Seismic Liquefaction Characteristics Of Coralline Sand And Its Ground Treatment Methods

Coralline sand(or coral sand)is a kind of bioclastic which is eroded,broken and deposited by coral reef rock,which is different from calcareous sand in academic circles.Dredging coralline rock and soil in the project site as engineering ground or foundation is often easy to obtain materials,which can greatly reduce the construction cost and effectively shorten the construction timescale.In recent years,more and more Chinese enterprises have undertaken more construction of coralline sand blowing and filling projects in maritime countries along the Sea Silk Road of "Belt and Road Initiative".Coralline sand is widely distributed all over the world,and the possibility of suffering from earthquake disaster is high.Coralline sand liquefaction phenomenon has occurred many times in historical earthquakes,and caused serious liquefaction geological hazard and engineering disasters.However,at present,there is a great controversy about the possibility of liquefaction of coralline sand.It is considered that coralline sand will not be liquefied or difficult to liquefy,so the liquefaction characteristics of coralline sand need to be further studied.In addition,the difficulty of liquefaction of coralline sand does not mean that it will not be liquefied,and meanwhile due to the lack of theoretical support,high anti liquefaction ground treatment measures are often adopt in engineering practice.But what kind of anti-liquefaction ground treatment measures are adopted and how to evaluate the effect of anti-liquefaction treatment? At present,there is a lack of liquefaction evaluation criteria for coralline sand and technical standards for formation and treatment of fill foundation.According to the technical means and methods based on land-based sand,it is likely to underestimate the anti-liquefaction ability of coralline sand,resulting in great waste.In this paper,dynamic triaxial test,permeability test and volume deformation test were carried out on coralline sand,and the anti-liquefaction strength,pore pressure growth and dissipation characteristics of coralline sand were analyzed.The feasibility,effectiveness and reliability of anti-liquefaction ground treatment measures such as encryption method and drainage method were explored,and the evaluation method and standard of coralline sand site liquefaction based on in-situ test index were established.The main research work of this dissertation are as follows:1.In view of the common phenomenon of dynamic stress attenuation during the dynamic triaxial test of coralline sand,it is easy to give the results which are obviously higher than the actual anti-liquefaction strength,and the measured vibrations are corrected according to the connotation and principle of equivalent cyclic vibrations.The gradation of sandy soil,effective confining pressures,consolidation ratios,relative density and rubber membrane embedding effect on the anti-liquefaction strength of coralline sand are analyzed.The purpose of this study is to provide experimental basis for the construction of evaluation standard of coralline sand liquefaction.2.Explore the law of pore pressure growth of coralline sand,analyze the characteristics and formation conditions of cyclic activity and its influence on the development of pore pressure,select the calculation model of pore pressure growth and give the model test parameters.3.The permeability and volume deformation tests are carried out,the factors affecting the permeability characteristics of coralline sand are analyzed,the permeability calculation model of coralline sand is established,and the volume compression coefficient of coralline sand is given.Experimental parameters for numerical calculation of pore pressure growth and dissipation of coralline sand are provided.4.Based on the construction,ground treatment and detection of several coralline sand dredged projects at home and abroad,such as Port Sudan,RSGT in Saudi Arabia,East Timor,etc,the strata feature of coral reef soil is explored,and the effective reinforcement depth and effect and ground bearing capacity treated by Dynamic Compaction and Vibro-impact Compaction are analyzed respectively.The empirical relationship or empirical calculation formula of effective reinforcement depth is established,and the evaluation method and standard of coralline sand liquefaction based on in-situ test index are established.The critical curve is calibrated according to the measured standard penetration number of Guam coralline sand seismic liquefaction.5.Based on the classical explanation of liquefaction mechanism and volume compatibility equation,the theoretical basis of anti-liquefaction treatment by drainage method is discussed,and the numerical calculation of pore pressure growth and dissipation under test conditions such as horizontal drainage and vertical gravel pile drainage is carried out respectively.The calculation and determination methods of different soil layer thickness and ground elevation of "binary geological structure" filling ground are given.The anti liquefaction design of gravel pile drainage method for coralline sand ground in East Timor is carried out,and construction parameters such as diameter and spacing of gravel pile are determined,and the effectiveness and influencing factors of drainage treatment measures are analyzed.