Study On Dynamic Characteristics Of Calcareous Sand Under Different Pile Driving Loads

The calcareous sand has the characteristics of friability,porosity and high compressibility.During the process of pile sinking,especially the calcareous sandwith long and large diameter will have serious particle breakage,which will have a significant impact on the installation process and bearing capacity of the pile foundation.Therefore,the mechanical properties of the calcareous sand in the process of pile driving need to be further studied.The common installation methods of driven pile include static pressure penetration,vibration penetration and hammer penetration.Corespondingly,the loads on calcareous sand around the pile are mainly static pressure load,vibration cyclic load and impact load.The static triaxial test,dynamic triaxial test and split Hopkinson pressure bar(SHPB)experiment are used to simulate the physical properties and mechanical response of calcareous sand under different pile driving loads.The particle breakage of calcareous sand under different loads and its influence on the driving ability and bearing capacity of pile sinking construction are compared.These works can be used as the basic work of theoretical system and design method for the prediction of bearing mechanism of steel tubular pile considering dynamic response mechanism of calcareous sand under different pile driving modes.The main research contents and conclusions are as follows:(1)The static triaxial shear tests under different confining pressures were carried out to study the relationship between stress and strain of calcareous sand with the change of stress level,to explain the change of shear expansion and shear contraction during the shear process,and to analyze the composition and mechanism of strength change of calcareous sand.The relative potential for breakage(Br)and breakage energy are used to describe the particle breakage before and after shearing,and to explain the reason for the change of axial force of pile body in calcareous sand foundation.(2)The dynamic triaxial tests under different confining pressure,dynamic shear stress ratio and frequency are carried out to describe the pore pressure development mode of calcareous sand under cyclic vibration load.The analysis shows that the dynamic strength increases with the increase of confining pressure and frequency.By checking the applicability of Seed pore pressure development model and its modified formula in calcareous sand,it can be seen that the dynamic shear stress ratio has the greatest influence on the shape of pore pressure development curve.The relationship between vibration time and vibration frequency,vibration frequency and vibration frequency is quantified,and the fitting formula is given.It is proposed that increasing the frequency in the construction process is conducive to shortening the construction time.(3)The dynamic mechanical properties of calcareous sand under impact load are studied by means of the improved SHPB experiment system.Considering the influence of strain rate,moisture content and relative compactness,the SHPB experiment system of scattered calcareous sand is established.The strain and strain rate,stress-strain relationship and energy absorption efficiency of calcareous sand under impact load are analyzed.The result shows that the transmissivity of calcareous sand is smaller than that of quartz sand,and the influence of relative density of calcareous sand gradually weakens after multiple impacts.(4)The particle breakage before and after the test of calcareous sand under different loads is compared with the distribution of each particle size group after crushing.It can be seen that the distribution rule of calcareous sand particles after crushing is similar.The particle size with stable relative content is around 0.25 mm,and the loading form and stress level will slightly change.In this paper,the calculation method of relative crushing potential of calcareous sand under the vibration cyclic load is put forward,which considers the frequency variation.The evolution law of particle crushing is described by the increment of relative crushing potential.In the actual project,the mechanical index corresponding to the extreme value of relative crushing potential can be selected as the reference of engineering design and construction.