Study On The Service Performance And Durability Limit Life Of Cantilever Anti-slip Pile Under Simulated Acid Rain Erosion Environment

With a great quantity of the cantilever anti-slide pile projects in the acid rain area of Southwest China gradually entering the middle ～ late service period,the piles inevitably produce damage accumulation,resistance attenuation and performance degradation under the combined action of long-term thrust load,pile aging characteristics and acid rain erosion environment,the cantilever anti-slide piles with degraded service performance have become a major risk source for retaining engineering.The basic research of degradation mechanism of service performance and prediction of durability limit life of cantilever anti-slide pile has important theoretical and engineering practical value for accurately evaluating the bearing performance and service life,ensuring the long-term safety,improving the toughness and technology development of cantilever anti-slide piles.Based on the key project of Chongqing Natural Science Foundation “Research on the long-term performance of anti-slide piles under the coupling effect of concrete deterioration and thrust”(Fund No: cstc2020jcyj-zdxmx0012),this paper focuses on the long-term performance evolution process of cantilever anti-slide piles under the acid rain erosion environment,the acid corrosion effect,rust expansion characteristics,long-term bearing performance and durability limit life prediction of cantilever anti-slide piles under acid rain erosion environment was systematically studied by means of theoretical analysis,physical model test and numerical simulation.The main research results are as follows:(1)The multi factor time-history evolution model of concrete neutralization depth under the combined action of bending tensile load and acid rain erosion was established.By carrying out physical and mechanical properties tests of concrete specimens under different acid rain corrosion environments and stresses,the evolution laws of physical and mechanical properties of concrete such as appearance,internal damage,mass,compressive strength and neutralization depth under acid rain corrosion environment were clarified.The deterioration effect of acid erosion and the synergistic effect of acidity,stress and time were illustrated by scanning electron microscope.Based on Fick’s first diffusion law,the multi factor time history evolution model of concrete neutralization depth is constructed by introducing the acidity factor and stress level factor.(2)The corrosion expansion mechanism and bearing performance degradation mechanism of cantilever anti-slide pile under the combined action of thrust load and acid rain erosion were revealed.Based on the model test of corrosion expansion deterioration and bearing performance of cantilever anti-slide pile under the combined action of different thrust loads and acid rain erosion,the load crack distribution,corrosion expansion crack distribution and reinforcement corrosion distribution of cantilever antislide pile under different load levels and reinforcement corrosion levels were analyzed.The calculation model of the reinforcement corrosion rate-corrosion expansion crack width of the cantilever anti-slide pile was established,and the mechanism of corrosion expansion deterioration of the cantilever anti-slide pile was clarified.The whole process of stress and deformation of the cantilever anti-slide pile under external trapezoidal load was revealed,the deterioration laws of the durability time-varying parameters such as the residual bearing capacity,pile displacement,pile bending moment,pile ductility and the crack distribution of the cantilever anti-slide pile under the combined action of thrust load and acid rain erosion were clarified,and the failure characteristics of the cantilever antislide pile were analyzed.The calculation method of optimized internal force and longterm stiffness of the cantilever anti-slide pile were proposed,and a calculation and evaluation method of the load-carrying capacity of the cantilever anti-slide pile was established,which comprehensively considers the damage of pile section,the deterioration of pile material and the degradation of bond performance between reinforcement and concrete.(3)The long-term performance limit state criterion and the deterministic prediction method of the durability limit life of the cantilever anti-slide pile were proposed,and the deterministic prediction system of the durability limit life of the cantilever anti-slide pile was constructed.Based on the deduction of the whole process of acid rain erosion of concrete and rust expansion of the reinforcement of the cantilever anti-slide pile,the durability limit state criterion of the cantilever anti-slide pile under the combined action of thrust load and acid rain erosion was established,and the prediction model of initial corrosion time of steel bar in the cantilever anti-slide pile under the combined action of thrust load and acid rain erosion was constructed.Based on Timoshenko elastic theory and Mohr-Coulomb failure criterion,the thick-walled cylinder model,Mazars damage model,and dispersed crack model were introduced,a time-varying model of corrosion current density of steel bar under acid rain erosion environment was proposed,a time prediction model for the expansion cracking on the surface of the concrete protective layer and a time prediction model for width limit of expansion crack of the concrete protective layer of the cantilever anti-slide pile were established by considering the external load effect of the pile,the filling process of corrosion products,and the confinement effect of stirrups,the error between the calculated results of the model and the existing empirical model is basically within ± 20 %.The damage process and degradation mechanism of the durability performance of the cantilever anti-slide pile in the whole life cycle were revealed,and the deterministic prediction system of the durability ultimate life of the cantilever anti-slide pile was constructed.(4)The probability analysis model of durability service life and the time-varying probability calculation model of bearing capacity in the whole life cycle of the cantilever anti-slide pile were established,and the probability prediction system for the durability ultimate life of the cantilever anti-slide pile was constructed.Considering the objective randomness of the service performance parameters during the service life of the cantilever anti-slid pile and subjective uncertainty of the calculation modes,the randomness of the service performance parameters and the calculation modes of the cantilever anti-slid pile were measured based on probability statistics theory and random field theory.Combined with the deterioration characteristics and the time-varying characteristics of pile material parameters,the Monte-Carlo simulation method is used to deduce the whole process of durability and bearing performance of the cantilever anti-slide pile in the whole life cycle.The time-varying probability prediction method of the durability ultimate life and the long-term performance of the cantilever anti-slide pile were proposed,and the probabilistic prediction system of the durability ultimate life of the cantilever anti-slide pile was constructed.