results show that the radius of tunnel anchor has a significant influence on the creep deformation;the influence of anchor location on the creep deformation mainly depends on the geological conditions,and the influence is not significant;the elastic modulus has a significant influence on the creep deformation,and the elastic modulus of maxell model is more sensitive than that of Kelvin model;the viscosity coefficient of Kelvin model is more sensitive than that of maxell model.(8)Based on the Boussinesq and Mindlin solutions,the stresses in the surrounding rock are calculated respectively.When the initial stress state of the surrounding rock is taken into consideration,it is obtained that the closer the shear stress is to the anchor body,the more obvious the stress redistribution is.The plastic circle radius of rock mass is 60 m according to Finner's formula,while the plastic circle radius is less than 60 m under the design load from the numerical results.Clearly,most rock mass is still under elastic working state after suffering the pullout force by the main cable.(9)Using the three dimensional numerical simulation to analyze the deformation of the anchor under the conditions of tunnel anchor excavation,design load and overload,it is concluded that the maximum displacement of the tunnel anchor is 5.7 mm under the design load,and the tunnel anchor belongs to the steady creep stage under the loads of all levels;the creep deformation of the anchor does not exceed the maximum allowable value under the normal design load and the creep deformation of the tunnel anchor would not affect the long-term stability of the suspension bridge.In addition,the In SAR technology is employed to monitor the surface deformation of the tunnel anchor.The results show that the tunnel anchor area is in the stable state without obvious deformation,and the deformation within one year is less than 10 mm.The numerical simulation results show that the creep deformation of the bridge tunnel anchor area is about 1.38 cm over a total duration of 100 years.The numerical and analytical results show that the tunnel anchor works well and the stability of the bridge could be guaranteed.