The Mechanical Behaviour Of Soil-nailing Under Pullout Loads

Soil nailing has rapidly spread and been applied to soil excavation and slope stability in China for its economic, reliable and fast construction as a new retaining technology. Numerous engineering examples show that the failure style of soil nail is mainly pulled out failure, and the failure style is progressive, but there is no perfect research result or specifications about the stress-distribution and ultimate pull resistance, because of the complexity of interaction between nail and soil. Therefore, based on the interface load transfer model of nail and soil, a theoretical study about stress distribution and ultimate pull resistance of soil nail under pull-out loads is carried out from the perspective of progressive failure of soil nail in this paper. The main research contents and results of this paper are as follows(1) The softening behavior of soil and the phenomenon of strain localization happens accordingly are considered in the shear failure process for nail-soil interface, then strain gradient plasticity theory is used in the tri-linear nail-soil interface load transfer model. The break phase of soil particles in the interface of nail-soil mainly happens is considered, then the elastic shear displacement is ignored in the softening stage, it is that the total shear displacement in softening stage equals to plastic shear displacement, therefore, a analytic solution of the shear displacement corresponding to the residual shear stress is derived, and also, theoretical value and experimental value of pull-out load and pull-out displacement are compared, then the utility and applicability of the tri-linear softening nail-soil interface load transfer model are verified by test indirectly, therefore, a theoretical basis for studying nail-soil interface load transferring characteristics and mechanical characteristics of soil nail directly is provided.(2) Parametric analysis is applied to the shear displacement corresponding to the peak shear stress and the shear displacement corresponding to the residual shear stress in the tri-linear interface load transfer model of nail-soil. The result is that: the shear displacement corresponding to the peak shear stress is relatively small, while the one corresponding to the residual shear stress is relatively large, then, the assumption that elastic shear displacement can be ignored in softening stage is verified; and the peak shear stress induced shear displacement is effected by the thickness of the shear zone largely, while the residual shear stress induced shear displacement is effected largely not only by the thickness of the shear zone but also by the soften modulus, then, the view that shear displacement in softening stage is determined by the volume deformation is verified.(3) Two slip coefficients are deduced respectively to represent combined slip effects caused by particle breakage and slippage caused by rigid body effects of nail compared to surrounding soil in softening stage and residual stage, the stress distribution and the ultimate pull-out resistance for soilnail in pullout process is derived from the progressive perspective of destruction in the interface of nail-soil based on tri-linear interface load transfer model of nail-soil, and a theoretical basis is provided for predicting mechanical properties of soil-nail in practical engineering.(4) Parametric analysis is carried out for ultimate pull-out resistance of soil-nail using the tri-linear interface load transfer model and the slip coefficients deduced in this paper for the interface of nail and soil. The results obtained are as follows: The impact of the parameters in tri-linear softening load transfer model, and the slip coefficients between nail and soil in both residual stage and softening stage to the ultimate pullout resistance is relatively larger; The smaller of slip coefficient between the extensible displacement of soil nail and shear diaplacement between nail and soil, the larger of the ultimate pullout resistance, then, in soften soil, the ultimate pull-out resistance will be overestimated by the ideal elastic-plastic interface transfer model of nail-soil and underestimated by the tri-linear ideal elastic- brittle interface transfer model of nail-soil. Therefore, the impact of tri-linear interface load transfer model and the slip coefficients deduced in this paper for the interface of nail and soil to ultimate pull-out resistance is explained from the theoretical perspective, and a theoretical basis is provided for the study of the ultimate pull-out resistance from progressive perspective of destruction and the enhancement of the ultimate pull-out resistance through improving the interfacial properties of nail and soil, and also, a theoretical basis is provided for research of soil nail by numerical simulation.