Experimental Research On Penetration Mechanism Of Driving Steel Pipe Pile In Coral Calcareous Sands

Many characteristics of coral calcareous sands,such as fragility,porosity and high compressibility,cause severe questions in the proceeding of pile running and on bearing capacity of steel pipe pile.At present,there are few studies at home and abroad on the development of crushing grain in coral calcareous sands,the composition of resistance and the bearing characteristics to driving piles.For this purpose,sponsored by NSFC project“Research on penetration mechanism and bearing behavior of steel pipe pile in coral reef”（51678145）,focuses on the key issue of the interaction between coral calcareous sand and steel pipe piles,based on the indoor model tests and numerical simulation methods of particle flow,considering the influencing factors of the hammer energy,the relative density of sand and the slenderness ratio of the pile,the author has conducted a systematical study on penetration mechanism of coral calcareous sands by driving steel pipe piles and the vertical bearing characteristics of steel pipe piles.The main contents and results are as follows:1.Through the simulation of the whole process of the pile driving,the indoor model test of driving steel pipe piles in the coral calcareous sand conducted.By comparing the effects of different hammer energy and the different relative density of calcareous sands to the impact on the penetration of steel pipe piles,the paper makes a discussion of the composition of resistance to driving piles and the energy transfer during the driving process.More importantly,the penetration mechanism of driving steel pipe pile in coral calcareous sands was prominently analyzed.2.Through conducting the vertical compressive static loading model tests for steel pipe piles in coral calcareous sands,the vertical bearing capacity and deformation properties of the pile foundation with the influencing factors of different relative density of calcareous sands and different slenderness ratio for driven piles and embedded piles were studied.The vertical bearing behavior,load transfer characteristics of steel-pipe piles in coral calcareous sands and the developing mechanism of pile-side resistance of the tip pile was also analyzed.3.Based on the particle flow simulation software PFC^2D,a microscopic model of driving piles in coral calcareous sands was established.The friction and tip resistance distribution of the steel pipe piles during the penetration process,the changing displacement field of soil around the piles,and the development of grain crushing in calcareous sands was also analyzed.By comparing the whole process of simulated driving piles with actual indoor test,the penetration mechanism of steel pipe piles in coral calcareous sands was deeply revealed.The analysis results of the indoor test and simulation were as follows:the velocity of driving pile in coral calcareous sand is faster;the greater the hammer energy and the smaller the relative density are beneficial for driving;the number of hammering and resistance of driving were increased with the pile position sinking;the resistance of tip pile mainly restrict the penetration of pile;the grain crushing accelerates the redistribution of the soil around the pile,and the compaction from driving plays a leading role in increasing the effective stress;the slenderness ratio,relative density of calcareous sand and piling method have significant influence on the vertical bearing capacity of pile foundations in coral calcareous sands;subjected to the vertical load,the axial force of steel piles decreases slowly,the frictional resistance of the piles quickly reaches its limit,the resistance of tip pile works in the initial stage of loading,and plays a major role in restrict of pile sinking.