Analysis Of Pullout Mechanical And Deformation Characteristics For Bolts In Non-homogeneous Ground

Bolts are widely used in geotechnical engineering due to their superiorities insafety, efficiency and stability and they will be applied broadly in future. However,during researches into transfer mechanism and stress deformation of bolts, rock-soilbody around anchorage segment is always regarded as homogeneous body. In fact, onaccount of the different type, formation time and consolidation degree of soil-rockbody, the heterogeneity of rock-soil medium is universal and the heterogeneity ofwhich has great impact on the interaction between rock-soil body and bolts. Yet, theresearch results based on this hypothesis may have some limitations and difficultiesand can hardly reflect the engineering practice. To solve this problem, with referenceto the National Natural Science Foundation of China (50878082), this paper aims tohave a further investigataton into the stress deformation of bolts in non-homogeneousground. Below are the main content and research results:(1) Considering the differences of mechanical properties of layered soil crossedby bolts, this paper obtains the unified mathematical expression which combined withfour kinds of load transfer model of the anchorage interfaces. Based on the loadtransfer method and transfer matrix method, analytic formulas of the displacement,axial force and shear stress of bolts are derived by using the suitable model. In thispaper the influence of the selected load transfer model and the sequence of soil layerto the displacement and stress of the bolts is also analyzed. The calculations indicatethat the mechanical properties and soil layer’s sequence have great impacts on bolts,and each soil layer should choose a suitable load transfer model which will directlyaffect the accurate estimation of ultimate bearing capacity and displacement of thebolts.(2) In consideration of the fact that the properties of soil mechanics varies indepth，it’s assumed that soil’s shear modulus, shear strength in the elastic limit stateand residual coefficient of shear strength would increase in depth by exponentialfunctions. Based on the shear displacement method, analytic formulas of thedisplacement, axial force and shear stress of bolts are derived when the surroundingsoil is under elastic or plastic stress state. This assumption also can be extended tolayered ground, expanding its application scope. Influences of foundation modelparameters and elastic modulus of anchorage segment on mechanical and deformation characteristics of bolts are analyzed as well. The results indicate that fact that theproperties of soil mechanics varies in depth will benefit the operation of bolts, withthe bearing potentials of rock-soil body more fully used. Besides, reasonableconsideration in the calculation can reflect the load transfer process of bolts directlyand accurately.(3) Based on the two types of foundation model, the elastic-plastic analyticalformula of main mechanical properties of bolt is obtained respectively in a foundationthat shear modulus changes in depth by power function.The influences of pullout loadand foundation model parameters on stress deformation of bolts are also studied. Thecalculations imply that the distributionof shear stress corresponding to two kinds ofground mode is quite different. The coefficient of shear modulus which varies indepth also has a great influence on the distribution of shear stress. When the plasticzone arrives at a certain extent, it makes pullout load and the length of the plasticzone show a nonlinear relationship. A slight increase in pullout load can make plasticzone advance rapidly to the deep. Therefore, bolts safety factor in practicalengineering should be reasonably set to avoid burst damage.