| Suspension bridge is one of the main forms of bridges.At present,almost all bridges with a span of more than 1000m are suspension bridges.Anchorage is the main loadbearing member of suspension bridges.Its load-bearing characteristics and stability are very important for the safety and stability of suspension bridges.At present,the design and calculation method for the pull-out resistance of tunnel anchorage in soft rock stratum is not simple and accurate.The cognition of the bearing characteristics of this type of tunnel anchorage is not clear enough,which limits the application of tunnel anchors to some extent.This article first simplifies the tunnel anchor mechanical model and deduces the clamping effect coefficients and the formula of bearing capacity of tunnel anchorage.Then based on a tunnel anchorage project in a soft rock stratum,this paper has carried out in-depth and systematic research on the bearing characteristics of the tunnel anchorage in soft rock stratum using laboratory rock physical and mechanical test,laboratory similar model test and numerical simulation.The main research work and results are as follows:①By establishing a two-dimensional plane analysis model,the bearing process of tunnel anchor in soft rock stratum formations can be roughly divided into three stages.Based on the stress characteristics of tunnel anchorage at different stages,the calculation formulas for the pull-out bearing capacity of tunnel anchorage in soft rock stratum considering the "clamping effect" are derived.The factors affecting the clamping effect coefficientsξ1 and ξ2 are analyzed.Based on the stress characteristics of the tunnel anchorage,the design process of the tunnel anchorage structure in soft rock stratum is proposed.Finally,the calculation method and calculation process of the pull-out bearing capacity of the tunnel anchorage in consideration of clamping effect are verified with the results of the in-situscale model test of a Yangtze River bridge in Chongqing.The result shows that the clamping effect coefficient ξ1 is mainly related to the friction coefficient f of the interface betweent anchor and rock,rock lateral pressure coefficient k,angle of anchorage plug body a and the extension angle β.The clamping effect coefficientξ2 is also related to the weight of surrounding rock weight γ,equivalent length L0,and shear strength c and φ of surrounding rock.The "clamping effect" of the surrounding rock is the main source of the bearing capacity of the tunnel anchorage.The pull-out resistance provided by it accounts for more than 95%of the ultimate pull-out bearing capacity of the anchor plug body.②Physical and mechanical parameters such as density,compressive strength,deformation modulus,elastic modulus,Poisson’s ratio,tensile strength,cohesion and internal friction angle of the mudstone at the anchor site were obtained through conventional physical and mechanical tests in the laboratory.At the same time,based on the above test results,the stability of the rock mass in the tunnel anchorage area of soft rock stratum in the background of the project is evaluated.It is considered to have the ability to maintain stability after withstanding the design pull-out load.③Based on engineering and similarity theory,indoor similar model test(low-load test and failure test)on the bearing characteristics of tunnel-type anchorages in a soft rock stratum with a geometric similarity ratio of 1:150 were designed and carried out.Through this similar model test,the load-bearing response law(deformation,stress),surface and profile failure characteristics andfailure modes of the anchor plug body and surrounding rock mass under different levels of load of the system are obtained.The studies have shown that ten tunnel anchorage bearing system in soft rock stratum still has a relatively high safety reserve.The failure mode is non-standard inverted wedge failure.The failure range is extended along the direction of tension with the anchor plane behind the plug body as the bottom plane.The failure degree of inter-anchor rock mass is greater than that of the anchor-side rock mass,and that of the upper rock mass if greater than that of the lower rock mass.④Based on the laboratory rock mechanics tests and the laboratory similarity model test,the finite difference software FLAC3D was used to establish the numerical analysis model.The bearing characteristics of tunnel anchorage structure in soft rock stratum from design load to failure load were studied.The research shows that the deformaion of tunnel anchor bearing system is very small under design load.The plastic zone does not appear until 5.0Pm.The surface surrounding rock of inter-anchor has the largest tensile deformation while that of anchor-side has the smallest tensile deformation,which makes the deformation is in the shape of "M".The surface surrounding rock above anchor has the largest vertical deformation while that of anchor-side has the smallest tensile deformation,which makes the deformation is in the shape of inverted "V".The horizontal displacement of the inclinometer hole is the largest at the axis of the anchor plug body,while upper rock mass is larger and lower rock mass is the smallest,which confirms the failure mode of the model test.⑤Based on the method of numerical simulation and analysis,the parameters of anchor plug body(expansion angle,length,buried depth,center axis distance,center axis dip angle and cross sectional dimension)that affect the characteristics of the tunnel anchorage in soft rock stratum.The research results show that within the scope of the study,the greater the parameters above,the smaller the deformation and plastic zone,which means that the bearing capacity of tunnel anchorage bearing system in soft rock stratum is stronger.At the same time,the parameter sensitivity analysis was carried out using the grey correlation method.The sensitivity of the displacement of the surface rock mass betweent the anchors at the front end of the front anchor chamber to the measurement point T1 to the design parameters is ranked as follows:the size of the front section of the anchor plug>the dip angle of the anchor axis>the distance between the anchor axes>anchor length>buried depth of the chassis base of the behind anchor surface>anchor extension angle. |
