The Pile-soil Interaction In Coral Sand Considering The Particle Breakage

The construction in the South China Sea Islands has been accelerated after the strategic goal"building a maritime power"been proposed.Coral sand is frequently encountered in the construction of the airport,highway,wharf and various military or civil buildings.Compared with silica sand,coral sand is breakable due to the rich corners and inner pores of the particle.The performance of pile foundation in coral reef sand is complex.Therefore,studying particle breakage of coral sand is beneficial to ensure the safety of engineering building and structures in the South China Sea.In previous study,distinguishing the broken particles and the unbroken particles in a particle sample is difficult for obtaining the detailed particle breakage.In this thesis,a method based on dyeing tracking and particle image segmentation was proposed to study the detailed particle breakage in coral sand,and new indexes based on the survival probability of particles in each particle size ranges were proposed to evaluate the true amount of particle breakage.Then,the pile-soil interaction tests including the pile load test and the penetration test were carried out to analyze the influence of particle breakage on bearing capacity and penetration performance of piles.Furthermore,a pile-soil interaction numerical model considering the particle breakage effects was built to analyze the interaction mechanism between pile and coral sand.The main contents and achievements are as follows:(1)The dyeing tracking and particle image segmentation method was proposed to obtain the detailed source and distribution of the broken particles quantitatively.The relationships between the absolute amount of particle breakage in each size range and pressure or impact energy were obtained after the compression and the repeated impact tests.Modes of particle breakage in the process of particle breakage were distinguished from the detailed amount of particle breakage in each particle size ranges.Compared with the original particle breakage indexes B_g and B_r,new indexes B_ag and B_ar based on the newly defined residual grading were proposed.The results show that new particle breakage indexes are a little larger than the original index,and new indexes are more sensitive to the compaction pressure.Besides,the new index B_sbased on the survival probability in the Markov chain was proposed,and is proved to be more reasonable than the original index B_g when evaluating the particle breakage of single sized particle samples.(2)The loading and penetration tests of single piles in coral sand are carried out compared with the tests in silica sand.Results show that the bearing capacity of piles in coral sand is obviously smaller than that of in silica sand.As the increase of loads on the pile top,the average friction resistance of pile side in coral sand tends to be stable after the average friction resistance increases to 13k Pa.The lateral resistance sharing ratio of the pile in coral sand is smaller than that in silica sand.The load sharing ratio of tip resistance of a pile in the coral sand is much larger than that in silica sand when the load on is large.The broken particles are distributed in the smaller particle size range,which means the main model of particle breakage around the pile is the edge loss of particles.The largest amount of particle breakage is in the second soil layer beneath the pile bottom.Besides,the influence depth of particle breakage at the pile bottom is about 2.5times of the pile diameter,which is obviously larger than that in silica sand.(3)A discrete-continuous numerical model of pile-soil interaction considering particle breakage effect was established.Bonded-particle model was used to construct the breakable particle clusters,and the specified graded clusters were generated to simulate the breakable coral sand particles around the pile.The pile is constructed by differential grids in FLAC,which is benefit for obtaining the continuous stress and strain of piles.In the numerical simulation of pile load tests,the gravity stress field is applied to the system,and the load is applied to the pile top in stages to simulate the load process.In the numerical simulation of pile penetration tests,the external load is applied to the top surface of the soil by the servo method,and the pile top is loaded with the rate control method.The load-settlement curves obtained from numerical simulation were verified by the laboratory test.(4)After establishing the numerical model,the numerical analysis of pile penetration test considering particle breakage was carried out.Results show that when the load is low and no particle breakage occurs,the load settlement curves of piles in breakable and the unbreakable granular soils are very close.When the load is large and particle breakage occurs,the settlement of piles in the unbreakable granular soil is obviously larger than that in breakable granular soils.Particle breakage reduced the ratio of lateral friction resistance of pile,which further lead to the increase in pressure under the pile bottom.Increasing in particle-pile friction can significantly reduce the amount of particle breakage;thus decreased the bottom pressure beneath the pile.Under the same load on the pile top,the travel distance of particles around the pile in the breakable granular soil is significantly larger than that in the unbreakable granular soil.Besides,there is a positive correlation between relative particle displacement and particle breakage,and the relationship between particle displacement and the average stress in local areas is relatively weak;furthermore,a positive feedback cycle mechanism is proposed to interpret the"abrupt settlement"of pile in coral sand.(5)The numerical study of pile penetration test considering particle breakage is carried out based on the established numerical model.Results show that before the obvious particle breakage,the soil squeezing effect is obvious as the increase in penetration depth,and the pressure needed for penetration is also increased.When particle breakage is obvious and the volume shrinkage of the soil occurs,and the pressure between soil and the pile decreased and the load needed for penetration decreased.The effective contact numbers around the pile have a negative correlation with numbers of broken bonds around the pile before particle breakage enters the stable stage.The effective contact number around the pile in the unbreakable granular soil is relatively stable in the whole process of pile penetration.The stress affected depth under the bottom of piles is about 4 times the pile diameter in the process of penetration.The compression zone adjacent to the pile bottom has relatively small amount of particle breakage;besides,there is a shear zone outside the compression zone,which has the largest amount of particle breakage.The bottom of the outer boundary of the particle breakage is about 2-2.5 times the pile diameter beneath the pile bottom.Furthermore,the particle breakage around pile tip not only can be divided into particle breakage zones but also has some evolution laws that can be interpreted from a microscopic view.