Liquefaction Characteristics Of Coralline Soils

Coralline soil is a wide grading cohesionless soil from silt to gravel or even giant grain group produced by erosion,decomposition,processing and deposition of coral and coral reef plate.It is also the main engineering material of China's island and reef engineering and the Silk Road Economic Belt and the 21st-Century Maritime Silk Road part of foreign projects.According to the analysis of global historical earthquake distribution and geological structure characteristics,such engineering sites usually face high seismic risk.Investigation of typical historical earthquake damage abroad shows that liquefaction is one of the main controlling factors of seismic damage in coralline soil site.At present,the liquefaction test of coralline soil is not mature in technology,and the research on the liquefaction risk of coralline soil is still controversial in China.In order to explore the liquefaction characteristics of coralline soil and to indicate the seismic liquefaction risk of coralline soil sites in China,this paper starts from the research on the liquefaction test technology of coralline soil and explores the liquefaction characteristics of coralline soil based on the laboratory test results.The main work and results are as follows:(1)By measuring and analyzing the B value of the saturation process of coralline soil sample preparation,it is pointed out that the saturation process has the phenomenon of ''unsteady saturation''.According to analysis of the saturation process and test results of the liquefaction test of coralline soil(also known as calcareous sand,coral sand,etc)reported in the literature,it is pointed out that the existence of ''unsteady saturation ''will overestimate the liquefaction resistance of coralline soil(including calcareous sand,coral sand,etc),and even affect the recurrence of liquefaction phenomenon in laboratory.Based on above analysis,this paper first proposes a saturation process optimization method in the test molding process,which limits the influence of ''unsteady saturation ''of coralline soil samples on the test results.Furthermore,a set of liquefaction test methods suitable for different gradations of coralline soil was proposed by integrating the derivation of relative density and compactness of the material containing internal pore particles and the membrane compliance correction technology.This set of test methods provided support for the standardization of the liquefaction test technology of coralline soil,and it was also the technical basis for the subsequent liquefaction test research of wide grading coralline soil containing large particles.(2)Four kinds of coralline soil samples with different gravel contents were prepared by using hydraulic fill coralline soil in an island and reef project in the South China Sea as the test material.The in-situ compaction degree was simulated.The liquefaction resistance of hydraulic fill coralline soil was studied by large-scale dynamic triaxial liquefaction test.Based on the Seed simplified method,the test results were compared with the seismic conditions of historical coralline soil liquefaction sites.The results show that hydraulic fill coralline soil can be liquefied.Under the above historical seismic conditions,the hydraulic fill coralline soil sites in the South China Sea islands and reefs engineering also face the risk of seismic liquefaction,and their liquefaction resistance is not better than that of historical liquefied coralline soil sites.Furthermore,the difference between the influence modes of gravel content on the liquefaction resistance of continental gravel soil and the hydraulic fill coralline soil was compared,and a modified model of gravel content on the liquefaction resistance was proposed.On the one hand,the influence of large particles on the liquefaction resistance of hydraulic fill coralline soil can be quantitatively described.On the other hand,a simplified estimation method for the liquefaction resistance of hydraulic fill coralline soil with different gravel contents was established.(3)The modified penetration numbers of standard penetration test(SPT)for typical coralline soil liquefaction sites in 1993 Guam earthquake are statistically analyzed by using the literature data.It was found that the test results of different sites and in general were in line with the logarithmic distribution,that is,the site were distributed from loose to dense.Thus,it was pointed out that the current evaluation method of liquefaction based on SPT index was not applicable to the coralline soil site.The liquefaction resistance test results of coralline soil with different relative densities and the same gravel content were summarized,and the liquefaction resistances of coralline soil with different compactness and continental cohesionless soil in the literature were compared and analyzed.It was found that the liquefaction resistance of coralline soil was lower than that of continental soil at high relative density,and the liquefaction resistance of coralline soil was higher than that of continental soil at low relative density.This paper points out that with the increase of relative density,the increase of liquefaction resistance of coralline soil is significantly lower than that of continental cohesionless soil.It explains the reason why the current liquefaction identification methods based on SPT index are not suitable for coralline soil sites,and the engineering risk of liquefaction resistance reinforcement of coralline soil foundation by strengthening soil layer is also pointed out.(4)The pore pressure test results of the isotropic consolidation undrained test(liquefaction test)under cyclic loading were statistically analyzed,and it was found that the pore pressure ratio of hydraulic fill coralline soil with different gravel contents can be developed to 1.0,that is,the liquefaction can be reached according to the initial pore pressure standard.At the same time,it is pointed out that the membrane compliance delays the development of pore pressure in the liquefaction test of coralline soil,and even changes its development mode.The pore pressure results of large-scale dynamic triaxial test which need to be corrected by the membrane compliance correction technology can be used as the experimental basis for establishing the pore pressure model of the hydraulic fill coralline soil.Further study shows that the normalized pore pressure ratio of coralline soil filled with large particles develops according to the '' inverse S '' form,and the classical Seed model can better describe this development form in addition to the conditions of high gravel content and high relative density.In this study,a two-parameter correction model based on Seed simplifed model was proposed to describe the pore pressure development of hydraulic fill coralline soil more reasonably,and a method for estimating the model parameters by using the gravel content and relative density of the sample without experimental conditions was given.