Study On Dynamic Bearing Characteristics Of Pile Foundation In Calcareous Sand Foundation

The calcareous sand on the coral reef which has the characteristics of breakable skeleton,multi-pores and prominent corners,exists in the marine dynamic environment,and the complex load action of the coral reef makes the calcareous sand foundation show high compressibility.As a common form of foundation in reef engineering,pile foundation is often subjected to the static load of the upper structure and also affected by dynamic load,which is affecting the bearing capacity of pile foundation and the stability of the upper structure.Moreover,the core issue is the interaction between pile and calcareous sand under dynamic loading.In this paper,a test system of dynamic load pile-sand interaction is developed,and the pile-sand interaction process is studied by two methods: model pile test and geotechnical simulation test.The settlement and bearing law of pile under dynamic load are analyzed via pile model test.The dynamic response characteristics of the calcareous sand around the pile under dynamic loading and the compressive deformation characteristics of the calcareous sand around the pile are analyzed,the non-destructive quantitative method of seepage method to describe the crushing of calcareous sand,the difference of the density and compressibility of different calcareous sand particle shapes are analyzed via geotechnical simulation test.The contents and results are as follows:This paper summarizes the engineering characteristics of calcareous sand and the literature of pile foundation engineering.Aiming at the issues of calcareous sand interaction under dynamic loading,a test system for pile-sand interaction under dynamic loading is developed.The test system is designed with similar theory,which consists of model test of pile foundation and geotechnical simulation test.The test system,which is reasonable in design and easy to operate,can apply cyclic load to the pile,obtain the settlement and bearing capacity data of pile,and analyze the dynamic response characteristics of calcareous sand around the pile.The model test of single pile under dynamic load was carried out to analyze the cumulative settlement and the bearing capacity law of pile,It is found that under different dynamic load ratio,there were three types of accumulated settlement of pile top: stable type,gradual type and destructive type,which have significant "threshold effect".When the number of cycles reaches "critical cycle",the rate of accumulated settlement of pile top tends to slow down.Based on this,the prediction formula of accumulative settlement of pile top was established.During dynamic loading,the pile end and pile side shared the upper load and the load sharing ratio was in dynamic change.The pile side friction gradually degenerates with dynamic loading,and there was a "accumulative damage" effect.The relationship between the weakening coefficient and the number of dynamic loading satisfy the Boltzmann function.After dynamic loading,the ultimate pile side friction decreases and the ultimate pile tip resistance amplitude decreases with the increase of dynamic load ratio.Through the geotechnical simulation test device,the interaction between the pile and the calcareous sand was carried out.The compression deformation,particle breakage,the strength and compressibility variation law of calcareous sand were analysed to reveal the dynamic response mechanism of calcareous sand under dynamic loading.The strength of pile-sand interface is weakened under dynamic loading and the calcareous sand particles are broken obviously.The compression deformation of calcareous sand in core area of pile end also appears similar phenomenon of the "threshold effect" and "critical cycle",which leads to the weakening of the strength enhancement amplitude.It is consistent with the test law of pile foundation model under dynamic load.The effect of particle shape,pressure direction,dynamic load ratio,loading times on particle deformation and particle strength were analysed.After dynamic loading,the particles show different degrees of damage,the greater the dynamic load,the more serious the damage is,the larger the reduction of particle strength is,the more uniform the flatness of each shape is.The crushing effect of calcareous sand particle around pile was developed by seepage method,the relationship between the permeability coefficient of calcareous sand around pile and the parameters such as particle gradation change,relative crushing rate,pore ratio,the crushing degree were analyzed.It is practicable to measure the breakage degree of calcareous sand through seepage method.Besides,the empirical formula of permeability coefficient of calcareous sand with particle size distribution,load level and relative crushing rate is fitted.The seepage method has the advantages of non-destructive,and can be used to monitor the particle breakage of calcareous sand.The influence of hammering load on the compression deformation,particle size distribution and strength of calcareous sand in the process of pile-sand interface tests are analyzed.It is found that calcareous sand is sensitive to dynamic load.A series confining compression tests of calcareous sand under static and dynamic loads are carried out.The load type and amplitude have a significant influence on the particle breakage of calcareous sand,leading to the changes of physical and mechanical properties of calcareous sand,particle size distribution,sand content,particle shape,etc.The particle shape change into a certain range of impact energy,based on these resulits,the construction advise are put forward.The image analysis and mathematical statistics were used to distinguish calcareous sands with different particle shapes.Three single shapes of calcareous sand sample were obtained: granular shape,dendritic shape and long strip shape.On this basis,the effect of particle shape and content on density of calcareous sand was analyzed by orthogonal design.The confining compression tests of calcareous sand under different amplitudes are carried out.Three kinds of calcareous sands have different compression properties and particle breakage rules,the shape and content affect the density and compressibility of pile end calcareous sand,which should be considered in engineering construction.The key points for design and construction of pile in calcareous sand were discussed from the four parts: the hydrogeological characteristics of the reef,the bearing capacity of pile foundation,the influence of construction load of pile foundation,operation monitoring and prediction of pile foundation.Some useful suggestions based on the test results for the engineering application are given.