Research On Experimental Design Method Of Liquefaction Centrifugal Model For Stone Columns Composite Foundation In Coral Soil

With the implementation and steady advancement of the "The Belt and Road"initiative and strategies such as a strong maritime nation and an energy power nation,a large number of port projects have been built in the countries along the "The Belt and Road" and China’ South Sea.Many of these port projects use coral soil as the foundation filling material.Historical earthquake damage shows that coral soil foundations in different areas often liquefy under strong earthquakes,causing serious damage and loss to port facilities and port buildings,such as the 1993 Guam earthquake,the 2006 Hawaii earthquake and the 2010 Haiti earthquake.At present,most of the research results of domestic and foreign scholars on the liquefaction characteristics of saturated foundations and anti-liquefaction treatment methods are based on terrestrial soils,and there are significant differences between coral soils and terrestrial soils in many mechanical properties.Therefore,the liquefaction characteristics and anti-liquefaction treatment measures of coral soil engineering sites are currently a hot topic that scholars and engineers are eager to answer in depth.Physical model testing is one of the important research methods to visually characterize the seismic response characteristics and failure patterns of foundations and geotechnical structures.In comparison,As the mainstream analysis method for complex problems in geotechnical engineering,the dynamic centrifuge test overcomes the defect that the conventional shaking table test cannot reasonably simulate the static and dynamic stress and strain fields of the prototype soil.However,coral soil has the characteristics of large particle size,wide gradation,abundant internal pores,irregular shape and strong adsorption,and the experience of dynamic centrifugation test of coral soil is lacking.Several cases of test soil particle size≤2.0mm or replaced by fine-grained soil cannot reflect the engineering mechanical properties of the coral soil foundation with large particle sizes and wide gradations.Therefore,systematic and in-depth research on the design method of dynamic centrifuge model tests on coral soil foundation is the primary work and prerequisite to ensure the rationality and science of dynamic centrifuge tests.This project relies on the construction demand of a Southeast Asian port project along the "The Belt and Road" as an opportunity,the project is located on the circum-Pacific seismic belt,and the design basic acceleration of ground motion is 0.53g.The coral soil obtained by site exploration,and explore the design method and characterization elements of test model materials such as coral soil,stone colmns and pile groups,compare the calculation methods for the fundamental period of the site and the natural vibration period of the group pile system,and answer the questions about the liquefaction strength of the coral soil engineering site,the anti-liquefaction performance of the stone colmns,and the safety of long-term service.Through the test results,the feasibility and rationality of the test design method and characterization elements are verified.The main research work and conclusions are as follows:（1）The research progress of dynamic centrifuge test of saturated soil foundation and anti-liquefaction treatment measures are systematically summarized,and the scientific issues of coral soil dynamic centrifugation testing with the goal of characterizing the engineering properties of in-situ site soils are condensed.The design method and similarity criterion of dynamic centrifuge model test of existing liquefiable soil are analyzed,respectively from the material level of particle size effect,permeability coefficient ratio,scale effect,and structural level of scale effect,natural vibration period,put forward the experimental design basic idea of coral soil and stone columns.（2）From the perspective of material properties,based on the particle size effect and drainage anti-liquefaction mechanism,a coral soil model material design method with gradation and permeability coefficient as dual control parameters is proposed.A coral soil model preparation method that it combinate with a 5cm falling distance pluviation and compaction is established,and 1.6 times d_maxis used as the optimal design parameter for the nozzle,which effectively solves the stratification phenomenon and controls the target density when preparing the wide-gradation soil model.The phenomenon of viscosity coefficient degradation of coral soil saturated with HPMC solution is discovered,and a vacuum cycle liquid injection saturation method is proposed,which effectively ensure the stability of the permeability coefficient and increase the saturation rate.Considering the size effect and permeability coefficient ratio requirements,the stone colmns model material design method using d₁₀as the characterization parameter is proposed,and the target permeability coefficient is accurately obtained.（3）From the perspective of structural characteristics,based on the scaling effect and stress concentration effect,the design method and calibration method for the similarity ratio of flexural and compressive stiffness of pile groups with optimal materials and sizes are proposed.The model piles adopt the optimal design of 6061-T6 aluminum tube material with an outer diameter of 26mm and an inner diameter of 22mm;considering the dynamic response of the site-foundation-pile group system and avoiding resonance,a tuning design method for the fundamental period of the site and the natural vibration period of the pile group system is proposed,effectively obtaining the target design period of the test model of the site and pile group system.Integrating the design criteria for small,medium and large deformations of the site and the classification standard of liquefaction earthquake damage index,a design method for the selection and sequence of test vibration loads is proposed.（4）The liquefaction response characteristics of the 0.05mm-5.0mm wide-graded coral soil are revealed,and the earthquake intensity triggering liquefaction is significantly higher than the conventional sandy soil foundation.The physical mechanism and effect ability of stone columns drainage and anti-liquefaction are clarified.The excavation profile images of stone columns,the particle gradation before and after the test and the dissipation rate of excess static pore pressure in the foundation under sequential earthquake loads verify that stone columns have no obvious clogging performance and have long-term service reliability and effectiveness.Meanwhile,the spectrum characteristics and development rules of the site transfer function and the dynamic response of the pile group system are given.