Triaxial Experimental Study On Creep And Permeability Response Of Saturated Coral Sand

With the rapid development of island and reef construction in the South China Sea,more and more structures have to be built on coral sand foundation.Coral sand has the characteristics of irregular shape,many edges and corners,rich internal pores,easy to break particles,low strength,high compressibility and high permeability.On the one hand,the coral sand foundation will produce a large amount of settlement under the load;on the other hand,the foundation will inevitably cause changes in the seepage field and the effective stress of soil particles in the process of settlement,which is a complex process of soil-water interaction.Therefore,the creep of coral sand and the change mechanism of permeability in the process of creep need to be deeply studied.In this paper,the related characteristics of coral sand are studied from both meso and macroscopic aspects,including X-ray diffraction analysis(XRD),electron microscope scanning(SEM),nuclear magnetic resonance,particle sieving,triaxial shear drainage permeability test,creep and permeability changes during creep,etc.the physical and chemical characteristics,creep characteristics and percolation characteristics of coral sand are analyzed,and the particles before and after creep are analyzed.An empirical creep model suitable for the creep characteristics of coral sand foundation is established.The main conclusions are as follows:(1)X-ray diffraction test shows that the main mineral composition of coral sand is aragonite and calcite.(2)The microstructure of a single coral sand particle was analyzed by scanning electron microscope(SEM)and nuclear magnetic resonance(NMR).It was found that the outer outline of a single coral sand particle was extremely irregular and rich in internal pores,and the pores in the particles were mainly concentrated in the surrounding area.(3)The seepage rate of coral sand has a good linear relationship with time,and the permeability of coral sand still conforms to Darcy's law;The triaxial drainage shear permeability test shows that under lower confining pressure,before the peak strength,the permeability coefficient of coral sand decreases at first with the increase of axial stress,and the decreasing rate is faster and then slower;after the peak strength,the axial stress decreases,but the permeability coefficient increases;under higher confining pressure,the permeability coefficient decreases gradually,and the decreasing rate is fast at first,then slow,and finally tends to be stable.(4)The tests of triaxial shear creep and percolation change during creep show that the creep of coral sand is attenuation-stable creep,and the volume change of coral sand in the creep stage is related to the volume change of the sample before creep.Under the condition of the same compactness,with the increase of stress,the time to reach stable creep is prolonged,and the permeability decreases continuously.under the same stress,with the increase of compactness,the creep effect increases and the permeability decreases.(5)Drying and sieving tests were carried out before and after the creep test,and the quantitative calculation and analysis of particle crushing was carried out by using the relative breaking potential theory put forward by Hardin.Under the same confining pressure,the relative breaking potential Br increases gradually with the increase of partial stress and increases with the increase of compactness.When the partial stress reaches a certain value,the relative crushing potential increases obviously,indicating that the partial stress under this condition has reached the crushing stress intensity of particles,resulting in an increase in the amount of particle fragmentation.(6)The fitting analysis of the test results is carried out by using the conventional Singh-Mitchell creep model and Mesri creep model,and it is found that the conventional creep model is not suitable to study the creep characteristics of coral sand.Based on two conventional models,the reasons for poor applicability are discussed,and a creep model suitable for coral sand is proposed.