Dynamic Properties Of Calcareous Sand Filler And Long-term Settlement Of Foundation Filled With Calcareous Sand Under Traffic Loads

With the prosperous development of the transportation in the South China Sea,the cyclic loads transmitted by the vehicles to the foundation of the transportation infrastructure filled with calcareous sand tend to have heavier amplitudes and higher speeds.This will aggravate the deformation accumulation and the service performance deterioration of calcareous sand filler,affecting the comfort and safety of driving.Compared with terrigenous siliceous sand particles,calcareous sand particles formed by marine biological deposition have the mechanical characteristics of irregular particle shape and being easily broken under external loads.Its long-term dynamic characteristics under traffic load are completely different from those of conventional siliceous sand.Therefore,to ensure the safety and stability of the structures in the calcareous sand foundation during the service period,investigating the long-term dynamic characteristics of saturated calcareous sand fillers under traffic loads,and establishing the long-term settlement prediction theory of the calcareous sand foundation are urgent scientific tasks.In this study,a series of monotonic and high-cyclic triaxial tests were carried out on saturated calcareous sands from the South China Sea to reveal the monotonic and long-term dynamic characteristics of calcareous sand fillers under complex stress paths.The samples after cyclic loading were detected by nuclear magnetic resonance,and the evolution of the microscopic pore structure of the calcareous sand samples was investigated.The elastic-plastic explicit calculation model of accumulated strain was proposed for calcareous sand.Then,this model was developed into the finite element software through a series of user-defined subroutines.Based on this,a long-term settlement numerical analysis model of an actual calcareous sand airport runway foundation engineering under traffic load was established.The main work and innovation achievements are as follows:（1）Drained and undrained monotonic triaxial tests were performed on saturated calcareous sand to study the effects of initial mean effective stress,initial void ratio and stress-induced consolidation anisotropy on the monotonic shear properties.Results show that there is a unique linear critical state line for calcareous sand in both the p’-q plane and the e-（p’/pa）^0.7 plane,regardless of the anisotropic consolidation and drainage conditions.In undrained shear,unlike quartz sands,calcareous sand reaches a"pseudo-steady state"along the non-flow stress path with rapid increase in p’and q before reaching a critical state,and then reaches the critical state along the stress path with simultaneous decrease in p’and q.The"pseudo-steady state"defines a transition state from a high dilatancy-dominated no-flow response to a particle breakage-dominated stress softening response.In drained shear,the degree of anisotropy has a significant effect on the stress ratio in the peak or phase transition state,which can be well characterized by the concept of state dependence.（2）Drained monotonic triaxial tests with the stress paths of Δq/Δp’=2,conventional compression,constant p compression,decreasing p compression and unloading were carried out on K0 and isotropically consolidated dense calcareous sand.The results show that under different consolidation and shear stress paths,there is a unique hyperbolic function relationship between the relative particle breakage index and the total input work of calcareous sand.With the clockwise deflection of the shear path and the increase of the initial mean effective stress,the peak state friction angleφpk and the critical state friction angleφcs of the calcareous sand both decrease due to the increased particle breakage of the calcareous sand.Theφpk andφcs of calcareous sand are negatively correlated with the relative particle breakage index.The calcareous sand has a unique linear critical state line in both the p’-q plane and the e-（p’/pa）^0.7plane,which is independent of the initial consolidation stress path and shear stress path.In the initial mean effective stress level range of this study（i.e.,50～1000k Pa）,particle breakage produced by calcareous sand does not cause the critical state lines in the p’-q and e-（p’/pa）^0.7planes to move and rotate,but causes the peak state line in the p’-q plane to bend significantly downward as the mean effective stress increases.（3）One-way long-term drained cyclic triaxial tests were conducted to investigate the effects of initial mean effective stress,cyclic stress ratio,void ratio and stress-induced anisotropy on the long-term dynamic characteristics of calcareous sand.The results show that the increase in void ratio and anisotropy leads to a significant increase in the accumulated axial strain of the calcareous sand.Under long-term cyclic loading,dense calcareous sand contracts first and then dilates,and there is a phase transition state line in the e-（p’/pa）^0.7 plane.There is a unique hyperbolic relationship between the relative particle breakage index and the total input work,regardless of stress conditions and initial void ratios.The dilated behavior of calcareous sands under long-term cyclic loading results in the formation of both larger and smaller pore structures in the samples.As the stress level increases,the smaller pores formed by dilation are filled by fine particles and disappear due to the increase of particle breakage and compressive deformation.（4）Drained high-cyclic triaxial tests with different load frequencies were carried out on dense calcareous sand samples to explore the effect of load frequency on the long-term dynamic characteristics.It was found that an increase in the load frequency leads to an increase in the accumulated axial strain and particle breakage of the calcareous sand.The higher the cyclic stress ratio,the more sensitive the deformation behavior of calcareous sand to the load frequency.An increase in the load frequency leads to a significant deterioration of the shakedown behavior.Therefore,a more accurate shakedown behavior discrimination boundary under different load frequencies was proposed for calcareous sand.In addition,the increase of the load frequency can improve the resilient modulus of calcareous sand.（5）Drained high-cyclic triaxial tests with constant and variable confining pressure conditions were carried out on dense calcareous sand samples to study the long-term dynamic characteristics of calcareous sand under the simultaneous vertical and horizontal vibration paths caused by traffic loads.The results show that at low cyclic stress ratios,the variable confining pressure path slightly increases the accumulated strain,while at high cyclic stress ratios,the variable confining pressure path significantly inhibits the generation of accumulated strain.The application of cyclic confining pressure leads to an increase in the contraction of the calcareous sand during the contraction stage,resulting in the disappearance of some big pore structures.In addition,with the increase of the cyclic confining pressure amplitude,the resilient modulus of the calcareous sand decreases significantly,and the damping ratio increases significantly.（6）Based on the cyclic triaxial test results and the plastic increment theory,an explicit high-cycle accumulation model（HCA model）considering the complex dilatancy and particle breakage of calcareous sand was established,where the functions controlling the influence of frequency and the fractional flow rule were introduced.Then,a global optimization parameter calibration method of HCA model based on simulated annealing algorithm was developed.The calculated results of the new model are in good agreement with the measured values.On this basis,the HCA model was embedded in the finite element software through a series of user-defined subroutines,and the numerical model of an actual calcareous sand airport runway project under the action of millions of traffic loads was established.The long-term cumulative deformation simulation of calcareous sand foundation was realized by combining implicit and explicit algorithms.Through the further development of the user-defined subroutines,the real-time change of soil stiffness with soil state during cyclic loading was incorporated in the numerical model.Then,according to the change of soil stiffness,the change of strain amplitude during cyclic loading was updated.Based on the finite element calculation,the long-term settlement behavior and key influencing factors of the calcareous sand foundation of the actual airport runway were systematically analyzed.